JPH055782A - Neutron absorption dose detector - Google Patents

Abstract

PURPOSE:To provide the title detector capable of simply measure a neutron absorption dose becoming the basis of the evaluation of the dose equivalent of a neutron according to a direct reading system within a real time. CONSTITUTION:A collection electrode 22a is arranged in an external electrode 20a and an absorbing layer 24 (composed of rubber containing boron B) limiting the transmission of a thermal neutron in a constant ratio is arranged to the neutron incident surface of a thermal neutron detector in which nuclear reaction gas is sealed. An epithermal neutron detector equipped with a deceleration material 26 and covered with a thermal neutron absorbing layer 28 is arranged to the rear of the thermal neutron detector and a proton recoil scintillation detector 30 is further arranged. A part of a thermal neutron transmits through the absorbing layer 24 to reach the thermal neutron detector to be detected thereby. An epithermal neutron is decelerated by the deceleration material 26 to be converted to a thermal neutron and reaches the epithermal neutron detector to be detected thereby. A fast neutron is detected by the proton recoil scintillation detector 30.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron absorption dose detector, and in particular, a neutron absorption dose detection for measuring neutrons in a wide energy range (2.5 x 10 ^-2 to 2 x 10 ⁷ eV) from thermal neutrons to fast neutrons. Regarding vessels.

[0002]

According BACKGROUND ART neutron equation ³ below neutrons and materials having no charge dose equivalent to measuring real time of ^{He + n → 3 T + 1} P + 765keV 10 B + n → 7 Li + 4 He (α) + 2.3MeV It is carried out by interacting with each other to generate charged particles due to nuclear reaction or recoil of ³ He or ¹⁰ B and electrically measuring the charged particles.

That is, thermal neutrons n mainly react with the nuclear reactants ³ He and ¹⁰ B to generate triton T, protons P, lithium Li and charged particles such as alpha particles α.
The charged particles have an energy of 765 keV or 2.3 MeV and fly through the substance to generate an ionization charge corresponding to the energy. The amount of neutrons is measured by measuring this ionized charge. The nuclear reaction is particularly effective for measuring thermal neutrons. On the other hand, the recoil of the nucleus is that neutrons collide with hydrogen nuclei and the like to recoil the nucleus. The ionization charge is measured and the neutron amount is obtained. Nuclear recoil is especially effective for fast neutrons.

As described above, thermal neutrons mainly cause nuclear reactions and fast neutrons mainly cause nuclear recoil. Therefore, in order to measure neutrons in the energy range of about 10 ⁹ by utilizing nuclear reactions, thermal neutrons are required. High-energy neutrons excluding (epithermal neutrons: 0.5 to 5 × 10 ⁴ eV, fast neutrons: 5 × 10 ⁴ eV or more)
It is necessary to decelerate and convert to thermal neutrons for measurement.

On the other hand, in the detector utilizing the nuclear recoil, the lower the neutron energy is, the lower the energy obtained by the recoil nucleus is. In addition, the neutron detection sensitivity is generally lower than in the case of nuclear reaction by thermal neutrons. Therefore, when detecting the amount of neutrons, a detector mainly utilizing a nuclear reaction is often used.

FIG. 5 shows an example of a detector utilizing such a nuclear reaction. 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 in 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 transmittance of the thermal neutron moderator 12 and the incident rate at which the epithermal neutrons and fast neutrons are heated and are incident on the proportional coefficient tube 10 for thermal neutrons are converted into 1 cm dose equivalents which depend on the energy of the neutrons. It is desired to match the ratio of the coefficient to the constant energy.

FIG. 6 shows the ratio of the 1 cm dose equivalent conversion coefficient depending on the energy of neutrons to constant energy. The horizontal axis is the neutron energy, and the vertical axis is the 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 factor, a conversion factor, a 3 mm dose equivalent conversion factor and a 70 μm dose equivalent conversion factor, 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 by the Radiation Hazard Prevention Law.
Therefore, the 1 cm dose equivalent can be obtained by measuring the absorbed dose of each neutron energy and multiplying it by the 1 cm dose equivalent conversion factor of each energy. It is difficult to measure time. Therefore, the ratio of the unit absorbed dose of neutrons of each energy to thermal neutrons and incident on the thermal neutron detector is adjusted to the energy dependence of the dose equivalent conversion coefficient.

For this reason, in the conventional detector shown in FIG. 5, the thermal neutron absorbing material 14 is arranged in the moderator 12 at a position about 20 mm from the proportional coefficient tube 10 for thermal neutrons.
The thermal neutron absorbing material 14 is provided with a hole that allows thermal neutrons to appropriately pass therethrough, and the arrangement position and the opening area are adjusted so that the thermal neutron of each energy has an incident rate or sensitivity energy of the detector. The dependence is adapted to the energy dependence of the dose equivalent conversion coefficient shown in FIG.

[0010]

As described above, since the moderator 12 and the absorber 14 are arranged in the conventional detector, there is a problem that the number of effective neutrons to be measured is extremely small.

