JP2957439B2 - Neutron detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron detector, and more particularly, to a neutron detector constituted by using a plurality of reactants.

[0002]

2. Description of the Related Art Neutron detectors are used for radiation control in particle acceleration facilities, nuclear fuel facilities, nuclear reactor facilities and the like. Conventionally, the most common configuration example of a neutron detector has a diameter of 3
3 centimeters for cylindrical containers such as aluminum
Boron fluoride (BF ₃ ) is sealed, and the surrounding area is
It was covered with a moderator such as polyethylene of about a centimeter. In this detector, the incident neutrons are first decelerated by a moderator before entering the cylindrical vessel. Here, an (n, α) reaction, which is a kind of nuclear reaction, is caused, and BF ₃ gas is ionized by α particles generated as a result, and this is observed as an electric signal. This utilizes the detection principle of ionization because neutrons themselves have no charge.

However, in this method, the structure of the detector is large due to the thickness of the moderator (in the case of the above example, the total width is about 23 cm), and the weight is as heavy as about 10 kg, which improves the portability and workability. There was room for it. The reason why the moderator is made thicker is the property of boron (B) that the reaction cross section decreases as the energy of the incident neutron increases. That is, the energy cannot be detected unless the energy of the neutrons incident on the cylindrical container is sufficiently reduced in advance.

Japanese Patent Publication No. 25313/1993 discloses a neutron detector which solves these disadvantages. The present invention (hereinafter referred to as "the prior invention") has been previously proposed by the present applicant, by its feature of using two kinds of reaction materials that BF ₃ and hydrogen compounds, the size of the detector It is intended. At this time, 1. Contrary to boron, the probability of releasing hydrogen atoms from hydrogen compounds increases as the energy of neutrons increases, so that not only the effect that most of the moderator becomes unnecessary, but also The amount of charge generated by the (n, p) reaction when a neutron collides with a hydrogen atom (equivalent to one proton due to recoil)
And the charge amount of α particles by boron (equivalent to two protons) is different, so that multiple dose equivalents with different neutron average energies and energy response characteristics can be obtained simultaneously from the counting ratio of the two reactions. It also created new effects. Here, this prior invention will be described with reference to the drawings.

FIG. 1 is a schematic structural view of the prior art. In the figure, BF ₃ gas is sealed inside the spherical detector 10, and a hydrogen compound 14 is formed in a film on the inner surface of the sphere. A first electrode 15 is provided inside the membrane, while a core-shaped second electrode 1 is provided on the center line of the detector 10.
2 are provided. A high voltage is applied between these two electrodes. The output terminal 13 is for extracting an electric signal generated by the reaction.

The state of detection in this configuration is as follows.

First, when low energy neutrons having low energy enter, a BF ₃ gas is ionized by causing an (n, α) reaction with boron, and a current flows between the first electrode 15 and the second electrode 12. This is output from the output terminal 13.

On the other hand, when fast neutrons enter, a (n, p) reaction occurs with hydrogen atoms in the hydrogen compound, and recoil protons are emitted into the detector 10. The protons similarly ionize the BF ₃ gas, and an electric signal is output from the output terminal 13. Because the amounts of charge involved in these two reactions are different,
Discrimination becomes possible.

FIG. 2 is a diagram showing the configuration of the discrimination circuit. In the figure, an electric signal output from an output terminal 13 is amplified by an amplifier circuit 22 and then input to a signal discrimination circuit 23. Here, the α signal (signal caused by α particles) and the p signal (signal caused by recoil protons) are discriminated by the voltage level or waveform of the signal, and the frequencies of these signals are counted by the counting circuits 24 and 25, respectively. The counting result is processed by the arithmetic processing circuit 26 by a method described later and output to the display unit 27.

FIG. 3 is a diagram showing the energy response characteristics of the (n, α) and (n, p) reactions. In the figure, the horizontal axis is the energy of the incident neutron, the vertical axis is the sensitivity,
The latter shows the characteristic of the upper right. Here, assuming that the counting results of the counting circuits 24 and 25 are C1 and C2, respectively.
Since the ratio of C2 / C1 is known, the average energy of the neutrons can be determined based on FIG. For example, C2
When / C1 = 2, E0 in FIG. Therefore, such a function may be programmed in advance and executed by the arithmetic processing circuit 26.

