Patent Publication Number: US-10777330-B2

Title: MgF2 system fluoride sintered body for radiation moderator and method for producing the same

Description:
TECHNICAL FIELD 
     The present invention relates to a MgF 2  system fluoride sintered body for a radiation moderator and a method for producing the same, and more particularly, to a MgF 2  system fluoride sintered body for a radiation moderator having a compact structure suitable for a moderator to restrict the radiation velocity and energy of radioactive rays of every kind such as neutrons and a method for producing the same. 
     BACKGROUND ART 
     Among fluorides, a calcium fluoride (CaF 2 ) single crystal body, a magnesium fluoride (MgF 2 ) single crystal body and the like have been used in the optical field limited to, for example, the vacuum ultraviolet region of wavelengths of 160 nm or less, or the extreme infrared region of wavelengths of 3 μm or more. Fluorides have been used as optical materials such as a window material, a lens and a prism for such specific wavelength regions wherein light does not pass through high-purity quart glasses and optical glasses widely used in the market. Therefore, the amount of demand thereof is naturally small. 
     Among them, a magnesium fluoride (MgF 2 ) noticeably shows a birefringence phenomenon, and therefore, it is not suitable for a lens or a prism. It is rarely used as a window material for transmitted light, and the demand thereof is very small. 
     In order to manufacture a MgF 2  single crystal body for optical use which is substantially only one usage thereof, the Bridgman-Stockbarger method (crucible descending method) being a kind of so-called zone melting, or the Czochralski method (single crystal pulling method) is adopted. 
     In the Bridgman-Stockbarger method, wherein a vertical heating furnace is used, a crucible is charged with a raw material powder. The crucible is allowed to slowly descend from the upper side in the heating furnace and pass through a soaking zone, by which the state of the raw material in the crucible is shifted from ‘heating’ to ‘melting’, and then to ‘solidification (crystallization)’, and then to ‘crystal growth’, gradually from the lower portion thereof toward the upper portion thereof. Meanwhile, vaporized gases generated through sublimation of the raw material or gases existing in voids among raw material particles are released from the top side of the crucible into the furnace so as to reduce residual bubbles in the crystal generated in solidification. 
     In the Czochralski method, wherein a heating furnace and a crucible are used, a raw material in the crucible is melted. A seed crystal is dipped from the upper side of the crucible in the upper surface portion of the melt, and the melt of the dipped portion is slowly pulled up with rotation and solidified (crystallized) so as to pull a single crystal upward. 
     Both of these methods are appropriate to reduce residual bubbles in the case of a raw material such as a fluoride which easily sublimate in the heating process. However, the above-described crystal growth takes a long time, for example, it takes several months to produce a single crystal of about 10 kg. Thus, the productivity thereof is extremely low, resulting in very high producing cost. 
     Generally speaking, there are very few cases wherein a fluoride is used for other than such optical uses. The CaF 2  single crystal body, or a single crystal body of lithium fluoride (LiF) and aluminum fluoride (AlF 3 ) has been rarely used as a shield to neutrons, one of radioactive rays. However, such single crystal bodies have plane orientation dependency of moderation performance originated in crystal orientation and ununiformity due to structural defects such as a subgrain or a subboundary. Moreover, in the case of MgF 2 , it has a large birefringence. Therefore, it is necessary to distinguish each characteristic of these materials for proper use, resulting in difficulties in use. 
     Since the above fluoride single crystal bodies include a defect portion such as a subgrain or a subboundary having a loose crystal structure and a slightly low density, in many cases, the density thereof is slightly lower than the theoretical density (i.e. true density). For example, in the case of MgF 2 , the true density thereof is 3.15 g/cm 3 , while the densities of actual single crystals are 3.130-3.145 g/cm 3 . Many of them have a relative density of 99.4%-99.8%. 
     Hereinafter, radioactive rays in relation to the usage of the present invention are briefly described. The radioactive rays are roughly classified into alpha (α)-rays, beta (β)-rays, gamma (γ)-rays, X-rays, and neutrons. The power passing through a substance (penetrability) gradually increases in this order. 
     The neutrons which have the highest penetrability among them are further classified into the below-described groups, for example, according to the energy level which they have. The energy level each type of neutrons has is shown in parentheses, and the larger the value is, the higher the penetrability is. In the order of the lowest penetrability, they are classified into cold neutrons (up to 0.002 eV), thermal neutrons (up to 0.025 eV), epithermal neutrons (up to 1 eV), slow neutrons (0.03-100 eV), intermediate neutrons (0.1-500 keV) and fast neutrons (500 keV or more). 
     Here, there are various views concerning the classification of neutrons, and the energy values in the parentheses are not precise. For example, there is a view that mentions 40 KeV or less, which is within the above energy region of intermediate neutrons, as the energy level of epithermal neutrons. 
     The typical effective utilization of neutrons is an application to the medical care field. In particular, the radiation therapy in which tumor cells such as malignant cancers are irradiated with neutrons so as to be broken has been coming into general use in recent years. However, in order to obtain medical effects in the present radiation therapy, neutrons of a certain high energy level must be used, so that in the irradiation of neutrons, the influence on a healthy part other than an affected part of a patient cannot be avoided, leading to side effects in some cases. Therefore, in the present situation, the application of the radiation therapy is limited to severe patients. 
     When a normal cell is exposed to high-energy-level neutrons, its DNA is damaged, leading to side effects such as dermatitis, anemia due to radiation and leukopenia. Furthermore, in some cases, a late injury may be caused some time after treatment, and a tumor may be formed and bleed in the rectum or the urinary bladder. 
     In recent years, in order not to cause such side effects and late injuries, methods of pinpoint irradiation on a tumor have been studied. Examples thereof are: “Intensity Modulated Radiation Therapy (IMRT)” in which a tumor portion is three-dimensionally irradiated accurately with a high radiation dose; “Motion Tracking Radiation Therapy” in which radiation is emitted to motions in the body of a patient such as breathing or heartbeat; and “Particle Beam Radiation Therapy” in which a baryon beam or a proton beam each having a high remedial value is intensively emitted. 
     As a characteristic of a neutron, the half-life thereof is short, about 15 min. The neutron decays in a short period of time, releases electrons and neutrinos, and turns into protons. And the neutron has no charge, and therefore, it is easily absorbed when it collides with a nucleus. The absorption of neutrons in such a manner is called neutron capture, and one example of an application of neutrons to the medical care field by use of this feature is the below-described “Boron Neutron Capture Therapy (hereinafter, referred to as BNCT)”, a new cancer therapy which is recently gaining attention. 
     In this BNCT, by causing tumor cells such as malignant cancers to react with a boron drug which is injected into the body by an injection, a reaction product of a boron compound is formed in the tumor portion. The reaction product is then irradiated with neutrons of an energy level which has less influences on a healthy part of the body (desirably comprising mainly epithermal neutrons, and low-energy-level neutrons being lower than epithermal neutrons). And a nuclear reaction with the boron compound is caused only within a very small range, resulting in making only the tumor cells extinct. Originally, cancer cells easily take boron into them in the process of vigorously increasing, and in the BNCT, by use of this feature, only the tumor portion is effectively broken. 
     This method has been attracting attention as an excellent radiation therapy since quite long before because of small influences on a healthy part of a patient, and has been researched and developed in varied countries. However, there are wide-ranging important problems on the development such as development of a neutron generator and a device for a selection of the types of neutrons to be remedially effective, and avoidance of influences on a healthy part other than an affected part of a patient (that is, formation of a boron compound only in a tumor portion). Therefore, the method has not come into wide use as a general therapy. 
     Significant factors in terms of apparatus why it has not come into wide use are necessity to develop a neutron generating device for exclusive use, insufficient downsizing of the whole apparatus including the generating device and insufficient enhancement of its performance. 
     For example, there is a latest system of the BNCT, which a group with Kyoto University as the central figure has been promoting (Non-Patent Document 1 and Non-Patent Document 2). This system comprises an apparatus for medical use only, having a cyclotron accelerator as a neutron generator which is exclusively installed without being attached to an existing nuclear reactor. One report says that the accelerator alone weighs about 60 tons, and its size is large. In the cyclotron system, protons are accelerated by use of a centrifugal force in a circular portion of the cyclotron. Accordingly, in order to efficiently generate neutrons, it is required to make the diameter of the circular portion large so as to obtain a large centrifugal force. That is one of the reasons why the apparatus is large. 
     Furthermore, in order to safely and effectively utilize the generated radiation (mainly neutrons), a radiation shield such as a shielding plate (hereinafter, referred to as a moderator) is required. As moderators, CaF 2  and polyethylene containing LiF, as well as Pb, Fe, Al and polyethylene, are selected. It cannot be said that the moderation performance of these moderators is sufficient, and in order to realize required moderation, the moderator becomes quite thick. Therefore, the moderation system device portion including the moderator is also one of the reasons why the apparatus is large. 
     In order to allow this BNCT to come into wide use in general hospitals, downsizing of the apparatus is necessary. In addition to further downsizing of the accelerator, to improve the remedial values by developing a moderator having high moderation performance and achieve downsizing of the moderation system device by the improvement of moderation performance is an urgent necessity. 
     The moderator which is important for downsizing a BNCT apparatus and improving remedial values is briefly described below. 
     As described above, in order to safely and effectively utilize radiation, it is necessary to arrange a moderator having the right performance in the right place. In order to effectively utilize neutrons having the highest penetrability among radioactive rays, it is necessary to accurately know the moderation performance of every kind of substances to neutrons so as to conduct effective moderation. 
     One example of the selection of particle beam types in order to effectively utilize neutrons for medical care is shown below. By removing high-energy neutrons which adversely influence the body (such as fast neutrons and a high-energy part of intermediate neutrons) as much as possible, and by further reducing extremely-low-energy neutrons having little medical effect (such as thermal neutrons and cold neutrons), the ratio of neutrons having high medical effects (such as a low-energy part of intermediate neutrons and epithermal neutrons) is increased. As a result, a particle beam effectively utilized for medical treatment can be obtained. 
     The low-energy part of intermediate neutrons and epithermal neutrons have a relatively high invasive depth to the internal tissues of a patient. Therefore, for example, in the case of irradiating the head with the low-energy part of intermediate neutrons and epithermal neutrons, as far as the tumor is not present in a considerably deep part, without craniotomy required, it is possible to carry out effective irradiation to an affected part in an unopened state of the head. 
     On the other hand, when the extremely-low-energy neutrons such as thermal neutrons are used in an operation, because of their low invasive depth, craniotomy is required, resulting in a significant burden on the patient. 
     In order to improve remedial values in the BNCT, it is required to irradiate an affected part with a large quantity of neutrons comprising mainly epithermal neutrons and some thermal neutrons. Specifically, an estimated dose of epithermal neutrons and thermal neutrons required in cases where the irradiation time is in the order of one hour, is about 1×10 9  [n/cm/sec]. In order to secure the dose, it is said that as the energy of an outgoing beam from an accelerator being a source of neutrons, about 5 MeV-10 MeV is required when beryllium (Be) is used as a target for the formation of neutrons. 
     The selection of particle beam types through moderators of every kind in a neutron radiation field for BNCT using an accelerator is described below. 
     A beam emitted from the accelerator collides with a target (Be, in this case), and by nuclear reaction, high-energy neutrons (fast neutrons) are mainly generated. As a method for moderating the fast neutrons, using lead (Pb) and iron (Fe) each having a large inelastic scattering cross section, the neutrons are moderated to some extent. In order to further moderate the neutrons moderated to some extent (approximately, up to 1 MeV), optimization of the moderator according to the neutron energy required in the radiation field is conducted. 
     As a moderator, aluminum oxide (Al 2 O 3 ), aluminum fluoride (AlF 3 ), calcium fluoride (CaF 2 ), graphite, heavy water (D 2 O) or the like is generally used. By injecting the neutrons moderated nearly to 1 MeV into these moderators, they are moderated to the epithermal neutron region of the energy suitable for BNCT (4 keV-40 keV). 
     In the case of the above Non-Patent Document 1 and Non-Patent Document 2, as moderators, Pb, Fe, polyethylene, Al, CaF 2  and polyethylene containing LiF are used. The polyethylene and polyethylene containing LiF among them are used as moderators for safety (mainly for shielding) which cover the outside portion of the apparatus in order to prevent leakage of high-energy neutrons out of the radiation field. 
     It can be said that it is appropriate to moderate the high-energy part of neutrons to some extent using Pb and Fe among these moderators (the first half of the stage of moderation), but it cannot be said that the second half of the stage of moderation using Al and CaF 2  after the moderation to some extent is appropriate. That is because the moderators used in the second half of the stage thereof have insufficient shielding performance to fast neutrons, and a high ratio of fast neutrons having a possibility of bad influences on healthy tissues of a patient is left in the moderated neutron type. 
     By reason of CaF 2  having insufficient shielding performance to the high-energy part of neutrons as a moderator used in the second half of the stage thereof, part of them passes without being shielded. The polyethylene containing LiF used with CaF 2  in the second half of the stage thereof covers over the entire surface except an outlet of neutrons on the treatment room side. It is arranged so as to prevent whole-body exposure of a patient to the fast neutrons, without having a function as a moderator on the outlet of neutrons. For information, the polyethylene among the moderators in the first half of the stage thereof covers over the entire surface of the periphery of the apparatus except the treatment room side, like the polyethylene containing LiF in the second half of the stage thereof, and it is arranged so as to prevent the fast neutrons from leaking to the surroundings of the apparatus. 
     Therefore, instead of CaF 2  as a shielding member to fast neutrons in the second half of the stage thereof, the development of a moderator which can shield and moderate high-energy-level neutrons while suppressing the attenuation of intermediate-energy-level neutrons required for treatment has been desired. 
     From various kinds of researches/studies, the present inventors found a MgF 2  sintered body or MgF 2  system substances, for example, a MgF 2 —CaF 2  binary system sintered body as a moderator which makes it possible to obtain neutrons (neutrons of the energy of 4 keV-40 keV) mainly comprising epithermal neutrons in anticipation of the highest remedial value, from the above neutrons moderated to some extent (the energy thereof is approximately up to 1 MeV). As the MgF 2  system substances, a MgF 2 —LiF binary system sintered body, a MgF 2 —CaF 2 —LiF ternary system sintered body other than the MgF 2 —CaF 2  binary system sintered body can be exemplified. 
     For information, as of now, there has been no report that magnesium fluoride (MgF 2 ) was used as a moderator to neutrons, not to mention that there has been no report that a MgF 2  sintered body or a MgF 2 —CaF 2  binary system sintered body was used as such neutron moderator. 
     The present inventors have filed an application of an invention relating to a sintered body of MgF 2  simple (a technical term related to raw material technology, a synonym for “single”) prior to this invention (Japanese Patent Application No. 2013-142704, hereinafter, referred to as Prior Application I). Furthermore, the present inventors have filed an application of an invention relating to a MgF 2 —CaF 2  binary system sintered body as a neutron moderator (Japanese Patent Application No. 2014-193899, hereinafter, referred to as Prior Application II). The Prior Application I and Prior Application II are described later in detail. 
     The present invention was achieved in order to further improve the characteristics of the MgF 2  sintered body and the MgF 2 —CaF 2  binary system sintered body according to the two prior applications. 
     A single crystal body of MgF 2  has high transparency, and high light transmittance within a wide range of wavelengths of 0.2 μm-7 μm, and it has a wide band gap and high laser resistance. Therefore, it has been mainly used as a window material for eximer laser. Or when a MgF 2  single crystal body is deposited on the surface of a lens, it shows effects of protection of the inner parts thereof or prevention of irregular reflection. In either case, it is used for optical use. 
     On the other hand, since the MgF 2  sintered body has low transparency because of its polycrystalline structure, it is never used for optical use. Since the MgF 2  sintered body has high resistance to fluorine gas and inert gas plasma, a few applications concerning an application thereof to a plasma-resistant member in the semiconductor producing process have been filed. However, there is no publication or report that it was actually used in the semiconductor producing process. 
     That is because the Bridgman-Stockbarger method (crucible descending method) or the Czochralski method (single crystal pulling method) is adopted in order to manufacture a MgF 2  single crystal body, as described above, resulting in the MgF 2  single crystal body&#39;s strong image of extremely high price, and because a MgF 2  sintered body produced by a general sintering method has a low density due to the foaming property of the MgF 2  raw material, leading to a tendency to have low mechanical strength, as described in the below-mentioned Patent Document 1. 
     As for a MgF 2  sintered body, according to the Japanese Patent Application Laid-Open Publication No. 2000-302553 (the below-mentioned Patent Document 1), the greatest defect of ceramic sintered bodies of fluoride such as MgF 2 , CaF 2 , YF 3  and LiF is low mechanical strength. And in order to solve this problem, the invention was achieved, wherein sintered bodies compounded by mixing these fluorides with alumina (Al 2 O 3 ) at a predetermined ratio can keep excellent corrosion resistance of the fluorides as well as obtain high mechanical strength. 
