JPH05281364A - Instrument for measuring neutron dose equivalent - Google Patents

Abstract

PURPOSE:To obtain an instrument for measuring neutron dose equivalent which can measure a wide range of neutrons from them neutrons to fast neutrons with high sensitivity and has a portable size and weight. CONSTITUTION:A first and second semiconductor detecting sections and 22 are put upon another and brought into contact with an object 11 to be inspected. The section 22 only detects thermal neutrons made incident to the object 11. Intermediate to fast neutrons are changed to thermal neutrons by the object 11 and back-scattered thermal neutrons are detected by means of the section 14. A Cd filter which blocks the thermal neutrons is provided between the sections 14 and 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron dose equivalent measuring device, and more particularly, to a neutron dose equivalent measuring device used for measuring neutrons in a wide energy range from thermal neutrons to fast neutrons in personal exposure management.

[0002]

2. Description of the Related Art In radiation control, generally, neutrons cannot be directly measured by ionization or the like because neutrons are not charged particles.

Therefore, neutrons are converted into charged particles or the like by some method and measured. That is,
Measurement of neutrons, for example, as fast neutrons, or by causing a nuclear reaction by decelerating thermal neutrons by decelerating with a moderator using a hydrogen-containing substance such as water or paraffin, the resulting α particles and protons etc. This is done by detecting charged particles and γ rays, and detecting radiation from RI generated as a result of the reaction.

That is, in the concrete measurement of neutrons, charged particles generated by the (n, α) reaction with respect to neutrons of ⁶ Li and ¹⁰ B are applied to a proportional counter, a scintillation detector or a TLD element (thermoluminescence dosimeter). Can be detected.

However, as shown in the characteristic diagram of the neutron reaction cross section with respect to the neutron energy shown in FIG. 4, the reaction cross section becomes smaller as the energy of the neutron becomes higher. Since it is limited to thermal neutrons in the low energy region, which has a relatively large area, it cannot be directly applied to the measurement of fast neutrons.

Therefore, in order to convert into thermal neutrons, an albedo type dosimeter is used, in which the combined nuclear reactant and the detector are covered with a moderator to decelerate to thermal neutron energy and converted into α particles or the like for measurement. Therefore, neutrons are measured by this.

As such a measuring device, there is a commercially available one, and a typical one is a rem using a BF3 counter (proportional counter) for measuring α particles and the like with the above-mentioned configuration. Counters and the like are widely used.

[0008]

However, in the neutron dose measurement by the conventional albedo type dosimeter as described above, the energy range of the neutron to be measured is wide, and therefore, the neutrons of all energies are decelerated. In order to do so, a large amount of moderator is required, which results in a heavy weight of 10 to 20 kg, which is disadvantageous in that the portable measurement is very inconvenient.

That is, in this type of measuring apparatus, the weight must be reduced in order to make it a pocket dose equivalent meter or a portable type. For this purpose, there is a method of substituting the moderator with a subject.

Further, since the energy distribution of thermal neutrons radiated to the outside of the subject due to neutron deceleration by hydrogen constituting the subject and backscattering changes depending on the energy of incident neutrons, the sensitivity of the dosimeter becomes neutron energy. There was a drawback that changed by.

As shown in FIG. 5, the backscattered neutrons from the subject have a thermal neutron reflectance from the subject with respect to incident neutron energy. According to the figure, the reflectance is low in the fast neutron region.

Therefore, it is necessary to devise a method of limiting low-energy neutrons that enter the nuclear reaction material, and a substance that absorbs thermal neutrons, such as cadmium, is appropriately disposed in the moderator to allow thermal neutrons to be injected from the beginning. It is necessary to take some measures such as absorbing some of the medium-speed neutrons and adjusting the energy of the neutrons incident on the nuclear reactant. Therefore, there is a drawback that the detector becomes very complicated.

In using the dosimeter, the energy spectrum of the neutrons in the working field is obtained in advance, and the dosimeter is calibrated based on the dose equivalent measured by the REM counter in that field and is suitable for that field. Although the dose evaluation formula is calculated, it is necessary to recalibrate if the energy distribution changes.

Further, in such a calculation, it was not possible to measure in real time because of a time delay until the calculation result was obtained.

