Patent Application
20180196146
Embodiments described herein generally relate to a scintillator array.
Conventionally, there have been proposed various neutron detectors (neutron two-dimensional detectors) used for neutron radiography, neutron imaging, neutron scattering experiments, and the like. Further, at present, in all the countries of the world, for basic experiments related to physical property research and atomic nuclei of various substances, construction of new high-intensity pulsed neutron sources is in progress.
As a commonly-used neutron detector, for example, there have been known a He-3 gas detector using 3He (hereinafter, mentioned as He-3.) gas which is an isotope of He (helium) and having high detection efficiency and a scintillation detector including a scintillator which reacts with neutrons directly or secondarily to emit light.
Because a neutron has no electric charge, a neutron converter which converts neutrons into charged particles, gamma rays, or the like is required for detecting the neutrons. As the neutron converter, there have been known He-3, Li-6, B-10, Cd-113, Gd-155, and Gd-157 having a large neutron absorption cross section, but at present, the He-3 gas detector which is a neutron detector using the He-3 gas is used due to low sensitivity to gamma rays. In order to efficiently detect fast neutrons and epithermal neutrons (also known as epithermal) having high energy, there also has been studied a structure in which a circumference of a neutron detector is covered with a neutron moderator such as polyethylene.
The scintillation detector has a high counting capacity. However, density is high and sensitivity to gamma rays is high due to a solid body. In order to detect neutrons at a high counting rate, it is essential to use a neutron detection scintillator having a short fluorescence lifetime. Therefore, for example, a neutron detector using a scintillator constituted of a Li2B4O7 single crystal for neutron detection and having a combination of its fluorescent property and a photomultiplier tube is under development. Further, particularly regarding a reduction in influence of gamma rays which is essential for the neutron detection or the neutron imaging, a scintillator constituted of light elements is preferable, and because Li, B, and O are the light elements, the scintillator constituted of the Li2B4O7 single crystal satisfies a demand also in terms of the above.
Furthermore, a neutron scintillator which is capable of having a configuration thinner compared with a conventional neutron scintillator and is more excellent in terms of gamma-ray sensitivity and position resolution compared with a conventional Li-based scintillator is under development. This is produced by, using a neutron scintillator formed into glass by adding Ce to an oxide constituted of B and Li as main components as a starting raw material, mixing Li2B4O7 and CeO2, thereafter heating the mixture at temperatures of at least 950° C. or higher and holding it for one hour or longer, and thereafter cooling it at a rate of 150° C./sec or more between temperatures of 800 and 400° C.
In LiBO3 and Li2B4O7 compounds constituted of only these light elements having low gamma-ray sensitivity, light emission by neutrons is very small. Furthermore, in single crystals obtained by adding Ce to these, an amount of Ce solid-dissolving in the crystals is very small, the light emission by neutrons is small, and it is difficult to use them as a two-dimensional detector for neutron imaging or for neutron radiography.
On the other hand, in Li and B, scintillator materials can be selected regardless of the gamma-ray sensitivity because a several MeV charged particle production reaction is used for the neutron detection. In particular, because B can be expected to have neutron detection efficiency about four times of that of the same amount of Li, it is possible to produce a thinner scintillator. Because this is very advantageous in terms of the gamma-ray sensitivity and the position resolution, an ideal neutron converter is enabled. However, B has about half charged particle energy to be produced of that of commercially available Li glass (Li-Glass) and is considered disadvantageous in terms of light emission output, and in most of the conventional neutron scintillators, Li is used as a converter.
Here, as a representative of a currently practically used neutron scintillator, for example, LiF/ZnS is present. This neutron scintillator has a high light emission amount and is also excellent in handleability, but is opaque and has limitations to detection efficiency and counting capacity.
In particular, a resolution when high-definition imaging is performed depends on a spread in emitting light by putting a reactant and a scintillator together or a resolution of an optical system or an image sensor which images the light. Recently, the number of pixels of a charge coupled device (CCD) element or a CMOS element used for an imaging system has been increasing dramatically, and therefore a configuration of a reaction film and a scintillator is considered to mainly determine the resolution. That is, when a reaction with neutrons in the reaction film generates charged particles and the charged particles and the scintillator react with each other to emit light, a flying distance (range) of the charged particles and a diffusion length of light emitted in the scintillator cause a blur related to the resolution.
In order to improve the resolution, it is necessary to make the reaction film thin and make a range of secondary charged particles to be generated short. In a case of LiF/ZnS, Li reacts with neutrons to emit alpha (a) rays, and the α rays make a ZnS phosphor emit light. An actual configuration, in which LiF/ZnS is granular powder, has a LiF/ZnS powder held by an organic binder on an Al plate which is used as a substrate in many cases.