     However, as for the corrosion resistance and mechanical strength of the sintered bodies produced by this method, in any combination, the sintered bodies are simply allowed to have just an intermediate characteristic between the characteristic of any of the fluorides and that of alumina. No sintered body having a characteristic exceeding one&#39;s characteristic superior to the other&#39;s has been obtained by compounding. In addition, their use is limited to high corrosion resistance uses, different from the uses of the present invention. 
     Another sintered body mainly comprising MgF 2  is described in Japanese Patent Application Laid-Open Publication No. 2000-86344 (the below-mentioned Patent Document 2), but its use is also limited to a plasma-resistant member. In the Patent Document 2, a sintered body comprises a fluoride of at least one kind of alkaline earth metals selected from the group of Mg, Ca, Sr and Ba, in which the total amount of metallic elements other than the alkaline earth metals is 100 ppm or less on a metal basis, the mean particle diameter of crystal grains of the fluoride is 30 μm or less, and the relative density is 95% or more. 
     However, the materials in the list (Table 1) of Examples of the Patent Document 2 were obtained by firing a fluoride of each single kind of the above four alkaline earth metals (i.e. MgF 2 , CaF 2 , SrF 2  and BaF 2 ), and no fired mixture of those fluorides is described. 
     Still another example of an application of a sintered body mainly comprising MgF 2  to a plasma-resistant member is the Japanese Patent Application Laid-Open Publication No. 2012-206913 (the below-mentioned Patent Document 3). The Patent Document 3 discloses an invention wherein, since a sintered body of MgF 2  simple has a defect of low mechanical strength, by mixing at least one kind of non-alkaline metallic dispersed particles having a lower mean linear thermal expansion coefficient than MgF 2  such as Al 2 O 3 , AlN, SiC or MgO, the defect of low mechanical strength thereof can be compensated for. When a sintered body of such mixture is used as the above moderator to neutrons, the moderation performance thereof is greatly different from that of MgF 2  simple because of the influence of the non-alkaline metal mixed into MgF 2 . Therefore, it is predicted that it is difficult to apply a sintered body of this kind of mixture to a use as a moderator. 
     In addition, an example of an application of a sintered body mainly comprising CaF 2  to a plasma-resistant member is the Japanese Patent Application Laid-Open Publication No. 2004-83362 (the below-mentioned Patent Document 4). The Patent Document 4 describes a method wherein using hydrofluoric acid, impurities other than Mg are removed from a low-purity raw material containing Mg, so as to precipitate high-purity CaF 2 , and a fluoride sintered body whose starting raw material is the high-purity CaF 2  containing Mg of 50 ppm or more and 5% by weight or less is produced. A problem here is a state of Mg contained in the starting raw material, though the state is not described at all. And there is no description concerning the technique by which the degree of purity of the low-purity raw material is raised using hydrofluoric acid. 
     Then, when presuming the process of raising the degree of purity of a low-purity raw material as a person skilled in the art, generally speaking, in the case of raising the degree of purity of a low-purity raw material using hydrofluoric acid, a method is often adopted, wherein impurities in the raw material are first dissolved into a hydrofluoric acid solution as many as possible, and if a component (Ca, here) desired to be a main raw material dissolved with impurities in this dissolution process, the component is precipitated and separated by use of the difference in solubility among the dissolved components. When further reviewing the invention, it is presumed that Mg exhibited different dissolution behavior from other impurities. In the specification, it is referred to as only “Mg”, and according to the descriptions of Examples in Table 1, as for high concentrations of impurity components other than Mg (such as Fe, Al, Na and Y), all of their concentrations were decreased by purity raising treatment, but only the concentration of Mg did not change, being 2000 ppm before the treatment and being also 2000 ppm after the treatment, leading to the above presumption. Hence, there is a high possibility that Mg might be in a state which is hard to dissolve in hydrofluoric acid, that is, a metal state. If CaF 2  containing metal Mg is a starting raw material, the sintering process thereof is very different from the case like the present invention wherein a mixture of CaF 2  and MgF 2  is a starting raw material, and the characteristics of the sintered bodies are also very different from each other. 
     On the other hand, inventions relating to a neutron moderator were disclosed lately. One of them is the Japanese Patent No. 5112105 (Patent Document 5). The Patent Document 5 discloses ‘a moderator which moderates neutrons, comprising a first moderating layer obtained by melting a raw material containing calcium fluoride (CaF 2 ), and a second moderating layer comprising metal aluminum (Al) or aluminum fluoride (AlF 3 ), the first moderating layer and the second moderating layer being adjacent to each other’. 
     In the Patent Document 5, the first moderating layer obtained by melting the raw material containing CaF 2  is disclosed, but raw material conditions such as the purity, components, particle size and treatment method thereof, and melting conditions such as heating temperatures, holding times thereof and the type of heating furnace are not mentioned at all, very insincerely described as a patent specification. In the Patent Document 5, there is no description suggesting that something related to MgF 2  should be used as a neutron moderator. 
     Another invention is disclosed in the document by KONONOV, O. E. et al. (Non-Patent Document 4). This Non-Patent Document 4 says that MgF 2  has good moderation performance as a neutron moderator. However, there is no description about the MgF 2  used therein except for ‘its density of 3.14 g/cm 3 ’. Even how to produce it, conditions for producing it, and whether the mineral texture thereof is a single crystal or polycrystalline or amorphous (vitreous), or a mixture of them, are not mentioned at all. Furthermore, whether it was produced by the writers thereof or by outsiders is not mentioned. 
     In cases where a polycrystalline article or an amorphous article which is not on the market is used, generally the descriptions such as its origin and the characteristics/physical properties such as its quality other than the density should be included. Therefore, in this case, it is presumed that a ‘single crystal’ article being on the market was used. 
     In case where the writers made a polycrystalline article of MgF 2  on an experimental basis by a sintering method, as described below, it should be concluded that an article having an extremely high density (3.14 g/cm 3 : relative density of 99.7%) could be made although MgF 2  has a high foaming property, and that an article of extremely excellent quality that had never been seen in the past could be made. In such case, naturally the description of such excellent quality should be included in the document. Hence, it can be concluded that as the MgF 2  article, a ‘single crystal’ article available in the market or that one can produce was used. 
     In the preceding documents, as described above, there is no description suggesting the use of a sintered body of MgF 2  as a moderator to neutrons, one kind of radioactive rays. In such situation, the present inventors found that it was possible to use a MgF 2  sintered body with a modification made thereto as a moderator to neutrons, one kind of radioactive rays, and achieved the invention of the Prior Application I. 
     In the invention of the Prior Application I, a high-purity MgF 2  raw material was pulverized and two-stage compressing and molding was conducted thereon. That is, after molding by a uniaxial press molding method, this press molded body was further molded by a cold isostatic pressing (CIP) method so as to form a CIP molded body. Then, by firing the same in three steps with different heating conditions using an atmospheric furnace in which the atmosphere can be adjusted, a sintered body having a compact structure was produced while suppressing foaming of MgF 2  as much as possible. However, since MgF 2  very easily foams, it was difficult to actually suppress foaming thereof. As a result, the range of the relative densities (i.e. 100×[bulk density of a sintered body]/[true density](%)) of the sintered bodies produced by this method was 92%-96%, and the mean value thereof was of the order of 94%-95%. 
     A characteristic desired for a sintered body for a neutron moderator is ‘the mean value of relative densities of 95% or more at least, desirably 96% or more should be stably secured’. 
     As described above, the fundamental performance required for a moderator used in the BNCT method is ‘to prevent high-energy neutrons such as fast neutrons from leaking, and to sufficiently secure epithermal neutrons necessary for therapy’, and a sintered body satisfying the above characteristic may be a sintered body having the above fundamental performance. 
     It could not be said that the relative density of the sintered body according to the Prior Application I should lead to the performance thereof sufficient as a moderator used in the BNCT method. Therefore, the present inventors worked toward further development and found out that when using a MgF 2 —CaF 2  binary system sintered body for radiation use, the relative density thereof could be easily improved, compared with a sintered body of MgF 2  simple, resulting in an anticipation to improve the performance as a moderator, which led to the invention of the Prior Application II. 
     In the Prior Application II, after mixing/pulverizing a high-purity MgF 2  raw material and a CaF 2  raw material, two-stage compressing and molding was conducted thereon. That is, after molding the same by a uniaxial press molding method, this press molded body was further molded by a cold isostatic pressing (CIP) method so as to form a CIP molded body. Then, it was fired in three steps with different heating conditions using an atmospheric furnace in which the atmosphere can be adjusted so as to produce a sintered body having a compact structure while suppressing foaming of MgF 2  as much as possible. 
     However, since a MgF 2 —CaF 2  binary system molded body also easily foams, it was difficult to actually suppress foaming thereof. Consequently, the range of the relative densities of the sintered bodies produced by this method was 94%-97%, and the mean value thereof was of the order of 95%-96%. 
     It was a level in which the request of ‘the mean value of relative densities of 95% or more at least, as a moderator’ was just cleared, and it was difficult to satisfy the request ‘desirably the mean value of relative densities of 96% or more should be stably secured’. 
     This MgF 2 —CaF 2  binary system sintered body had a relative density about 1% higher than the MgF 2  sintered body in the Prior Application I. However, it was admitted that open pores remained in the periphery portion of the sintered body, judging from a phenomenon in which pure water penetrated the sintered body when it was soaked in pure water for measuring the bulk density thereof. Thus, even in the invention of the Prior Application II achieved in order to improve the invention of the Prior Application I, it was predicted that open pores still remained in the sintered body, and it was proved that there should be yet room for improvement. 
     As described above, since MgF 2  very easily foam, it is not easy to actually suppress foaming thereof. Consequently, it is desired that even the sintered bodies produced by the methods according to the Prior Application I and Prior Application II should have further improved moderation performance to radiation, especially to neutrons, and a higher relative density (i.e. 100×[bulk density of the sintered body]/[true density](%)). 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2000-302553 
         Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2000-86344 
         Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2012-206913 
         Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2004-83362 
         Patent Document 5: Japanese Patent No. 5112105 
       
    
     Non-Patent Document 
     
         
         Non-Patent Document 1: H. Tanaka et al., Applied Radiation and Isotopes 69 (2011) 1642-1645 
         Non-Patent Document 2: H. Tanaka et al., Applied Radiation and Isotopes 69 (2011) 1646-1648 
         Non-Patent Document 3: Hiroaki Kumada, Tetsuya Yamamoto, Dose Evaluation of Neutron Capture Therapy in JRR-4, Health Physics, 42(1), (2007) 23-37 
         Non-Patent Document 4: KONONOV, O. E. et al., “ACCELERATOR-BASED SOURCE OF EPITHERMAL NEUTRONS FOR NEUTRON CAPTURE THERAPY”, Atomic Energy, November, 2004, Vol. 97, No. 3, pp. 626-631 
       
    
     SUMMARY OF THE INVENTION 
     Solution to Problem and Advantageous Effect of Invention 
     The present invention was developed in order to solve the above problems, and it is an object of the present invention to provide a MgF 2  system fluoride sintered body for a radiation moderator, having excellent characteristics as a moderator used for moderating the energy of neutrons, a kind of radiation, in the good use of the neutrons for therapy, which makes it possible to enhance remedial values and downsize an apparatus for therapy, and which is inexpensive unlike a single crystal body, and a method for producing the same. 
     In other words, it is an object of the present invention to provide a MgF 2  system fluoride sintered body for a radiation moderator having a very compact structure, without plane orientation dependency of the moderation performance originated in crystal orientation which a single crystal body has, and without ununiformity based on the structural defects such as subgrains, and a producing method by which such sintered bodies can be stably produced. 
     The MgF 2  system fluoride sintered body according to the present invention is mentioned mainly as a moderator to neutrons, but the sintered body has excellent performance as a member for shielding to not only neutrons but also other radioactive rays such as X-rays or gamma-rays. 
     In the Prior Application I and Prior Application II, the present inventors gave basic consideration to the selection of moderators (material substances) suitable for shielding (i.e. moderation) to neutrons, and found a MgF 2  system fluoride sintered body, specifically a MgF 2  sintered body and a MgF 2 —CaF 2  binary system sintered body. 
     As the basic characteristics required for a moderator other than moderation performance, a characteristic of keeping the shape of the product is exemplified. It is important to be excellent in mechanical strength with which damage in mechanical processing and in handling during manufacturing the product can be prevented. 
     The mechanical strength of a sintered body is determined by micro strength of bonding parts between particles, the compactness of the sintered body, and moreover, the brittleness originated from a crystal structure (such as polycrystal or single crystal or amorphous) of the parent thereof. 
     The compactness of the sintered body is determined by the defoaming state such as the sizes, shapes, distribution and number of bubbles, in other words, the shape such as the width and length of the bonding parts and a bound body (parent) of ex-particles. 
     Basic technical ideas of the present invention are: 
     (1) relaxing the sintering conditions by preparing a starting raw material to satisfy a proper particle size condition so as to make it possible to conduct low-temperature sintering, that is, increasing the packing density by making the particle sizes of the starting raw material smaller than those in the Prior Application I and Prior Application II and setting a smaller range of particle sizes, leading to promoting the sintering reaction, particularly flocculation of particles through solid phase reaction and bonding thereof (low-temperature sintering) (mainly a phenomenon in the preliminary sintering step and primary sintering step); 
     (2) allowing the sintered body after the completion of this primary sintering to be a high-density sintered body through the formation of a solid solution, having strong cohesion between particles; 
     (3) when a fluoride system raw material is heated at a high temperature, part of the raw material vaporizes (mainly part of the fluoride is thermally decomposed (sublimates) and generates fluorine gas), leading to the formation of bubbles (foaming) By sintering at a temperature low enough to avoid this foaming and making the heating process (sintering heat pattern) proper, a compact sintered body is obtained; and 
     (4) although an atmospheric sintering method is adopted both in the primary and secondary sintering steps in the Prior Application I and Prior Application II, a pressure sintering method such as a uniaxial hot press method or a hot isostatic pressing (HIP) method is adopted both in the primary and secondary sintering steps as needed in the present invention. Furthermore, another option of conducting the tertiary sintering step is added thereto, wherein a pressure sintering method such as the uniaxial hot press method or the hot isostatic pressing (HIP) method is adopted so as to further reduce bubbles, voids and the like in the sintered body, resulting in a more compact sintered body. 
     By combining the above technical ideas (1)-(4), the present invention aimed to stably produce MgF 2  system fluoride sintered bodies for a radiation moderator excellent in moderation performance required as a member for a moderator to radiation, especially to neutrons and mechanical strength (shape keeping) characteristic important as a fundamental characteristic other than the moderation performance. 
     Concerning the technical idea (1), by making the particle size condition of the starting raw material proper, low-temperature sintering was promoted. 
     Using a high-purity MgF 2  raw material powder and a CaF 2  raw material powder, the mean diameter of which is about 140 μm in median diameter (hereinafter, simply referred to as median diameter), the MgF 2  raw material powder was pulverized by the below-mentioned pulverization method in the case of MgF 2  simple. On the other hand, in the case of MgF 2 —CaF 2  binary system, a prescribed quantity of MgF 2  raw material powder and that of CaF 2  raw material powder each were scaled, mixed using a V mixer and pulverized by the below-described pulverization method. Thereafter, the same was provided to the subsequent processing step, and the characteristic evaluation of the completed sintered body was conducted so as to find proper particle size conditions of the starting raw material. 
     As a result, 
     (i) the particle size distribution range should be small, specifically, the maximum particle diameter should be 50 μm or less, desirably 30 μm or less; 
     (ii) as a preferable state of the particle size distribution, the shape of the particle size distribution curve drawn with particle diameter (μm) as the abscissa and particle diameter ratio (ratio of every particle diameter: wt. %) as the ordinate, should be not ‘2-peak type’ or ‘3-peak type’, but ‘sub-1-peak type’ or ‘1-peak type’, preferably 1-peak type which closely resembles that of normal distribution; 
     (iii) the median diameter should be 6 μm or less, desirably 3 μm or less; and 
     by simultaneously satisfying these conditions of the items (i), (ii) and (iii), the sintered body could be allowed to have a high density, leading to a noticeable improvement of moderation performance as a neutron moderator as shown below. 
     The foaming phenomenon in the above technical idea (3) is described below a little in detail. Using a differential thermal analyzer, alterations in weight and in endothermic and exothermic amount of the sample of the starting raw material were examined while heating. As a result, a minute quantity of weight decrease was found at approximately 800° C.-850° C., though there were slight differences depending on the mix proportion of the starting raw material. It appears that fluorine attached to a parent of a preliminary sintered body or fluorine resolving in the parent, for example, with a weak bonding property, dissociated and decomposed first of all. After further heating, a point of inflection of the weight decrease curve appeared at approximately 850° C.-900° C., and the weight decrease became noticeable. 