Further, in the albedo type dosimeter, since the backscattering of neutrons by the subject is utilized, the sensitivity changes as shown in FIG. 6 depending on the distance between the subject and the dosimeter.
Since the relative sensitivity decreases depending on the distance between the surfaces, it was necessary to devise a technique such that the dosimeter main body was closely attached to the body surface and worn.

Further, there are some which use the blackening of emulsions in photographs and the like, but it is difficult to determine that the developing conditions are not constant and it is neutrons, and therefore a specialist is required for the measurement. There were drawbacks such as.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to detect thermal neutrons and fast neutrons of neutrons incident on a subject with separate semiconductor detectors, and to detect each neutron. The purpose of the present invention is to provide a small and lightweight neutron dose equivalent measuring device capable of directly converting a dose of neutron into a dose equivalent with high accuracy.

[0018]

In order to achieve the above object, according to the present invention, the neutron detection surface is placed in contact with the subject, fast neutrons in the middle and high energy range irradiated to the subject is A first neutron conversion material that converts thermal neutrons that are turned into thermal neutrons by backscattering inside the object to be re-emitted to the outside of the object to be converted into specific charged particles by a nuclear reaction,
A medium-fast neutron detection section comprising a first semiconductor detector formed by being laminated on the first neutron conversion material, which injects the converted charged particles and outputs a detection signal corresponding to the dose, and a neutron detection surface. To the outside of the subject,
A second neutron conversion material that converts low-energy thermal neutrons incident on an object into specific charged particles by a nuclear reaction, and a second neutron conversion material that is formed by stacking the converted neutrons A second semiconductor detector that outputs a detection signal according to a dose, and a thermal neutron detector disposed adjacent to the medium-fast neutron detector, the medium-fast neutron detector, and the thermal neutron detector. Between
A thermal neutron absorbing section which is provided on the back surface of the first semiconductor detector and the second semiconductor detector, and which absorbs thermal neutrons that pass through each of the detectors and prevent mutual incidence on the semiconductor detectors; , Are included.

The second neutron converting material is an absorber having a predetermined thickness, and a part of the absorber has a small thickness or a hole is provided in a part thereof, and It is characterized in that conversion to charged particles is restricted and a detection signal of the second semiconductor detector is adjusted.

Further, the second neutron converting material and the second neutron converting material
A second semiconductor having an absorber having a predetermined thickness, which is provided between the semiconductor detector and a hole formed in a part thereof to partially absorb the charged particles converted by the second neutron converting substance. It is characterized in that the detection signal of the detector is adjusted.

Furthermore, the medium-fast neutron detector is characterized in that the first neutron converting substance is integrally formed on the radiation incident surface of the first semiconductor detector by vapor deposition or thermal diffusion.

[0022]

According to the neutron dose equivalent measuring apparatus of the present invention having the above-described structure, first, the first neutron converting substance is brought into contact with the object to which neutrons are emitted.

Here, the neutrons incident on the subject are changed into low-energy thermal neutrons due to scattering in the subject.

As a result, the first neutron conversion material brought into contact with the subject absorbs thermal neutrons and the thermal neutrons are converted into α particles or the like by a nuclear reaction to emit the α particles or the like.

Then, the α particles and the like emitted from the first neutron converting substance are detected by the first semiconductor detector and the neutron detection signal is output.

Further, among the thermal neutrons incident on the second detector, the thermal neutron blocking unit prevents the nuclear neutrons from being absorbed by the nuclear reaction material and passing through the second detector to enter the first semiconductor detector and not be detected. Blocked by.

On the other hand, the thermal neutron of the incident neutron is measured by using the second semiconductor detector in combination with the nuclear reactant.

That is, thermal neutrons are converted into α-particles or the like by a nuclear reaction substance and are incident on the second semiconductor detector to output a detection signal.

Here, each detection signal obtained by each of the semiconductor detectors is counted through an amplifier, a wave height discriminator and the like, the counting result is displayed on a display, and the neutron dose is measured.