As Li which reacts with neutrons, an enriched isotope Li-6 is normally used in order to increase reaction efficiency. However, because an atom density with respect to all including the binder or the like is low, a thickness of a coated layer is about several hundreds Therefore, the resolution is determined by the thickness of the coated layer and is not high. When energy of neutrons increases in particular, a rate of reaction with Li-6 decreases further, and efficiency also becomes poor. The layer is considered to be made thicker in order to increase the rate of reaction, but because LiF/ZnS is opaque and emitted light scatters in LiF/ZnS and is not transmitted, the efficiency does not increase even though the layer is made thicker.
As a method of solving the above, there has been proposed a two-dimensional detector in which a neutron detector is constituted of a capillary plate having a plurality of openings passing through in a thickness direction and filled with a liquid scintillator which reacts with neutrons in a plurality of these openings and an imaging detector, and which measures scintillation light. However, because the capillary plate portion does not react and the neutrons pass through this portion, high-definition and high-efficiency two-dimensional detector is not obtained. Further, it is also difficult in manufacturing to hold the scintillator uniformly in all holes of the capillary plate, which does not yet reach practical use.
Meanwhile, an imaging intensifier (or electron multiplier) which combines a reaction film and a scintillator, converts light from the scintillator into electrons by using a photoelectric conversion film, and amplifies the electrons is also under development in order to achieve high definition and increase sensitivity. However, in this structure, a thickness of the reaction film is about 5 μm for the purpose of high definition, and in a case of B-10, reaction efficiency with neutrons is about 10%, and remaining 90% of the neutrons is transmitted and is not used. Furthermore, when energy of neutrons increases, the reaction efficiency decreases further. Therefore, the reaction efficiency is poor despite high definition, and when the number of generated neutrons is small (flux is small), integration is to be performed by spending time.
As described above, in the neutron detector for performing imaging and detecting scattered neutrons two-dimensionally with high definition and high sensitivity in a non-destructive manner by using neutrons and transmitting conditions of substances and the inside of a structure, development of a neutron detector capable of detecting a high energy neutron region with high definition and high efficiency is desired in particular.
A scintillator array to be used for a neutron detector of an embodiment includes a structure including a plurality of stacks layered each other. Each of the stacks has in sequence: a light reflector including ceramics or single-crystal silicon; a first film to react with a neutron incident along a direction intersecting a lamination direction of the stacks and thus emit a radiation ray; a second film including a material to reflect light; and a scintillator to emit light in response to the radiation ray. The light from the scintillator is reflected by the reflector and the second film to propagate an inside of the scintillator and thus to be led to an outside of the structure.
Hereinafter, embodiments of a scintillator array to be used for a neutron detector according to the present invention will be described referring to the drawings.
The scintillator array to be used for the conventional neutron detector illustrated in
A basic mechanism of neutron detection is as follows. A neutron (n) 1 is transmitted through the aluminum substrate 17 and reacts with Li of the granular scintillators 4. Strictly speaking, Li is an isotope of Li-6, and this Li-6 and the neutron (n) 1 react with each other to emit alpha (α) rays 2. This reaction is mentioned as (n, α). The emitted α rays 2 make a ZnS phosphor of the granular scintillators 4 emit the light 5.
The α rays 2 are emitted in all directions in the granular scintillators 4 and have a range (a flying distance of radiation rays) of a degree of about 5 μm to 10 μm. A value of a particle size of the granular scintillators 4 is substantially the same as a value of the range of these α rays 2, and the light 5 emitted inside the granular scintillators 4 is absorbed and attenuated in the granular scintillators 4 and comes out of the granular scintillators 4 at the same time. This light 5 is transmitted in other granular scintillators 4 and reflects on the granular scintillators 4, and further is transmitted in the binder 6 and comes out of the neutron detector. When a proportion of the number of atoms of a Li-6 atom to react with neutrons which is occupied in all the granular scintillators 4 is considered as LiF/ZnS which is a ratio of LiF to ZnS, LiF/ZnS is ¼, and when the number of atoms of the binder 6 is taken into consideration, the whole reaction efficiency becomes poor.
Accordingly, in order to increase the efficiency, it is necessary to make a thickness of a coated portion of the granular scintillators 4 thick. However, because the light 5 emitted in a region of the granular scintillators 4 on a side on which the neutron 1 is incident is transmitted in the granular scintillators 4 and reflects on the granular scintillators 4, and further is transmitted in the binder 6 and comes out of the neutron detector, a transmittance of light decreases, and because it diffuses further to be transmitted, resolution also becomes poor.
