Patent ID: 12196899

DETAILED DESCRIPTION

A neutron detector according to each embodiment of the present invention is a scintillation detector for detecting neutrons. Accordingly, similarly to the neutron detector described in Patent Document 2, for example, and the like, it uses a scintillator that absorbs neutrons and thereby emits fluorescent light and a photodetector that detects the fluorescent light in combination. The present invention is characterized by a structure of the scintillator or by a form of combining the scintillator with the photodetector. Hereinafter, two embodiments distinguished based on the basic structure of the scintillator will be described.

First Embodiment

FIG.1Ashows a plan view representing a structure of a scintillator10used in a neutron detector according to a first embodiment, andFIG.1Bshows a cross-sectional view representing that. Here, neutrons to be detected enter the scintillator10from the negative side in the z direction in the diagrams. The scintillator10has a layered structure in which a phosphor layer11and a light transmission layer12are alternately laminated in the z direction. The phosphor layer11and the light transmission layer12each have a thin film form extending in the x- and y-directions in the diagrams. While five phosphor layers11and four light transmission layers12are provided inFIGS.1A,1B, the numbers of layers are appropriately set.

The phosphor layer11is made of a phosphor that emits fluorescent light by absorbing the energy of charged particles, and the charged particles can be detected by detecting the fluorescent light. Here, to make the phosphor have sensitivity particularly to neutrons having no charge, a phosphor to which a neutron-absorbing isotope is added is used, for example. Examples of such a phosphor are an already known scintillation glass for neutron detection as6Li-glass:Ce3+. Specifically, GS20, KG2 (manufactured by Scintacor) and the like are used. To obtain sufficient n/γ discrimination ability, the amount of luminescence caused by absorbing neutrons is preferably 1.5 MeVee or larger in terms of electron equivalent energy (MeVee). The light transmission layer12is made of a material having high transmittance for the fluorescent light emitted by the phosphor and absorbing neutrons only slightly. Further, it is also preferable that the light transmission layer12has a refractive index for the fluorescent light close to that of the phosphor layer11, as will be described later, and is transparent to the fluorescent light, and materials preferably used to form the light transmission layer12include synthetic quartz, lead glass slightly containing lead oxide, and the like.

With such a configuration, the scintillator10is increased in n/γ discrimination ability in neutron detection, and highly efficient neutron detection can be performed with the increased n/γ discrimination ability. This point will be described below. In general, a phosphor forming a scintillator emits fluorescent light by absorbing the energy of charged particles. When the fluorescent light is detected by a photodetector (photomultiplier tube) or the like having high temporal resolution, photoelectrons are generated on the photocathode by the fluorescent light and are amplified, thereby producing a pulsed electrical output according to temporal distribution of the luminescence. The number of the photoelectrons corresponds to the luminescence intensity, which corresponds to the output pulse, so-called pulse height or an integrated charge value.

There, while there is a slight difference between neutrons and gamma rays in the output pulse waveform, caused by a difference in the form of energy transfer to the phosphor from them, it generally is not easy to distinguish between the output pulse produced by fluorescence at a time of neutron absorption and the output pulse produced by fluorescence at a time of energy transfer from gamma rays, in a case the amount of light (total photon number) is about the same for the two types of fluorescence. For example, even when the emission energy at a time of neutron absorption in the phosphor is 4.78 MeV, as will be described later, electron equivalent energy for the corresponding luminescence amount is about 1.6 MeVee in GS20 described above. On the other hand, for example, gamma rays of 2.2 MeV mainly undergo Compton scattering in the phosphor, where they transfer continuous energy of about 2.0 MeV and lower to electrons in the phosphor. Therefore, in a region where pulse heights due to the two types of fluorescence overlap each other, it is theoretically impossible to distinguish between the two types of fluorescence from the pulse heights. It is demanded to detect neutrons by discriminating them from gamma rays even in such a situation.

In this respect, the scintillator10of the present invention is configured to have a structure in which, when neutrons and gamma-ray photons are incident, luminescence intensity (pulse height) is largely different between the neutrons and the gamma-ray photons. Thereby, it becomes possible to easily discriminate between them from the pulse heights,

First, a description will be given of output (pulse height distribution of output pulses) in a case where a general type of scintillator is used and both neutrons and gamma rays are present.FIG.2is a diagram schematically illustrating the situation. There, the horizontal axis represents pulse height of output pulses, which corresponds to energy absorbed by the phosphor from a single neutron or gamma-ray photon. The vertical axis represents detection frequency of neutrons or gamma-ray photons when they are detected in large numbers.

InFIG.2, D1 is a pulse height distribution of output pulses due to neutrons in a case of using a usual scintillator whose entire body is made of a thick phosphor. Here, the peak energy of the distribution corresponds to a constant emission energy due to nuclear reaction between6Li and neutrons, which will be described later, (electron equivalent energy of the luminescence amount is 1.6 MeVee in GS20, as described above). On the other hand, D2 is a pulse height distribution for gamma rays (2.2 MeV) when using the same scintillator. Here, in contrast to that D1 due to neutrons has a single peak as described above, the distribution due to gamma rays is a broad one, because gamma rays generate a continuous spectrum caused by Compton scattering and the like extending from near a maximum energy the gamma rays have toward the low energy side. As indicated here, it is difficult, in the region with overlap of D1 and D2, to distinguish (discriminate) between neutrons and gamma rays from only the pulse heights.

In general, a phosphor constituting the scintillator becomes luminous (emits fluorescent light) by absorbing energy of charged particles having sufficiently higher energy than that necessary to raise electrons in the phosphor from the ground state into the excited state. A wavelength of the luminescence corresponds to the energy difference between the excited state and the ground state, and photons having the luminescence wavelength are generated in numbers according to the energy absorbed by the phosphor and the amount of absorbed energy per unit length or the like.

