JPH0513042A - Neutron detector - Google Patents

Abstract

PURPOSE:To ensure high sensitivity detection of only epithermal neutrons. CONSTITUTION:A rod-like collector electrode 22 is arranged on the inside of a box-like outside electrode 20, and helium -3<3>He is sealed therein. The output of the collector electrode 22 is connected to a measurement circuit 30. A moderator 32 such as polyethylene is provided on the lower part of the outside electrode 20, and an absorption layer 34 made of rubber including boron is arranged on the entire circumference of a detector. Thermal neutrons are absorbed by the absorption layer 34 and are not detected thereby. Accelerated neutrons are transmitted through the absorption layer 34 as well as the moderator 32 and are not detected thereby. Since only epithermal neutrons of 0.5-5X10<4>eV are transmitted through the absorption layer 34 and is moderated by the moderator 32 into thermal neutrons, nuclear reaction occurs and ionized charges are generated, which are detected as pulses.

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 which measures only epithermal neutrons.

[0002]

2. Description of the Related Art Neutron measurement is performed in a wide energy range of 9 decad (2.5 × 10 ^-2 to 1.
4 × 10 ⁷ eV), reliable, convenient and economical. Since neutrons have no electric charge, it is necessary to convert them into charged particles for electrical measurement.

In general, there are two methods used for measurement: one is a nuclear reaction effective for thermal neutrons, and the other is a method utilizing recoil of nuclei effective for fast neutrons. Therefore, low energy and high energy can be measured by using both methods in the wide energy range of 9 decade, but 0.5 eV to 5 × 1.
Neither method can be used for measuring 0 ⁴ eV epithermal neutrons.

Here, when neutrons collide with a light element such as hydrogen nucleus, energy is given to the light element. Therefore, each time the collision is repeated, the neutron loses energy and finally becomes a thermal neutron. Therefore, this thermal neutron can be converted into charged particles by a nuclear reaction and electrically measured. For this reason, substances that contain many light elements such as hydrogen nuclei and are effective for thermal neutronization (hereinafter referred to as moderators), such as polyethylene, paraffin, and water, are placed, and epithermal neutrons and even fast neutrons are heated. Neutronization and measurement are performed.

FIG. 3 shows an example of a detector using such a moderator. In the figure, the thermal neutron detector is configured such that a proportional coefficient tube for thermal neutrons (diameter 30 mm, length 50 mm) ¹⁰ BF ₃ gas or ³ He gas is enclosed at the center of a moderator 12 made of polyethylene or the like. To be done. The thickness of the moderator 12 up to the proportional coefficient tube 10 for thermal neutrons is set to about 90 mm, and epithermal neutrons and fast neutrons incident from the outside of the moderator 12 are decelerated to the proportional coefficient tube 10 for thermal neutrons. It is measured as thermal neutrons before it is incident. On the other hand, thermal neutrons incident from the outside pass through the moderator 12 and are measured by the proportional coefficient tube 10 for thermal neutrons.

Here, the transmissivity of the moderator 12 for thermal neutrons and the incident rate at which epithermal neutrons and fast neutrons enter the proportional coefficient tube 10 for thermal neutrons are constant 1 cm dose equivalent conversion factors depending on the energy of neutrons. It is desired to match the ratio to energy.

FIG. 4 shows the ratio of the 1 cm dose equivalent conversion coefficient depending on the energy of neutrons to constant energy. The horizontal axis is neutron energy, and the vertical axis is 1 cm dose equivalent value per unit absorbed dose, that is, 1
The cm dose equivalent conversion coefficient value is shown. In the figure, A,
B, C, and D are a 1 cm dose equivalent conversion coefficient, a conversion coefficient, a 3 mm dose equivalent conversion coefficient, and a 70 μm dose equivalent conversion coefficient, respectively. The 1 cm dose equivalent conversion factor is ICR
It is based on P (International Radiation Protection Organization) Recommendation 51 and is also adopted in the Radiation Hazard Prevention Law.
Therefore, the 1 cm dose equivalent can be obtained by measuring the absorbed dose for each energy of neutrons and multiplying it by the 1 cm dose equivalent conversion coefficient for each energy. It is difficult to measure time. Therefore, the ratio of the unit absorbed dose of neutrons of each energy converted into thermal neutrons and incident on the thermal neutron detector is adjusted to the energy dependency of the dose equivalent conversion coefficient.

Therefore, in the conventional detector shown in FIG. 3, the thermal neutron absorbing material 14 is disposed in the moderator 12 at a position approximately 20 mm from the proportional coefficient tube 10 for thermal neutrons.
The thermal neutron absorbing material 14 is provided with a hole for appropriately transmitting thermal neutrons, and the arrangement position and this opening area are adjusted to adjust the incident rate or the sensitivity energy of the thermal neutron of each energy to the detector. The dependence is adapted to the energy dependence of the dose equivalent conversion coefficient shown in FIG.