     The results of this differential thermal analysis, and the examination results of the sintering conditions and the structure of the sintered body in the below-mentioned preliminary sintering test, specifically, the examination results such as: 
     1. the situation of bubble generation in the sintered body; 
     2. the situation of organizational structure of the sintered portion; and 
     3. the bulk density of the sintered body, 
     were comprehensively considered. It was anticipated that when heated at a temperature of the point of inflection of the weight decrease curve or higher, part of bonded fluorine element in MgF 2  or CaF 2  would start to decompose, and cause the generation of fluorine gas, leading to the formation of fine bubbles. 
     Then, the temperature of the point of inflection of the weight decrease curve, 850° C.-900° C. is referred to as the starting temperature of foaming (Tn). The temperature at which vaporization started was slightly different depending on the composition. In the case of a composition mainly comprising MgF 2  (MgF 2  of 70-99.8% by weight, and CaF 2  of the rest), vaporization started at about 800° C. and it became quite brisk at about 850° C. (the point of inflection, that is, the starting temperature of foaming Tn was decided to be 850° C.). In the case of a composition mainly comprising CaF 2  (MgF 2  of 10-40% by weight, and CaF 2  of the rest), vaporization started at about 850° C. and it became quite brisk at about 900° C. (similarly, Tn was decided to be 900° C.). In the case of MgF 2  of 40-70% by weight and CaF 2  of the rest, vaporization started at around the intermediate temperature between the above two cases, that is, in the temperature limits of about 825° C. or more, and it became quite brisk at about 875° C. (similarly, Tn was decided to be 875° C.). 
     That is, in the case of the composition mainly comprising MgF 2  (MgF 2  of 70-99.8% by weight, and CaF 2  of the rest), sublimation starts at about 800° C., and it becomes brisk and foaming starts at about 850° C. In the case of the composition mainly comprising CaF 2  (MgF 2  of 10-40% by weight, and CaF 2  of the rest), sublimation starts at about 850° C. and it becomes brisk and foaming starts at about 900° C. In the case of MgF 2  of 40-70% by weight, and CaF 2  of the rest, the mix proportion intermediate therebetween, sublimation starts at about 825° C., and it becomes brisk and foaming starts at about 875° C. 
     Thus, when a fluoride sublimates (a phenomenon in which a solid phase changes into a gas phase without passing through a liquid phase. In this case, a synonym for “vaporize”), fluorine gas is generated, resulting in generation of fine bubbles in the sintered body. 
     When observing the broken-cross section of the sintered body with an electron microscope (SEM), the sizes of the bubbles range from small ones of several μm to large ones of 20 μm-40 μm in diameter seen on the broken-cross section. 
     The shapes of the small ones of several μm are approximately circles and the shapes of the large ones are rarely circles. Most of them are irregular such as long and narrow, or angular. 
     Judging from these shapes, it is considered that the small ones are bubbles which have just been generated through sublimation of the above fluoride, and that the large ones are aggregates of some of the generated bubbles or residuals originated from voids among particles or the like which could not defoam in the sintering process. 
     The reason why the value of relative density is shown by range corresponding to one value of bulk density is because in the case of a binary system sintered body of MgF 2  and CaF 2 , the true densities of the both are different (that of MgF 2  is 3.15 g/cm 3 , while that of CaF 2  is 3.18 g/cm 3 ), and therefore, depending on the mix proportion thereof, the true density of the mixture varies slightly. Here, the value of true density of the mixture is decided as shown below, so as to calculate the relative density thereof. 
     It is decided that: 
     (a) the true density is 3.15 g/cm 3 , in the case of a composition mainly comprising MgF 2 , that is, MgF 2  of 70% by weight or more and 99.8% by weight or less (referred to as 70-99.8% by weight in the present application) and CaF 2  of the rest; 
     (b) the true density is 3.16 g/cm 3 , in the case of MgF 2  of 40% by weight or more and less than 70% by weight (referred to as 40-70% by weight) and CaF 2  of the rest; and 
     (c) the true density is 3.17 g/cm 3 , in the case of MgF 2  of 10% by weight or more and less than 40% by weight (referred to as 10-40% by weight) and CaF 2  of the rest. 
     Concerning the technical idea (4), in the primary and secondary sintering steps, an atmospheric firing method using an atmospheric firing furnace in which the atmosphere can be adjusted was adopted as a basic technique, similarly to the Prior Application I and Prior Application II. 
     In the primary and secondary sintering steps, in place of this atmospheric firing method, a hot press method using a uniaxial pressing hot press furnace and a hot isostatic pressing (HIP) method using a hot isostatic pressing furnace were adopted and conducted. 
     On the sintered body fired by an atmospheric firing method or a pressure firing method, the tertiary sintering step with the hot press method or the HIP method applied thereto was further conducted so as to improve the density of the sintered body. 
     In the hot press method, a sintered body is pressed from one axial direction in the sintering process when using a uniaxial hot press furnace. With such pressing, compacting of the sintered body can be further promoted, compared with the atmospheric firing method. 
     In the HIP method, a sintered body can be pressed from three axial directions in the sintering process. With such pressing, compacting of the sintered body can be further promoted, compared with the atmospheric firing method. 
     In order to achieve the above object, a MgF 2  system fluoride sintered body for a radiation moderator according to a first aspect of the present invention is characterized by comprising MgF 2  having a compact polycrystalline structure with a bulk density of 3.07 g/cm 3  (a relative density of 97.5%) or more. 
     Using the MgF 2  system fluoride sintered body for a radiation moderator according to the first aspect of the present invention, a sintered body having excellent moderation performance as a neutron moderator and enhanced mechanical strength, which satisfies all the characteristics required for a neutron moderator, can be obtained. 
     The MgF 2  system fluoride sintered body for a radiation moderator according to a second aspect of the present invention is characterized by having a bending strength of 12 MPa or more and a Vickers hardness of 100 or more as regards mechanical strengths in the MgF 2  system fluoride sintered body for a radiation moderator according to the first aspect of the present invention. 
     Using the MgF 2  system fluoride sintered body for a radiation moderator according to the second aspect of the present invention, a sintered body having more excellent moderation performance as a neutron moderator and extremely excellent mechanical strength, can be provided. 
     A method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a first aspect of the present invention is characterized by comprising the steps of: 
     pulverizing a high-purity MgF 2  raw material to control the particle size, so as to allow the maximum particle diameter in a particle size distribution to be 50 μm or less, the shape of the particle size distribution curve to be of sub-1-peak type or 1-peak type and the median diameter to be 6 μm or less; 
     adding 0.02-1% by weight of a sintering aid to the particle-size-controlled raw material to mix; 
     molding the compound at a molding pressure of 5 MPa or more using a press molding device; 
     molding the press molded article at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device; 
     conducting preliminary sintering by heating the CIP molded article in a temperature range of 600° C.-700° C. in an air atmosphere (preliminary sintering step); 
     conducting atmospheric sintering or pressure sintering by heating the preliminary sintered body in a temperature range from (Tn−100)° C. to (Tn)° C. when the starting temperature of foaming of the preliminary sintered body is (Tn)° C., in an air atmosphere or in an inert gas atmosphere or in a vacuum atmosphere (primary sintering step); and 
     forming a sintered body having a compact structure by heating the same in a temperature range of 900° C.-1150° C. under atmospheric pressure or under pressure in the same atmosphere as the preceding step (secondary sintering step). 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the first aspect of the present invention, by pulverizing a raw material and controlling the particle size thereof, it becomes possible to allow the sintering reaction with low-temperature heating to easily make progress and to make the degree of progress of sintering in every part of the sintered body more uniform so as to make it hard to cause a difference in density in the whole sintered body. As a result, a high-density sintered body can be obtained. 
     Moreover, the difference between parts of the organizational structure of the sintered body is small, and the quantity of generated melt is held down. The crystal growth of a solid solution is suppressed, leading to decreased frequency of occurrence of brittle portions. As a result, the strength of the sintered body can be enhanced. 
     Therefore, the sintered body fired by this method has strong cohesion between particles, and high micro strength of the bonding part. The mechanical strength which was a problem to be solved is remarkably improved, and the sintered body can be used as a member for a neutron moderator without problems for actual use. 
     The crystalline structure of the sintered body fired by this method is polycrystalline, resulting in remarkable improvement of the brittleness compared with a single crystal. 
     And the highest attained relative density thereof within a range of good sintering conditions can be increased, and by making wider the temperature limits of the secondary sintering temperature thereof, in which the bulk density thereof becomes high, the stable sintering conditions can be easily realized. 
     In the case of pressure sintering, compared with atmospheric sintering, the sintered body has stronger cohesion between particles and higher micro strength of the bonding part, resulting in remarkable improvement of the mechanical strength. 
     Particularly with the improvement of the particle size condition and the pressure application in the heating process, the degree of compactness of the sintered body can be noticeably improved. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a second aspect of the present invention is characterized by further comprising the tertiary sintering step of reheating in a temperature range of 900° C.-1150° C. under pressure in an inert gas atmosphere or in a vacuum atmosphere in the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the first aspect of the present invention. 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the second aspect of the present invention, it becomes easy to defoam in the sintering process and it becomes far easier to enhance the relative density of the sintered body. The sintered body can be allowed to have more excellent moderation performance as a neutron moderator, a density remarkably improved compared with the sintered body on which only primary and secondary sintering were conducted, and mechanical strength remarkably enhanced. 
     A MgF 2  system fluoride sintered body for a radiation moderator according to a third aspect of the present invention is characterized by being a MgF 2 —CaF 2  binary system fluoride sintered body having a compact polycrystalline structure, containing CaF 2  of 90% by weight at the maximum with a relative density of 95.2% or more. 
     Using the MgF 2  system fluoride sintered body for a radiation moderator according to the third aspect of the present invention, the difference between parts of the organizational structure of the sintered body is small, and the quantity of generated melt is held down. The crystal growth of a solid solution is suppressed, leading to decreased frequency of occurrence of brittle portions. As a result, the strength of the sintered body can be enhanced. Therefore, a sintered body having excellent moderation performance as a neutron moderator and enhanced mechanical strength, which satisfies all the characteristics required for a neutron moderator, can be obtained. 
     The MgF 2  system fluoride sintered body for a radiation moderator according to a fourth aspect of the present invention is characterized by having a bending strength of 13 MPa or more and a Vickers hardness of 100 or more as regards mechanical strengths in the MgF 2  system fluoride sintered body for a radiation moderator according to the third aspect of the present invention. 
     Using the MgF 2  system fluoride sintered body for a radiation moderator according to the fourth aspect of the present invention, a sintered body having more excellent moderation performance as a neutron moderator and extremely excellent mechanical strength can be provided. 
     A method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a third aspect of the present invention is characterized by comprising the steps of: 
     mixing a high-purity MgF 2  powder with a high-purity CaF 2  powder in such a manner that the content of CaF 2  is 90% by weight at the maximum, and then pulverizing the same to control the particle size, so as to allow the maximum particle diameter in a particle size distribution to be 50 μm or less, the shape of the particle size distribution curve to be of sub-1-peak type or 1-peak type and the median diameter to be 6 μm or less; 
     adding 0.02-1% by weight of a sintering aid to the particle-size-controlled raw material to mix; 
     molding the compound at a molding pressure of 5 MPa or more using a press molding device; 
     molding the press molded article at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device; 
     conducting preliminary sintering by heating the CIP molded article in a temperature range of 600° C.-700° C. in an air atmosphere (preliminary sintering step); 
     conducting atmospheric sintering or pressure sintering by heating the preliminary sintered body in a temperature range from (Tn−100)° C. to (Tn)° C. when the starting temperature of foaming of the preliminary sintered body is (Tn)° C., in an air atmosphere or in an inert gas atmosphere or in a vacuum atmosphere (primary sintering step); and 
     forming a sintered body having a compact structure by heating the same in a temperature range of 900° C.-1150° C. under atmospheric pressure or under pressure in the same atmosphere as the preceding step (secondary sintering step). 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the third aspect of the present invention, the sintered body fired by this method has strong cohesion between particles, and high micro strength of the bonding part. The mechanical strength which was a problem to be solved is remarkably improved, and the sintered body can be used as a member for a neutron moderator without problems for actual use. 
     The crystalline structure of the sintered body fired by this method is polycrystalline, resulting in remarkable improvement of the brittleness compared with a single crystal. 
     And the highest attained relative density thereof within a range of good sintering conditions can be increased, and by making wider the temperature limits of the secondary sintering temperature thereof, in which the bulk density thereof becomes high, the stable sintering conditions can be easily realized. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a fourth aspect of the present invention is characterized by further comprising the tertiary sintering step of reheating in a temperature range of 900° C.-1150° C. under pressure in an inert gas atmosphere or in a vacuum atmosphere in the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the third aspect of the present invention. 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the fourth aspect of the present invention, it becomes easy to defoam in the sintering process and it becomes far easier to enhance the relative density of the sintered body. A sintered body having more excellent moderation performance as a neutron moderator and extremely excellent mechanical strength can be provided. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a fifth aspect of the present invention is characterized by the inert gas atmosphere in the two sintering steps (primary and secondary sintering steps) comprising one kind of gas or a mixture of plural kinds of gases, selected from among nitrogen, helium and argon, wherein using a hot press furnace or a hot isostatic pressing furnace, hot molding work is conducted in the heating process in the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the first or third aspect of the present invention. 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the fifth aspect of the present invention, it is possible to make it far easier to defoam in the sintering process, and to make it far easier to increase the relative density of the sintered body. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a sixth aspect of the present invention is characterized by the inert gas atmosphere in the three sintering steps (primary, secondary and tertiary sintering steps) comprising one kind of gas or a mixture of plural kinds of gases, selected from among nitrogen, helium and argon, wherein using a hot press furnace or a hot isostatic pressting furnace, hot molding work is conducted in the heating process in the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the second or fourth aspect of the present invention. 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the sixth aspect of the present invention, it is possible to make it far easier to defoam in the sintering process, and to make it far easier to increase the relative density of the sintered body. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to a seventh aspect of the present invention is characterized by in the two sintering steps (primary and secondary sintering steps), in a vacuum atmosphere of less than 100 Pa, conducting hot molding work using a hot press furnace in the heating process in the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the first or third aspect of the present invention. 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the seventh aspect of the present invention, it is possible to make it far easier to defoam in the sintering process, and to make it far easier to increase the relative density of the sintered body. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to an eighth aspect of the present invention is characterized by in the three sintering steps (primary, secondary and tertiary sintering steps), in a vacuum atmosphere of less than 100 Pa, conducting hot molding work using a hot press furnace in the heating process in the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the second or fourth aspect of the present invention. 
     Using the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the eighth aspect of the present invention, it is possible to make it far easier to defoam in the sintering process, and to make it far easier to increase the relative density of the sintered body. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing an example of a particle size distribution when a raw material of MgF 2  simple was pulverized; 
         FIG. 2  is a block flow diagram showing the steps of producing a MgF 2  system fluoride sintered body; 
         FIG. 3  is a phase diagram of the MgF 2 —CaF 2  binary system; 
         FIG. 4  is a diagram showing the relationship between the secondary sintering temperatures and the relative densities of sintered bodies of MgF 2  simple with varied particle size conditions of the compound; 
         FIG. 5  is a diagram showing the relationship between the secondary sintering temperatures and the relative densities of MgF 2 —CaF 2  binary system sintered bodies with varied particle size conditions of the compound; 
         FIG. 6  is a diagram showing the relationship between the relative densities of the sintered bodies produced by conducting primary and secondary sintering on a MgF 2  simple raw material using three kinds of firing furnaces and the secondary sintering temperatures; 
         FIG. 7  is a diagram showing the relationship between the relative densities of the sintered bodies produced by conducting primary and secondary sintering on a MgF 2 —CaF 2  compound using three kinds of firing furnaces and the secondary sintering temperatures; 
         FIG. 8  comprises SEM photographs showing an original raw material and the raw material after pulverized for three weeks; 
         FIG. 9  is Table 1 showing the measurement results of the neutron moderation performance of MgF 2 —CaF 2  binary system sintered bodies having varied raw material mix proportions; 
         FIG. 10  is Table 2 showing measured data of Examples; and 
         FIG. 11  is Table 3 showing measured data of Comparative Examples. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The MgF 2  system fluoride sintered body (including a MgF 2  simple sintered body and a MgF 2 —CaF 2  binary system sintered body, hereinafter, referred to similarly) for a radiation moderator having a compact polycrystalline structure excellent in radiation moderation performance, especially neutron moderation performance according to the preferred embodiments of the present invention and the producing method thereof are described below by reference to the Figures. 
     The method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the preferred embodiment of the present invention, as shown in  FIG. 2 , comprises the steps of raw material mixing, particle size controlling, uniaxial press molding, CIP molding, preliminary sintering, primary sintering and secondary sintering. 