Here, since gamma-ray photons transfer their energy to electrons in the phosphor by undergoing electromagnetic interaction with the electrons, the above-described charged particles are the electrons in that case, and fluorescent light is emitted by the electrons transferring their kinetic energy to the phosphor. At that time, the electrons ejected by gamma rays tend to progress in the forward direction according to the law of conservation of momentum, and in particular, the progressing direction of electrons having received a large amount of energy being closer to the pulse height due to neutrons becomes closer to the initial incident direction of the gamma rays.

In contrast, neutrons scarcely undergo electromagnetic interaction, and accordingly neutron absorption probability of general substances is low. However, when the phosphor contains a neutron-absorbing isotope that emits high-energy secondary charged particles by absorbing neutrons, high-energy secondary charged particles are generated by the neutron-absorbing isotope when absorbing neutrons. For example, when a well-known neutron-absorbing isotope6Li is used, the reaction is as expressed by an equation (1).
[Equation 1]
n+6Li→α+3H+4.78 MeV  (1)

That is, the secondary charged particles in this case are α particles (4He nuclei) and tritium (3H nuclei), which have no direction dependence in their emission distribution, unlike electrons in the above-described gamma ray case, and are emitted in opposite directions while having kinetic energies of 2.05 MeV and 2.73 MeV respectively and totally about 4.78 MeV, according to the law of conservation of momentum. Subsequently, electrons in the phosphor are excited by the secondary charged particles, and fluorescent light is emitted in a similar way. It is the same for other neutron-absorbing isotopes (for example,10B) that energy is transferred from secondary particles, while the secondary particles may be different in kind and energy from the above-described ones.

Here, since [mass of secondary charged particle (nucleus)]>>[mass of electron] (for example, [mass of α particle]:[mass of electron]=7300:1), when they have almost the same energy, [velocity of secondary charged particle]<<[velocity of electron] stands. Accordingly, there arises a difference between the situation where electrons generated by gamma-ray photons cause luminescence in the phosphor and the situation where secondary charged particles generated by neutrons cause luminescence in the phosphor.FIG.3is a diagram schematically showing these situations. There, both gamma-ray photons and neutrons are assumed to be incident from the left side in the diagram.

InFIG.3, (A) in a top row schematically shows a situation where high-energy electrons generated by gamma-ray photons transfer their energy and thereby cause luminescence in a scintillator100made of a thick phosphor. On the other hand, (B) in a middle row and (C) in a bottom row show similar situations for secondary charged particles generated by neutrons, in comparison. Here, the diagram is the one illustrating a region or the like of luminescence, but not the one illustrating the total amount of luminescence. In addition to the fact that the speed of electrons is higher than that of secondary charged particles, as described above, the charge amount of electrons is half that of a particles (secondary charged particles), for example. Since transfer energy per unit transit distance (dE/dx) of charged particles (electrons, secondary charged particles) is proportional to the product of transit time (inversely proportional to velocity) and square of the charge amount, [dE/dx of secondary charged particles]>> [dE/dx of electrons] stands. Accordingly, in the scintillator100. [range of secondary charged particles]<<[range of electrons] stands, and for example, a maximum range of 1.5 MeV electrons in the above-described phosphor GS20 is about 2.5 mm. However, because the electrons are light (or have the same mass as electrons in the phosphor), they accordingly are easily scattered, have complicated routes, as depicted inFIG.3(A), and practically have a shorter range. When the scintillator100is thinner than the range of the electrons, the probability that the electrons cannot transfer all the energy to the scintillator100becomes high, and the pulse height is reduced compared to in a case the scintillator100is sufficiently thick.

On the other hand, since high-energy secondary charged particles generated by neutron absorption transfer a large amount of energy per unit transit distance, they need only a short distance for transferring all the energy, for example, several μm for α particles and several tens of μm for3H, in the phosphor GS20.

Accordingly, when a neutron-absorbing isotope is contained in a phosphor, the thickness of the phosphor may be set at a level of enabling transfer of almost all kinetic energy of secondary charged particles in neutron absorption, and enabling immediate departure of high-energy electrons generated in interaction with gamma-ray photons. When the thickness is thus set, inFIG.2, in contrast to that the pulse height distribution due to neutrons D1 ideally is maintained with no change, the pulse height distribution due to gamma rays changes from D2 to D3 present in the lower energy side. There, by setting a threshold value T between the distributions D1 and D3, it is possible to determine that neutrons have been detected when a detected pulse height is equal to or larger than T, and that the gamma rays have been detected when the pulse height is smaller than T.

Since, as described earlier, the progressing direction of high-energy electrons is substantially equal to the incident (progressing) direction of gamma rays, it is preferable, in order to enhance the above-described effect, that the incident direction of gamma rays is equal to the thickness direction of the phosphor when the phosphor is of a thin film form. In many of the cases where both neutrons and gamma rays are present, the neutron source and the gamma ray source overlap, and accordingly the condition is satisfied.

Since the probability of neutron absorption is not high, there may be a case, as shown in (B) inFIG.3, where neutrons are absorbed near the surface and secondary charged particles are generated there, and there may be also a case, as shown in (C), where neutrons are absorbed at a deep position in the phosphor and secondary charged particles are generated there. When the phosphor is thin and composed of only a single layer, neutrons can be detected in the case of (B), but neutrons cannot be detected in a case of (C) corresponding to a case where neutrons pass through the phosphor without undergoing interaction, where the neutron detection efficiency may accordingly be reduced. In contrast, by employing a multilayer structure composed of phosphor layers11and light transmitting layers12as shown inFIG.1Bto maintain a total thickness of the phosphor layers11, it becomes possible to detect neutrons even in the case of (C), and thereby keep a high neutron detection efficiency.