As described above, in the conventional detector, by decelerating the neutrons and absorbing the thermal neutrons, the measurement sensitivity of each energy is matched with the energy dependence of the dose equivalent conversion coefficient. Since the signal of 1 is based on the dose equivalent, the dose equivalent can be obtained in a direct-reading manner only by introducing the signal from the electric signal derivation terminal 16 into a normal measurement circuit system.

[0010]

However, although such a detector can easily measure the dose equivalent, there is a problem that energy information (information distinguishing between thermal neutrons, epithermal neutrons, and fast neutrons) cannot be obtained. there were.

As described above, thermal neutrons mainly cause nuclear reaction, and fast neutrons mainly cause nuclear recoil, so a method of measuring charged particles by these is also conceivable, but epithermal neutrons cannot be measured in this case as well. There is a problem. Further, in recoil particle measurement used for fast neutron measurement, there are problems that a scintillator or the like that causes recoil has a large volume and increases in weight, separation from γ-rays is difficult, and sensitivity is low.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a neutron detector capable of reliably and simply measuring epithermal neutrons in real time. is there.

[0013]

In order to achieve the above object, the neutron detector according to claim 1 has a box-shaped outer electrode in which a collecting electrode is arranged and which is nuclear-reacted with thermal neutrons. In a neutron detector formed by enclosing a nuclear reaction gas that produces an ionizing charge, a moderator that decelerates epithermal neutrons having an energy of 0.5 eV to 5 × 10 ⁴ eV into thermal neutrons is a neutron of the external electrode. 2 mm to 12 mm is arranged on the incident surface side or the neutron emission surface side, and an absorption layer that absorbs thermal neutrons is arranged to cover the entire circumference of the external electrode and moderator, and only epithermal neutrons are converted into thermal neutrons and detected. Characterize.

Further, in order to achieve the above object, in the neutron detector according to a second aspect of the present invention, a collecting electrode is arranged in a box-shaped external electrode, and a nuclear reaction with thermal neutrons is generated inside to generate an ionizing charge. In a neutron detector in which a nuclear reaction gas is sealed, a moderator that decelerates epithermal neutrons having an energy of 0.5 eV to 5 × 10 ⁴ eV into thermal neutrons is provided on the neutron incident surface side of the external electrode and It is characterized in that it is arranged at 1 mm to 6 mm on the neutron emission surface side, an absorption layer that absorbs thermal neutrons is covered around the outer electrodes and moderator, and only epithermal neutrons are converted into thermal neutrons and detected.

[0015]

As described above, in the neutron detector of the present invention, a moderator having an appropriate thickness is arranged on one side (neutron incident surface or emission surface) or both sides of the ionization chamber composed of the external electrode and the collecting electrode. The detector is covered with a thermal neutron absorption layer.

Most of the thermal neutrons coming from the outside are absorbed by the absorption layer, so that they do not react with the nuclear reaction gas sealed in the ionization chamber and are not detected.

Further, fast neutrons having an energy of 5 × 10 ⁴ eV or more are not absorbed in the absorption layer, and are transmitted almost without being slowed down by the moderator, so that thermal neutrons do not undergo nuclear reaction with the nuclear reaction gas. Not detected.

On the other hand, epithermal neutrons having energies of 0.5 eV to 5 × 10 ⁴ eV are not absorbed by the absorption layer, but are moderated by a moderator having an appropriate thickness and converted into thermal neutrons. A nuclear reaction with the nuclear reaction gas sealed in produces an ionization charge, which is collected in a collecting electrode and pulse-measured.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the neutron detector according to the present invention will be described below with reference to the drawings.

First Embodiment FIG. 1 shows the configuration of the present embodiment, in which a rod-shaped collecting electrode 22 is arranged inside a box-shaped external electrode 20. The rod-shaped collector electrode 22 is supported in an airtight structure via a cylindrical guard ring 26 and an annular insulator 24. The helium--3 ³ the He gas for the nuclear reaction is sealed in the interior,
It constitutes an ionization chamber. The guard ring 26 is grounded, and a negative voltage is applied to the external electrode 20 by the bias electrode 28. Then, the electric signal from the rod-shaped collecting electrode 22 is connected to the measuring circuit 30, and the neutron amount is pulse-measured from the amount of charges collected in the rod-shaped collecting electrode 22.

Here, in this embodiment, two moderators 32 made of polystyrene are provided below the box-shaped external electrode 20.
mm to 12 mm thick, and the external electrode 2
0 and an absorbing layer 34 made of rubber containing boron B are arranged so as to surround the moderator 32. This absorption layer 34
Covers the entire circumference of the detector, has no openings, etc. and absorbs almost all incoming thermal neutrons.

As described above, in this embodiment, since the entire circumference is covered with the thermal neutron absorbing layer 34, thermal neutrons do not reach the detector, and only epithermal neutrons and fast neutrons reach. In FIG. 1, the symbol T represents thermal neutrons and is shown as being absorbed by the absorption layer 34.

