JP2002006055A - Radiation measuring method and device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation measuring method and a device using it that can measure a neutron beam dose in a normal low dose rate and a neutron beam dose in a high dose rate. SOLUTION: Detectors (14, 15), which are sensitive to both γ and a neutron beam, and a substance (12) that radioactivates by the neutron beam are combined. Neutron beam doses in both of the low dose and the high dose rate are determined by measuring a pulse in the low rate and by estimating a neutron beam dose by a radioactivating method in the high dose rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation measuring method and apparatus, and more particularly, to a radiation measuring method and apparatus for measuring a neutron dose which is always small and irregularly large in a burst.

[0002]

2. Description of the Related Art Conventionally, methods for measuring a neutron dose mainly include a method using a detector for directly measuring a neutron beam,
In addition, there is a method of estimating a neutron dose by measuring the amount of a substance activated by using activation due to a nuclear reaction by a neutron beam.

[0003]

In the event that a large amount of neutrons are generated in a burst, the output of a conventional neutron dose monitor is saturated when the dose rate exceeds a certain level, and the neutron dose during the saturation is reduced. If the detection efficiency is reduced so as not to be understood or to prevent saturation, there is a problem that it is not useful as a normal neutron dose monitor.

Accordingly, an object of the present invention is to provide a radiation measuring method and apparatus capable of measuring a neutron dose at a low dose rate at normal times and a neutron dose at a high dose rate.

[0005]

Means for Solving the Problems In order to solve the above problems, the invention of claim 1 combines a detector sensitive to both γ-rays and neutrons with a substance activated by neutrons. The neutron dose rate is calculated for both low and high dose rates by estimating the neutron dose by pulse measurement for dose rate and activation method for high dose rate. I do. According to the invention,
Neutron dose can be measured from a small dose rate to a large dose rate.

According to a second aspect of the present invention, there is provided a radiation detector comprising an activation foil made of a substance activated by a neutron beam and a detector provided behind the activation foil and sensitive to γ-rays and neutron rays. A signal processing unit that receives the output of the radiation detection unit and performs parallel processing on the signal based on the neutron beam and the signal based on the γ-ray; and on the output of the signal processing unit, the dose rate of the neutron beam incident on the radiation detection unit. And a data analysis unit that calculates According to the present invention, it is possible to measure the small dose rate in the normal state and the large dose rate in the abnormal state with one measuring device, and obtain the respective measured values without omission.

According to a third aspect of the present invention, in the radiation measuring apparatus according to the second aspect of the present invention, the signal processing section discriminates a signal based on a neutron beam and a signal based on a γ-ray based on a peak value. According to the present invention, a signal due to a neutron beam and a signal due to a gamma ray can be clearly distinguished.

According to a fourth aspect of the present invention, in the radiation measuring apparatus according to the second aspect of the present invention, the data analyzing section determines a neutron level based on a time change of the signal by the neutron beam and the signal by the γ ray input from the signal processing section. It is configured to determine the occurrence of saturation of the detector at the dose rate and to estimate the neutron dose during the dead period of the detector. According to the present invention, the saturation of the detector can be determined in accordance with each event of the radiation measurement.

According to a fifth aspect of the present invention, in the radiation measuring apparatus according to the second aspect of the present invention, the data analysis section detects a difference between a waveform caused by neutron incidence and a waveform caused by γ-ray incidence, thereby obtaining an incident light. Of the type of radiation
The configuration is such that the immediate neutron dose rate is estimated from the neutron count, and the amount of activated material generated is estimated from the count for each γ-ray energy range. According to the present invention, the signal from the neutron beam and γ
It is easy to distinguish the signal by the line, and it is easy to remove noise superimposed on the signal.

According to a sixth aspect of the present invention, in the radiation measuring apparatus according to the second aspect of the present invention, the activation foil is made of a substance which causes a nuclear reaction having a threshold energy of fast neutrons, and the data analysis unit is provided for each neutron energy. It is configured to estimate the existence ratio of. According to the present invention, the energy distribution of the neutron source can be obtained.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A neutron detector is usually sensitive to gamma rays, but in the prior art, the contribution of gamma rays is generally removed as a noise component. However, the radiation measurement method and apparatus of the present invention utilize both of them,
The current dose rate determined by counting the neutron incidence on the detector and the γ-rays from the radioactive material activated by the neutron incidence are measured. In the latter, radioactive substances with a large reaction cross section for thermal neutrons placed around the detector, such as Au and In, are activated by neutrons, which are released by their decay A radiation detector that is sensitive to both γ-rays and neutrons
For example, it is measured by a detector in which ⁶ Li is mixed in a ZnS (Ag) scintillator.