Hereinafter, a description will be given of a result of specific investigation performed to make clear the above-described matter.FIG.4shows a result of calculating the probability that most of the energy of secondary charged particles (α particles,3H nuclei) generated by neutrons is absorbed in the phosphor. Here, it is assumed that the neutron-absorbing isotope is6Li as described earlier, accordingly the secondary charged particles are α particles and3H nuclei, and that the phosphor is GS20 already described. While the phosphor is preferred to be thick in order to sufficiently increase the absorption probability (to nearly 1), it is recognized from the result that the absorption rate is 0.98 when the phosphor thickness is 1.0 mm, about 0.94 even when the thickness is 0.25 mm, but is drastically reduced for smaller thicknesses.

On the other hand, as described earlier, the electron equivalent energy corresponding to the luminescence amount due to neutrons (secondary charged particles described above) when using GS20 is 1.6 MeVee. Accordingly, the pulse height in output becomes substantially the same for electrons having that energy and for secondary charged particles, it accordingly is impossible to discriminate between gamma rays and neutrons (or between high-energy electrons and secondary charged particles) by the pulse height, as indicated by D1 and D2 inFIG.2.

To make it possible to discriminate between neutrons and gamma-ray photons generating electrons having the above-described level of energy by the pulse height of output pulses, the distribution of D3 inFIG.2can be realized by reducing the energy transferred from gamma rays (electrons generated by them) to the phosphor to be sufficiently smaller than 1.6 MeV. For example, a case considered here is that of using a general detector that is composed of a photomultiplier tube and the already-described GS20 phosphor (scintillator) whose energy resolution (a value obtained by dividing the peak value of a pulse height distribution by the full width of half maximum of the pulse height distribution) is about 16% for secondary charged particles and about 24% for electrons of 1.2 MeV. In this case, by setting T, in the pulse height distribution ofFIG.2, such that 99% of the distribution D1 for neutrons is included in a range equal to or larger than T, and making energy transfer caused by gamma rays in the phosphor layer 1.2 MeV or lower, almost no pulse height values for the gamma rays becomes included in the range equal to or larger than the threshold T, and accordingly the n/γ discrimination ability by the pulse height is increased. Therefore, the phosphor layer11may be made thin such that the transferred energy from electrons to the phosphor becomes 1.2 MeV or lower.

Since, as described earlier, the absorption probability of neutrons having an energy equal to or higher than that of epithermal neutrons, a total thickness of the phosphor layer11is required to be equal to or larger than a certain value in order to detect neutrons with high efficiency, and such a multilayer structure as shown inFIG.1Bis effective for this purpose. However, by adopting the multilayer structure, the probability of interaction with gamma rays is also increased, and even if their pulse height distribution is shifted toward the low energy side as described above, it may also occur that the frequency itself increases to cause increase in the sensitivity to gamma rays. For this reason, investigation was performed on the sensitivity to gamma rays (probability of transfer of energy exceeding 1.2 MeV to the phosphor) when the number of the phosphor layers11(and the thickness of individual layer) is varied, while keeping the total thickness constant.FIG.5shows a result of calculating relative values of the sensitivity to gamma rays when the total thickness of the phosphor layers11is fixed at 5 mm, and is distributed to a single layer (with a thickness of 5 mm), among three layers (with each individual phosphor layer11having a thickness of 1.67 mm), among 5 layers (with each individual layer11having a thickness of 1 mm), among 10 layers (with each individual layer11having a thickness of 0.5 mm), and among 20 layers (with each individual layer11having a thickness of 0.25 mm), where, in the multilayered cases, the light transmission layer12made of 2 mm thick synthetic quartz is inserted between the phosphor layers11, thus forming the multilayer structure ofFIGS.1A,1B. There, the energy of gamma rays was assumed to be 2.2 MeV corresponding to that to be emitted by reaction between moderated neutrons and hydrogen in the neutron moderator material, as described above. Such gamma rays of 2.2 MeV may be a major background event in various measurement conditions, and accordingly are one of gamma rays to which attention needs to be paid most in practical use.

InFIG.5, the case of 5 mm thickness (single layer) corresponds to that of a usual scintillator entirely and uniformly made of a phosphor without having no light transmission layer12. FromFIG.5, it is obvious that, by adopting the multilayer structure with the light transmission layers12inserted therein, the luminescence intensity due to gamma-ray photons can be substantially reduced compared to the usual scintillator.

The amount of electron energy absorption greatly depends on the product of the density and the thickness of the phosphor (referred to as density length). Therefore, to discuss the result more generally, it is preferable to use the density length obtained by multiplying the abscissa inFIG.5by the density of GS20 (2.5 g/cm3) as an index, and discussing in that way, it is considered that even when a phosphor other than GS20 is used, a similar result to that on GS20 can be obtained by using a phosphor having an equivalent density length. For example, compared with a case of using a single phosphor layer11with a density length of 1.25 g/cm2(equivalent to 5 mm thick GS20), energy absorption is reduced by adopting the above-described layered structure including five phosphor layers11each with 0.25 g/cm2density length (equivalent to 1 mm thick GS20), where the rate of absorbing energy of 1.2 MeV or higher becomes about 1/10. When the density length is used as the index, a preferred range of the density length of the phosphor layer11to obtain the above-described effect becomes 0.0625 to 0.5 g/cm2.

As a result, even in cases where the total thickness of the phosphor layers11is identical, when the thickness of each individual phosphor layer11is made smaller and the total number of the layers is made larger, a difference between the pulse height of output pulses due to neutrons and that due to gamma-ray photons is increased.