[0011]

As described above, according to the prior invention, it is possible to reduce the size of the detector and to improve the measurement efficiency. However, it has been found that the present invention has room for further improvement in the following points.

[0012] [Problem 1] because it was configured to encapsulate a gas, such as BF ₃ in maintaining a conventional common detector and the prior invention of the detection characteristics, it is necessary to consider the deterioration due to leakage and high pressure application of the gas. Therefore, a detector that maintains constant characteristics for a long period of time has been desired.

[Problem 2] Size of Structure When conditions for good detection according to the prior invention are analyzed by experiments or the like, it is preferable to provide a moderator of about 1 cm around the detector 10. all right. At this time, the diameter of the spherical portion of the detector 10 is about 5 cm, and the entire diameter is about 7 cm. The smaller the structure, the wider the application, so if possible, we want to make it even smaller.

[Object of the present invention] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to maintain a detection characteristic by using a solid reactant, and to detect the reactant, the reactant and the reactant. A neutron detector that can be further downsized by realizing the moderator with the same member is disclosed.

[0015]

In order to achieve the above object, a neutron detector according to the present invention comprises a first solid reactant which emits charged particles by a nuclear reaction with respect to an incident low-speed neutron; A second solid reactant that emits charged particles by a recoil action on neutrons, a light-shielding material that shields at least the first and second solid reactants, and the first and second solid reactants A detecting unit for detecting the charged particles emitted.

Further, in the present invention, the first solid reactant is formed of a substance which emits charged particles by the (n, α) reaction, and the second solid reactant is charged particles by the (n, p) reaction. Formed of substances that release

Further, in the present invention, the first solid reactant is at least one selected from the group consisting of lithium and boron.
Wherein the second solid reactant is a hydrogen compound.

On the other hand, the detection unit in the present invention includes a scintillator that emits scintillation light when charged particles are detected, and a photoelectric conversion unit that converts the scintillation light into an electric signal.

At this time, the scintillator is a plastic scintillation fiber, and the scintillator doubles as the second solid reactant.

Further, in the present invention, the first solid reactant is formed into a thin film covering the second solid reactant.

Further, the present invention further includes a neutron moderator covering the outer periphery of the first and second solid reactants and formed of the same substance as the second solid reactant.

Further, the present invention has a plurality of sets of the first solid reactant and the second solid reactant, and tightly bundles these sets.

At this time, the shape of the set of the first solid reactant and the second solid reactant is a triangular prism, a quadrangular prism or a hexagonal prism, and these sets are bundled with their side surfaces in close contact with each other. It is.

Further, the photoelectric conversion section of the present invention is provided for each of the plurality of sets.

[0025]

According to the present invention having the above-described structure, the first solid reactant emits charged particles due to nuclear reaction with respect to the incident low-speed neutrons. On the other hand, the second solid reactant emits charged particles due to recoil action on fast neutrons. here,
A detection unit detects charged particles emitted from the first and second solid reactants.

According to the present invention, the first solid reactant emits charged particles by the (n, α) reaction, and the second solid reactant emits charged particles by the (n, p) reaction. I do.

Here, since the first solid reactant contains at least one substance selected from the group consisting of lithium and boron, the (n, α) reaction to slow neutrons is likely to occur, while Since the solid reactant is a hydrogen compound, an (n, p) reaction to fast neutrons is likely to occur.

Further, in the present invention, when the scintillator of the detecting section detects charged particles, it emits scintillation light, and the photoelectric conversion section converts this into an electric signal.

Here, since the scintillator is a plastic scintillation fiber, a (n, p) reaction to fast neutrons is likely to occur, and can also serve as the second solid reactant.

On the other hand, the first solid reactant is formed in the form of a thin film covering the second solid reactant, and α particles generated as a result of the (n, α) reaction are the second solid reactant. Incident on the material.