A radiation measuring apparatus according to an embodiment of the present invention comprises:
As shown in FIG. 1, it comprises a radiation detecting section 1, a signal processing section 2, a data analyzing section 3, and a data recording section 4. The neutron source to be measured is to the left of the radiation detector 1.

The radiation detecting section 1 is a shielding box made of a lead plate.
11, a radiation foil 12 made of Au or In, a β-ray shielding sheet 13 made of Al, a scintillator 14 made of ZnS (Ag) or glass mixed with ⁶ Li, and a photomultiplier tube 15 They are arranged in this order. The photomultiplier tube 15 has a high voltage source 16 for driving and a preamplifier 17 for amplifying the radiation detection signal.
Is connected.

The signal processing unit 2 has separate systems for the signal based on the neutron beam and the signal based on the γ-ray.
n, 21γ, wave height discriminators 22n, 22γ, and counting circuit 23n,
23γ.

[0015] In the radiation measuring apparatus having such a configuration, ⁶ L as a converter for converting neutrons into charged particles is used.
Neutron beam and γ incident on scintillator 14 containing i
The signal by the line is converted into an electric signal by the photomultiplier tube 15.

The scintillator 14 is formed by the ⁶ Li (n, α) T reaction.
Energy is about 4.8 keV,
Γ emitted by ¹⁹⁸ Au generated by activation of gold ( ¹⁹⁷ Au)
The lines are 412 keV, 676 keV, and 1088 keV, and the γ-ray emitted by ¹¹⁶ min produced by the activation of ¹¹⁵ In is 2 from 138 keV.
The range is 112 keV. Therefore, a signal wave is generated from energy with respect to α-rays, such as a scintillator 14 having a large α / β ratio, which represents a ratio of luminous efficiency due to α-rays to luminous efficiency due to γ-rays (and secondary electrons generated by the interaction with the scintillator 14). If a scintillator having a high conversion efficiency to a high value is used, a difference occurs between the peak values of the neutron beam and the γ-ray signal as shown in FIG. Therefore, each of the counting circuits 23
n, 23γ are amplified by linear amplifiers 21n, 21γ whose amplification factors are set so as to have appropriate peak values, and in the neutron beam counting circuit 23n, only the neutron beam is counted, and the γ-ray counting circuit is used.
At 23γ, counting is performed after passing through pulse height valves 22n and 22γ in which the peak value level is set so as to count only γ-rays due to activation, thereby obtaining a count using only neutron beams or only γ-rays.

As described above, in the radiation measuring apparatus according to the present embodiment, when the counting rate due to neutrons is small, the neutrons incident on the radiation detecting unit 1 are not considered, without taking into account the counting accompanying the incidence of gamma rays due to activation. The neutron dose rate is determined from the line count alone.

The neutron dose during the period when the counting rate becomes large and the counting is saturated and the measurement cannot be performed is determined by a circuit (21γ, 22
(γ, 23γ), the amount of activation of the activation foil 12 is measured, and the amount is determined by going back to the saturation period. The activation measurement circuit (21γ, 22γ, 23γ) used for neutron dose estimation in the past saturation period is a different system from the one for immediate dose rate measurement by neutron incidence (21n, 22n, 23n). ,
Even during activation measurement, the neutron dose at that time can be obtained in real time.

When a substance having a low energy / signal peak conversion efficiency with respect to α rays, for example, a substance having a small signal peak value with respect to α rays such as NaI (Tl) having a small α / β ratio is used as the scintillator 14, a neutron beam is used. It is necessary to use a substance that causes a higher-energy reaction because the difference between the applied energy due to γ rays and the applied energy due to γ rays needs to be larger. For this purpose, it is preferable to use a fission material, for example, a trace amount of ²³⁵ U. ^The energy due to the fission of ²³⁵ U is about 200 MeV, and even a detector having a small conversion efficiency from the energy for α-rays to the peak value of a signal can easily be distinguished from the count by γ-rays of several MeV. ²³⁵ mixed amount of U may be a very small amount, also concentrated ²³⁵
There is no need to use U, and ²³⁵ U naturally is sufficient.