However, if neutrons are absorbed in the light transmission layer12, the neutrons are not detected because energy due to this reaction does not contribute to luminescence. As a result, in the scintillator10having the structure ofFIGS.1A,1B, the neutron detection efficiency is reduced by an amount according to the neutron absorption in the light transmission layer12. Therefore, it is desirable that a material having a low neutron absorption probability is used for the light transmission layer12.

It accordingly is preferable that the light transmission layer12is made of a material not containing the neutron-absorbing isotope described earlier and being transparent to the fluorescent light. However, in a neutron detector that will be described later, fluorescent light extracted from the scintillator10is detected outside, where reflection at the interface between the phosphor layer11and the light transmission layer12becomes an obstacle in the extraction of the fluorescent light to the outside of the scintillator10. To suppress such reflection at the interface, it is preferable that the refractive index of the phosphor layer11and that of the light transmission layer12is close to each other for the fluorescent light. Specifically, it is preferable that a rate of the refractive index of the light transmission layer12to that of the phosphor layer11is in a range from 0.90 to 1.10 at the wavelength of the fluorescent light. When, for example, the above-described GS20 is used for the phosphor layer11, synthetic quartz may be used as a material satisfying the above-described requirement. That is, by using materials whose main component is silicon dioxide (SiO2) as materials to form respective ones of the phosphor layer11and the light transmission layer12, the reflection at the interface can be suppressed.

As has been described above, when using the scintillator10ofFIGS.1A,1B, it is possible to significantly reduce only the pulse height of pulse output due to gamma-ray photons, without reducing that due to neutrons and the neutron detection efficiency. As a result, it becomes possible to discriminate between neutrons and gamma-ray photons only by the pulse height. Here, since the decay time constant of output pulses is determined by the material constituting the phosphor layer11, and the decay time constant is not increased at least by employing the above-described configuration, use of a material having a small value for the decay time constant, for the phosphor layer11, enables measurement in a high dose condition, similarly to usual scintillators.

Next, a description will be given of an aspect of practically using the scintillator10in a detector.FIGS.6A and6Bschematically show two types of neutron detectors1and2in which the above-described scintillator10is used. In the diagrams, an arrow A represents the incident direction of neutrons to detect or of gamma rays to be an obstacle to neutron detection, and arrows B, C and D represent the progressing directions of respective portions of fluorescent light emitted by neutrons or gamma rays in the scintillator10that are to be detected by a photodetector.

A photodetector used here is the one having high temporal resolution and being capable of issuing output pulses by receiving the above-described fluorescent light emitted by the phosphor layer11, and specifically is a photomultiplier tube or the like.

In the neutron detector1ofFIG.6A, both the incident direction of neutrons (arrow A) and that of light to be detected by the photodetector21(arrow B) are set to be in the layering direction inFIG.1B(z-axis direction). Accordingly, when the scintillator10is of a planar shape extending in the x-y plane inFIGS.1A,1B, the configuration can be implemented particularly easily.

While the single photodetector21is used inFIG.6A, a multi-channel photodetector or the like may be used with respect to the x-y plane ofFIGS.1A,1B. In that case, neutron absorption positions (luminescence positions) in the x-y plane of the scintillator10can be recognized with a resolution nearly equal to the channel interval of the photodetector. There, as well as the photodetector, the scintillator10may be similarly arranged in a divided form.

As shown by (B) and (C) inFIG.3, positions at which neutron absorption occurs vary in depth owing to a not high probability of neutron absorption, and accordingly in the present case, the phosphor layers11and the light transmission layers12are configured into the multilayer structure, as described above. InFIG.6A, fluorescent light emitted by the phosphor layer11on the most surface side (the leftmost side inFIG.6A) reaches the photodetector21after passing through all the layers located closer to the photodetector21than the phosphor layer11is (four light transmission layers12and four phosphor layers11), as indicated by a path R1. In contrast, fluorescent light emitted by the phosphor layer11on the closest side to the photodetector21(the rightmost side) reaches the photodetector21directly at a short distance, as indicated by a path R2. As a result, when absorption, decay, and reflection between layers of fluorescent light cannot be neglected in the phosphor layers11and the light transmission layers12, the former fluorescent light is detected as having a lower intensity (lower pulse height) than the latter one in the photodetector21. For this reason, even when the same energy is transferred, the luminescence intensity to be detected (pulse height) may differ depending on in which phosphor layer11inFIG.1Bthe energy is transferred. This broadens D1 inFIG.2in the lateral direction.

FIG.7shows a result of practically measuring the influence of the light transmission layer12in such detection of fluorescent light. It is a result of measuring pulse height distributions in output of the photodetector21corresponding to D1 inFIG.2, by using the configuration ofFIG.6A, where, for the purpose of investigating the influence, only the leftmost one of the phosphor layers11illustrated inFIG.1Bwas provided, with no other ones of the phosphor layers11being provided, and the number of the light transmission layers12provided on the right side of the single phosphor layer11was varied. There, the difference in the number of the light transmission layers12causes a difference in influence of the total thickness of light transmission layers which the fluorescent light is to pass through, and of reflection between the layers, and the like. There, the light transmission layers12were made of synthetic quartz, and the thickness of each individual layer was set at 2.5 mm. InFIG.7, a case represented by “NO GLASS LAYER” corresponds to that where fluorescent light emitted by the phosphor layer11reaches the photodetector21without passing through any light transmission layer12, and cases represented by “4 GLASS LAYERS” and “12 GLASS LAYERS” correspond to, respectively, that with four light transmission layers12(with 10 mm total thickness) provided therein, and that with twelve light transmission layers12(with 30 mm total thickness) provided therein. From this result, it is noticed that even when using such a material exhibiting sufficiently high light transmittance as synthetic quartz, a decrease in the detected pulse height is not negligible. When the scintillator10having the structure ofFIG.1Bis used inFIG.6A, from which phosphor layer11the fluorescent light detected by the photodetector21was emitted is variable, which causes variation in the number (total thickness) of layers the fluorescent light passed through, and as a result, a pulse height distribution practically obtained by the photodetector21becomes the sum of the pulse distributions shown inFIG.7. In such a case, even when broadening of the pulse distribution (full width of half maximum) shown inFIG.7is small for each individual phosphor layer, broadening of the summed pulse height distribution is thus increased. This is undesirable in respect of making the distributions D1 and D3 inFIG.2apart from each other.