     A great difference between a method for producing a MgF 2  simple sintered body and that for producing a MgF 2 —CaF 2  binary system sintered body is that the method for producing a MgF 2  simple sintered body does not require the “raw material mixing” step. After the raw material mixing step, in both methods, every step may be advanced almost in the same manner. 
     In the method for producing a MgF 2  system fluoride sintered body for a radiation moderator according to the preferred embodiment, a high-purity (purity of 99.9% by weight or more) MgF 2  raw material was mixed with a high-purity (purity of 99.9% by weight or more) CaF 2  raw material in the proportion of 0-90% by weight (included in a total of 100). (Here, 0% by weight means a case where a CaF 2  raw material is not mixed, that is, a MgF 2  simple raw material. In producing a MgF 2  simple sintered body, the “raw material mixing” step is not required as mentioned above.) Mixing of the raw materials was conducted by mixing for 12 hours using a V mixer. 
     Then, particle size control of the raw material powder was conducted. 
     As the particle size control, using a ball mill (a container of a pot mill made of alumina: an inside diameter of 200 mm and a length of 250 mm), balls made of alumina, ϕ20 mm:3000 g and ϕ30 mm:2800 g, and about 3000 g of the raw material powder were filled therein and thereafter, rotated for a prescribed period of time. The rotation was stopped every two or three days so as to take sample powders and measure the same. 
     The particle size distribution were measured using ‘a laser diffraction particle size analyzer (type number: SALD-2000)’ made by Shimadzu Corporation according to JIS R1629 ‘Determination of particle size distributions for fine ceramic raw powders by laser diffraction method’. The sample preparation at that time was conducted according to JIS R1622 ‘General rules for the sample preparation of particle size analysis of fine ceramic raw powder’. 
     As the light source of the SALD-2000, a semiconductor laser of a wavelength of 680 nm is used. The sensitivity to particles having a diameter larger than this wavelength (about 1 μm or more) was good and the measurement accuracy was high. On the other hand, as for the sensitivity to fine particles of the order of submicron, it was considered that the measurement accuracy was low compared with the particles having a large diameter though some way to improve the measurement accuracy was devised. 
     Therefore, it is considered that the actual number of fine particles of the order of submicron may be larger than the analysis result. In other words, ‘there is a high possibility that the ratio of fine particles may be larger than the analysis result in the actual particle size distribution, and that the mean particle size may be smaller than the shown value thereof.’ However, in the present application, the values of the particle sizes measured according to the above measurement method are shown as they are. 
       FIG. 1  shows a particle size distribution in the case of the above particle size control (pulverization) of a MgF 2  raw material. It was proved that the median diameter could be about 10 μm after three-day pulverization, about 8 μm after five-day pulverization, about 6 μm after one-week pulverization, about 5 μm after two-week pulverization, about 4 μm after three-week pulverization, and about 3 μm after four-week pulverization. Even if a MgF 2  raw material and a CaF 2  raw material were mixed, the particle size distribution similar to that of the raw material of MgF 2  simple could be obtained. 
     SEM photographs of particles of the original raw material powder and the above particles whose median diameter became about 4 μm after three-week pulverization are shown in  FIG. 8 . In the particles of the original raw material powder, some irregular-shaped particles, mainly angular particles were seen, while most of the particles after three-week pulverization were rounded. It was found that most of the angular portions of the particles of the original raw material powder were worn by pulverization so as to be approximated to sphere shapes. 
     Judging from the particle size distribution curve of the powder after particle size control, the curve of three-day pulverization and that of five-day pulverization obviously showed a high ratio of coarse particle portions, respectively. When the shape of the curve looks like as if “two peaks” or “three peaks” run in a line, which is called ‘2-peak type’ or ‘3-peak type’ by being likened to the shape of a mountain range, they were regarded as ‘2-peak type’ or ‘3-peak type’. 
     In the case of one-week pulverization and two-week pulverization, respectively, the ratio of the coarse particle portions substantially decreased, the coarse particle portion of 30 μm or more remained several % by weight, but that of 50 μm or more almost disappeared, and the shape of the mountain range was reaching almost the 1-peak type having a small gently inclined portion around the particle diameter of 10 μm-15 μm (this state is called ‘sub-1-peak type’). And it was found that in the case of three-week or more pulverization, the coarse particle portion of 30 μm or more almost disappeared and that the shape of the particle size distribution curve could be approximately similar to a normal distribution (this state is called ‘1-peak type’. In this state, the mean diameter and the median diameter are about to be equal.). 
     Thus, the particle shapes were rounded and approached sphere shapes by pulverization of the raw material, and the ratio of coarse particles decreased, resulting in a great change in shape of the particle size distribution curve from ‘2-peak type’ or ‘3-peak type’ to ‘sub-1-peak type’, and further to ‘1-peak type’. This change exerted a noticeable influence on sintering reaction in the sintering process, which is described below with comparative examinations (the results of the comparative examinations are shown in  FIGS. 4 and 5 ). 
     Furthermore, as a sintering aid, a carboxymethyl cellulose (CMC) solution was added thereto in the proportion of 0.02-1% by weight (not included in 100) to 100 of the mixture and mixed. The mixture was used as a starting raw material (raw material mixing step). 
     In the uniaxial press molding step, using a wooden mold form, the starting raw material of a prescribed quantity is filled in the mold form and thereafter, a prescribed molding pressure of 5 MPa or more is uniaxially applied thereto using a press jig for molding (a method wherein by moving the press jig downward from the upper side while applying pressure, the starting raw material in the mold form is compressed). By this press molding, without causing deformation in the handling process of the press molded body such as taking from the mold form for molding, it is possible to secure the hardness of the molded body for keeping the form thereof. 
     In the CIP molding step, this press molded body is molded at a molding pressure of 5 MPa or more using a cold isostatic pressing (CIP) device so as to form a CIP molded body. 
     In this step of molding using the CIP device, the press molded body after uniaxial press molding is put into a thick vinyl bag, which is then deaired and sealed, and it is put into the molding part of the CIP device having a structure wherein the device body can be split to the upper and lower portions, which is then sealed. The space between the vinyl bag with the press molded body therein and the sealed molding part is filled with clean water, and then, isostatic pressing is conducted at a hydraulic pressure of 5 MPa or more, which is the CIP molding. 
     Through this CIP molding, it is possible to conduct compression on the press molded body which is a uniaxially compressed molded body from all the directions of three axes so as to make the spaces between the raw material particles smaller in all directions. Accordingly, sintering reaction in the following preliminary, primary and secondary sintering steps, or preliminary, primary, secondary and tertiary sintering steps can be promoted. 
     Preliminary sintering is conducted by heating this CIP molded body in a temperature range of 600° C.-700° C. in an air atmosphere (preliminary sintering step). 
     In the primary sintering step, the preliminary sintered body sintered in the preliminary sintering step is sintered in a temperature range just below the starting temperature of foaming Tn: 
     (a) using an atmospheric firing furnace in which the atmosphere can be adjusted, in an air or in an inert gas atmosphere, for example, in a nitrogen gas atmosphere; 
     (b) using a uniaxial pressing hot press furnace, by a hot press method in an inert gas atmosphere or in a vacuum atmosphere; or 
     (c) using a hot isostatic pressing furnace, by a hot isostatic pressing (HIP) method in a pressurized inert gas atmosphere. 
     The sintering temperature in the primary sintering step was set to range from (Tn−100° C.) to Tn when the starting temperature of foaming was Tn. As described above, the starting temperature of foaming Tn was determined from the measured temperatures using a differential thermal analyzer, and the temperature varied in a range from about 850° C. to 900° C. depending on the mix proportion of the raw materials of MgF 2  and CaF 2 . 
     Specifically, the temperature range of 750° C.-850° C. in the case of a composition mainly comprising MgF 2 , that of 800° C.-900° C. in the case of a composition mainly comprising CaF 2 , and that of 775° C.-875° C. in the case of an intermediate composition of the both, were determined depending on the composition. 
     In the primary sintering step, by heating in these temperature ranges for a relatively long period of time (4-16 hours), sintering is allowed to make progress uniformly all over the sintered body. 
     In the secondary sintering step, by heating in the same furnace and the same atmosphere as the primary sintering step, in a temperature range of 900° C.-1150° C. in the vicinity of the temperature limits in which a solid solution starts to be formed (the temperature limits in the vicinity of 980° C., being a temperature at which a solid solution starts to be formed in the MgF 2 —CaF 2  binary system phase diagram in  FIG. 3 ) for a relatively short period of time (0.5-8 hours), a sintered body having a compact structure is formed. 
     ‘A solid solution’ here is a state of a solid in single phase in which components of a plurality of compounds having similar crystal state, for example, those of MgF 2  and CaF 2  in the case of this sintered body, are uniformly and indiscriminately distributed in a crystal. 
     In some cases, after conducting the above primary and secondary sintering steps, by the hot press method or the HIP method, another step of firing with the primary or secondary sintering conditions, the same with the case of the atmospheric firing furnace so as to form a MgF 2  system fluoride sintered body having a more compact structure (tertiary sintering step) may be conducted. 
     Here, as an inert gas, other than nitrogen (N 2 ), helium (He) or neon (Ne), argon (Ar) or the like is used. 
     The reason why the sintering step was divided into two steps, primary and secondary, is in order to suppress foaming as much as possible, and make the difference of the degree of sintering progress in every part (such as a periphery portion and a center portion) of the sintered body as small as possible. 
     Particularly, in order to produce a large-size compact sintered body, the technique is important. The large size here is applied to press molded bodies in the below-described Examples having the size of about 220 mm×220 mm×H85 mm, while the small size is applied to the below-mentioned press molded bodies having the size of dia. 80 mm×H50 mm. 
     In a test conducted in order to roughly grab proper heating conditions of the sintering step, the starting raw materials (three kinds of particle sizes were selected) comprising MgF 2  simple and MgF 2 —CaF 2  binary system, respectively, were used, the sample size was the above large size, and both of the two stages of sintering were conducted in a nitrogen gas atmosphere. In the primary sintering, the temperature was held at 840° C. for 6 hours and in the subsequent secondary sintering, the heating time was set to be 2 hours with varied heating temperatures so as to measure the relative densities of the sintered bodies. 
     As a result, as shown in  FIGS. 4 and 5 , the relative densities of 95% or more could be secured in a wide range of heating conditions when two-stage sintering step was conducted, in either case of MgF 2  simple and MgF 2 —CaF 2  binary system, even in the test on large-size samples. 
     Particularly, in the case of MgF 2 —CaF 2  binary system, with the three kinds of particle sizes of the raw material shown in the figure, the relative densities of 96%-97% could be obtained in the good condition range (heating at 1025° C.-1075° C.). 
     On the other hand, it is not shown in the figure, but when only one-stage sintering step was conducted on the samples of large size as in the case of the two-stage sintering, the relative densities were 94% or less. 
     The aim of mixing a CaF 2  powder being a secondary raw material into a MgF 2  powder being a main raw material to form MgF 2 —CaF 2  binary system is to cause the sintering reaction which allows the region of the formation of a solid solution on the phase diagram shown in  FIG. 3  to become clearer, since MgF 2  simple has a high melting point of 1252° C. and the temperature region of the formation of a solid solution is partially unclear, shown with dot lines. 
     By mixing the right quantity of CaF 2 , being a fluoride of Ca which is presumed to have similar characteristics to Mg since Ca belongs to the same group as Mg on the periodic table of elements and its period is next to Mg, the melting point can be lowered and the temperature conditions of the formation of a solid solution can be clarified. 
     By mixing CaF 2 , the melting point can be moved from the dot line region on the left end portion of the line indicating the temperature region of starting of the formation of a solid solution in  FIG. 3  toward the solid line region of the intermediate mix proportions positioned on the right hand. As a result, it becomes easy to make the sintering temperature conditions proper. 
     As a material to be mixed into MgF 2  other than CaF 2  being a fluoride of Ca, LiF being a fluoride of Li can be exemplified. 
     The mix proportion of MgF 2  being a main raw material and CaF 2  being a secondary raw material were varied in a range of 0-97.5% by weight (included in the total). The raw material and balls were filled into the ball mill, which was rotated for a week so as to make the median diameter about 6 μm. The ball mill made of alumina having an inside diameter of 280 mm and a length of 400 mm was used, and balls of ϕ5 mm:1800 g, ϕ10 mm:1700 g, ϕ20 mm:3000 g and ϕ30 mm:2800 g, made of alumina were filled therein. 
     As the sintering aid added to the particle-size-controlled raw material of 3000 g, two kinds, the CMC and the calcium stearate (SAC), were selected. With various adding proportions of each of them in the proportion of 0-2% by weight (not included in the total), the effects of addition thereof were confirmed. For comparison, a test with no sintering aid was also conducted. After adding the sintering aid, the raw material was mixed using a V mixer for 12 hours to be a compound. 
     This compound of a prescribed quantity was filled into a wooden mold form, and using a uniaxial press device, compressed and molded at a uniaxial press pressure of 5 MPa or more. The inside size of the mold form for a large-size test was 220 mm×220 mm×H150 mm, and the inside size of the mold form for a small-size test was 80 mm in diameter and 100 mm in height. 
     This press molded body was put into a thick vinyl bag, which was then deaired and sealed, and it was put through a cold isostatic pressing (CIP) device. The press molded body was put into the molding part having a two-split structure (inside diameter 350 mm×H120 mm), which was sealed. The space between the vinyl bag with the press molded body therein and the molding part was filled with clean water, and then, isostatic pressing was conducted at a hydraulic pressure of 5 MPa or more so as to form a CIP molded body. 
     The preliminary sintering step was conducted on the CIP molded bodies in an air atmosphere with various kinds of conditions in a heating temperature range of 500° C. to 750° C. and in a heating time range of 3 to 18 hours. 
     After observing the appearance of the preliminary sintered bodies, the preliminary sintered bodies were sintered with the conditions which were regarded as good sintering conditions in the preceding preliminary test. The sintering step was conducted with the conditions wherein, in a nitrogen gas atmosphere, the temperature was raised from room temperature to 600° C. at a fixed rate for 6 hours, and held there for 8 hours, and then, it was raised to 1000° C. at a fixed rate for 2 hours and held there for 1 hour. And thereafter, it was lowered to 100° C. for 20 hours. 
     By observing the appearance of the taken-out sintered bodies, the state of compactness of the inside thereof and the like, proper raw material mix proportions, raw material processing conditions, preliminary sintering conditions and the like were investigated. 
     As a result, in cases where the mix proportion of the secondary raw material CaF 2  to the main raw material MgF 2  was 90.1% by weight or more, a larger number of large bubbles were left in the inside portion of the sintered body, compared with the periphery portion thereof, resulting in insufficient compactness. It was presumed that the reason was because the sintering speed was too fast. 
     Judging from these situations, the mix proportions of CaF 2  to MgF 2 , in which the difference in compactness between the inside portion and the periphery portion of the sintered body was small, that is, the sintering performance was in a good state, were 0-90% by weight. It was confirmed that the more desirable mix proportions thereof in which the difference in compactness between the inside portion and the periphery portion of the sintered body was smaller, resulting in an excellent degree of uniformity, were 0-50% by weight. Hence, the proper range of mix proportions of CaF 2  was judged to be 0-90% by weight, more desirably 0-50% by weight. 
     There was no big difference between the effects of the two kinds of sintering aids, but when the mix proportion of the sintering aid was less than 0.02% by weight, the shape keeping performance of the molded body was poor. And when the mix proportion thereof exceeded 1.1% by weight, coloring which appeared to be a residual of the sintering aid was noticed on the preliminary sintered body or the sintered body in some cases. Hence, the proper range of mix proportions of the sintering aid was judged to be 0.02-1% by weight. 
     In a uniaxial press test using the above wooden mold form for a small-size test, when the molding pressure of the uniaxial press device was less than 5 MPa, the press molded body easily lost its shape in handling. As the molding pressure was gradually increased from 5 MPa, the bulk density of the press molded body gradually increased, and it was recognized that the bulk densities of the preliminary sintered body and the sintered body also tended to increase though slightly. The test was conducted with the molding pressure gradually increased to 100 MPa. However, even if the molding pressure was raised to 20 MPa or more, no improvement of performance of the preliminary sintered body or the sintered body was recognized. Hence, the proper value of the molding pressure of the uniaxial press device was decided to be 5 MPa or more, desirably 20 MPa. 
     When the molding pressure of the CIP device was less than 5 MPa, the CIP molded body easily lost its shape in handling. As the molding pressure was gradually increased from 5 MPa, the bulk density of the CIP molded body gradually increased, and it was recognized that the bulk densities of the preliminary sintered body and the sintered body also tended to increase though slightly. The test was conducted with the CIP molding pressure gradually increased to 60 MPa. However, even if the molding pressure was raised to 20 MPa or more, no great improvement of performance of the preliminary sintered body or the sintered body was recognized. Hence, the proper value of the molding pressure of the CIP device was decided to be 5 MPa or more, desirably 20 MPa. 