Scintillators which are used for imaging actually and have this configuration have a thickness of about several hundreds μm. In a fast neutron whose energy of a neutron is high, a reaction cross section with Li-6 is several digits smaller than a reaction cross section with Li-6 in a thermal neutron. Therefore, because the thickness is to be increased further in order to increase the reaction efficiency, the resolution becomes poorer and poorer.
As a method of increasing the resolution to obtain bright images with high luminance, a method illustrated in
However, also in B-10, similarly to Li-6, a range of the α rays 2 emitted by a (n, α) reaction is a degree of about 4 to 5 μm, and therefore when the thickness is more than 5 μm, the α rays 2 emitted on an incident side by a reaction with a neutron 1 cannot pass through the film 7 and reach a CsI phosphor 8. When the thickness of the film 7 is 5 μm, looking overall, about 80% of thermal neutrons is transmitted and only a degree of about 20% of the thermal neutrons is effectively utilized. However, an atom density per unit volume in a reaction surface is higher compared with a case of the configuration in
The needle-shaped CsI phosphor 8 having high transparency sheds the light 5 in accordance with the α rays 2, and the photoelectric conversion film 18 generates an electron 10 in accordance with the light 5. Amplification of this electron increases efficiency of conversion and transmission and makes it possible to obtain sensitivity about 100 or more times compared with a case of photographing by using an image sensor as the configuration illustrated in
Next, a configuration of a scintillator array to be used for a neutron detector according to one embodiment of the present invention will be described referring to
This multi-layered structure has a multi-layered structure having stacks in which a layered structure constituted of a film 12 which is a ceramic film composed of the white ceramics such as AlN (aluminum nitride) or a silicon single-crystal film, the film 7 which is 10B4C (enriched boron carbide obtained by enriching a boron-10 isotope) vapor-deposition film, a film 13 which is a ceramic vapor-deposition film composed of the white ceramics such as AlN (aluminum nitride) or an Al (aluminum) vapor-deposition film, and a scintillator 11 is layered repeatedly in multistage (for example, several hundreds to several thousands stages) in a first embodiment. This multi-layered structure is alternately arranged in a double-cross shape (grid shape) so that a difference in lamination direction becomes 90 degrees when it is seen from an incident surface side of the neutron 1, and thus the detection part (two-dimensional neutron reaction scintillator) of the scintillator array which has a two-dimensionally expanding neutron incident surface and is used for the neutron detector is constituted.
The film 12 has a thickness (a length in an upper and lower direction in
The film 7 is formed on the film 12 by the vapor deposition and extends along the incident direction of the neutron 1. The film 7 has a thickness (a length in the upper and lower direction in
The film 13 has a thickness (a length in the upper and lower direction in
The scintillator 11 is composed of a plastic scintillator or the like. The scintillator 11 is arranged adjacently to the film 13 so as to extend along the incident direction of the neutron 1. This scintillator 11 has a thickness (a length in the upper and lower direction in
The scintillator 11 also includes either type of a rare earth oxysulfide phosphor or garnet, and further includes at least one selected from praseodymium, terbium, europium, cerium, zirconium, and phosphorus as an activator in these.
A scintillator layer includes a material represented by a general formula Y3Al5O12:Ce, a general formula Gd3(Al, Ga)5O12:Ce, a general formula Lu3(Al, Ga)5O12:Ce, a general formula Gd3(Al, Ga)5O12:Tb, a general formula Lu3(Al, Ga)5O12:Tb, or a general formula (Gd, Lu)3(Al, Ga)5O12:Ce.
A large number of the above-described layered structures are layered further, thereby constituting the detection part of the scintillator array to be used for the neutron detector. In this embodiment, in the above-described layered structure, the respective layers such as the film 7 are each formed to have an inclination in the lamination direction with respect to the incident direction of the neutron 1, namely, formed to extend along a direction inclined with respect to a direction perpendicular to the substrate 3 and make a rear end side (a right side in the figures) rise toward an upper side in
As described above, the scintillator array to be used for the neutron detector of this embodiment has the scintillator array 11 sandwiched between the film 13 and the film 12. Further, the film 7 is also formed to be sandwiched by either of the film 13 and the film 12.
The film 7 is black and has low reflectance of light, and therefore when the film 7 and the scintillator 11 are layered directly, the light 5 emitted in the scintillator 11 cannot be efficiently transmitted. In contrast, making a sandwich structure in which the scintillator 11 is sandwiched by the film 13 and the film 12 as described in this embodiment makes it possible to efficiently transmit the light 5 emitted in the scintillator 11 by using reflection by the film 13 and the film 12 and take out the light 5 to the outside.