Accordingly, in the case of the configuration ofFIG.6A, it is particularly preferable that the light transmittance is high for the phosphor layer11and the light transmission layer12and the reflectance at the interface of these layers is low (the difference in refractive index is small between the layers). Further, it is preferable not to increase the total number of laminated layers more than necessary.

Thus, as an influence of the light transmission layer12on the detection of fluorescent light by the photodetector21, variation of the pulse height distribution in neutron detection depending on the number of light transmission layers12which the fluorescent light is to pass through has been shown inFIG.7. Next, a description will be given of the sensitivity to gamma rays when the thickness of the light transmission layer12is varied.

FIGS.8and9each show a result of calculating the sensitivity to gamma rays when varying the thickness of the light transmission layer12, where, similarly to the case ofFIG.5, the total thickness of the phosphor layers11is fixed at 5 mm, and is distributed to a single layer (with a thickness of 5 mm), among three layers (with each individual layer having a thickness of 1.67 mm), among 5 layers (with each individual layer having a thickness of 1 mm), among 10 layers (with each individual layer having a thickness of 0.5 mm), and among 20 layers (with each individual layer having a thickness of 0.25 mm), andFIGS.8and9respectively show results for cases of gamma ray energy being 2.2 MeV and 5 MeV. The light transmission layer12is assumed to be made of synthetic quartz. UnlikeFIG.5, the vertical axis is represented in a logarithmic scale.

From the results ofFIGS.8and9, it is noticed that, in the multilayer structure, the sensitivity to gamma rays (luminescence intensity) decreases with increasing the total thickness of the light transmission layers12not contributing to the luminescence. In this respect, when neutron absorption by the light transmission layer12can be neglected, the light transmission layer12is preferred to be thick in order to reduce the luminescence intensity due to gamma-ray photons. However, while neutron absorption by the light transmission layer12is slight, the neutron detection efficiency gradually decreases with increasing the total thickness of the light transmission layers12. Further, particularly in an aspect shown inFIG.6B, which will be described later, a photo-sensing area of the photodetector needs to be made large when the light transmission layer12is thick, which causes another disadvantage in cost. Therefore, it is undesirable to increase the thickness of the light transmission layer12more than necessary, and specifically, the thickness of the light transmission layer12is preferred to be about 6 mm (1.3 g/cm2in terms of density length) or smaller. As shown inFIG.8, when the phosphor layer11is set at 0.25 mm in thickness (0.0625 g/cm2in terms of density length), sufficient effect of suppressing the sensitivity to 2.2 MeV gamma rays is expected even when the light transmission layer12is 1 mm in thickness (0.2 g/cm2in terms of density length). However, when the phosphor layer11is set at 0.25 mm in thickness, the total number of the phosphor layers11needs to be made large in order to secure the neutron detection efficiency, which causes broadening of the pulse height distribution in the configuration ofFIG.6A, as described above, and accordingly is undesirable. Even in the case of using the configuration ofFIG.6B, when the light transmission layer12is too thin, the light propagation efficiency in the in-plane direction is decreased, which also causes broadening of the pulse height distribution, and accordingly is undesirable in respect of making the distributions D1 and D3 inFIG.2apart from each other. Therefore, when expressed in terms of density length as in the above-described case of the phosphor layer11, a preferred range for the light transmission layer12is 0.2 g/cm2to 1.3 g/cm2.

Meanwhile, in the neutron detector2ofFIG.6B, incident directions (arrows C and D) of fluorescent light to be detected by photodetectors are different from that in the case ofFIG.6Aby 90 degrees, where two photodetectors, a first photodetector31A and a second photodetector31B, opposing each other are used, with the scintillator10inserted between them in the y direction. Since luminescence occurring when neutrons are absorbed in the phosphor layer11has no specific directionality, and light is emitted in all directions, the emitted light can be detected also by the photodetectors31A and31B.

There, light emitted by the leftmost phosphor layer11inFIG.6Band light emitted by the rightmost phosphor layer11, which have been mentioned earlier, each pass through the phosphor layer11having emitted the light and light transmission layers12neighboring the phosphor layer11, along the y direction, and there is no difference between them in path length from their emission to their arrival at the photodetectors31A and31B. Accordingly, in the present case, unlike in the case ofFIG.6A, it does not occur that the pulse height differs depending on which one of five phosphor layers11has emitted the light (has absorbed neutrons), and as a result, there occurs no broadening of the pulse height distribution due to the employment of the multilayer structure.