The radiation detector 1 has a shielding structure using a lead plate having a thickness of about 10 cm in order to prevent gamma rays from being counted by an external background. By adopting such a structure, it is possible to reduce the influence of the surrounding environment and external γ-rays at the time of criticality, and at the same time, to the outside of the γ-rays emitted from the activation foil 12 in the detection system during neutron detection. Also acts as a shield. The structural material of the detection unit 1 includes a substance that does not hinder the determination of the amount of activation, for example, aluminum (0.235 barn), which is a substance having an activation cross section smaller than gold (activation cross section 98.8 barn). By using, it is possible to measure the neutron dose rate with higher accuracy.

The time change of the count in the signal processing unit 2 is recorded in the data recording unit 4. If the count of γ-rays exceeds the threshold value, it is determined that saturation has occurred immediately before, and the dose is calculated from the activation amount of the radioactive substance obtained by measuring γ-rays in the period in which the saturation occurred. If there is no saturation in the counting of γ-rays, the neutron dose rate is obtained from the conversion factor obtained by the preliminary calibration only from the neutron counting.

The threshold value of the counting rate of γ-rays used to determine whether or not saturation has occurred is based on the transition of the neutron dose up to that time, and the amount of activation is always grasped by the data analyzer, and is changed accordingly. Let it. Its value is determined as follows. Activation amount R (t) (unit ato) at a certain time point t (unit: second)
m) is the previous neutron dose rate D (t) (unit: Sv / sec),
Activation rate p (unit: atom / Sv) and decay constant λ (decay / second
/ atom) and

(Equation 1) Because of the relationship

(Equation 2) Is obtained. However, the dose rate during the period when saturation occurs
The dose during saturation is approximated by the average of the length of the saturation period.

Therefore, if the threshold value of the γ-ray count at the time of no activation is Th (0), the threshold value Th (t) of the γ-ray count at a certain time t is

(Equation 3) It can be expressed as. Here, ε is the detection efficiency (unit: counts / decay) of γ-rays in the detection unit 1 per decay.

As described above, by changing the threshold value, a large amount of neutrons are generated again in a burst form within a time period within several half-lives of the radioactive material, and even when the counting is saturated, the large amount of neutrons is generated. The saturation can be detected every time, and the dose can be corrected.

There is a method of discriminating neutron rays and γ rays from discrimination by waveforms other than using peak values. The output waveform of the radiation detection unit 1 using some scintillators, for example, most scintillators such as NaI (Tl) and CsI (Tl) differs depending on the type of incident particles as shown in FIG. Utilizing this property, as shown in FIG. 5, the output waveform of the preamplifier 17 is digitized by the digitizing device 5 and sent to the data analysis unit 3, where the waveform analysis is performed. It can discriminate between rays and gamma rays, determine energy, measure count rate, and the like.

By analyzing the waveforms, it is also possible to remove noise from individual waveforms and to eliminate superimposition of waveforms when the waveform approaches saturation, thereby raising the upper limit of the saturation counting rate when counting neutron pulses. be able to.

In the discrimination based on only the peak value of the output wave of the radiation detection unit 1, a combination in which the peak value of the output signal due to γ-rays and the peak value of the output signal due to neutrons are similar, for example, α / beta ratio and a scintillator, such as NaI (Tl) having a ^{small, 10 B (n, α)} 7 Li could not be used a combination of materials to produce a small nuclear reaction generates energy, such as (2.8MeV), waveform analysis In such discrimination, combinations of such substances can also be used.

As the scintillator 14 used to cause a reaction with the thermal neutrons in the radiation detecting section 1, the above-described scintillator ZnS (A) containing ⁶ Li as a converter is used.
g), but as a converter, ¹⁰ B (n, α) ⁷ Li
(2.8 MeV), ¹⁴ N (n, p) ¹⁴ C (0.63 MeV), ³⁵ Cl (n, p) ³⁵ S
A substance such as (0.62 MeV) was difficult to discriminate only by the peak value, but it can be discriminated by analyzing the waveform and can be used. FIG. 5 shows a circuit of the measuring apparatus in this case.

