Patent Publication Number: US-8111801-B2

Title: Radioisotope production gas target having fin structure

Description:
RELATED APPLICATIONS 
     This application claims priority to Korean Application No. KR10-2008-0040632 filed Apr. 30, 2008, the teachings of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to a gas target for producing gas isotopes such as C-11 and, more particularly, to a radioisotope production gas target, in which a fin structure is formed in an internal space, i.e. a target cavity, in which stable isotopes that are target materials cause a nuclear reaction with protons, thereby stably and remarkably increasing a yield of the production of the isotopes. 
     2. Description of the Related Art 
     Generally, isotopes are produced by irradiating protons or neutrons to stable isotopes. In this manner, a mechanism or an apparatus that makes it possible to irradiate the protons or the neutrons to the stable isotopes refers to a target. 
     A radioactive medicament called 2-[ 18 F]fluoro-2-deoxy-D-glucose ([ 18 F]FDG) (hereinafter, referred to as “FDG”) that synthesizes fluorine (F) into glucose is used in positron emission tomography (PET) for the diagnosis of tumors or cancer. In the case of image diagnosis of a brain or a heart, gas isotopes such as C-11 are used for high reliability. The representative gas isotope, C-11, is converted into a radioactive compound such as methyl iodide (MeI) or acetate, and then is used for diagnosis. 
     The gas isotopes such as C-11 are produced by irradiating accelerated protons to gaseous stable isotopes. An apparatus for accelerating the protons is an accelerator called a cyclotron, which is widely used for research and diagnosis in many institutes which use the PET. 
     The gas target is basically configured of a target window that is an entrance into which the protons accelerated by the cyclotron are sent, a target cavity that is a space in which the accelerated protons cause a nuclear reaction with the target materials (or stable isotopes) so as to produce radioisotopes, a target cooling system that collects heat generated by energy absorption at the target cavity, and a targetry system that supplies the stable isotopes to the target and collects the produced radioisotopes. 
     The gas isotopes, C-11, which are to be used in the PET are produced through a nuclear reaction,  14 N(p,α) 11 C, by generating a beam of protons from the accelerator, that is, the cyclotron, and irradiating the protons generated from the cyclotron to the stable isotopes, N 2 , that are the target materials. 
     The protons accelerated by the cyclotron are characterized in that the energy thereof is sharply reduced according to the density of material. Thus, the target window, which is a target incident section for producing the isotopes, is designed to least have only a mechanism so as to be able to maintain the proton energy at the maximum extent. For this reason, a thin metal sheet is used in the front of the target window through which the proton beam passes, and a structure such as a grid structure is installed together so as to be able to withstand high pressure. 
       FIG. 1  illustrates one example of a conventional gas target that is designed and used according to the aforementioned principle and basic configuration. A target window  10  onto which the proton beam is incident has a diameter of about 20 mm, which is designed to an appropriate size so that the proton beam can pass through when the proton beam generated from the cyclotron is widened to the maximum extent. A support structure  12  is installed adjacent to the target window  10  so as to support a thin metal sheet  14 . 
     The gas target used for producing the isotopes is divided into two types, a cylindrical type and a conical type, according to a shape thereof. The conical type gas target is adapted to the spatial shape of proton beam locus increasing its cross section by scattering as it approaches the second half thereof in the gas target (see  FIG. 2 ). 
     A portion where the nuclear reaction is produced by the proton beam undergoes a phenomenon called density reduction caused by compressibility of gas as well as generation of heat. Here, the density reduction refers to an effect where the heat is generated from portion where the nuclear reaction occurs by the application of the proton beam, and thus the portion where the nuclear reaction occurs is subjected to a reduction in density, whereas a surrounding portion remote from the portion where the nuclear reaction occurs is subjected to an increase in density. For this reason, a length of the proton beam passing through the gas is varied, and secondary beam scattering takes place at a rear end where the nuclear reaction occurs ( The International Journal of Applied Radiation and Isotopes , Volume 33, Issue 8, August 1982, Pages 653-659, Sven-Johan Heselius, Peter Lindblom, Olof Solin;  The International Journal of Applied Radiation and Isotopes , Volume 35, Issue 10, October 1984, Pages 977-980, Sven-Johan Heselius, Peter Lindblom, Ebbe M. Nyman, Olof Solin). 