As a result, the neutron beam with which the subject is irradiated is separately detected by the first and second semiconductor detectors for fast neutrons and thermal neutrons, respectively, and the arithmetic circuit has a sensitivity corresponding to energy. By processing, the appropriate neutron dose equivalent can be measured with high accuracy.

Therefore, according to the present invention, it is possible to directly measure the dose converted into the dose equivalent based on each detection signal.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIGS. 1 and 2 are schematic configuration diagrams of a neutron dose equivalent measuring apparatus according to the present invention, and FIG. 3 shows various conversion coefficient characteristics for dose equivalent as a function of neutron energy.

A feature of the present invention is that among the neutrons irradiated or incident on the subject, medium-fast neutrons are decelerated by scattering in the subject, converted into thermal neutrons by scattering in the subject, and backscattered. And, it is to measure separately what is irradiated to the subject with thermal neutrons, by this, thermal neutrons and medium fast neutrons irradiated to the subject are detected separately from thermal neutrons to fast neutrons It is a highly accurate measurement by converting the dose into the dose equivalent and reading it directly.

The configuration of this embodiment will be described below with reference to FIG.

In FIG. 1, a neutron dose equivalent measuring apparatus as a main component of the present embodiment is provided with an object 11 such as a human body.
Contact with, when neutrons incident medium fast neutrons to thermal neutrons of in the object, is reacted with the nuclear reactants example ¹⁰ B, alpha particles, and converted into ⁷ Li particle, the first semiconductor detecting the energy level The detection unit 10 and the second semiconductor detection unit 12 that detects only the original thermal neutrons that are incident on the site to be inspected are joined, and both detection units are connected in a column.

Specifically, as shown in FIG. 1, first, in the first semiconductor detector 10, for example, each layer is laminated in the order of Al (aluminum) -Si (silicon) -Au (gold) in a sandwich form. The first semiconductor detector 14 thus formed is used.

In order to detect thermal neutrons from the object 11, the first semiconductor detector 14 is made of, for example, Al--Si--Au, and has its incident window surface 14a, that is, , Au (gold) joined to form
^A boron layer of ¹⁰ B or a deposited boron film is provided.

That is, in the measurement of the medium fast neutrons converted into thermal neutrons, as shown in the figure, the first semiconductor detector 1
The ¹⁰ B layer 16 is provided on the incident window surface 14a of No. 4 and the Cd (cadmium) filter 18 is provided on the rear surface 14b of the first semiconductor detector 14.

A reverse bias voltage is supplied to the Al layer and the Au layer. For example, a positive voltage of a DC power source E1 is applied to the Al layer, and the negative voltage is connected in series to the Au layer. The voltage is applied via the connected resistor R1 and ammeter A1.

As a result, from the first semiconductor detector 10, α particles corresponding to the dose of the incident medium-fast neutrons,
^{The 7} Li detection signal 10a is output.

Next, the second semiconductor detector 12 is bonded to the rear surface of the first semiconductor detector 10, that is, the Cd filter 18 as shown in the figure, and the first semiconductor detector 1 is connected.
It is integrated with 0.

The second semiconductor detector 12 is formed by stacking layers in the order of Au (gold) -Si (silicon) -Al (aluminum) like a sandwich, like the first semiconductor detector 10. The thermal neutron incident part 12a is provided with a boron layer of ¹⁰ B or a vapor deposited boron film 20 laminated on the Au (gold).
As a result, only thermal neutrons of the neutrons emitted to the subject 11 can be absorbed.

A reverse bias voltage is supplied to the Al layer and the Au layer as in the case of the first semiconductor detecting section 10, but a positive voltage of a DC power source E2 is applied to the Al layer, so that the Au layer is supplied. A negative voltage is applied to the layer via a resistor R2 and an ammeter A2 connected in series. As a result, the second semiconductor detector 12 outputs a detection signal 22a of α particles, ⁷ Li, which corresponds to the dose of the incident thermal neutrons.

In this embodiment, the detection signals 10a and 22a from the semiconductor detectors 14 and 22 are measured by the ammeters A1 and A2, but of course, instead of this, the drawings are shown. As the well-known circuit configuration, the detection signals 10a and 22a are provided via an amplifier, an MCA (multi-channel analyzer), a counter, a display device, and the like.
It is also possible to measure the neutron dose from.