     The research of preliminary sintering conditions of the CIP molded body in an air atmosphere was conducted under the below-described conditions. By mixing MgF 2  with CaF 2  of 3% by weight, and adding CMC of 0.1% by weight as a sintering aid thereto, a starting raw material was prepared. Using the wooden mold form for a small-size test, by setting the molding pressure of a uniaxial press device to be 20 MPa and setting the molding pressure of a CIP device to be 20 MPa, CIP molded bodies were formed. Using the CIP molded bodies formed under such conditions, the preliminary sintering conditions were researched. 
     As a result of the research, at heating temperatures of less than 600° C., shrinkage was small compared with the size of the molded body, while at heating temperatures of 710° C. or more, the shrinkage rate was too high and therefore, shrinkage was difficult to control. Hence, the proper range of the preliminary sintering temperatures was decided to be 600° C.-700° C. 
     Concerning the heating time, at 600° C., it was judged that 8-9 hours were optimal, and that 4-10 hours were proper, judging from the evaluation of the shrinkage rate. At 700° C., it was judged that 6-8 hours were optimal, and that 4-10 hours were proper. From these results, the heating conditions in the preliminary sintering step were decided to be at 600° C.-700° C. for 4-10 hours in an air atmosphere. 
     What is likely to give most influence on the performance of a sintered body in producing the MgF 2  system fluoride sintered body for a radiation moderator is the sintering step. From the above researches and tests, the proper conditions until just before the sintering step were clarified. 
     Before clarifying the proper conditions in the sintering step, the sintering step and the sintering mechanism which appear to be desirable to a MgF 2  system fluoride sintered body for a radiation moderator are put in order. 
     The terms “primary flocculation process” and “secondary flocculation process” which express the degrees of progress of the sintering step, are described below. The “primary flocculation process” refers to an event in the first half of the stage of sintering, and in the initial stage thereof, the intervals between particles gradually become narrower and the voids among particles also become smaller. With further progress of sintering, the particle-to-particle contact portions become thick and the voids among them become further smaller. Here, the majority of the voids are open pores connecting to the surrounding atmosphere. Up to this stage is called “primary flocculation process”. 
     On the other hand, after the end of the primary flocculation process, with further progress of sintering, the open pores gradually decrease and turn into closed pores. The stage of turning into closed pores and the subsequent stage of defoaming and compacting are generically called “secondary flocculation process”. 
     In the producing method according to the preferred embodiment, due to raw material mixing, particle size control, sintering aid adding/mixing, two-stage molding (uniaxial press molding and CIP molding), preliminary sintering and the like, it was noticed that the voids among particles of the preliminary sintered body were small, and that the voids almost uniformly scattered without gathering (the first half stage of the primary flocculation process). 
     In the heating process of the next sintering step (corresponding to the primary sintering step), the heating temperature is gradually raised. Around the temperature limits (500° C.-550° C.) slightly lower than the preliminary sintering temperatures (600° C.-700° C.), particles start to gather, and thereafter, solid phase reaction starts in the temperature limits far lower than 980° C. at which a solid solution starts to be formed. With that, flocculation of particles makes progress, leading to shorter particle-to-particle distances and smaller voids. 
     It is generally said that the solid phase reaction starts in the temperature limits lower by the order of 10% or further lower than the temperature at which a solid solution starts to be formed. From the observation results in the preliminary test by the present inventors, it was considered that the solid phase reaction started in far lower temperature limits than the above generally said temperature limits, in the order of 500° C.-550° C. 
     It can be said on the ground that at 600° C., the lowest limit of the preliminary sintering temperature, sintering by the solid phase reaction has already made progress considerably so that the preliminary sintered body considerably shrinks compared with the CIP molded body. 
     It is considered that the solid phase reaction makes progress at a low reaction rate in the temperature limits and that it makes progress at a quite high reaction rate in the temperature limits in the vicinity of 750° C., or more up to 980° C. Here, in the case of heating at relatively low temperatures (600° C.-700° C.) like assumed preliminary sintering for a short period of time, most of the voids remain in a state of open pore (which is the first half stage of the primary flocculation process). 
     What attention should be paid to here are the solid phase reaction varying depending on the mix proportion of the raw material and behavior of fine bubbles (foaming bubbles) generated through vaporization of part of the raw material in the temperature limits of about 850° C.-900° C. or more, as mentioned above. In the case of heating at about 1000° C. or more, the heating time should be as short as possible, since this formation of foaming bubbles comes to be noticeable. 
     In the producing method according to the preferred embodiment, the sintering step is divided into two. In the primary sintering step, by heating in the relatively low temperature limits in which no foaming bubbles are formed for a long period of time, sintering of the whole body is allowed to make progress almost uniformly. The micro structure of the sintered body comprises mainly open pores, but part of them is turned into closed pores (after finishing the second half stage of the primary flocculation process, partially in the secondary flocculation process). 
     In the secondary sintering step, heating is conducted in the relatively high temperature limits in the vicinity of 980° C. at which a solid solution starts to be formed for a minimum required period of time. As the micro structure of the sintered body, the formation of foaming bubbles is suppressed as much as possible, while the sintering reaction is allowed to make progress so as to turn almost all the open pores into closed pores, that is, the secondary flocculation process is finished, resulting in obtaining a high-density sintered body. 
     Micro behavior of raw material particles is described here. As the micro behavior of raw material particles in the sintering step, in the case of a raw material of MgF 2  simple, it is considered that the solid phase reaction starts from the portion at which particles make contact with each other, and that the particle-to-particle contact area gradually increases. 
     On the other hand, in the case of a raw material of MgF 2 —CaF 2  binary system, it is presumed that particles of CaF 2  being a secondary raw material are present around particles of MgF 2  being a main raw material and promote interface reaction with the particles of MgF 2  at a higher reaction speed than the case of MgF 2  simple. 
     Around a heating temperature exceeding 980° C. at which a solid solution starts to be formed, the reaction starts in the vicinity of a particle interface where the particles of CaF 2  are present around the particles of MgF 2 , and a solid solution of a MgF 2 —CaF 2  binary system compound starts to be formed. It is presumed that this solid solution fills the voids among particles and that in some part, finer voids are also filled therewith through capillary phenomenon. 
     On the other hand, even if the heating temperature is lower than 980° C., by heating and holding at about 750° C. or more for a relatively long period of time as described above, the solid phase reaction easily makes progress, the voids gradually decrease with the elapse of time so as to be closed pores. Parallel with that, a gas component within the closed pores diffuses within the bulk (parent) of the sintered body, leading to the progress of defoaming so as to make the sintered body compact with few bubbles (this state is the secondary flocculation process). 
     Also here, in heating at temperatures not lower than the starting temperature of foaming Tn (as described above, the starting temperature of foaming differs depending on the mix proportion of the raw materials MgF 2  and CaF 2 ), that is, temperatures exceeding 850° C.-900° C., attention should be paid to the formation of fine bubbles (foaming bubbles) generated through vaporization of the raw material. 
     That is because it is presumed that the foaming bubbles contain fluorine gas, and it is considered that this gas is a relatively heavy element and difficult to diffuse in the bulk of the sintered body. As measures for that, to avoid heating in the temperature limits of vaporization as much as possible, and if necessary, to heat at a temperature as low as possible or to heat for a short period of time are considered. 
     The difference in appearance between such foaming bubbles and bubbles left after pores became closed but could not be defoamed in the sintering step (hereinafter, referred to as residual bubbles) is described below. The sizes of the foaming bubbles generated by heating for a relatively short period of time are approximately several μm in diameter, and the shapes thereof are almost perfect spheres. 
     On the other hand, the shapes of the residual bubbles are not perfect spheres but irregular, and the sizes thereof are all mixed up, large, medium and small. Therefore, it is possible to distinguish the both according to the difference in shape. Here, in the case of high-temperature heating at temperatures far exceeding 1160° C., or heating at temperatures exceeding 1160° C. for a long period of time, a foaming bubble and a foaming bubble, or a residual bubble and a foaming bubble gather and grow to a large irregular bubble in some cases, resulting in difficulty in judging its origin. 
     With the progress of the secondary flocculation process, the voids among particles become smaller, and all or most of the voids are surrounded by particles or a bridge portion of the sintered body so as to be closed pores (bubbles). Depending on the conditions, there is another case wherein gases are released through the voids (open pores), or gases within the bubbles permeate into the bulk (parent) such as the particles or the bridge portion of the sintered body to degas, resulting in no bubbles (referred to as ‘a defoaming phenomenon’ or simply ‘defoaming’). 
     Whether the voids among particles are left as closed pores, that is, bubbles, or by degassing, no bubbles are formed, is a significant element for deciding the degree of achievement of compactness of the sintered body, leading to deciding the characteristics of the sintered body. 
     Particularly in the case of sintering in a light element gas atmosphere such as He or Ne among inert gases, it is considered that the lighter element more easily diffuses within the pores or the bulk of the sintered body, leading to promoting the capillary phenomenon and defoaming phenomenon, so that bubbles are difficult to remain, leading to easy compacting. 
     Thus, in order to make the whole compact, it is important to advance the primary flocculation process (in detail, it is presumed that the primary flocculation process is divided into the first half stage and the second half stage) and the secondary flocculation process almost simultaneously and almost uniformly on the whole in each process. 
     In the producing method according to the preferred embodiment of the present invention, the preliminary sintering step chiefly equivalent to the first half stage of the primary flocculation process, the primary sintering step chiefly equivalent to the second half stage of the primary flocculation process, and the secondary sintering step chiefly equivalent to the secondary flocculation process are separately conducted, so as to make the two flocculation processes easy to make progress almost uniformly throughout the sintered body. 
     However, Even if the sintering step is divided into two steps of preliminary sintering and sintering like this, a noticeable difference in degree of compactness is caused without proper heating conditions. For example, in the case of heating at high temperatures exceeding the proper limits in the preliminary sintering step, in the case of rapidly heating at the temperature raising stage of the sintering step, or in cases where the holding temperature in the sintering step is a high temperature exceeding the proper limits, a remarkable difference in degree of compactness is caused between the periphery portion and the inside portion of the sintered body. By improper heating, degassing becomes difficult in the process of compacting of the inside portion of the sintered body, and the compactness of the inside portion thereof is likely to be insufficient. 
     It means that it is important to make the heating temperature pattern in the sintering step proper according to the size. Particularly, when producing a large-size sintered body, it is necessary to strictly control the heating conditions since a great difference in degree of compactness between the periphery portion and the inside portion of such sintered body is easily caused. 
     In order to clarify the relationship between the sample size and the sintering state, the present inventors conducted a small-size test using samples molded in a mold form of a uniaxial press device the inside size of 80 mm in diameter and 100 mm in height, and a large-size test using samples molded in a mold form thereof the inside size of 220 mm×220 mm×H150 mm. 
     As a result, in the small-size test, there were cases where a high-density sintered body having a relative density exceeding 95% was obtained depending on the heating conditions even if one sintering step was conducted. On the other hand, in the large-size test, with one sintering step, any of the sintered bodies had a low density of less than 94% under the same heating conditions as the small-size test. 
     What is important here is that the whole of the preliminary sintered body has already advanced almost uniformly to the first half stage of the primary flocculation. Only preliminary sintered bodies in a state in which the whole body has already advanced to the first half stage of the primary flocculation were provided to these tests of the sintering step. 
     The Description of the Sintering Step Test 
     A mixture of a main raw material MgF 2  with CaF 2  of 3% by weight, and a raw material of MgF 2  simple were used as starting materials. CMC of 0.1% by weight was added thereto as a sintering aid. And using the above mold form for a large-size test, the compounds were molded at a molding pressure of 20 MPa of a uniaxial press device and at a molding pressure of 20 MPa of a CIP device. 
     The CIP molded bodies were preliminary sintered at 650° C. for 6 hours in an air atmosphere so as to form preliminary sintered bodies. 
     In a nitrogen gas atmosphere, as primary sintering step, the preliminary sintered bodies were heated to 840° C. and the temperature was held there for 6 hours and then raised to a secondary sintering temperature for 2 hours. 
     The secondary sintering temperature was varied from 700° C. to 1250° C., at an interval of every 50° C., and the temperatures each were held for 2 hours. 
     Thereafter, the heating was stopped and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours, and when reaching 100° C. or lower at which time it was previously set to take out the sintered body, it was taken out. 
     As a result of the sintering test with such two-stage sintering step, in the case of a sintering temperature range from 900° C. to 1150° C., most of the bulk densities of the sintered bodies exceeded 2.96 g/cm 3 , which were high. The true density of the binary system compound was 3.15 g/cm 3  and the relative density thereof was 94.0%, while the true density of the raw material of MgF 2  simple was also 3.15 g/cm 3  and the relative density thereof was also 94.0%. 
     In either case of sintering temperatures of less than 900° C., and those of 1160° C. or more, the relative densities were lower than 94.0% (the bulk density of 2.96 g/cm 3 ). The sintered bodies of the MgF 2 —CaF 2  binary system raw material tended to have a higher relative density by the order of 0.5%-1.5% than those of MgF 2  simple in a range of good sintering conditions. 
     When observing the sections of those sintered bodies, in the case of sintered bodies sintered at temperatures lower than 900° C., not many but some open pores were noticed in some of them, wherein the bridge width of the sintered portion was narrow, so that it could be regarded as absolutely insufficient progress of sintering. 
     In the case of sintered bodies sintered at temperatures of 1160° C. or more, especially 1200° C. or more, those had a porous pumiceous structure as if bubbles were innumerably formed inside. Fine bubbles which were almost perfect spheres of several to dozen μm in diameter were observed all over the sintered body and innumerable irregular bubbles (foaming bubbles and aggregates thereof) of 10 μm or more in diameter were found all over the sections. 
     From another examination using a differential thermal analyzer by the present inventors, it was found out that when heating the compound of MgF 2 —CaF 2  binary system, the weight clearly started to decrease at a temperature of about 800° C.-850° C. (the temperature becomes gradually higher within the temperature range as the mix proportion of CaF 2  to MgF 2  increases), and that the weight started to drastically decrease at about 850° C.-900° C. This means that a sublimation phenomenon in which MgF 2  or CaF 2  dissolves/vaporizes to generate fluorine starts due to heating at about 800° C.-850° C. or more. 
     A foaming phenomenon through this fluorine sublimation is noticeably caused by heating at about 850° C.-900° C. or more, and fine bubbles are formed all over the sintered body. The behavior of the foaming bubbles such as defoaming or remaining as bubbles is decided according to the degree of progress of the sintering step, in which portion of the sintered body they were formed and the like. In the primary flocculation process, for example, since the whole sintered body contains mainly open pores, the majority of foaming bubbles can be defoamed through the open pores, leading to few bubbles left. In the secondary flocculation process, since the sintered body contains mainly closed pores, a large number of foaming bubbles cannot be defoamed, leading to remaining as bubbles. To swiftly complete the sintering in the secondary flocculation process leads to suppressing foaming and reducing residual bubbles. 
     Hence, it is preferable that the transition from the primary flocculation process to the secondary flocculation process should be advanced in the whole sintered body with as small a difference of the degree of progress as possible among the portions thereof. However, it is not easy to undergo the transition from the primary flocculation process to the secondary flocculation process in the whole sintered body without a difference of the degree of progress among the portions thereof. 
     Then, the present inventors considered the below-described method. 
     Heating at a relatively low temperature in the temperature limits just below the starting temperature of foaming Tn (850° C.-900° C.), specifically in the temperature limits between (Tn−100° C.) and Tn for a relatively long period of time was conducted, so that the primary flocculation process and the first half of the secondary flocculation process were completed. And then, by heating at a temperature in the vicinity of the temperature (980° C.) at which a solid solution starts to be formed for a relatively short period of time, the second half of the secondary flocculation process was completed. By such sintering, the degree of progress of sintering could be made uniform in the whole sintered body, and the formation of bubbles could be suppressed as much as possible. 
     How the proper sintering conditions were decided is described below. 
     In the same manner as the above sintering condition change test, a main raw material MgF 2  was mixed with CaF 2  of 3% by weight. CMC of 0.1% by weight was added thereto as a sintering aid. The same was molded using a mold form for a large-size test at a molding pressure of 20 MPa of a uniaxial press device and a molding pressure of 20 MPa of a CIP device. Preliminary sintering was conducted on this CIP molded body at 650° C. which was held for 6 hours in an air atmosphere. 
     As the conditions of the sintering step, the atmosphere was set to be a nitrogen gas atmosphere. Preliminary tests concerning each of heating and cooling conditions in the heating pattern were conducted in three cases of the required time of 4, 6 and 8 hours. As a result, in the case of 4 hours, small cracks occurred in the sintered body, while in the other cases, the results were good. Therefore, the required time was set to be 6 hours, shorter one selected from 6 and 8 hours. 