On the other hand, epithermal neutrons having an energy of 0.5 eV to 5 × 10 ⁴ eV reach the detector without being absorbed by the absorption layer 34. are arranged moderator 32 at the bottom, this is reduced at the speed reduction member 32 is thermalized, the collector electrode 22 occurs ionization charge according to the following nuclear reactions with helium -3 ³ the He gas sealed inside Will be collected. That is, ³ He + n → ³ T + ¹ P + 765 keV In FIG. 1, the symbol E represents epithermal neutrons, penetrates the absorption layer 34 and enters the moderator 32, and M 1,
It is shown that they collide with M2 and M3, turn into thermal neutrons, and undergo a nuclear reaction at D.

Fast neutrons are similar to thermal neutrons in the absorption layer 3
4 is not absorbed and is transmitted through the detector with little loss of energy even by the moderator 32,
It will never be detected. In FIG. 1, the symbol F represents fast neutrons, and is shown to be transmitted through the detector.

As described above, in this embodiment, only the epithermal neutrons can be detected by disposing the thermal neutron absorption layer around the entire circumference of the detector and disposing the moderator under the external electrode.

In this embodiment, the moderator 32 is provided on the lower side of the external electrode 20, that is, on the emission side. However, the same effect can be obtained by arranging the moderator 32 on the neutron incident side, that is, on the upper side of the external electrode 20 in the figure. Obtainable.

Second Embodiment FIG. 2 shows the configuration of the second embodiment of the present invention.
The configuration is almost the same as that of the first embodiment described above, but the second embodiment
In the embodiment, the moderator 32 is not arranged only under the external electrode 20, but the moderators 32a,
32b is arranged to form a sandwich structure. here,
The moderators 32a and 32b are set to have a thickness half that of the moderator 32 shown in the first embodiment, that is, 1 mm to 6 mm, and epithermal neutrons are used for the moderators 32a and 32b.
It loses energy at b and is converted to thermal neutrons. Then, the ionized charges generated by the nuclear reaction with the enclosed nuclear reaction gas are collected at the collecting electrode and pulse-detected. Further, thermal neutrons are absorbed by the absorption layer 34 as in the first embodiment, and fast neutrons pass through the detector without being converted into thermal neutrons by the moderator. Therefore, even in the second embodiment, only epithermal neutrons can be detected.

As described above, according to the first and second embodiments, thermal neutrons and fast neutrons are almost insensitive, and only epithermal neutrons are converted into thermal neutrons, which enables highly sensitive and reliable detection. In addition, since it is a thermal neutron measurement method that measures charged particles generated by a nuclear reaction, a conventional thermal neutron measurement circuit system can be used as the measurement circuit system, and its output signal is large, so it can be used as a signal by γ rays or X rays. It can also be separated. Although polyethylene is shown as the material of the moderator in the first and second embodiments, materials such as paraffin, various plastics, water, graphite, etc. that can moderate neutrons and easily turn into thermal neutrons can be used.

Further, as the material for the absorption layer, cadmium Cd can be used in addition to boron B.

[0030]

As described above, the neutron detector according to the present invention has an effect that only epithermal neutrons can be measured with high sensitivity.

Further, since the moderator in the present invention only needs to decelerate epithermal neutrons, the moderator requires a small amount as compared with the case of measuring fast neutrons, and the size and weight can be reduced.

Further, since the absorption layer of the present invention covers the entire circumference of the detector, there is an advantage that it is not necessary to provide an opening having a constant area or to arrange it in a mosaic manner.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a second embodiment of the present invention.

FIG. 3 is a configuration diagram of a conventional detector.

FIG. 4 is a graph showing a ratio of a dose equivalent conversion coefficient to constant energy.

[Explanation of symbols]

20 external electrodes 22 Collection electrode 32, 32a, 32b moderator 34 Absorption layer

Claims (2)

[Claims] 1. A neutron detector comprising a box-shaped outer electrode in which a collecting electrode is arranged, and a nuclear reaction gas which nuclear-reacts with thermal neutrons to generate an ionizing charge is enclosed. A moderator that decelerates epithermal neutrons having an energy of 5 × 10 ⁴ eV into thermal neutrons is arranged on the neutron incidence surface side or the neutron emission surface side of the external electrode by 2 mm to 12 mm, and absorbs thermal neutrons. The neutron detector is characterized in that the outer electrode and the moderator are covered and disposed on the entire circumference thereof, and only epithermal neutrons are converted into thermal neutrons and detected. 2. A neutron detector comprising a box-shaped outer electrode in which a collecting electrode is arranged, and a nuclear reaction gas that nuclear-reacts with thermal neutrons to generate an ionizing charge is enclosed. A moderator that decelerates epithermal neutrons having an energy of 5 × 10 ⁴ eV into thermal neutrons is 1 m on the neutron incidence surface side and the neutron emission surface side of the external electrode.
A neutron detector characterized in that it is arranged m to 6 mm, an absorption layer that absorbs thermal neutrons is provided so as to cover the outer electrodes and the entire circumference of the moderator, and only epithermal neutrons are converted into thermal neutrons and detected.