The operation of this embodiment will be described below with reference to FIG.

The specific operation of the schematic configuration shown in FIG. 1 is such that the incident neutrons are divided into thermal neutrons and intermediate fast neutrons and the doses are measured separately by using the two semiconductor detectors 10 and 12. However, first, the ¹⁰ B layer 16 provided in the front incident window 14 a by the first semiconductor detection unit 10 is directly brought into close contact with and brought into contact with the surface 11 a of the subject 11.

As a result, only the thermal neutrons that are decelerated in the subject 11 and reflected and emitted are made incident (medium fast neutrons at the time of incidence).

The ¹⁰ B layer 1 as a thermal neutron converting material
It is very easy to bring 6 into close contact with the body surface 11a because it is a semiconductor detector and is small.

The neutrons incident on the subject 11 are decelerated at the hydrogen nuclei in the subject by elastic scattering or the like to become thermal neutrons. This thermal neutron backscatters outside the body while repeating scattering, and
It measures thermal neutrons that are scattered back and slowed down.

That is, as shown in the characteristics shown in FIG. 4, the reaction cross section of the ¹⁰ B layer 16 of the incident window surface 14a with respect to thermal neutrons is 1000 times or more that of fast neutrons. Only thermal neutrons can be detected at 14.

That is, the ¹⁰ B layer 16 of the entrance window surface 14a
In, the thermal neutrons are converted into α particles and ⁷ Li particles by the (n, α) reaction (nuclear reaction) of the ¹⁰ B layer 16 and emitted to the first semiconductor detector 14.

Therefore, due to the charged particles generated by this, in the first semiconductor detector 14, due to the rectifying action (ionization) between Si and Au by the reverse bias voltage E1,
As shown in FIG. 1, an electric current flows according to the α particles and ⁷ Li particles generated by the nuclear reaction in the ¹⁰ B layer 16.

Then, as described above, the detection signal 10a of the pulse crest value corresponding to the energy level of the converted α particles and ⁷ Li particles is output through the resistor R1 and measured by the ammeter A1. ..

Further, the detection signal 10a can be discriminated because the peak value of the pulse is larger than the peak value of γ-rays, for example.

Therefore, a wave height discriminator (MCA) not shown
With this, the detection signal 10a can be discriminated at a predetermined wave height level via the amplifier, the discrimination result is counted by the counter, and the dose is calculated by the counted value.

Here, since the Cd filter 18 is located behind the first semiconductor detector 14, there is no influence of thermal neutrons from outside the subject, and only thermal neutrons are incident on the incident window. What is incident on the inside of the first semiconductor detector 14 through the ¹⁰ B layer 16 of the surface 14 a is measured.

That is, the Cd filter 18 specifically absorbs thermal neutrons and allows them to pass therethrough without acting on medium-fast neutrons, and thus serves to prevent thermal neutrons from reaching the first and second semiconductor detection parts. Thus, it becomes possible to measure all the irradiated neutrons by measuring the medium fast neutrons and the thermal neutrons of the irradiated neutrons independently.

That is, when thermal neutrons are incident on the boron layer 20 of the second semiconductor detector 12, the thermal neutrons are α particles, ⁷ Li due to the nuclear reaction in the boron layer 20, as in the case of the first semiconductor detector 14. The particles are converted into particles and emitted to the second semiconductor detector 22.

Therefore, in the second semiconductor detector 22, the reverse bias voltage E is generated by the charged particles generated thereby.
1 by the rectifying action (ionization) between Si and Au by
Α produced by the nuclear reaction in the boron layer 20 as
An electric current depending on the particles and ⁷ Li particles flows.

Therefore, as described above, the pulse crest value detection signal 22a corresponding to the energy levels of the α particles and ⁷ Li particles converted through the resistor R2 is output, and this is detected by the ammeter A2. ..

Of course, this detection signal 22a can be discriminated from the detection signal 22a at a predetermined wave height level through an amplifier by a wave height discriminator (MCA), which is not shown in the same manner as described above. The dose can be calculated by counting the discrimination results with a counter.