In neutron dosimetry, not only the number of neutrons but also their energy is an important factor in dose determination. Therefore, if the abundance ratio of thermal neutrons / epithermal (fast) neutrons is known, more accurate dose calculation becomes possible.

[0030] As estimation of the thermal neutrons / epithermal neutron abundance by counting directly neutron detector, as well as a substance having high thermal neutron cross-sectional area of ⁶ Li or the like to the converter material, the threshold in response to formation reaction ²⁴ Mg (reaction: ²⁴ Mg (n, p) ²⁴ Na, threshold energy 6.0 MeV), and ⁵⁴ Fe
(Reaction: ⁵⁴ Fe (n, p) ⁵⁴ Mn, threshold energy 2.2 MeV), ⁵⁸
Ni (reaction: ⁵⁸ Ni (n, p) ⁵⁸ Co, threshold energy: 2.9 MeV)
By using the waveform analysis, it is possible to discriminate particles produced by thermal neutrons from particles produced by epithermal neutrons, and to know the abundance ratio of thermal neutrons / epineutrons.

As the activated substance, a substance having a threshold reaction is used, and γ released when the substance is activated and decays.
By setting the window only for the energy of the line and counting, not only the dose due to thermal neutrons but also the dose due to epithermal neutrons can be obtained at the same time.

[0032]

According to the radiation measuring method and apparatus of the present invention,
Since both neutron counting and activation gamma ray counting are used, a wider range of dose rates can be measured than with a normal neutron monitor.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a radiation measuring apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating discrimination between a neutron beam and a γ-ray based on a peak value of a generated pulse.

FIG. 3 is a diagram showing detection of counting saturation when a plurality of burst neutron beams are generated.

FIG. 4 is a diagram showing a difference in a generated pulse waveform depending on an incident particle.

FIG. 5 is a diagram showing a configuration of a radiation measuring apparatus according to another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Radiation detection part, 2 ... Signal processing part, 3 ... Data analysis part, 4 ... Data recording part, 5 ... Digitizing device, 11 ... Shielding box, 12 ... Activation foil, 13 ... Beta ray shielding plate, 14 ... Scintillator, 15 photomultiplier tube, 16 high voltage source, 17 preamplifier, 21n, 21γ linear amplifier, 22n, 22γ wave height discriminator, 23n, 23γ counting circuit.

Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court II (Reference) G01T 3/00 G01T 3/00 G (72) Inventor Noriyuki Seki 1 Toshiba-cho, Fuchu-shi, Tokyo Toshiba Fuchu Works F term (reference) 2G088 FF09 GG20 JJ01 JJ29 KK01 KK12

Claims (6)

[Claims] 1. A combination of a detector sensitive to both gamma rays and neutrons and a substance activated by neutrons. Pulse measurement is performed at a low dose rate, and activation is performed at a high dose rate. A radiation measurement method characterized by determining a neutron dose rate in both a low dose rate and a high dose rate by estimating a neutron dose by a method. 2. A radiation detector comprising: an activation foil made of a substance activated by a neutron beam; and a detector provided behind the activation foil and sensitive to γ-rays and neutron rays; A signal processing unit that receives the output of the unit and processes the signal of the neutron beam and the signal of the gamma ray in parallel, and receives the output of the signal processing unit and calculates the dose rate of the neutron beam incident on the radiation detection unit And a radiation measuring device. 3. A signal processing unit, comprising:
3. The radiation measuring apparatus according to claim 2, wherein a signal based on the line is discriminated by a peak value. 4. A data analysis unit judges the occurrence of saturation of a detector at a high dose rate of neutrons from a time change of a signal by a neutron beam and a signal by a γ-ray input from a signal processing unit, and makes the detector insensitive. 3. The radiation measuring apparatus according to claim 2, wherein a neutron dose in the period is estimated. 5. The data analysis unit detects a difference between a waveform due to neutron incidence and a waveform due to γ-ray incidence, identifies a line type of the incident radiation, and immediately calculates a neutron from a neutron count. 3. The radiation measuring apparatus according to claim 2, wherein the dose rate is estimated from the count of each energy range of γ-rays to estimate the amount of the activated substance generated. 6. The activation foil according to claim 2, wherein the activation foil is made of a substance that generates a nuclear reaction having a threshold energy of fast neutrons, and the data analysis unit estimates an abundance ratio for each energy of the neutrons. Radiation measurement device.