     Further, when beam divergence is larger than the diameter of an interior of the target, i.e. a target cavity, according to a characteristic of the proton beam, this leads to a loss of the energy of the proton beam, and thus serves as a factor that reduces production yield of radioisotopes. Accordingly, the loss of the proton beam energy is prevented by a conical gas target, which has been recently manufactured according to a shape corresponding to a shape of the beam divergence. Thereby, the conical gas target is being studied beginning from the concept that the conical gas target obtains a yield higher than that of the cylindrical gas target. 
     However, the production of the radioisotopes using the cylindrical or conical gas target is basically accompanied with a generation of high pressure, so that it causes a problem with safety of the thin metal sheet installed as the target window. Further, such production fails to effectively inhibit the effect of the density reduction caused by the nuclear reaction, so that it increases instability of the production yield. In other words, only the conversion of the shape of the gas target from the cylindrical type to the conical type has a limitation to improving the production yield of the radioisotopes and maintaining production stability of the radioisotopes. 
     In order to ensure a stable production yield of the radioisotopes, it is necessary to effectively cool the gas target. Thus, the conventional gas targets as illustrated in  FIG. 1  have employed a method of lowering a temperature of a coolant flowing through a cooling channel  18  installed outside a target chamber  16  in order to inhibit the gas in the target chamber  16  from being raised in temperature, or a method of increasing a heat transfer area by forming cooling fins (not shown) on an outer surface of the target chamber  16  that is in contact with the coolant. 
     However, the cooling fins are based on a basic concept that the cooling fins are installed when heat exchange and heat transfer effects of a fluid can be expected to be improved by increasing a heat radiation surface area in a direction in which the heat transfer from the fluid does not sufficiently occur. As such, the configuration in which the cooling fins are formed on the outer surface of the target cavity as in the conventional gas target is estimated to be not quite optimal. 
     In other words, when the cooling fins are formed on the outer surface of the target cavity, the outer surface of the target cavity that is in contact with the coolant may be sufficiently cooled. However, since the capacity of heat transmitted from the target gas of the target cavity which is generally no more than one several hundredth of the liquid to the outer surface of the target cavity is not sufficient, it will be difficult to expect the cooling of the target gas. As such, it is necessary to design the gas target based on a new concept so that the target material, i.e. the gas, in the target cavity itself can be effectively cooled. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an aspect of the present invention to provide a new concept of radioisotope production gas target, which can be applied to a conventional cylindrical gas target as well as a conical gas target considering a divergence phenomenon of a beam of protons, and thus improve yield and stability in the production of isotopes. 
     According to exemplary embodiments of the present invention, the radioisotope production gas target can directly and more efficiently cool gases in a gas chamber in order to accomplish the yield and stability of the production of isotopes. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
     The foregoing and/or other aspects of the present invention are achieved by providing a radioisotope production gas target, which includes: a target chamber that is shaped of a hollow cylinder and has a plurality of inner fins protruding from an inner surface thereof along a length thereof; and a body that is shaped of a hollow cylinder enclosing the target chamber, has a target gas inlet for feeding target gas to a hollow region of the target chamber (the hollow region being the interior volume of the target chamber), a target gas outlet for collecting the target gas after a nuclear reaction occurs, and a first coolant inlet and a first coolant outlet for feeding and discharging a coolant flowing along an outer surface of the target chamber, and includes a thin metal sheet in the front thereof through which a beam of protons passes. 