In the α rays 2 emitted by the (n, α) reaction at each point in the film 7, a component emitted in a direction (a 4 to 5 μm thickness direction) nearly perpendicular to the neutron 1 is emitted as the light 5 in the scintillator 11. The light 5 can be propagated while being reflected by the film 13 and the film 12 in the scintillator 11 having a high transparency, led to the outside of the multi-layered structure, and taken out.
The film 12 with a thickness of 5 μm or more absorbs the α rays 2 coming out downward in the film 7 in
In this embodiment, the film 7 is formed so as to extend along the incident direction of the neutron 1 as described above. Then, because the neutron 1 goes not perpendicularly to but nearly horizontally to this film 7, and so as to move on a diagonal line in the inclined arranged film 7, the reaction efficiency can be greatly improved.
As described above, the scintillator array to be used for the neutron detector according to this embodiment makes it possible to greatly improve use efficiency of neutrons and perform propagation of the emitted light 5 with efficiency and without diffusing the light. This makes it possible to obtain the scintillator array to be used for a two-dimensional detector with respect to neutrons which is capable of imaging efficiently with high definition.
Thus, the scintillator array of the embodiment is capable of high-definition and high-efficiency detection with respect to a wide range of energy of neutrons, particularly with respect to high energy thereof.
Next, a scintillator array to be used for a neutron detector according to a second embodiment will be described referring to
The scintillator array to be used for the neutron detector of the second embodiment makes it possible to increase the reaction efficiency with a neutron 1 in a neutron input surface in addition to actions and effects in the neutron detector of the first embodiment. This makes it possible to make a size in a direction (a left and right direction in
Next, a scintillator array which is suitable for measurement of fast neutrons and is used for a neutron detector according to a third embodiment will be described referring to
As the scintillator 16, for example, a plastic scintillator can be used. Further, when a glass scintillator, a polycrystalline scintillator, or a ceramic scintillator of a rare earth oxysulfide phosphor, garnet, or the like, which does not include hydrogen, is used as the scintillator 16, the scintillator is preferably covered with a resin including hydrogen. The scintillator array includes the film 12 having the thickness of 5 μm or more, the film 7, and a film 13 having a thickness of about 0.1 μm to 0.5 μm so as to extend along an incident surface of a neutron 1 on an incident side (a left side in
In the scintillator array which has the above-described configuration and is used for the neutron detector according to the third embodiment, a fast neutron 9 which is a neutron having a high energy component reacts in the film 7 to emit α rays 2, and these α rays 2 react in the scintillator 16 to be emitted as a light 5. However, in a (n, α) reaction in the film 7, when energy of neutrons increases, an absorption cross section declines in the order of digits as indicated by a line of a neutron absorption cross section of enriched boron illustrated in
Therefore, attention is focused on hydrogen which scatters at an almost constant rate particularly with respect to the neutron energy in a reaction with neutrons, and in the third embodiment, the scintillator 16 is used and the fast neutron 9 is slowed down by hydrogen. Slowed-down neutrons 15 diffuse in an isotropic manner from the scintillator 16. The slowed-down neutrons 15 react in the film 7 on a lower side of the scintillator 16, the α rays 2 are emitted, and these α rays 2 react with the scintillator 16 to emit the light 5.
On the other hand, when the slowed-down neutrons 15 react in the film 7 on an upper side of the scintillator 16, the light 5 is emitted in the upper and lower scintillators 16, and therefore the resolution becomes poor. Accordingly, in order not to react with the film 7 on the upper side of the scintillator 16, a wraparound of the neutrons on the lower side can be prevented by providing the film 14 which is a Gd2O3 vapor-deposition film including gadolinium (Gd) having a large absorption cross section in a thermal neutron region under the film 7 on the upper side.
In the third embodiment as described above, particularly in the case of the high energy neutron, a reaction distance with the film 7 is long, and therefore the fast neutron 9 is converted into the slowed-down neutrons 15 by hydrogen atoms of the scintillator 16 and made to react with the film 7. Since the slowed-down neutrons 15 have a range of several centimeters or more and diffuse, the diffusing slowed-down neutrons 15 are absorbed by forming the film 14 (a thickness of about 5 μm to several tens μm) on one side (a lower side in
While certain embodiments of the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-185490 | Sep 2015 | JP | national |
This application is a continuation of prior International Application No. PCT/JP2016/004247, filed on Sep. 16, 2016 which is based upon and claims the benefit of priority from Japanese Patent Application No. 2015-185490, filed on Sep. 18, 2015; the entire contents of all of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2016/004247 | Sep 2016 | US |
| Child | 15907594 | US |