Here, the detection signal 10a as described above is used.
The respective dose values of the thermal neutrons (fast neutrons at the time of incidence) and the thermal neutrons by the detection signal 22a are converted into dose equivalents based on the conversion coefficient of the characteristic shown in FIG.

That is, FIG. 3 shows the conversion coefficient characteristics according to the neutron energy level for obtaining the dose equivalent of the incident neutron beam.

This neutron energy conversion coefficient characteristic is that the neutron fluence incident as a parallel beam perpendicular to the plane in contact with the ICRU sphere is on the main axis of the sphere per unit fluence in free space (passes through the contact point). 1 cm, 3 mm, and 70 μm are given as dose equivalents at respective depths.

As a result, each of the semiconductor detectors 14 and 2 is
The dose equivalent of neutron can be measured by weighting the count value which is the detection output of No. 2 based on the conversion coefficient characteristic as shown in FIG.

That is to say, this is a coefficient that, when evaluated as the exposure dose to the human body, takes into account the neutron energy such as neutron scattering and absorption in the subject in addition to free space dose measurement. By multiplying by the coefficient, the neutron dose in the subject can be evaluated.

As shown in FIG. 3, for example, the dose of neutrons is evaluated by the dose equivalents at a depth of 1 cm, 3 mm, and 70 μm of the subject, and 1 cm dose equivalent (a) and 3 mm dose equivalent ( A), obtained by multiplying the free space dose value by the conversion factor of each curve of 70 μm dose equivalent (c).

By approximating the energy dependence of neutrons to the energy dependence of the conversion coefficient as shown in the figure, each dose equivalent, for example, 1 cm dose equivalent (a)
Can be read directly.

Specifically, this can be displayed on the display unit by connecting an arithmetic unit for multiplying the conversion coefficient between the counter unit and the display unit.

Next, FIG. 2 shows a second embodiment according to the present invention.

The same members as those in FIG. 1 described above are designated by the same reference numerals, and the description of the structure and operation will be omitted.

In FIG. 2, the first semiconductor detector 14 is shown.
Is provided with a Cd filter 24 having a hole 24 'for transmitting thermal neutrons in the front thereof.

As a result, the sensitivity of the irradiated thermal neutrons and the sensitivity of the thermal neutrons (fast neutrons at the time of incidence) that are decelerated and reflected by the inside of the subject are matched so as to match the REM response curve (REM response) shown in FIG. can do.

As described above, according to the configuration and operation of the embodiment of the present invention, the first and second semiconductor detectors 1 are provided.
Since 4 and 22 are used, the measuring device itself can be made extremely small and lightweight.

As a result, the measuring device can be constructed in the size of, for example, a pencil or a fountain pen, and the measurement can be performed directly on the subject 11
It is possible to measure the dose of the neutron beam by contacting with, and to obtain the dose value converted into the dose equivalent by direct reading. Therefore, the measurement can be performed very easily, quickly and with high accuracy.

The neutron dose equivalent measuring apparatus shown in FIG. 1 has the first semiconductor detector 10 and the second semiconductor detector 1 described above.
2 is integrated with each other, each of these semiconductor detectors 10 and 12 is provided separately and independently, for example, as in the second embodiment of FIG. Dose may be measured.

[0078]

As described above, since the neutron dose equivalent measuring apparatus according to the present invention is composed of a semiconductor detector, a moderator or the like is used specially as compared with the conventional measuring apparatus such as a REM counter. Instead of using the subject, it is possible to reduce the size and weight and is convenient to carry.

Therefore, there is an advantage that the fast neutrons can be decelerated and measured by directly adhering the thermal neutron converting substance directly to the subject.

Further, thermal neutrons from the body passing through the first semiconductor detector are absorbed by the thermal neutron absorbing section so as not to be incident on the second semiconductor detector. I try not to measure.

On the other hand, thermal neutrons pass through the second semiconductor detector, and thermal neutrons heading for the first semiconductor detector are absorbed by the thermal neutron absorbing section as described above, and are not measured by the first semiconductor detecting section. There is.

Therefore, the neutron beam emitted to the subject can be separately detected by the first and second semiconductor detectors, respectively, for the medium fast neutrons and the thermal neutrons. And it becomes possible to measure with high accuracy.