On the other hand, when such light absorption or the like in the phosphor layer11and the light transmission layer12as described above cannot be neglected, it affects the pulse height of output pulses obtained by the photodetectors31A and31B. InFIG.6B, when neutrons are absorbed on the side of the photodetector31A (upper side in the diagram) in a phosphor layer11, thus emitted light reaches the photodetector31A via a short path R3and does the photodetector31B via a long path R4. In contrast, when neutrons are absorbed on the side of the photodetector31B (lower side in the diagram) in a phosphor layer11, thus emitted light reaches the photodetector31A via a long path R5and does the photodetector31B via a short path R6. Accordingly, in a case where light absorption cannot be neglected, when neutrons are absorbed on the side of the photodetector31A, the pulse height in the photodetector31A becomes higher and that in the photodetector31B becomes lower, relative to each other, and they become in a reverse relation when neutrons are absorbed on the side of the photodetector31B. That is, there occurs distribution in the pulse height of output pulses in each of the photodetectors31A and31B depending on the neutron incident position in the y direction.

In this respect, in the neutron detector2, an output pulse of the photodetector31A (first output pulse) PAand that of the photodetector31B (second output pulse) PBare input to a coincidence counting circuit (coincidence counting unit)32. When it recognizes the output pulse PAand the output pulse PBsimultaneously, the coincidence counting circuit32outputs their sum PA+PB. The pulse height of PA+PBis almost independent of the neutron incident position in the y direction, and corresponds to the energy absorbed by neutron absorption in the phosphor layer11. Accordingly, by using such a coincidence counting circuit32and thereby obtaining output pulses independent of the neutron incident position in the y direction, even in a case where light absorption occurs in the phosphor layer11, it is possible to suppress broadening of the pulse height distribution in neutron detection. As a result, discrimination between neutrons and gamma rays becomes easy to perform. However, since decay of light due to absorption relates nonlinearly with the distance from the incident position to the photodetector, for example, a simple sum of PAand PBis not exactly a quantity independent of the incident position. More exactly, it is preferable to use a pulse height independent of the incident position appropriately calculated using PAand PBwith the above-described point taken into consideration. Here, the coincidence counting circuit32may be configured in the form of an electric circuit, or may be configured using a computer or the like that performs processing on digitized output pulses. Particularly, in the case of using a computer, the processing does not necessarily need to be performed in real time at the time of detection, and may be performed by the computer collectively on a series of output pulse data after storing the data during a certain time period, for example. In that case, the processing may be performed in an offline state separated from the measurement environment by arranging the coincidence counting circuit32apart from the photodetectors and the like.

Further, even in the configuration ofFIG.6B, by providing photodetectors31A in the form of an array along the x direction, similarly providing photodetectors31B, and providing coincidence counting circuits32in accordance with the photodetectors, for example, neutron absorption positions (luminescence positions) in the x direction can be recognized.

In the configuration ofFIG.6B, since fluorescent light generated by neutron absorption theoretically is always detected by the photodetectors31A and31B at the same time, a noise component irrelevant to detection of radiation such as neutrons is removed from outputs of the photodetectors31A and31B and is not included in the output of the coincidence counting circuit32, unless its simultaneous output occurs accidentally. Here, with respect to synchronicity of outputs from the photodetectors31A and31B in the coincidence counting circuit32, those both of which occur within an appropriately determined short time period are recognized to be synchronous with each other.

In the case as shown inFIG.6Bwhere light emitted along the y direction is detected by the photodetectors, when absorption of the light in the phosphor layer11can be neglected, a configuration not including the coincidence counting circuit32inFIG.6Band including only either of the photodetectors31A and31B may be employed. Further, it is particularly effective to employ a configuration as described earlier, the phosphor layers11are formed to be thin in the z direction, and the light transmission layers12thicker than the phosphor layers11are provided adjacent to the respective phosphor layers11, thereby enabling detection of not only light propagating in the phosphor layers11but also light propagating in the light transmission layers12, along the y direction.

Second Embodiment

Next, a description will be given of a second embodiment that uses a scintillator having a different structure from that of the scintillator10shown inFIGS.1A,1B.FIG.10is a cross-sectional view showing a structure of such a scintillator50used in the present embodiment, which corresponds toFIG.1B. In the scintillator50, it is the same as in the scintillator10that five phosphor layers11and five light transmission layers12are provided, and that the phosphor layers11emit fluorescent light by interacting with neutrons (or gamma rays).

However, in the present case, a thin light shielding layer13that does not transmit but does reflect fluorescent light is formed on the right side of each of the light transmission layers12in the diagram. It is preferable that the light shielding layer13is made of a material not transmitting the fluorescent light but negligibly absorbing neutrons (for example, such as aluminum). In general, it is difficult to absorb neutrons but easy to shield visible light and ultraviolet light by using a thin metal, and it accordingly is easy to provide such a light shielding layer13. Further, energy of gamma rays or of high-energy electrons generated by gamma rays may be absorbed by the light shielding layer13. Meanwhile, in the scintillator50, light detection is performed in terms of each of segments, as will be described later, where the light shielding layer13serves as a boundary between the segments. There, the thickness of the light shielding layer13may be set thick in order to facilitate the light detection in terms of each segment. Such setting is also easy because neutron absorption by aluminum or the like, for example, is slight.

InFIG.10, light emitted by one phosphor layer11enters the adjacent light transmission layer12, but its incidence on a phosphor layer11next to the adjacent light transmission layer12is prevented by the light shielding layer13. Accordingly, the scintillator50is divided into five segments S1to S5in the z direction with the light shielding layer13being each boundary, with respect to fluorescent light. InFIG.10, one phosphor layer11and one light transmission layer12are provided in each segment, and light emitted by one of the phosphor layers11progresses only within the segment including the phosphor layer11particularly along the y direction. Further, because of the existence of the light shielding layer13, light emitted by the phosphor layer11is sent out only in the directions of arrows C and D illustrated inFIG.6B, but not in the direction of an arrow B inFIG.6A.