     According to an embodiment of the present invention, the body may include: a front adaptor that is shaped of a ring, a central part of which is bored, and which has a circular groove on a radial outer side of the central part, that has the target gas inlet communicating with the bored central part in a front surface of the front adaptor and the first coolant inlet communicating with the groove in a rear surface of the front adaptor, and that is coupled to a front end of the target chamber such that the bored central part communicates with a hollow region of the target chamber with the groove facing the target chamber; a rear adaptor that is coupled to a rear end of the target chamber, includes the target gas outlet in an outer circumference thereof which communicates with the hollow region of the target chamber, and at least one slot in an inner circumference thereof at a portion where the rear adaptor is coupled with the target chamber; casings coupled between the front adaptor and the rear adaptor so as to enclose an outside of the groove of the front adaptor and an outside of the slot of the rear adaptor; a front flange having a grid structure supporting a thin metal sheet and coupled to a front surface of the front adaptor; and a rear flange having the first coolant outlet and coupled to a rear surface of the rear adaptor. Further, the thin metal sheet may be disposed between the front adaptor and the front flange. 
     According to another embodiment of the present invention, the target chamber may be formed by coupling a plurality of target chamber units having at least one of the inner fins. Particularly, the target chamber units may be coupled with each other by welding. 
     According to another embodiment of the present invention, the target chamber may include a plurality of outer fins protruding from the outer surface thereof along the length thereof. In this case, the target chamber may be formed by coupling a plurality of target chamber units having at least one of the inner fins and at least one of the outer fins. Particularly, the target chamber units may be coupled with each other by welding so as to be able to maintain internal airtightness. 
     According to another embodiment of the present invention, the front flange may include a groove formed around the grid structure, and a cover member covering a front of the groove, and have second coolant inlet and outlet in an outer circumference thereof. Here, the second coolant inlet and outlet may be formed so as to be opposite to each other. 
     According to another embodiment of the present invention, the rear adaptor may include a concave space recessed from a portion where the rear adaptor is coupled to the rear end of the target chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating one example of a conventional gas target; 
         FIG. 2  is a photograph taken of divergence of a beam of protons in a target chamber (Optical Studies of the Influence of an Intense Ion Beam on High-pressure Gas Targets—The International Journal of Applied Radiation and Isotopes, Volume 33, Issue 8, August 1982, Pages 653-659, Sven-Johan Heselius, Peter Lindblom, Olof Solin); 
         FIG. 3  is a perspective view illustrating a gas target according to an embodiment of the present invention; 
         FIG. 4  is an exploded perspective view illustrating the gas target of  FIG. 3 ; 
         FIG. 5  is an exploded perspective view illustrating an example in which the target chamber of the gas target of  FIG. 3  is configured of target chamber units; 
         FIG. 6A  is a graph showing a change in pressure in a target chamber when beams of protons having capacities of current of 10 μA and 20 μA are irradiated in an initial state in which target gas is filled in the target chamber; and 
         FIG. 6B  is a graph showing a production yield of isotopes in the cases of  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in greater detail to a radioisotope production gas target according to an exemplary embodiment of the invention with reference to the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
     The radioisotope production gas target  100  according to an exemplary embodiment of the invention generally includes: a target chamber  110  that is shaped of a hollow cylinder and has a plurality of inner fins  112  protruding from an inner surface thereof along a length thereof in a radial inward direction; and a body that is shaped of a hollow cylinder enclosing the target chamber  110 , which has a target gas inlet  124  for feeding target gas to a hollow region in the target chamber  110 , a target gas outlet  134  for collecting the target gas after a nuclear reaction occurs, and a first coolant inlet  122  and a first coolant outlet  172  for respectively feeding and discharging a coolant flowing along an outer surface of the target chamber  110 , and is provided with a thin metal sheet  162  in a front thereof through which a beam of protons passes. 