According to the present invention, the neutron moderator, which covers the outer periphery of the first and second solid reactants and is formed of the same substance as the second solid reactant, slows down incident neutrons.

Further, the present invention has a plurality of sets of the first solid reactant and the second solid reactant, and since these sets are tightly bundled, neutrons can be detected with high sensitivity.

Here, the shape of the set of the first solid reactant and the second solid reactant is a triangular prism, a quadratic prism or a hexagonal prism, and these bundles are bundled with their side surfaces in close contact with each other. Can be

According to the present invention, the photoelectric conversion unit independently converts scintillation light generated in each of the plurality of sets into an electric signal.

[0035]

【Example】

Embodiment 1 FIG. Here, preferred embodiments of the present invention will be described with reference to the drawings as appropriate.

FIG. 4 is a diagram showing a configuration of the neutron detector according to the first embodiment. A feature of the present invention is that the two reactants are both formed of solid reactants, and neutrons are detected by a scintillation reaction instead of ionization. In determining the shape of this embodiment, 14 MeV
The calculation is performed under the condition that a wide range of neutrons up to the fast neutron level is detected with a sensitivity of 1 microsievert to a worst case of about 10 millisievert.

In the same figure, the length is 10 cm,
Many plastic scintillation fibers 1 (hereinafter, referred to as “PSF1”) each having a thickness of about 1 mm are tightly bundled. As an example, the core of PSF1 is made of polystyrene, and the clad is made of polymethyl methacrylate (PMMA). Lithium fluoride LiF or boron B (hereinafter referred to as “LiF
Etc.) are formed by vapor deposition or coating. The thickness of the whole bundle may be about 1 cm.

Since LiF and the like have a large reaction cross section with respect to slow neutrons, when slow neutrons enter (n,
The α) reaction releases α particles or tritium ( ³ H) nuclei. Therefore, in this embodiment, the thin film layer 3 becomes the first solid reactant.

On the other hand, PSF1 plays three roles. That is, the PSF 1 not only functions as a scintillator that emits scintillation light when charged particles are incident, but also serves as a second solid reactant and also functions as a moderator. Since PSF1 is a hydrogen compound containing a large amount of hydrogen, it mainly has the effect of emitting recoil protons to fast neutrons by the (n, p) reaction and slowing down the neutrons by the reaction. According to the configuration of the present embodiment, both α particles generated in the thin film layer 3 and protons generated in the PSF 1 are detected by the PSF 1.

One end of the bundle of PSFs 1 has a photoelectric conversion element 5
Is attached. The photoelectric conversion element 5 is a photomultiplier tube (PMT) or an avalanche photodiode (AP).
D) and the like, and converts the scintillation light into an electric signal. The PSF 1 and the entire photoelectric conversion element 5 are covered with a light-shielding film 7 and a moderator 9 having a thickness of about 1 cm surrounding the light-shielding film 7. As the moderator 9, a hydrogen compound such as polyethylene can be used. For the reason described later, a metal film or the like that sufficiently reflects light is used as the light shielding film 7.

With respect to the electric signal thus converted,
As in the prior invention, a configuration may be adopted in which the signal is finally given to the display circuit via an amplification, discrimination, arithmetic processing circuit, and the like.

The configuration of the neutron detector according to the first embodiment has been described above. The manner in which neutrons are detected by this configuration will be described in more detail.

FIG. 5 is a diagram showing a reaction when low-speed neutrons and high-speed neutrons enter one PSF 1. As shown in the figure, slow neutrons cause an (n, α) reaction in the thin film layer 3 to release α particles. If the emission direction is an arrow A in the figure, this excites a scintillation substance in the PSF 1 and emits scintillation light. Arrow B
In the case of (1), scintillation light is emitted in the adjacent PSF 1 not shown. In this embodiment, since a large number of PSFs 1 are bundled, the total area of the thin film layer 3 is dramatically improved, and the sensitivity to low-speed neutrons is improved.