In particular, when exposed to high energy fast neutrons, the measurement can be performed immediately.

Further, according to the present invention, since it is possible to directly measure the dose converted into the dose equivalent based on each of the detection signals, it is possible to immediately measure the individual exposure dose of neutron rays. ..

This is very useful as a personal dosimeter (neutron) complying with the new law.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of a neutron dose equivalent measuring apparatus according to the present invention.

FIG. 2 is a schematic configuration diagram showing a second embodiment of the neutron dose equivalent measuring apparatus according to the present invention.

FIG. 3 is a characteristic diagram showing conversion factor characteristics according to the energy level of neutrons for calculating the dose equivalent of incident neutron rays.

FIG. 4 is a characteristic diagram showing a neutron reaction cross section with respect to neutron energy of a nuclear reaction.

FIG. 5 is a characteristic diagram showing the dependency of thermal neutron ray reflectance on incident neutron energy.

FIG. 6 is a characteristic diagram showing a change in sensitivity depending on the distance between the albedo type dosimeter and the surface of the phantom.

FIG. 7 is a characteristic diagram showing energy dependence of an albedo type dosimeter.

[Explanation of symbols]

 10 1st semiconductor detection part 12 2nd semiconductor detection part 14 1st semiconductor detector 16, 20 Boron layer 18 Cd filter (thermal neutron absorption part) 22 2nd semiconductor detector

Claims (4)

[Claims] 1. A neutron in a wide energy range from low-energy thermal neutrons irradiated to an object to medium- and high-energy fast neutrons is converted into specific charged particles by a nuclear reaction,
In the neutron dose equivalent measuring device for detecting the ionization by the charged particles and measuring the dose of the neutron beam, the neutron detection surface is placed in contact with the subject, fast neutrons in the middle-high energy range irradiated to the subject A first neutron conversion substance that converts thermal neutrons that are turned into thermal neutrons by backscattering inside the subject and re-emitted to the outside of the subject into specific charged particles by a nuclear reaction, and the first neutron conversion substance A first semiconductor detector, which is formed by stacking and receives the converted charged particles and outputs a detection signal according to this dose, and a medium-fast neutron detection section, and a neutron detection surface outside the subject. A second neutron conversion material that is arranged toward and that converts low-energy thermal neutrons incident on the subject into specific charged particles by a nuclear reaction, and is formed by stacking the second neutron conversion material, A second semiconductor detector for injecting the converted charged particles and outputting a detection signal according to this dose; and a thermal neutron detector arranged adjacent to the medium-fast neutron detector, Between the neutron detection section and the thermal neutron detection section, provided on the back surface of the first semiconductor detector and the second semiconductor detector, passing through each of the detectors and mutually incident on the semiconductor detectors. And a thermal neutron absorber that absorbs thermal neutrons to block the fast neutrons irradiated to the subject are converted into thermal neutrons in the subject, which is then detected by the medium fast neutron detector. Along with detecting by converting to charged particles, the low-energy thermal neutrons irradiated to the subject are converted into charged particles in the thermal neutron detector and detected, and the neutrons are converted into dose equivalents based on the respective detection signals. Measuring dose Neutron dose equivalent measuring device according to claim. 2. The neutron dose equivalent measuring device according to claim 1, wherein the second neutron conversion material is an absorber having a predetermined thickness, and a part of the absorber has a small thickness or a hole. A neutron dose equivalent measuring device, characterized in that the conversion signal of the second semiconductor detector is adjusted by limiting conversion of the low energy thermal neutrons to charged particles. 3. The neutron dose equivalent measuring device according to claim 1 or 2, wherein the predetermined thickness is provided between the second neutron conversion material and the second semiconductor detector, and is formed by forming a hole in a part thereof. A neutron dose equivalent measuring device having an absorber for absorbing a part of the charged particles converted by the second neutron converting substance and adjusting the detection signal of the second semiconductor detector. 4. The neutron dose equivalent measuring device according to claim 1, 2 or 3, wherein in the medium-fast neutron detector, the first neutron conversion material is vapor-deposited or heat-treated on a radiation incident surface of the first semiconductor detector. A neutron dose equivalent measuring device characterized by being integrally formed by diffusion.