     The atmosphere was set to be a nitrogen gas atmosphere, and the heating temperature was varied in a range of 700° C. to 1250° C. In eleven cases of the holding time of 2, 3, 4, 5, 6, 8, 10, 12, 14, 16 and 18 hours, the tests were conducted. 
     As a result, in the case of less than 750° C., the compactness was insufficient, regardless of the holding time. In the case of heating at 750° C., the compactness was insufficient with a holding time of 4 hours or less. On the other hand, in the case of heating at 1160° C. or more, a large number of bubbles were generated due to too fast sintering speed, regardless of the holding time. In the case of a holding time of 18 hours, in some cases, foaming occurred in part of the periphery of the sintered body, leading to getting out of shape in appearance. 
     Reviewing the results, in the case of heating at 750° C., the sintering state was good with a holding time of 14 and 16 hours. 
     In the case of heating at 800° C., the sintering state was good with a holding time of 10 and 12 hours, while slightly insufficient with 6 and 8 hours, and beyond decision of quality with 14 hours or more. 
     In the case of 830° C., the sintering state was good with a holding time of 10 and 12 hours. 
     In the case of 850° C., the sintering state was good with a holding time of 8, 10 and 12 hours, while slightly insufficient with 5 hours, and beyond decision of quality with 14 hours or more. 
     In the case of 900° C., the sintering state was good with a holding time of 5 to 12 hours, while slightly insufficient with 4 hours, and beyond decision of quality with 14 hours or more. 
     In the case of 1000° C., the sintering state was good with a holding time of 5 to 12 hours, while slightly insufficient with 4 hours, and much foaming was seen with 14 hours or more. 
     In the case of 1050° C., the sintering state was good with a holding time of 5 to 10 hours, while slightly insufficient with 4 hours, and much foaming was seen with 12 hours or more. 
     In the case of 1100° C., the sintering state was good with a holding time of 4 to 8 hours, while slightly insufficient with 3 hours or less, and much foaming was seen with 10 hours or more. 
     In the case of 1150° C., the sintering state was good with a holding time of 2 and 3 hours, while much foaming was seen with 4 hours or more. 
     In the case of 1160° C. or more, much foaming was seen with any holding time, and the results were beyond decision of quality or poor because of too much melting. 
     Here, when the heating temperature was a comparatively low temperature of 750° C. to 850° C., the sintering state was good with a holding time of 6 to 12 hours, while that was slightly insufficient with a holding time of 3 to 5 hours. Since the method according to the preferred embodiment has the subsequent secondary sintering step, with the evaluation in this step (equivalent to the primary sintering step), the holding time of 3-12 hours was regarded as a good heating condition. 
     In order to examine the relationship between the heating temperature and the bulk density of the sintered body, using the same preliminary sintered bodies as the above, the heating temperature was varied within a range of 600° C. to 1300° C. (with a holding time of 6 hours in any case). 
     As a result, in the case of a heating temperature of 850° C., the bulk density was approximately 2.96 g/cm 3  (the relative density of 94.0%). The sintered body having a bulk density of that value or more was judged to have sufficient compactness without troubles such as losing its shape in the treatment of the second step. On the other hand, in the case of heating temperatures of 1160° C. or more, in some cases, foaming occurred in part of the periphery of the sintered body, resulting in a trouble such as getting out of shape in appearance. 
     From the above examination results of the sintering conditions and the relationship between the heating temperature and the bulk density, it was judged that, if the sintering step was one heating step, the heating temperature of 850° C. to 1150° C. and the holding time of 3 to 12 hours were proper. 
     What was clarified here is, when relatively long time heating, such as at 900° C. for 14 hours or more, at 1000° C. for 14 hours or more, at 1100° C. for 10 hours or more, or at 1150° C. for 8 hours or more, was conducted, a large number of foaming bubbles were generated and part of those gathered and grew to large bubbles. Such sintered body involved defects which would cause cracks to occur from a large bubble portion or cause splitting in processing of the next mechanical processing step. 
     From these situations, as a fundamental plan of the sintering step, it was decided that foaming should be suppressed as much as possible, while the sintering reaction should be allowed to sufficiently make progress, leading to producing a sintered body having a good processability in the subsequent mechanical processing step. 
     At the first stage of the sintering step (the primary sintering step), it was aimed to suppress foaming to a minimum, to allow the sintering to make slow progress, and to minimize a difference of the degree of progress of sintering between the inside portion and the periphery portion of the sintered body. 
     Therefore, the heating temperature was decided to be within the above range of 700° C. to 1150° C. Since the starting temperature of foaming Tn is 850° C. in the case of a raw material mainly comprising MgF 2 , it was judged that the heating temperature should be 850° C. or less, not exceeding the temperature. On the other hand, since the sintering state was insufficient in the case of heating at temperatures lower than the Tn by 100° C. or more, it was judged that the heating temperature at the first stage of the sintering step should be between (Tn−100° C.) and Tn, between 750° C. and 850° C. in the case of a raw material mainly comprising MgF 2 . 
     The proper heating conditions in the primary sintering step were the heating temperature between (Tn−100° C.) and Tn, and the holding time of 3-12 hours. The same tendency was found even in cases where the mix proportion of CaF 2  to MgF 2  varied between 0-90% by weight. 
     Heating at the stage of enhancing the sintering reaction of the sintered body, that is, heating in the secondary sintering step, was decided to be conducted properly in the temperature limits in the vicinity of 980° C. at which a solid solution starts to be formed, that is, 900° C. to 1150° C. It was aimed to make the holding time as short as possible in order to enhance the sintering reaction and suppress foaming. The proper holding time was decided to be 0.5 to 8 hours, since the enhancement of the sintering reaction was poor in the case of less than 0.5 hour, and too many bubbles were formed in the case of 9 hours or more. 
     The examination of the proper conditions of the heating temperature and the holding time in the secondary sintering process when the atmospheric gas was changed from nitrogen gas to helium gas is described below. 
     A mixture of a main raw material MgF 2  with CaF 2  of 3% by weight was used as a starting material, to which CMC of 0.1% by weight was added as a sintering aid. 
     Using a mold form of press molding for a large-size test, the material was molded at a molding pressure of 20 MPa of a uniaxial press device and at a molding pressure of 20 MPa of a CIP device. This CIP molded body was preliminary sintered at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     Using helium gas as the atmospheric gas in the primary and secondary sintering processes, the preliminary sintered body was heated to 840° C. which was held for 6 hours as primary sintering. Then, it was raised to each of secondary sintering temperatures varying in a range of 700° C. to 1250° C., at an interval of every 50° C. for 2 hours, and the target temperature was held for 2 hours. And then, the heating was stopped and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours, and when reached a predetermined taking-out temperature of 100° C. or lower, the sintered body was taken out. 
     As a result of the sintering test with the above two-stage sintering step, in the case of a temperature range of 900° C. to 1150° C., most of the sintered bodies had a high bulk density exceeding 2.96 g/cm 3 . The true density of this binary system compound (mixed with CaF 2  of 3% by weight) was 3.15 g/cm 3  and the relative density thereof was 94.0%. The true density and the relative density thereof were the same as the true density and the relative density of the raw material of MgF 2  simple. 
     In either case of sintering temperatures lower than 900° C., and those of 1160° C. or more, the relative density was lower than 94.0% (the bulk density of 2.96 g/cm 3 ). The sintered bodies sintered in a helium gas atmosphere tended to have a higher relative density within a range of good sintering conditions (a temperature range of 900° C. to 1150° C.) by the order of 0.5%-1% than in a nitrogen gas atmosphere. 
     It is considered that the reason why the bulk density becomes high in a helium gas atmosphere is because the diffusion velocity of helium gas within the bulk (parent) of the sintered body is higher than that of nitrogen gas. It is presumed that, since helium gas more easily diffuses within the bulk than nitrogen gas, when voids become closed pores with the progress of sintering in the sintering process, part of the closed pores disappear without becoming bubbles, or the sizes of the closed pores become smaller. 
     However, helium gas showed better effects within a range of the above proper sintering conditions, while the effects were not all-around, being not noticeably seen in the region other than the proper sintering conditions. 
     As the reasons of such result, it was considered that under the sintering conditions outside the proper range, for example, there was a limit in improving too slow sintering speed due to an insufficient heating condition, or in the case of an excessive heating condition, the ununiformity of the sintering speed of every part of the sintered body could not be improved by enhancing the diffusivity of helium gas in the bulk. 
     In the case of helium gas, when the heating temperature in the sintering step was less than 900° C., regardless of the holding time, or in the case of a holding time of 4 hours or less, the compactness was insufficient. When the heating temperature was 1160° C. or more, the sintering speed was too high, regardless of the holding time, as is the case with nitrogen gas, resulting in occurrence of a large number of bubbles, and in the case of a holding time of 16 hours or more, because of foaming, the appearance got out of shape in some cases. 
     Accordingly, in the case of a starting raw material made by mixing mainly MgF 2  with CaF 2 , it was judged that the proper range of sintering temperatures was 900° C.-1150° C., regardless of the kind of inert atmospheric gas in the sintering step. Furthermore, in the case of sintering temperatures of 930° C.-1100° C., even when the sintered body was provided to the mechanical processing, structural defects such as cracks were difficult to occur, leading to good mechanical processability. As a result, it was judged that the sintering temperature was more preferably in a temperature range of 930° C.-1100° C. 
     Therefore, as proper heating conditions of the sintering step in a helium gas atmosphere, as is the case with the above nitrogen gas atmosphere, the proper condition of the primary sintering step was in a range of 750° C. or more and less than the starting temperature of foaming, while that of the secondary sintering step was in a temperature range of 900° C.-1150° C. 
     The inert gas is not limited to nitrogen and helium. In the case of argon or neon, the same effects can be obtained. Moreover, since neon is expected to have high solubility or high diffusivity in the parent of the sintered body, like helium, the defoaming phenomenon can be more promoted and effects equal to those of helium can be expected. 
     In this invention, on the basis of the above technical ideas, the raw material particles were further pulverized, and using a uniaxial pressing hot press furnace or a hot isostatic pressing (HIP) furnace in addition to an atmospheric firing furnace as furnaces used in the sintering step, defoaming of the sintered body was further promoted, and as a result, a higher-density sintered body could be successfully obtained. 
     Particularly what was found is: 
     (i) when the particle size distribution of the raw material is narrow and the particle size distribution curve thereof is approximate to a normal distribution, the smaller the mean particle diameter thereof is, the higher packing density the molded body thereof (for example, compared with that of the raw material having a median diameter of about 8 μm or more, that of the raw material having a median diameter of about 6 μm or less noticeably) tends to have, and the easier it becomes to compact the preliminary sintered body and the sintered body; 
     (ii) in sintering in a light element gas atmosphere such as He or Ne among inert gases, the lighter the element is, the more easily it diffuses in pores or in the bulk of the sintered body, so that a capillary phenomenon and a defoaming phenomenon are promoted. Bubbles are hard to remain and compacting becomes easy to conduct; and 
     (iii) in the sintering step using a uniaxial hot press furnace and a HIP furnace, particularly in the secondary sintering step, by applying pressure to the sintered body in heating, defoaming of the sintered body can be promoted. 
     By proving these findings and further promoting defoaming of sintered bodies, higher-density sintered bodies could be successfully obtained. 
     The Description of the Particle Size Distribution Performance Test 
     In the case of a raw material of MgF 2  simple, selecting the raw materials having the following three kinds of particle size distributions, respectively, the test was conducted. That is, as shown in  FIG. 4 , as the three kinds of particle size distributions, particle size distribution A (about 8 μm in median diameter), particle size distribution B (similarly about 6 μm) and particle size distribution C (similarly about 4 μm), were adopted. 
     As a sintering aid, CMC of 0.1% by weight was added thereto and mixed to obtain a starting raw material. 
     This starting raw material was filled into the mold form for a large-size test of a press device, and press molded at a uniaxial press pressure of 20 MPa. 
     This press molded body (the size about 220 mm×220 mm×t85 mm) was put into a thick vinyl bag, which was then deaired and sealed, and it was put into the molding part of a cold isostatic pressing (CIP) device (inside size: 350 mm in diameter×120 mm in height). The space between the vinyl bag with the press molded body therein and the molding part of the CIP device was filled with clean water, and then, isostatic pressing was conducted at a hydraulic pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     This CIP molded body was heated (preliminary sintered) at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     As primary sintering, using an atmospheric firing furnace in which the atmosphere can be adjusted, in a He gas atmosphere, the preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours, and the temperature was held there for 6 hours (the primary sintering condition was fixed). 
     Then, the temperature was raised to a secondary sintering temperature (varied in a range of 700° C. to 1250° C., at an interval of every 50° C.) at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours, and after confirming that the temperature reached a predetermined taking-out temperature of 100° C. or lower, the sintered body was taken out. 
     The sintering test by such two-stage sintering step was conducted. Here, since it has been proved that the proper range of the heating and holding time in this secondary sintering step is 0.5-3 hours as described above, the holding time can be changed within this range. The results in the case of a holding time of 2 hours are shown in  FIG. 4 . 
     The bulk densities of the sintered bodies, the starting raw materials of which had the particle size distributions B and C, were high to the extent that the relative densities exceeded 96% in the case of sintering temperatures in a range of 900° C. to 1150° C. In both cases of sintering temperatures less than 900° C. and those of 1160° C. or more, the relative densities were lower than 96%. 
     As shown in  FIG. 4 , the difference caused by the different particle size conditions of the starting raw materials was made clear. In the range of good sintering conditions, the sintered bodies having the particle size distribution B (about 6 μm in median diameter) tended to have relative densities higher by the order of 1% than the sintered bodies having the particle size distribution A (about 8 μm in median diameter). The sintered bodies having the particle size distribution C (about 4 μm in median diameter) tended to have relative densities higher by the order of 2% than the sintered bodies having the particle size distribution A (about 8 μm in median diameter). The highest attained value of relative density of the sintered bodies having the particle size distribution C exceeded 97%, which was high. 
     When observing the sections of those sintered bodies, in the case of sintered bodies sintered at temperatures lower than 900° C., not many but some open pores were noticed in some of them, wherein the bridge width of the sintered portion was narrow, so that it could be regarded as absolutely insufficient progress of sintering. 
     On the other hand, in the case of sintered bodies sintered at temperatures of 1160° C. or more, especially 1200° C. or more, those had a porous pumiceous structure as if bubbles were innumerably formed inside. Fine bubbles which were almost perfect spheres of several to dozen μm in diameter were observed all over the sintered body and innumerable irregular bubbles of 10 μm or more in diameter were found all over the sections. That is, a large number of bubbles which appeared to be foaming bubbles and aggregates thereof were noticed. 
     In the case of a raw material of MgF 2 —CaF 2  binary system, as is the case with a raw material of MgF 2  simple, selecting the raw materials having three kinds of particle size distributions, respectively, the test was conducted. That is, as shown in  FIG. 5 , as the three kinds of particle size distributions, particle size distribution A (about 8 μm in median diameter), particle size distribution B (similarly about 6 μm) and particle size distribution C (similarly about 4 μm), were adopted. 
     As a sintering aid, CMC of 0.1% by weight was added thereto and mixed to obtain a starting raw material. 
     This starting raw material was filled into the mold form for a small-size test of a press device, and press molded at a uniaxial press pressure of 20 MPa. 
     This press molded body was put into a thick vinyl bag, which was then deaired and sealed, and it was put into the molding part of a cold isostatic pressing (CIP) device. The space between the vinyl bag with the press molded body therein and the molding part of the CIP device was filled with clean water, and then, isostatic pressing was conducted at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     This CIP molded body was heated (preliminary sintered) at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     As primary sintering, using an atmospheric firing furnace in which the atmosphere can be adjusted, in a He gas atmosphere, the preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours, and the temperature was held there for 6 hours (the primary sintering condition was fixed). 
     Then, the temperature was raised to a secondary sintering temperature (varied in a range of 700° C. to 1250° C., at an interval of every 50° C.) at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours, and after confirming that the temperature reached a predetermined taking-out temperature of 100° C. or lower, the sintered body was taken out. 
     The sintering test by such two-stage sintering step was conducted. Here, since it has been proved that the proper range of the heating and holding time in this secondary sintering step is 0.5-8 hours as described above, the holding time can be changed within this range. The results in the case of a holding time of 2 hours are shown in  FIG. 5 . 
     The bulk densities of the sintered bodies, the starting raw materials of which had the particle size distributions B and C, were high to the extent that the relative densities exceeded 96% in the case of sintering temperatures in a range of 900° C. to 1150° C. In both cases of sintering temperatures less than 900° C. and those of 1160° C. or more, the relative densities were lower than 96%. 