Here, similarly to the case ofFIG.1, the number of laminated layers including the phosphor layers11and the like is appropriately set in practice. Further, while each segment is composed of one phosphor layer11and one light transmission layer12in the example ofFIG.10, a plurality of phosphor layers11and a plurality of light transmission layers12may be provided in each segment (each of regions separated by the light shielding layers13).

As described above, in contrast to that absorption of a single neutron causes luminescence only in a single phosphor layer11, a single gamma-ray photon may cause luminescence in more than one phosphor layers11. In this respect, when the segments S1to S5are arranged as in the structure ofFIG.10and detection of luminescence is thereby performed in terms of each of the segments, influence of luminescence in other segments is removed with respect to luminescence due to gamma-ray photons, which enables further reduction of luminescence intensity due to gamma-ray photons.

In the present case, the scintillator50cannot be used in place of the scintillator10inFIG.6Abecause light cannot be extracted in the direction of the arrow B ofFIG.6A, but it can be used in place of the scintillator10inFIG.6B. In addition, the scintillator50can be used in a different aspect from that ofFIG.6B.FIG.11shows a configuration of such a neutron detector3in a manner of corresponding toFIG.6B. There, the scintillator50is depicted in a simplified manner where it is represented by only the segments S1to S5.

In the configuration, in a manner of corresponding to the photodetectors31A and31B inFIG.6B, photodetectors61A and61B are provided in the segment S1, photodetectors62A and62B in the segment S2, photodetectors63A and63B in the segment S3, photodetectors64A and64B in the segment S4, and photodetectors65A and65B in the segment S5. Further, in a manner of corresponding to the coincidence counting circuit32inFIG.6B, outputs P1Aand P1Bof the photodetectors61A and61B are input to a coincidence counting circuit (coincidence counting unit)71, outputs P2Aand P2Bof the photodetectors62A and62B to a coincidence counting circuit (coincidence counting unit)72, outputs P3Aand P3Bof the photodetectors63A and63B to a coincidence counting circuit (coincidence counting unit)73, outputs P4Aand PAB of the photodetectors64A and64B to a coincidence counting circuit (coincidence counting unit)74, and outputs P5Aand P5Bof the photodetectors65A and65B to a coincidence counting circuit (coincidence counting unit)75.

Accordingly, the coincidence counting circuit71outputs P1that is a sum of the output pulses P1Aand P1Brecognized to be synchronous in the photodetectors61A and61B, and similarly, the coincidence counting circuits72to75respectively output P2to P5each of which is a sum of output pulses of two photodetectors connected to the corresponding one of the coincidence counting circuits. That is, in the neutron detector3, the configuration of the neutron detector2ofFIG.6Bis implemented in each of the segments, and P1to P5are obtained as outputs from respective ones of different channels. Here, P1(and the like) does not necessarily need to be determined to be a sum of P1Aand P1B(and the like) as described above, but a value of P1(and the like) may be appropriately calculated from P1Aand P1B(and the like) by making correction on a nonlinear component independent of incident positions and then be used.

Thus, in the case of using the scintillator50ofFIG.10, the output pulses P1to P5respectively corresponding to the segments S1to S5are extracted from respective ones of different channels CH1to CH5, where PA+PBextracted as an output inFIG.6Bcorresponds to each of P1to P5in the present case.

FIG.12shows a result of calculation performed in respect of that the luminescence intensity due to gamma-ray photons can be particularly reduced in the configuration ofFIG.11, in a manner of corresponding toFIG.5. Shown there is the sensitivity to gamma rays when the number of phosphor layers11(or the thickness of each individual layer) is varied, while keeping the total thickness of the phosphor layers11constant, in terms of with or without segmentation.

InFIG.12, with respect to the above-described result ofFIG.5on the scintillator10(the sensitivity to gamma rays of 2.2 MeV), a result of performing similar calculation on the scintillator50obtained by applying segmentation to the scintillator10using the light shielding layers13(with segmentation) is shown, along with the result ofFIG.5(without segmentation). There, the case of 5 mm thickness corresponds to that of using only a single phosphor layer11, and accordingly the case is practically identical for both with and without segmentation. For the cases of the multilayer structure, a calculation result on the first one of the phosphor layers11(segment S1), whose luminescence intensity (energy absorption) due to gamma-ray photons is highest, is shown. It is noticed from the result that, when the segmentation is applied, the luminescence intensity due to gamma-ray photons can be greatly reduced, compared to the cases without segmentation, particularly in the cases of a larger number of thinner phosphor layers11. On the other hand, when absorption of neutron energy by the light shielding layer13can be neglected, the luminescence intensity due to neutron absorption is the same as that in the scintillator10. As a result, the n/r discrimination ability can be particularly increased by using the configuration ofFIG.11.

In the configuration ofFIG.11, since outputs are extracted from the five channels (CH1to CH5), neutron discrimination and detection may be performed in terms of each of the channels, where the discrimination is particularly easily performed. In addition, when outputs are thus extracted separately from the five channels, for example, the counting rate of each segment is reduced, and accordingly the configuration is also effective in a high dose condition. Further, the configuration is also effective in a case where precise measurement of the speed of neutrons is required, such as in neutron TOF measurement, because it is possible to recognize in which phosphor layer (segment) neutrons have undergone reaction, which reduces uncertainty in the distance.

FIG.13shows a configuration of a neutron detector4corresponding to a modification of the neutron detector3ofFIG.11. In contrast to the case of the neutron detector3ofFIG.11where outputs are extracted for respective ones of the five channels, an anti-coincidence counting circuit (anti-coincidence counting unit)81is used for generating a single output in the neutron detector4.