     Thus, the gas target  100  having this structure is configured in such a manner that, because the hollow body encloses the outside of the hollow target chamber  110 , an annular space is defined between an inner surface of the body and the outer surface of the target chamber  110 , and a circular cross-sectional space is defined by an inner surface of the target chamber  110 . The annular space, which is defined between the inner surface of the body and the outer surface of the target chamber  110 , functions as a coolant channel through which a coolant, for instance, a fluid such as water, flows. The circular cross-sectional space, which is defined by an inner surface of the target chamber  110 , functions as a channel which is filled with target gas, for instance with a target material such as stable isotopes, N 2 , for producing gas isotopes, C-11. 
     Particularly, it should be noted that the target chamber  110  of the gas target  100  is provided with the plurality of inner fins  112 , which protrudes from the inner surface of the target chamber  110  along the length of the target chamber  110  in a radial inward direction. In this manner, since the inner fins  112  are formed on the inner surface of the target chamber  110  along the length of the target chamber  110  in a radial inward direction, the gas target  100  according to the embodiment of the present invention can more effectively and directly transmit heat, which is generated by the nuclear reaction of the target gas in the process of the nuclear reaction and is a property of the gas in the target chamber  110  including gas isotopes, to the outside. 
     Preferably, the inner fins  112  are formed on the inner surface of the target chamber  110  in a radial inward direction. This is because the gas heated in the target chamber  110  undergoes convention around the proton beam having the conical locus as illustrated in  FIG. 2  in a lengthwise direction of the proton beam. Thus, the configuration in which the inner fins  112  are formed on the inner surface of the target chamber  110  in a radial inward direction is more efficient from the viewpoint of heat transfer. 
     The target chamber  110  may be configured in such a manner that a plurality of target chamber units  110 ′ having at least one of the inner fins  112  is coupled with each other. In this manner, the configuration in which the individual target chamber units  110 ′ are coupled with each other without manufacturing the target chamber  110  in an integrated shape is advantageous in that it can not only improve convenience of manufacturing but also makes it easier to adjust the length of the target chamber by means of adjustment of the number of target chamber units  110 ′ so as to be able to manufacture the target chamber  110  in various sizes. Preferably, these target chamber units  110 ′ are mutually coupled by welding in order to maintain good airtightness. Of course, any other replaceable coupling method can be used as long as the airtightness of the target chamber  110  can be maintained. 
     Further, the target chamber  110  may be additionally provided with a plurality of outer fins  112 ′ protruding from the outer surface thereof along the length thereof in a radial outward direction in addition to the inner fins  112 . This is equally applied to each target chamber unit  110 ′. Thus, each target chamber unit  110 ′ may be additionally provided with at least one outer fin  112 ′ protruding from the outer surface thereof along the length thereof in a radial outward direction. The outer fins  112 ′ are for improving heat transfer efficiency. Owing to this heat transfer efficiency, the heat transmitted from the gases in the target chamber to the outer surface of the target chamber  110  through the inner fins  112  is transmitted to the coolant flowing along the outer surface of the target chamber  110 . Making reference to the target chamber units  110 ′ in greater detail, the inner and outer fins  112  and  112 ′ are formed on the inner and outer surfaces of each target chamber unit  110 ′, respectively. 
     The body enclosing the target chamber  110  having the aforementioned configuration has the shape of a hollow cylinder, and includes the target gas inlet  124  feeding the target gas to the hollow region in the target chamber  110 , the target gas outlet  134  collecting the target gas after the nuclear reaction occurs, and the first coolant inlet  122  and the first coolant outlet  172  feeding and discharging the coolant flowing to and from the annular coolant channel, which is defined between the inner surface of the body and the outer surface of the target chamber  110 . Preferably, the target gas inlet  124  and the first coolant inlet  122  are formed at a first end of the body, while the target gas outlet  134  and the first coolant output  172  are formed at the second end of the body. 
     Further, the body is provided with a grid structure  168  through which the proton beam passes at one end thereof at which the target gas inlet  124  and the first coolant inlet  122  are formed together which the thin metal sheet  162 . The configuration of the body will be described below in detail. In this case, in consideration of the ease of description together with a function of the body, the first end of the body having the grid structure  168 , the entrance through which the proton beam passes, and the thin metal sheet  162  will be called a front portion, and the second end of the body, that is, the exit through which the coolant and the gas isotopes are discharged, will be called a rear portion. 