As described above, the proportional coefficient tube 10 for thermal neutrons has a diameter of about 30 mm and a length of about 50 mm. Therefore, the minimum detectable limit of neutrons is small and real-time continuous measurement of neutron background is possible. There was a problem that it was difficult.

Further, the outer dimensions of the moderator 12 are about 20 in diameter.
Since it is 0 mm and length is about 300 mm, it is large and weighs about 2
There was a problem that it was heavy as kg and was not excellent in transportability and operability.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to measure the absorbed dose, which is the basis of the dose equivalent evaluation of neutrons, in a direct-reading type simply and economically in real time. It is to provide a neutron absorption dose detector capable of

[0014]

In order to achieve the above object, the neutron absorption dose detector according to claim 1 is disposed in the back surface of the thermal neutron detector and the thermal neutron detector. 0.5 on the neutron entrance side or exit side of
Epithermal neutron detection constructed by arranging a moderator for decelerating epithermal neutrons having an energy of eV to 5 × 10 ⁴ eV to generate thermal neutrons by 2 mm to 12 mm and coating with an absorption layer absorbing thermal neutrons A vessel,
Located on the back of this epithermal neutron detector, 5x
And a scintillation detector for detecting fast neutrons having an energy of 10 ⁴ eV or more.

In order to achieve the above object, the neutron absorbed dose detector according to claim 2 is a thermal neutron detector,
It is placed on the back surface of this thermal neutron detector, and 0.5 eV to 5 × on the neutron incident surface side and the emission surface side of the thermal neutron detector.
An epithermal neutron detector configured by disposing a moderator for decelerating epithermal neutrons having energy of 10 ⁴ eV into thermal neutrons by 1 mm to 6 mm and covering with an absorption layer absorbing thermal neutrons, A scintillation detector arranged on the back surface of the epithermal neutron detector for detecting fast neutrons having an energy of 5 × 10 ⁴ eV or more.

Further, in order to achieve the above object, the neutron absorption dose detector according to claim 3 is the neutron absorption dose detector according to claim 1 or 2, wherein the neutron absorption surface side of the thermal neutron detector is on the neutron incident surface side. It is characterized in that a second absorption layer that restricts and transmits a fixed ratio of thermal neutrons is arranged.

[0017]

As described above, the neutron absorption dose detector of the present invention skillfully combines the thermal neutron detector and the scintillation detector to individually measure thermal neutrons, epithermal neutrons and fast neutrons. That is, thermal neutrons are detected by a thermal neutron detector, but epithermal neutrons are provided with a moderator in this thermal neutron detector, and thermal neutrons are detected by an epithermal neutron detector whose entire circumference is covered with a thermal neutron absorption layer. Are detected and detected.

On the other hand, here, the moderator is arranged around the thermal neutron detector, that is, on the neutron incident surface side or the emission surface side of the thermal neutron detector, or on both surfaces thereof, with a predetermined thickness, whereby only the epithermal neutrons are provided. It is converted into thermal neutrons and can be detected by a thermal neutron detector.

On the other hand, fast neutrons pass through these thermal neutron detectors and epithermal neutron detectors, reach the scintillation detectors, and are detected by measuring the emitted light due to nuclear recoil.

In order to match the ratio of incidence to the thermal neutron detector with the energy dependence of the dose equivalent conversion coefficient, the detection signal from the thermal neutron detector may be electrically processed. An absorption layer that restricts and transmits a certain ratio of thermal neutrons may be provided on the incident surface side, and the ratio may be adjusted by limiting the time of incidence.

[0021]

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

First Embodiment FIG. 1 shows the configuration of the first embodiment. The thermal neutron detector of this embodiment has a collector 2 inside an external electrode 20a.
2a was placed, ionization chamber enclosing the nuclear reaction gas helium -3 ³ the He and the like are used therein. External electrode 20a
Is biased to a negative voltage, and the collecting electrode 22a is connected to a measuring circuit (not shown).

An absorption layer 24 made of rubber containing boron B and having a constant opening area is disposed on the neutron incident surface side of the external electrode 20a, and absorbs a part of incoming thermal neutrons to generate a constant proportion of heat. Only neutrons penetrate.

The thermal neutrons that have flown in cause a nuclear reaction represented by the following equation with this nuclear reaction gas to generate an ionizing charge,
It is collected by the collecting electrode 22a and detected. That is, ³ He + n → ³ T + ¹ P + 765 keV.

On the back surface of this thermal neutron detector is 0.5 eV
An epithermal neutron detector is provided for detecting epithermal neutrons having an energy of ˜5 × 10 ⁴ eV. This epithermal neutron detector is arranged with a thickness of 2 mm to 12 mm on the back surface of the external electrode 20b, the collecting electrode 22b, the nuclear reaction gas sealed inside and the external electrode 20b which constitute the thermal neutron detector described above. The moderator 26 is made of polyethylene moderator 26, and the collector electrode 22b is connected to a measuring circuit (not shown). The moderator 26 has a function of decelerating epithermal neutrons by collision to convert them into thermal neutrons. The above-mentioned nuclear reaction is caused to generate ionized charges, which are detected by the collecting electrode 22b.