     As shown in  FIG. 5 , the difference caused by the different particle size conditions of the starting raw materials was made clear. In the range of good sintering conditions, the sintered bodies having the particle size distribution B (about 6 μm in median diameter) tended to have relative densities higher by the order of 1% than the sintered bodies having the particle size distribution A (about 8 μm in median diameter). The sintered bodies having the particle size distribution C (about 4 μm in median diameter) tended to have relative densities higher by the order of 2% than the sintered bodies having the particle size distribution A (about 8 μm in median diameter). The highest attained value of relative density of the sintered bodies having the particle size distribution C exceeded 97%, which was high. 
     When observing the sections of those sintered bodies, in the case of sintered bodies sintered at temperatures lower than 900° C., not many but some open pores were noticed in some of them, wherein the bridge width of the sintered portion was narrow, so that it could be regarded as absolutely insufficient progress of sintering. 
     On the other hand, in the case of sintered bodies sintered at temperatures of 1160° C. or more, especially 1200° C. or more, those had a porous pumiceous structure as if bubbles were innumerably formed inside. Fine bubbles which were almost perfect spheres of several to dozen μm in diameter were observed all over the sintered body and innumerable irregular bubbles of 10 μm or more in diameter were found all over the sections. That is, a large number of bubbles which appeared to be foaming bubbles and aggregates thereof were noticed. 
     Thus, it was proved that by controlling the particle size distribution condition of the starting material, further compacting of the sintered body could be achieved. 
     The Description of the Pressure Firing Test 
     As firing furnaces used in these primary and secondary sintering steps, three kinds of firing furnaces, an atmospheric firing furnace in which the atmosphere can be adjusted, a uniaxial pressing hot press furnace and a hot isostatic pressing (HIP) furnace, were prepared. By using them, with the same heating temperature conditions, heating was conducted. 
     Specifically, heating was conducted: 
     (a) using the atmospheric firing furnace, in an air or in an atmospheric inert gas atmosphere; 
     (b) using the uniaxial pressing hot press furnace, in an inert gas atmosphere or in a vacuum atmosphere of less than 100 Pa; and 
     (c) using the HIP furnace, in an inert gas atmosphere. 
     The reason why the degree of vacuum of the uniaxial pressing hot press furnace was set to be less than 100 Pa is because with low degrees of vacuum of 100 Pa or more, the compactness of the sintered bodies tended to be insufficient, compared with in an inert gas atmosphere or in a high vacuum of less than 100 Pa. 
     It was considered that the presence or absence of pressure application or pressure reduction to the sintered bodies during firing, and the difference of the mode of pressure application caused differences in density of the sintered bodies. 
     That is, in the case of the uniaxial pressing hot press furnace, by uniaxially driving a press jig, pressure can be applied, differently from the atmospheric firing furnace. In the driven press jig side portion of the sintered body, defoaming can be promoted. 
     In the case of the HIP furnace, by increasing the gas pressure therein to a prescribed pressure, the sintered body can be pressed from triaxial directions. Therefore, it was considered that defoaming of the sintered body could be further promoted compared with the atmospheric firing furnace and the uniaxial pressing hot press furnace. 
     As starting materials, raw materials having the particle size distribution B (about 6 μm in median diameter) shown in  FIG. 4  (MgF 2  simple raw material) and in  FIG. 5  (MgF 2 —CaF 2  binary system raw material) were used. Preliminary sintered bodies formed with the same conditions in uniaxial press molding, CIP molding and preliminary sintering were used. In the subsequent firing, in place of an atmospheric firing furnace, primary sintering and secondary sintering using an uniaxial pressing hot press furnace or a HIP furnace were conducted. 
     The heating conditions in the primary sintering were fixed to a heating temperature of 840° C. in a nitrogen gas atmosphere and a holding time of 6 hours, while the heating temperature in the secondary sintering were varied (the heating and holding time was fixed to 2 hours) so as to see changes in relative density of the sintered bodies. 
     Concerning pressure application in the uniaxial pressing hot press furnace and the HIP furnace, in the uniaxial pressing hot press furnace, a press pressure of 50 MPa was uniaxially applied during the heating and holding time, while in the HIP furnace, the gas pressure therein was set to be 200 MPa during the heating and holding time, by which a pressure of 200 MPa was substantially applied in the triaxial directions so as to conduct pressure sintering. The results showing the relationship between the secondary sintering temperatures and the relative densities are shown in  FIG. 6  (MgF 2  simple raw material) and  FIG. 7  (MgF 2 —CaF 2  binary system raw material). 
     As is obvious from  FIGS. 6 and 7 , which kind of firing furnace was used greatly influenced the relative densities of the sintered bodies. It was proved that regardless of which raw material was used, by using the atmospheric furnace, the uniaxial pressing hot press furnace, and the HIP furnace in ascending order, the relative densities of the sintered bodies increased. By using the uniaxial pressing hot press furnace, the heating temperature limits (proper heating temperature limits) could be widened, and those could be further widened by using the HIP furnace. 
     As described above, it was found out that with the proper conditions in the secondary sintering, that is, a heating temperature of 900° C.-1150° C., and the pressure condition of 50 MPa or more during the heating and holding time in the uniaxial pressing hot press furnace, or 200 MPa or more, preferably 300 MPa or more during the heating and holding time in the HIP furnace, high-density sintered bodies having a relative density of 95% or more could be obtained. Consequently, here, these heating condition and pressure condition are called ‘the proper conditions’ in the pressure sintering method. 
     The proper conditions with which on the sintered body obtained through the primary and secondary sintering steps (hereinafter, referred to as ‘a secondary sintered body’), tertiary sintering using the uniaxial pressing hot press furnace or the HIP furnace is further conducted are described below. 
     Depending on whether the relative densities of the ‘secondary sintered bodies’ are (i) 95% or more, or (ii) less than 95%, the densities of the sintered bodies obtained after the tertiary sintering are greatly different. 
     In the case of (i) 95% or more, when the tertiary sintering was conducted with the proper conditions, the relative densities of the sintered bodies could be 97.5%-100%, being roughly approximate to the theoretical density (true density) and high. 
     On the other hand, in the case of (ii) less than 95%, even if the tertiary sintering was conducted with the proper conditions, the relative densities of the sintered bodies were far lower than the theoretical density (true density). Some sintered bodies had low relative densities not reaching 95%. 
     In the case of (i) 95% or more, the number of bubbles was small because of such high density, the number of open pores on the surface of the sintered body was also small, and the number of closed pores within the sintered body was also small. It was considered that by the pressure sintering in the tertiary sintering, most of those bubbles disappeared. 
     On the other hand, in the case of (ii) less than 95%, the number of bubbles was large because of such low density, and there were many large-size bubbles. It was considered that even if the pressure sintering in the tertiary sintering was conducted under the proper conditions, many of these bubbles remained, resulting in insufficient densification. 
     EXAMPLES 
     The MgF 2  system fluoride sintered body for a radiation moderator and the producing method thereof according to the present invention are described below in more detail by reference to Examples. The scope of the present invention is not limited to the below-described Examples. 
     Here, a characteristic evaluation test conducted on sintered bodies is described. Samples for evaluation were prepared by prototyping large-size sintered bodies (rough size of the sintered body: about 205 mm×about 205 mm×H about 70 mm) and conducting mechanical processing such as cut-out in the shape of a required sample thereon. 
     In order to evaluate the neutron moderation performance, as shown in the above Non-Patent Documents 1 and 2, a beam emitted from an accelerator was allowed to collide with a plate made of Be being a target, and by nuclear reaction, high-energy neutrons (fast neutrons) were mainly generated. 
     Then, using Pb and Fe each having a large inelastic scattering cross section as a moderator in the first half of moderation, the fast neutrons were moderated to some extent (approximately, up to 1 MeV) while suppressing the attenuation of the number of neutrons. 
     The moderated neutrons were irradiated to a moderator to be evaluated (a moderator in the second half of moderation), and by examining the neutrons after moderation, the moderator was evaluated. 
     The examination of the neutrons was conducted according to the method described in the above Non-Patent Document 3′. 
     The moderators to be evaluated were made of raw materials MgF 2  and CaF 2  in some varied mix proportions. Through the above-described raw material mixing step, molding step and sintering step, a high-density MgF 2 —CaF 2  binary system sintered body having a relative density in a fixed range (96.0±0.5%) was produced. The total thickness of a moderator in the second half was set to be 600 mm in any case. 
     What was evaluated here is the dose of epithermal neutrons having intermediate-level energy which is effective for therapy, and how many fast neutrons and gamma-rays having high-level energy which has a high possibility of adversely influencing a patient (side effects), remained in the neutrons moderated by the moderator. The results are shown in Table 1 of  FIG. 9 . 
     The dose of epithermal neutrons effective for therapy tended to slightly increase, as CaF 2  was added more to MgF 2 , but the digit of the neutron flux (dose) of epithermal neutrons was the ninth power in any case, so that regardless of the mix proportion of CaF 2 , the dose thereof sufficient for therapy was secured. 
     On the other hand, the mix rate of fast neutrons having a high possibility of adversely influencing a patient (the ratio of fast neutron dose in the total neutron dose after passing through a moderator) was the lowest in the case of mixing CaF 2  of 0 to 10% by weight (0 wt. %, 2 wt. %, 5 wt. %, 8 wt. %. and 10 wt. % as the data). It gradually increased as the mix proportion thereof far exceeded these mix proportions and increased to 20% by weight, and to 40% by weight. It was the highest when CaF 2  was 100% by weight. 
     The mix rate of gamma-rays having the next highest possibility of adversely influencing a patient after fast neutrons (the ratio of gamma-ray dose in the total neutron dose after passing through a moderator) was a low digit of E −14  (the minus 14th power), regardless of the mix proportion of CaF 2  to MgF 2 . It can be said that the mix rate of gamma-rays was not greatly affected by the mix proportion of CaF 2 . 
     From these results, it was proved that when MgF 2  was mixed with CaF 2  of 0-10% by weight, it had the most excellent performance as a moderator. Even if the mix proportion was other than such mix proportions, for example, 10.1% by weight or more and 90% by weight or less, the neutrons on the level usable for therapy were included, resulting in holding required characteristics in moderation performance. 
     The evaluation results are limited to the cases where the relative density of the sintered body is roughly within a fixed range (96.0±0.5%). The higher relative density the sintered body has, the lower the residual dose and the mix rate of fast neutrons are. Conversely, the lower relative density the sintered body has, the higher the residual dose and the mix rate of fast neutrons are. Accordingly, the improvement of the density of the sintered body is effective for a moderator. 
     As described above, concerning the moderation performance of a moderator to neutrons, it was sufficient that sintered bodies of MgF 2  simple and MgF 2 —CaF 2  binary system having a compact structure should have just a relative density of 96% or more. 
     The moderator to neutrons is required to have mechanical strength other than moderation performance. According to the below-described examination of mechanical strength, the MgF 2  system fluoride sintered body for a radiation moderator according to the Examples had sufficient mechanical strength, with which it could be used without problems in processing and molding such as cutting-off, grinding, polishing, cleaning and drying as a moderating member in a moderation system device for BNCT, and further in handling such as the installation thereof in the moderation system device. Even if it was irradiated with neutrons, it was capable of resisting their irradiation impacts, being extremely excellent. 
     As mechanical strengths, bending strength and Vickers hardness were examined. The samples for bending strength, having a size of 4 mm×46 mm×t3 mm with the upper and lower surfaces optically polished were prepared according to JIS C2141, and tested according to the three-point bending test JIS R1601. 
     To obtain the Vickers hardness, according to JIS Z2251-1992, using ‘Micro Hardness Tester’ made by Shimadzu Corporation, an indenter having a load of 100 g was pressed for 5 seconds of loading time so as to measure the diagonal length of the impression, which was converted into hardness in the following manner.
 
Vickers hardness=0.18909× P /( d ) 2  
 
     Here, P: load (N) and d: diagonal length of impression (mm) 
     The pressure applied in the pressure sintering step was 50 MPa in uniaxial pressing hot press, or 200 MPa in HIP in every below-described Example. 
     The pressure applied in the pressure sintering step was 25 MPa in uniaxial pressing hot press, or 100 MPa in HIP in every Comparative Example. 
     Example 1 
     A high-purity MgF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) was pulverized using the above alumina pot mill and alumina balls for two weeks (median diameter of the pulverized simple raw material: about 5 μm and purity of 99.9% by weight or more). 
     Thereafter, a carboxymethyl cellulose (CMC) solution was added thereto as a sintering aid in the proportion of 0.5% by weight to 100 of the compound, which was mixed using a V mixer for 12 hours so as to obtain a starting raw material. 
     This starting raw material was filled into a mold form (inside size of the mold form of 220 mm×220 mm×H150 mm) of a uniaxial press device, and compressed and molded at a uniaxial press pressure of 20 MPa. 
     This press molded body (size of about 220 mm×220 mm×t85 mm), which was put into a thick vinyl bag and sealed after deairing, was put in the molding part (inside size: dia. 350 mm×H120 mm) of a cold isostatic pressing (CIP) device. Clean water was filled into the space between the vinyl bag with the press molded body therein and the CIP molding part, and by isostatic pressing at a molding pressure of 20 MPa at room temperature, CIP molding was conducted. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a helium gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1050° C. at a fixed rate for 4 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to 100° C. or lower at which time it was previously set to take out the sintered body. After confirming that, it was taken out. 
     The bulk density of the sintered body, that is, ‘the secondary sintered body’ was calculated at 3.075 g/cm 3  (the true density of this compound is 3.15 g/cm 3  and the relative density thereof is 97.6%. Hereinafter, referred to as “true density of 3.15 g/cm 3  and relative density of 97.6%”) from the bulk volume of the appearance thereof and the weight thereof. The sintering state thereof was good. 
     Since the appearance of the sintered body was a square form, the “bulk density” here was obtained by a method wherein the bulk volume was calculated from the measured two sides of the square and thickness, and the weight separately measured was divided by the bulk volume. This also applied to the following. 
     Using a sample taken from this sintered body, evaluation tests of neutron moderation performance and characteristics of every kind were conducted. The results are shown in Table 2 of  FIG. 10 . 
     As for the following Examples and Comparative Examples, the evaluation tests of neutron moderation performance and characteristics of every kind were conducted in the same manner and the results are shown in Table 2 of  FIG. 10 . Here, concerning a sintered body of CaF 2  simple, being a comparative material, the neutron moderation performance and mechanical strengths were measured in the same manner. 
     The sintered body according to Example 1 showed excellent neutron moderation performance, and the mechanical strengths thereof were also good enough not to cause problems in handling in the next step. 
     Example 2 
     Using a MgF 2  simple powder (median diameter of about 5 μm and purity of 99.9% by weight or more) obtained by pulverizing a high-purity MgF 2  raw material powder with the same pulverization conditions as in the above Example 1, a sintering aid was added thereto so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. By conducting isostatic pressing of 20 MPa on this press molded body (the size of about 220 mm×220 mm×t85 mm) using a CIP device in the same manner as in the above Example 1, a CIP molded body was formed. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     As HIP treatment, this preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 8 hours. It was then raised to 1020° C. at a fixed rate for 4 hours while the gas pressure therein was held at 200 MPa, and the temperature was held there for 2 hours. 
     The heating was then stopped, while the gas pressure therein was gradually decreased, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body, that is, the secondary sintered body was taken out. 
     The relative density of the secondary sintered body was 98.4% (bulk density of 3.11 g/cm 3  and true density of 3.16 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 3 
     A high-purity MgF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight, and the compound was pulverized using the above alumina pot mill and alumina balls for two weeks (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more). 
     Thereafter, a sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. By conducting isostatic pressing of 20 MPa on this press molded body (the size of about 220 mm×220 mm×t85 mm) using a CIP device in the same manner as in the above Example 1, a CIP molded body was formed. 
     Heating was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1050° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body (secondary sintered body) was 96.5% (bulk density of 3.04 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 4 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 1.5% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device, and by conducting CIP molding thereon at a molding pressure of 20 MPa at room temperature, a CIP molded body was formed. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 820° C. at a fixed rate for 6 hours in a vacuum the degree of which is 50 Pa using a uniaxial hot press furnace, and the temperature was held there for 6 hours. It was then raised to 930° C. at a fixed rate for 2 hours, and the temperature was held there for 4 hours. In each of these holding times, pressing with a load of 50 MPa was uniaxially conducted. 