     The configuration of the body, which has been described above in brief, will be described below in greater detail with reference to  FIGS. 3 through 5 . 
     The body generally includes a front adaptor  120  coupled to a front end of the target chamber  110 , a front flange  160  coupled to a front surface of the front adaptor  120 , a rear adaptor  130  coupled to the rear end of the target chamber  110 , a rear flange  170  coupled to a rear surface of the rear adaptor  130 , and casings  140  and  140 ′ coupled between the front adaptor  120  and the rear adaptor  130 . For the sake of ease of manufacturing, the casings  140  and  140 ′ are preferably made by cutting a cylindrical pipe into two pieces in a lengthwise direction. 
     The front adaptor  120  is shaped of a ring, a central part  126  of which is bored, and which has a circular groove  123  on a radial outer side of the central part  126 , particularly between outer and inner circumferences thereof. The bored central part  126  communicates with the target gas inlet  124  through a front surface of the front adaptor  120 , while the groove  123  communicates with the first coolant inlet  122  through a rear surface of the front adaptor  120 . Here, since the central part  126  and the groove  123  of the front adaptor  120  are separated from each other, the target gas inlet  124  and the first coolant inlet  122  are also separated from each other. 
     This front adaptor  120  is coupled to the front end of the target chamber  110  such that the groove  123  is opposite to the target chamber  110 . Thus, the bored central part  126  of the front adaptor  120  communicates with the hollow region of the target chamber  110 , and the groove  123  of the front adaptor  120  is exposed to the rear side of a portion where the front adaptor  120  is coupled with the target chamber  110 . 
     The rear adaptor  130  coupled to the rear end of the target chamber  110  includes the target gas outlet  134  in an outer circumference thereof which communicates with the hollow region of the target chamber  110 , and at least one slot  132  in an inner circumference thereof at a portion where the rear adaptor  130  is coupled with the target chamber  110 . Thus, similar to the groove  123  of the front adaptor  120 , the slot  132  is also exposed to the front side of the portion where the rear adaptor  130  is coupled with the target chamber  110 . 
     Further, the rear adaptor  130  includes a concave space  138 , into which the gas in the target chamber  110  can be collected, and which is recessed from the portion where the rear adaptor  130  is coupled to the rear end of the target chamber  110 . In this case, the target gas outlet  134  preferably communicates with the concave space  138 . 
     The casings  140  and  140 ′ in the shapes of a pipe are coupled between the front adaptor  120  and the rear adaptor  130  in such a manner that they enclose the outside of the groove  123  of the front adaptor  120  and the outside of the slot  132  of the rear adaptor  130 . Thus, the outside of the target chamber  110  is airtightly sealed by the casings  140  and  140 ′, and the coolant channel through which the groove  123  of the front adaptor  120  and the slot  132  of the rear adaptor  130  are coupled is defined by the casings  140  and  140 ′. 
     The front surface of the front adaptor  120  is coupled with the front flange  160  having the grid structure  168  that is the entrance to which the proton beam is applied. The thin metal sheet  162  is disposed between the front adaptor  120  and the front flange  160 . The grid structure  168  is formed in a substantially cylindrical shape, and serves to support the thin metal sheet  162  disposed between the front adaptor  120  and the front flange  160 . 
     The front adaptor  120  is provided with a plurality of through holes  128 , which pass between the outer circumferences of the front and rear surfaces of the front adaptor  120 , and the front flange  160  is provided with a plurality of through holes  166 , which pass between the outer circumferences of the front and rear surfaces of the front flange  160 . Thus, the front adaptor  120  and the front flange  160  are coupled with each other through the through-holes  128  and  168 . For example, the through-holes  128  and  166  are provided with screw threads on inner circumferences thereof, and then are fastened with bolts corresponding to the screw threads. 