On the other hand, when high-speed neutrons fly in, the probability of causing a reaction in the thin film layer 3 is very low, and the neutrons normally enter the PSF 1 as they are. Here, an (n, p) reaction is caused to emit scintillation light. These scintillation lights propagate to both ends of the PSF 1, but the photoelectric conversion elements 5
Since the scintillation light traveling in the opposite direction is reflected by the light shielding film 7 employing the metal film at the end face of the PSF 1 as described above, the energy loss of light to be captured can be minimized.

The above is an outline of the detection. In these two reactions, the difference between the charge amounts makes it possible to discriminate the reactions.

Although the range of the α particles is usually on the order of 0.1 mm depending on the energy, the following considerations are made in this embodiment.

1. About the formation of the thin film layer 3 The thin film layer 3 is formed with a sufficiently thin and uniform thickness. Naturally, the thickness must be smaller than the range of the α-particle.

2. How to bundle PSF1 It is necessary to bundle PSF1 without gaps. FIG. 6 is a diagram illustrating an example of a cross section in a state where the PSFs 1 are bundled. As shown in the figure,
In this case, a regular hexagonal prism is used as the PSF 1. When the thin film layer 3 is formed uniformly, P
Since the cross section of the SF1 is maintained as a regular hexagonal prism, it is possible to bundle the PSFs 1 in a state where the side surfaces are in close contact with each other without any gap as shown in FIG. For this purpose, a regular triangular prism, a regular square prism, or the like may be employed as the PSF 1. In determining the shape, the packing density of LiF, etc., P
What is necessary is just to consider the viewpoint of easy acquisition or manufacture of SF1.

As a result of simulation and experiment, it has been found that the entire width of the detector may be about 3 cm, so that it is possible to further reduce the size of the detector compared to the prior invention. In addition, since a solid reactant is used, the problem of gas leakage and deterioration can be solved, and the conventional problem can be solved.

This embodiment has another effect. That is, since the total area of the thin film layer 3 is greatly improved by forming the bundle, the sensitivity to the (n, α) reaction is improved. In this case, the present embodiment is more advantageous than a method in which, for example, LiF or the like is powdered and mixed with the scintillator. This is because if LiF or the like is mixed, the transmittance of the scintillator is deteriorated, and the sensitivity is reduced. In this embodiment, since the scintillation light propagates along the wall surface inside the PSF 1, there is no possibility that LiF or the like will block the light path.

In the present embodiment, one photoelectric conversion element 5 is provided at one end of the bundle of PSFs 1. However, one photoelectric conversion element 5 may be arranged for each PSF 1. In this case, the scintillation light generated in each PSF 1 can be independently detected in addition to the detection of the sum of the scintillation lights by the entire detector. As a result, the position where the neutrons are incident can be specified, and the function of the detector is improved.

In the present embodiment, the thin film layer 3 is formed around the PSF 1. However, for example, when a thin shaft hole or slit is provided in the PSF 1, this gap may be filled with LiF or the like. Good. In this case, the filling using the capillary phenomenon is also possible, which is convenient in terms of the management of the filling amount and the filling region shape.

Embodiment 2 FIG. In the first embodiment, a hydrogen compound such as polyethylene is used as the moderator 9, but in this embodiment, a configuration in which this portion is changed will be described.

FIG. 7 is a configuration diagram of a neutron detector according to the second embodiment. In the figure, the same reference numerals are given to the same components as those of the first embodiment, and the description will be omitted.

The feature of the second embodiment is that the moderator 9 uses PSF
The point is to use. That is, the same PSF used as a scintillator is extended as shown in FIG. However, at this time, the PSF used as the moderator 9 is not provided with a thin film layer such as LiF. This is only for the purpose of deceleration. In the figure, the area of the PSF 1 having the thin film layer 3 is indicated by oblique lines.

As described above, the configuration of the second embodiment can be applied to the first embodiment.
Similar effects can be obtained. In the case of the second embodiment, it is not necessary to prepare a new substance as the moderator 9, which is convenient for manufacturing the detector.

[0057]

As described in detail above, according to the neutron detector of the present invention, since the first solid reactant and the second solid reactant are used in combination, the detector can be downsized and the length can be increased. A decrease in detection efficiency due to the use of the period can be avoided.