     Then, the pressing and heating were stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 97.5% (bulk density of 3.07 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 5 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 1.5% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. As HIP treatment, this preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure therein was set at 200 MPa, and the temperature was held there for 5 hours. It was then raised to 1150° C. at a fixed rate for 2 hours while keeping the gas pressure, and the temperature was held there for 0.5 hour. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 97.8% (bulk density of 3.08 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 6 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 1.5% by weight and pulverized for three weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1020° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 97.1% (bulk density of 3.06 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 7 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 1.5% by weight and pulverized for one week in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 6 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1050° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 96.2% (bulk density of 3.03 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 8 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 10% by weight and pulverized for one week in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 6 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 800° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 960° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 95.2% (bulk density of 3.00 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 9 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 20% by weight and pulverized in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in an air atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1150° C. at a fixed rate for 2 hours, and the temperature was held there for 0.75 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 96.8% (bulk density of 3.05 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 10 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 20% by weight and pulverized for one week in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 6 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 810° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 960° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 95.4% (bulk density of 3.005 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 11 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 50% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 6 hours. It was then raised to 1080° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 96.2% (bulk density of 3.04 g/cm 3  and true density of 3.16 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 12 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 50% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. This preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a helium gas atmosphere using a uniaxial hot press furnace, and the temperature was held there for 8 hours. It was then raised to 1080° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. In each of these holding times, pressing with a load of 50 MPa was uniaxially conducted. 
     Then, the pressing and heating were stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 97.2% (bulk density of 3.07 g/cm 3  and true density of 3.16 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 13 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 50% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     As HIP treatment, this preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure was set at 200 MPa, and the temperature was held there for 8 hours. Thereafter, while keeping the gas pressure, it was raised to 970° C. at a fixed rate for 2 hours, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 97.5% (bulk density of 3.08 g/cm 3  and true density of 3.16 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 14 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 90% by weight and pulverized for three weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound: about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a uniaxial hot press furnace, and the temperature was held there for 9 hours. It was then raised to 1080° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. In each of these holding times, pressing with a load of 50 MPa was uniaxially conducted. 
     Then, the pressing and heating were stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 97.2% (bulk density of 3.08 g/cm 3  and true density of 3.17 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 15 
     A MgF 2  powder being a main raw material was pulverized with the same pulverization conditions as in the above Example 11, and the pulverized MgF 2  simple powder (median diameter of the pulverized raw material: about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     On this starting raw material, uniaxial press molding was conducted with the same molding conditions as in the above Example 1, and CIP molding was further conducted thereon so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1000° C. at a fixed rate for 4 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 96.5% (bulk density of 3.04 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This ‘secondary sintered body’ was heated from room temperature to 940° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 1.5 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the ‘tertiary sintered body’ was 99.4% (bulk density of 3.13 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 16 
     A MgF 2  powder being a main raw material was pulverized with the same pulverization conditions as in the above Example 11, and the pulverized MgF 2  simple powder (median diameter of the pulverized raw material: about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     On this starting raw material, uniaxial press molding was conducted with the same molding conditions as in the above Example 1, and CIP molding was further conducted thereon so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1050° C. at a fixed rate for 4 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the ‘secondary sintered body’ was 97.3% (bulk density of 3.065 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 940° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 1.5 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the ‘tertiary sintered body’ was 100% (bulk density of 3.15 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 17 
     A MgF 2  powder being a main raw material was pulverized with the same pulverization conditions as in the above Example 11, and the pulverized MgF 2  simple powder (main raw material: median diameter of about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     On this starting raw material, uniaxial press molding was conducted with the same molding conditions as in the above Example 1, and CIP molding was further conducted thereon so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1000° C. at a fixed rate for 4 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the ‘secondary sintered body’ was 96.5% (bulk density of 3.04 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 940° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 1.5 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the ‘tertiary sintered body’ was 99.5% (bulk density of 3.135 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 18 
     A MgF 2  powder being a main raw material was pulverized with the same pulverization conditions as in the above Example 2, and the pulverized MgF 2  simple powder (main raw material: median diameter of about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     On this starting raw material, uniaxial press molding was conducted with the same molding conditions as in the above Example 1, and CIP molding was further conducted thereon so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a hot press furnace, and the temperature was held there for 8 hours. It was then raised to 1020° C. at a fixed rate for 4 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the ‘secondary sintered body’ was 95.2% (bulk density of 3.00 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 920° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 2 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the ‘tertiary sintered body’ was 99.0% (bulk density of 3.12 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 19 
     Using a MgF 2  simple powder pulverized with the same pulverization conditions as in the above Example 11 (the pulverized raw material: median diameter of about 4 μm and purity of 99.9% by weight or more), a sintering aid was added thereto so as to obtain a starting raw material like the above Example 1. 
     On this starting raw material, uniaxial press molding was conducted with the same molding conditions as in the above Example 1, and CIP molding was further conducted thereon so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 800° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 960° C. at a fixed rate for 4 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the sintered body was 93.3% (bulk density of 2.94 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 1100° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 0.75 hour. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the sintered body was 97.6% (bulk density of 3.075 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 20 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 1.5% by weight and pulverized for three weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound of about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a hot press furnace, and the temperature was held there for 5 hours. It was then raised to 1020° C. at a fixed rate for 4 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the sintered body was 97.1% (bulk density of 3.06 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 940° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 1.5 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the sintered body was 99.8% (bulk density of 3.145 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 21 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 1.5% by weight and pulverized for three weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound of about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 810° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 970° C. at a fixed rate for 4 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the sintered body was 93.7% (bulk density of 2.95 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 1100° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 0.5 hour. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the sintered body was 97.8% (bulk density of 3.08 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 22 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 50% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (mean particle diameter of the pulverized compound of about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1060° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. 
     As HIP treatment, this secondary sintered body was further heated from room temperature to 1060° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 2 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the tertiary sintered body was 97.2% (bulk density of 3.07 g/cm 3  and true density of 3.16 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 23 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 50% by weight and pulverized for two weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound of about 5 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 850° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1020° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 95.2% (bulk density of 3.01 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 950° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a hot press furnace, and the temperature was held there for 1.5 hours. In this holding time, the press jig was driven to uniaxially apply a load of 50 MPa for pressing. 
     Then, the pressing using the press jig and heating were stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the tertiary sintered body was 98.4% (bulk density of 3.11 g/cm 3  and true density of 3.16 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     Example 24 
     A MgF 2  powder being a main raw material was mixed with a CaF 2  powder of 90% by weight and pulverized for three weeks in the same manner as in the above Example 1. The pulverized compound (median diameter of the pulverized compound of about 4 μm and purity of 99.9% by weight or more) was mixed with a sintering aid so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 850° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1050° C. at a fixed rate for 4 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 97.2% (bulk density of 3.08 g/cm 3  and true density of 3.15 g/cm 3 ), and the sintering state thereof was good. 
     This secondary sintered body was heated from room temperature to 950° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 200 MPa, and the temperature was held there for 1.5 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. The relative density of the tertiary sintered body was 99.8% (bulk density of 3.165 g/cm 3  and true density of 3.17 g/cm 3 ), and the sintering state thereof was good. 
     Any of the evaluation results of the neutron moderation performance and mechanical strengths were good as shown in Table 2 of  FIG. 10 . 
     COMPARATIVE EXAMPLE 
     Comparative Example 1 
     A high-purity MgF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) was pulverized with the same conditions as the three-day pulverization case of the above particle size control test so as to obtain a MgF 2  powder (median diameter: about 10 μm and purity of 99.9% by weight or more), to which a sintering aid was added so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 890° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1170° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 91.7% (bulk density of 2.89 g/cm 3  and true density of 3.15 g/cm 3 ), which was low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as bubbles in the sintering process since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 2 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1180° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 93.0% (bulk density of 2.93 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, some bubbles of several tens of μm in diameter were noticed. It was considered that many bubbles remained since the median diameter of the compound was about 8 μm, which was relatively large, and the secondary sintering temperature was 1180° C., which was too high. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 3 
     A high-purity MgF 2  raw material powder was mixed with a high-purity CaF 2  raw material powder of 1.5% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls in the same manner as in the Comparative Example 2. The pulverized compound had a median diameter of about 10 fu m and a purity of 99.9% by weight. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was pressed and molded at a pressure of 20 MPa using a uniaxial press device in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 900° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 5 hours. It was then raised to 1050° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 92.1% (bulk density of 2.90 g/cm 3  and true density of 3.15 g/cm 3 ), which was low. When observing the inside of the sintered body under a microscope, there were a large number of fine bubbles of several μm in diameter and bubbles of several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as bubbles since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 4 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a uniaxial hot press furnace, and the temperature was held there for 5 hours. It was then raised to 880° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. In each of these holding times, the press jig was driven to uniaxially apply a load of 25 MPa for pressing. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 92.1% (bulk density of 2.90 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, some irregular-shaped bubbles were noticed. It was considered that many bubbles remained due to the insufficient degree of progress of sintering since the median diameter of the compound was about 10 μm, which was large, and the secondary sintering temperature was 880° C., which was low. 
     Although the hot press method was used as the firing method in the primary and secondary sintering steps, the insufficient sintering property due to the coarse particle size of the compound and the low secondary sintering temperature could not be covered. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 5 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 20% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1200° C. at a fixed rate for 2 hours, and the temperature was held there for 0.75 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 92.1% (bulk density of 2.90 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several tens of μm in diameter. It was considered that these bubbles were part of voids which remained as large bubbles in the sintering process since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 6 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 50% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 900° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 1080° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 92.1% (bulk density of 2.91 g/cm 3  and true density of 3.16 g/cm 3 ), which was low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 7 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 50% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 860° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 8 hours. It was then raised to 880° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 91.1% (bulk density of 2.88 g/cm 3  and true density of 3.16 g/cm 3 ), which was low. When observing the inside of the sintered body under a microscope, there were a large number of irregular-shaped bubbles. It was considered that many bubbles remained due to the insufficient degree of progress of sintering since the median diameter of the compound was about 10 μm, which was large, and the secondary sintering temperature was 880° C., which was low. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 8 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 90% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 910° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 9 hours. It was then raised to 1080° C. at a fixed rate for 2 hours, and the temperature was held there for 2 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 92.1% (bulk density of 2.92 g/cm 3  and true density of 3.17 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 9 
     A high-purity MgF 2  powder (median diameter: about 8 μm and purity of 99.9% by weight or more), which was obtained by pulverizing with the same conditions as the five-day pulverization case in the above particle size control test, was mixed with a sintering aid so as to obtain a starting raw material in the same manner as in the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 730° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 6 hours. It was then raised to 880° C. at a fixed rate for 2 hours, and the temperature was held there for 3 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 90.5% (bulk density of 2.85 g/cm 3  and true density of 3.15 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 1160° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 91.7% (bulk density of 2.89 g/cm 3  and true density of 3.15 g/cm 3 ), which was low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 10 
     A high-purity MgF 2  powder (median diameter: about 10 μm and purity of 99.9% by weight or more), which was obtained by pulverizing with the same conditions as the three-day pulverization case in the above particle size control test, was mixed with a sintering aid so as to obtain a starting raw material in the same manner as in the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 890° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 3 hours. It was then raised to 1180° C. at a fixed rate for 2 hours, and the temperature was held there for 1 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 91.4% (bulk density of 2.88 g/cm 3  and true density of 3.15 g/cm 3 ), which was low. 
     This secondary sintered body was heated from room temperature to 740° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 93.0% (bulk density of 2.93 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 11 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 730° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 6 hours. It was then raised to 880° C. at a fixed rate for 2 hours, and the temperature was held there for 3 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 91.1% (bulk density of 2.87 g/cm 3  and true density of 3.15 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 1170° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 92.4% (bulk density of 2.91 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 12 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 890° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 3 hours. It was then raised to 1190° C. at a fixed rate for 2 hours, and the temperature was held there for 0.5 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 92.1% (bulk density of 2.90 g/cm 3  and true density of 3.15 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 740° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 94.0% (bulk density of 2.96 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 13 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 20% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 730° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 6 hours. It was then raised to 880° C. at a fixed rate for 2 hours, and the temperature was held there for 3 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 91.1% (bulk density of 2.87 g/cm 3  and true density of 3.15 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 730° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 92.7% (bulk density of 2.92 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 14 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 20% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 890° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 3 hours. It was then raised to 1190° C. at a fixed rate for 2 hours, and the temperature was held there for 1 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 92.4% (bulk density of 2.91 g/cm 3  and true density of 3.15 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 730° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 94.6% (bulk density of 2.98 g/cm 3  and true density of 3.15 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 15 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 50% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 730° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 6 hours. It was then raised to 900° C. at a fixed rate for 2 hours, and the temperature was held there for 3 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 92.1% (bulk density of 2.91 g/cm 3  and true density of 3.16 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 760° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 94.3% (bulk density of 2.98 g/cm 3  and true density of 3.16 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 16 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 50% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 910° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 3 hours. It was then raised to 1200° C. at a fixed rate for 2 hours, and the temperature was held there for 0.5 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 92.7% (bulk density of 2.93 g/cm 3  and true density of 3.16 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 760° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 95.3% (bulk density of 3.01 g/cm 3  and true density of 3.16 g/cm 3 ). When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm in diameter. The sizes of these bubbles were those of foaming bubbles. It was considered that foaming briskly occurred in the sintering process since the median diameter of the compound was about 10 μm, which was large, and the heating temperatures in the primary and secondary sintering were relatively high. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 17 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 90% by weight, and the compound was pulverized for five days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 8 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 730° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 7 hours. It was then raised to 900° C. at a fixed rate for 2 hours, and the temperature was held there for 3 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 92.4% (bulk density of 2.93 g/cm 3  and true density of 3.17 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 760° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 93.7% (bulk density of 2.97 g/cm 3  and true density of 3.17 g/cm 3 ), which was relatively low. When observing the inside of the sintered body under a microscope, there were a relatively large number of bubbles of several μm and several tens of μm in diameter. It was considered that the former of these bubbles were foaming bubbles, and that the latter were part of voids which remained as many bubbles in the sintering process since the median diameter of the compound was about 8 μm, which was relatively large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     Comparative Example 18 
     A high-purity MgF 2  raw material powder (main raw material: median diameter of about 140 μm and purity of 99.9% by weight or more) was mixed with a high-purity CaF 2  raw material powder (median diameter: about 140 μm and purity of 99.9% by weight or more) of 90% by weight, and the compound was pulverized for three days using the above alumina pot mill and alumina balls. The pulverized compound had a median diameter of about 10 μm and a purity of 99.9% by weight or more. A sintering aid was added to the pulverized compound so as to obtain a starting raw material like the above Example 1. 
     This starting raw material was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 910° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 4 hours. It was then raised to 1200° C. at a fixed rate for 2 hours, and the temperature was held there for 0.5 hour. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (secondary sintered body) was taken out. The relative density of the secondary sintered body was 93.1% (bulk density of 2.95 g/cm 3  and true density of 3.17 g/cm 3 ). 
     This secondary sintered body was heated from room temperature to 760° C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIP furnace, while the gas pressure in the furnace was set at 100 MPa, and the temperature was held there for 4 hours. 
     Then, while gradually decreasing the gas pressure in the furnace, the heating was stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 10 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body (tertiary sintered body) was taken out. 
     The relative density of the sintered body was 94.6% (bulk density of 3.00 g/cm 3  and true density of 3.17 g/cm 3 ). When observing the inside of the sintered body under a microscope, there were a large number of bubbles of several μm in diameter and some bubbles of several tens of μm in diameter. The sizes of the former were those of foaming bubbles. It was considered that foaming briskly occurred in the sintering process since the heating temperatures in the primary and secondary sintering were relatively high. The latter varying in size and shape were part of voids which remained as bubbles in the sintering process since the median diameter of the compound was about 10 μm, which was large. 
     In the evaluation results of the neutron moderation performance and mechanical strengths, as shown in Table 3 of  FIG. 11 , some insufficient levels of neutron moderation performance and mechanical strengths were noticed. 
     [Comparative Material 1] 
     The above CaF 2  powder being a secondary raw material (median diameter of about 6 μm), which was prepared in the same manner as in the above Example 12 as a starting raw material, was filled into a mold form of a press device, and compressed and molded at a uniaxial press pressure of 20 MPa in the same manner as in the above Example 1. This press molded body was put in the molding part of a cold isostatic pressing (CIP) device and CIP molding was conducted thereon at a molding pressure of 20 MPa at room temperature so as to form a CIP molded body. 
     Preliminary sintering was conducted on this CIP molded body at 650° C. for 6 hours in an air atmosphere so as to obtain a preliminary sintered body. 
     This preliminary sintered body was heated from room temperature to 880° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using an atmospheric firing furnace, and the temperature was held there for 6 hours. It was then raised to 1100° C. at a fixed rate for 2 hours, and the temperature was held there for 1.5 hours. 
     The heating was then stopped, and the temperature was lowered by self-cooling (so-called furnace cooling) for about 20 hours to a predetermined taking-out temperature of 100° C. or lower. After confirming that, the sintered body was taken out. 
     The relative density of the sintered body was 95.3% (bulk density of 3.03 g/cm 3  and true density of 3.18 g/cm 3 ). 
     The mechanical strengths were good, but a large quantity of fast neutrons remained after moderation, that is, the neutron moderation performance was insufficient. 
     INDUSTRIAL APPLICABILITY 
     It is possible to be used as a moderator to restrict the radiation velocity of radioactive rays of every kind such as neutrons.