     Further, the front flange  160  includes a channel for cooling the surroundings of the grid structure  168 , and a second coolant inlet  164  and a second coolant outlet (not shown) which communicate with the channel and are formed so as to be opposite to each other. A method of forming the channel connected to the second coolant inlet  164  and the second coolant outlet in the front flange  160  will be described by way of example. A groove  165  is formed around the grid structure  168  of the front flange  160 , and then is covered with an annular cover member  167  in the front thereof. A coolant, which is identical or similar to the coolant flowing along the outer surface of the target chamber  110 , flows through the second coolant inlet  164  and the second coolant outlet. 
     The rear surface of the rear adaptor  130  is coupled with the rear flange  170  having the first coolant outlet  172 . The first coolant outlet  172  of the rear flange  170  communicates with the slot  132  of the rear adaptor  130 . Preferably, in the case in which there are two or more slots  132 , the first coolant outlet  172  must be formed in the center of the group of slots  132 . Thus, according to the embodiment of the present invention, the first coolant outlet  172  is formed in the center of the rear surface of the rear flange  170 . 
     Further, the first coolant outlet  172  preferably communicates with the slot  132  through a storage space  174 , which is recessed from the front surface toward the rear surface of the rear flange  170 . This is because the storage space  174  functions to absorb shock so as to help the coolant smoothly flow from the slot  132  to the first coolant outlet  172 . 
     Here, the rear adaptor  130  is provided with a plurality of through-holes  136 , which pass between the outer circumferences of the front and rear surfaces of the rear adaptor  130 , and the rear flange  170  is provided with a plurality of through-holes  176 , which pass between the outer circumferences of the front and rear surfaces of the rear flange  170 . Thus, the rear adaptor  130  and the rear flange  170  are coupled with each other through the through-holes  136  and  176 , which is equal to the coupling method of the rear adaptor  130  and the rear flange  170 . In detail, the through-holes  136  and  176  are provided with screw threads on inner circumferences thereof, and then are fastened with bolts corresponding to the screw threads. 
     In the embodiments of the prevent invention, the radioisotope production gas target shows the following effects as can be clearly seen from  FIGS. 6A and 6B . 
       FIG. 6A  is a graph showing a change in pressure in a target chamber when beams of protons having capacities of current of 10  82  A and 20 μA are irradiated in an initial state in which target gas is filled in the target chamber, and  FIG. 6B  is a graph showing a yield of production of isotopes in the cases of  FIG. 6A . 
     Comparing pressure data of the gas target of the embodiment of the present invention with that of a conventional gas target in the case in which the target chamber has a length of 150 mm, it can be found that the increase in pressure is remarkably reduced in the case of the embodiment of the present invention. Of course, in the case in which the target chamber has a length of 250 mm, the gas target of the embodiment of the present invention in which the target chamber has a length of 150 mm shows that the pressure thereof is higher than that of the conventional gas target. However, this is responsible for an initial pressure difference in the target chamber. As such, from the viewpoint of the production yield of the isotopes, it can be seen that the gas target of the embodiment of the present invention is very stable regardless of the length of the target chamber regardless of the production yield as well as the capacity of current of the proton beam, as compared to the conventional gas target. 
     The improvement of the stable production yield of the gas target in the embodiment of the present invention can be regarded to result from inhibiting the pressure in the target chamber from being increased. 
     Thus, the gas target of the embodiment of the present invention shows a lower increase in pressure in the case of the same capacity of current, as compared to the conventional gas target. As such, the proton beam having a higher capacity of current can be irradiated, so that the production yield can be increased, and be maintained with higher reliability. Thereby, the gas target of the embodiment of the present invention can obtain the gas isotopes desired by a user at a larger quantity over a shorter time. 
     Further, when the pressure increase in the target chamber is inhibited, the thin metal sheet vulnerable to high pressure can be used for a longer time in addition to the improvement of the production yield, so that the durability of the entire gas target having the thin metal sheet can be improved. 
     Although exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.