Here, since the first solid reactant contains lithium or boron, an (n, α) reaction to low-speed neutrons is likely to occur. On the other hand, the second solid reactant is a hydrogen compound and thus has a high speed. (N, p) reaction to neutrons is likely to occur. This results in improved detection sensitivity over a wide energy range while reducing moderators.

Further, according to the present invention, since the detection principle is based on the scintillation function, there is no need to use a gas reaction material based on ionization, and it is not necessary to consider the problem of gas leakage and deterioration.

Here, in the present invention, since plastic scintillation fibers are used as scintillators,
This also acts as an (n, p) reactant, contributing to downsizing of the detector.

On the other hand, the first solid reactant is formed in a thin film covering the second solid reactant. Accordingly, the sensitivity can be improved without missing the α particles generated here.

Further, according to the present invention, the same substance as the first solid reactant can be used as the moderator, which is advantageous in the manufacture of the detector.

Further, according to the present invention, since the set of the first solid reactant and the second solid reactant is tightly bundled, neutrons can increase the detection sensitivity.

Here, since the shape of the set is a triangular prism, a quadrangular prism, or a hexagonal prism, these can be tightly bundled.

Further, according to the present invention, since the photoelectric conversion unit independently converts the scintillation light generated in each of the sets into an electric signal, a position-sensitive detector can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a prior invention.

FIG. 2 is a diagram illustrating a configuration of a discrimination circuit that discriminates two reactions.

FIG. 3 is a diagram showing energy response characteristics of (n, α) and (n, p) reactions.

FIG. 4 is a diagram illustrating a configuration of a neutron detector according to the first embodiment.

FIG. 5 is a diagram showing a reaction when low-speed and high-speed neutrons are incident on one PSF1.

FIG. 6 is a diagram showing an example of a cross section in a state where PSFs 1 are bundled.

FIG. 7 is a diagram illustrating a configuration of a neutron detector according to a second embodiment.

[Explanation of symbols]

1 PSF, 3 thin film layer, 5 photoelectric conversion element, 7 light shielding film, 9 moderator.

Claims (10)

(57) [Claims] 1. A first solid reactant that emits charged particles by nuclear reaction with respect to incident slow neutrons, and a second solid reactant that emits charged particles by recoil action with respect to incident fast neutrons And a light-shielding material that shields at least the first and second solid reactants, and a detector that detects charged particles emitted from the first and second solid reactants. Neutron detector. 2. The neutron detector according to claim 1, wherein the first solid reactant is formed of a substance that emits charged particles by an (n, α) reaction, and the second solid reactant is (N, α). A neutron detector formed of a substance that emits charged particles by an (n, p) reaction. 3. The neutron detector according to claim 1, wherein the first solid reactant includes at least one substance selected from the group consisting of lithium and boron, and the second solid reactant includes a hydrogen compound. A neutron detector, characterized in that: 4. The neutron detector according to claim 1, wherein said detection unit emits scintillation light when said charged particles are detected, and a photoelectric converter which converts said scintillation light into an electric signal. A neutron detector, comprising: a converter; 5. The neutron detector according to claim 4, wherein the scintillator is a plastic scintillation fiber, and the scintillator doubles as the second solid reactant. 6. The neutron detector according to claim 5, wherein the first solid reactant is formed in a thin film covering the second solid reactant. 7. The neutron detector according to claim 1, wherein the detector further covers an outer periphery of the first and second solid reactants, and the second solid reactant A neutron detector comprising a neutron moderator formed of the same material as the above. 8. The neutron detector according to claim 6, wherein the detector has a plurality of sets of the first solid reactant and the second solid reactant, and the sets are closely arranged. A neutron detector characterized by being bundled. 9. The neutron detector according to claim 8, wherein a shape of a set of the first solid reactant and the second solid reactant is a triangular prism, a quadratic prism, or a hexagonal prism, and the pair is A neutron detector characterized in that the neutron detectors are bundled with their side surfaces in close contact. 10. The neutron detector according to claim 8, wherein the photoelectric conversion unit is provided for each of the plurality of sets, and independently converts scintillation light generated in each of the pairs. A neutron detector, which converts an electric signal into an electric signal.