The anti-coincidence counting circuit81accepts CH1(P1) to CH5(P5) and outputs only one of them whose synchronicity has not been recognized, inversely to the coincidence counting circuit32described above. Accordingly, any one of P1to P5is output from the anti-coincidence counting circuit81. That this output's synchronicity has not been recognized means that none of other segments than the segment corresponding to the output pulse to be output (P1to P5) has emitted light simultaneously with the corresponding segment. While discrimination between neutrons and gamma-ray photons is performed based on the pulse height, as already described, gamma-ray photons may cause simultaneous luminescence in more than one of the segments, and accordingly the use of the anti-coincidence counting circuit81also enables suppression of detecting gamma rays and further increasing of the n/γ discrimination ability. It is the same as the case of the coincidence counting circuit32described earlier that the anti-coincidence counting circuit81may be configured using a computer, and that the computer's processing does not necessarily need to be performed in real time but may be performed in an offline state. Particularly in a high dose condition, the probability that more than one signals are counted accidentally at the same time in each segment increases, to which attention needs to be paid when using the anti-coincidence counting circuit81.

The above-described effect of the segmentation varies depending on the energy of gamma rays.FIG.14shows a result of calculation for gamma rays of 5.0 MeV performed in a similar way to that performed for obtaining the result for gamma rays of 2.2 MeV shown inFIG.12. When incident gamma rays are of high energy, the probability is high that high-energy electrons scattered in a phosphor layer11penetrate through the phosphor layer11, subsequently penetrate through also a light transmission layer12, and then enter a phosphor layer11located next to the light transmission layer12, thus contributing to luminescence therein. Additionally, the probability is very high that high-energy electrons scattered in a light transmission layer12are not completely absorbed in the light transmission layer12and are absorbed also in a phosphor layer located next to the light transmission layer12. For this reason, in the case without segmentation inFIG.14, the sensitivity to gamma rays is higher when the thickness of the phosphor layer11is 1 mm (0.25 g/cm2in terms of density length) or 1.67 mm (0.42 g/cm2in terms of density length) than when it is 5 mm (1.25 g/cm2in terms of density length) (prior art). However, when the segmentation is adopted, luminescence occurring in a plurality of phosphor layers11separately generates output of each segment, and accordingly output of each segment decreases and the sensitivity is greatly reduced. Therefore, the effect of segmentation is larger when the energy of gamma rays is higher. Further, for such high-energy gamma rays, the above-described anti-coincidence counting circuit81is particularly effective.

In the second embodiment, a preferred range of the thickness (density length) of the phosphor layer11is the same as that in the first embodiment. On the other hand, in the case of the second embodiment, since fluorescent light does not propagate between segments, restriction on the thickness (density length) of the light transmission layer12is relaxed. However, in the second embodiment, when each segment: is thick, a photo-sensing area of the photodetectors needs to be large, which causes a disadvantage in cost. Therefore, it is undesirable to make the light transmission layer12thicker than necessary, and is preferable to make the thickness about 6 mm (1.3 g/cm2in terms of density length) or smaller.

In the second embodiment, while the light transmission layer12has a function to propagate fluorescent light to the photodetectors, also the phosphor layer11can similarly guide fluorescent light to the photodetectors, and accordingly, particularly when the segmentation is adopted, the light transmission layer12does not necessarily be provided in the segments. However, since the phosphor layer11is set to be thin as described above, its efficiency of propagating light (fluorescent light) to the photodetectors is not high in the in-plane direction. In this respect also, it is preferable to provide also the light transmission layer12in the segments.

In the technology described in Patent Document 2, inorganic phosphor particles and a resin material are used, and it may be considered that the inorganic phosphor particles correspond to the phosphor layer11and a layer made of the resin material corresponds to the light transmission layer12. However, unlike in the light transmission layer12described above, the probability that neutrons are scattered and thereby thermalized or absorbed in the resin material containing hydrogen is high, and as a result, the neutron detection efficiency in the prior technology is lower than that of the present invention. In addition, while the inorganic phosphor particles and the resin material are generally made of completely different materials, their densities are required to be close to each other in order to uniformly mix them. Under such a condition, to further make their refractive indices close to each other in order to suppress reflection at an interface between them as in the case of between the phosphor layer11and the light transmission layer12in the present invention, restriction on the resin material or the material for the inorganic phosphor particles becomes tight. Therefore, it is not easy to select and use such materials in practice. In contrast, in the present invention, such restriction is not placed on densities of the phosphor layer11and the light transmission layer12, and accordingly the degree of freedom of material selection is high.

Furthermore, in the case of the present invention, by forming the phosphor layer11to have a thin film form with a small thickness in the incident direction of gamma rays (neutrons) and employing the layered structure, the sensitivity to gamma rays can be greatly reduced without reducing that to neutrons, as described above, but in contrast, in the technology described in Patent Document 2, since the size of the inorganic phosphor particles is isotropic and independent of the incident direction, the effect of reducing the sensitivity to gamma rays without reducing that to neutrons is small.

Here, as long as the same operation can be performed, a specific configuration of the neutron detector is optional. For example, as long as the same functions as that described above are possible, any combination may be used for the phosphor layer and the light transmission layer, and a main component may be different between the layers. Further, a configuration of the photodetectors may be appropriately set.

REFERENCE SIGNS LIST

1to4: neutron detector10,50,100: scintillator11: phosphor layer12: light transmission layer13: light shielding layer21: photodetector31A,61A,62A,63A,64A,65A: photodetector (first photodetector)31B,61B,62B,63B,64B,65B: photodetector (second photodetector)32,71to75: coincidence counting circuit (coincidence counting unit)81: anti-coincidence counting circuit (anti-coincidence counting unit)S1to S5: segment