Further, this epithermal neutron detector is
Absorption layer 2 made of polyethylene similar to the absorption layer 24
The entire circumference is covered with 8 to prevent thermal neutrons from reaching the detector. Therefore, only epithermal neutrons will be detected by this detector.

On the back surface of this epithermal neutron detector, a proton recoil scintillation detector 3 is provided.
0, the light guide 32, and the photoelectron amplifying tube 34 are arranged to detect fast neutrons having energy of 5 × 10 ⁴ eV or more. That is, when fast neutrons enter this proton recoil scintillation detector, they cause nuclear recoil with the protons to give energy to the protons, and the emitted light is condensed by the light guide 32 and converted into an electric signal by the photomultiplier tube 34. To do.

As described above, in the first embodiment, the thermal neutron detector, the epithermal neutron detector, and the fast neutron detector are independently provided and combined with each other, so that a wide range of energy from thermal neutrons to fast neutrons can be obtained. Neutrons can be detected reliably and easily.

As described above, in this embodiment, thermal neutrons,
Since the epithermal neutrons and the fast neutrons are detected individually, the absorption layer 24 arranged to limit the incidence ratio of thermal neutrons in FIG. By performing signal processing so that the detection ratio of thermal neutrons matches the conversion factor, it is possible to obtain the same detection result as that of the detector of FIG.

Further, in this embodiment, the moderator 26 in the epithermal neutron detector is arranged on the back surface of the external electrode 20b. However, as shown in FIG. 2, the moderator 26 is arranged on the neutron incident surface side of the external electrode 20b. Even so, the epithermal neutrons can be decelerated into thermal neutrons.

Second Embodiment FIG. 3 shows the configuration of the second embodiment of the present invention.
Almost the same as in the first embodiment described above, a thermal neutron detector composed of an ionization chamber, an epithermal neutron detector composed of an ionization chamber provided with a moderator 26, and a proton recoil scintillation detector 30 for detecting fast neutrons are combined. However, in the second embodiment, the moderator 26 is not arranged on either the incident side or the emission side of the external electrode 20b, but the moderators 26a and 26b are respectively 1 mm or 2 mm on both the incident side and the emission side. It is arranged 6 mm to form a sandwich structure. Thus, even when the moderators 26a and 26b are arranged on both sides with a predetermined thickness, the epithermal neutrons can be decelerated by both moderators to be converted into thermal neutrons.

In the second embodiment as well, the same detection result can be obtained by arranging the absorption layer 24 and performing signal processing of the detected amount of thermal neutrons in the measuring circuit connected to the collecting electrode 22a.

In the first and second embodiments, the moderator 2 is used.
Polyethylene was used for 6, but materials such as paraffin, various plastics, water, and graphite that can easily decelerate neutrons and turn into thermal neutrons can be used.

Further, in the first and second embodiments, rubber containing boron B was used for the absorption layer, but cadmium Cd can also be used, and the absorption layer 24 also has a structure as shown in FIG.
Various modes such as (a) a rubber layer containing a constant thickness of boron, (b) uniform distribution of openings, and (c) thinning of a part can be used.

[0035]

As described above, the neutron absorbed dose detector according to the present invention can easily measure the absorbed dose of neutrons in a wide energy range from thermal neutrons to fast neutrons in real time.

[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 another arrangement of the moderator of the embodiment.

FIG. 3 is a configuration diagram of a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of an absorption layer of the present invention.

FIG. 5 is a configuration diagram of a conventional detector.

FIG. 6 is a graph showing a ratio of a dose equivalent conversion coefficient to constant energy.

[Explanation of symbols]

 20 External Electrode 22 Collector Electrode 24, 28 Absorption Layer 26, 26a, 26b Moderator 30 Proton Recoil Scintillation Detector

Claims (1)

Claim: What is claimed is: 1. A thermal neutron detector and a rear surface of the thermal neutron detector. The thermal neutron detector has 0.5 eV to 5 × 10 ⁴ on the neutron incident surface side or the emission surface side of the thermal neutron detector. An epithermal neutron detector configured by arranging a moderator for decelerating epithermal neutrons having eV energy into thermal neutrons by 2 mm to 12 mm and covering with an absorption layer absorbing thermal neutrons, and the epithermal neutron detector A scintillation detector, which is disposed on the back surface of the neutron detector and detects fast neutrons having an energy of 5 × 10 ⁴ eV or more, and a neutron absorbed dose detector. 2. A thermal neutron detector and an epi disposed on the back surface of the thermal neutron detector and having an energy of 0.5 eV to 5 × 10 ⁴ eV on the neutron incidence side and the emission side of the thermal neutron detector. An epithermal neutron detector configured by arranging a moderator for decelerating thermal neutrons into thermal neutrons 1 mm to 6 mm and covered with an absorption layer that absorbs thermal neutrons, and a back surface of the epithermal neutron detector And a scintillation detector for detecting fast neutrons having an energy of 5 × 10 ⁴ eV or more, and a neutron absorbed dose detector. 3. The neutron absorption dose detector according to claim 1 or 2, wherein a second absorption layer that restricts and transmits a fixed ratio of thermal neutrons is arranged on the neutron incident surface side of the thermal neutron detector. Characteristic neutron absorbed dose detector.