JP3358617B2 - Neutron dose rate meter - 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 dose rate meter for monitoring neutrons in a radiation handling facility such as a nuclear power plant or a reprocessing facility, and more particularly to a device using a semiconductor detecting element as an element for detecting neutrons.

[0002]

2. Description of the Related Art Radiation Protection Dosimetory is a conventional neutron personal dosimeter for semiconductor detectors.
(Radiation protection dosimetry),
Vol.27, No.3, P145-156 (1989)
(Hereinafter referred to as prior art 1) and US Pat.
No. 76 (hereinafter referred to as “prior art 2”). Since the semiconductor detecting element cannot directly detect neutrons, neutrons are caused to act on other substances to generate charged particles, and neutrons are detected indirectly by detecting the charged particles. For this reason, the former structure of the neutron detector that detects neutrons forms a boron layer on the surface of the semiconductor detection element in order to detect thermal neutrons with low energy, and to detect fast neutrons with high energy. , A polyethylene layer is provided on the boron layer. Further, a neutron moderator for moderating neutron energy is arranged in front of them. The latter neutron detector is disclosed in which a boron layer is formed on the surface of a semiconductor detection element and the periphery thereof is surrounded by a neutron moderator in order to detect fast neutrons, similarly to the former.

On the other hand, as a conventional technology of a neutron dose rate meter for monitoring neutrons in a radiation handling facility, Japanese Patent Application No.
As described in Japanese Patent Publication No. 235646 (hereinafter referred to as "prior art 3"), many use a BF ₃ counter or a ³ He counter. Further, in the same publication, in order to cut thermal neutrons and detect only high-speed neutrons, the counter tube is wrapped with a neutron moderator, the periphery thereof is further wrapped with a moderator, and the outside thereof is further wrapped with a neutron moderator. Is disclosed.

[0004]

[0003] Each country has adopted various international radiation detectors as ICRP (International Committee on Radar).
Energy response performance that enables effective dose equivalent evaluation in accordance with the recommendations of diological protection) is required. Japan also revised its national radiation hazard prevention ordinance in April 1989 in accordance with the recommendations of the ICRP. In general, even if radiations having the same energy are applied to a substance, the damage (exposure dose) to each substance varies. What is the effective dose equivalent?
A dose value that accurately evaluates the amount of exposure when the human body receives neutrons. In order to realize this effective dose equivalent, it is necessary to evaluate the exposure of the human body for each energy over a wide energy range existing in the radiation handling facility. The energy range of neutrons is 0.5 eV to 0.5 eV.
V and extends to fast neutrons over 10 MeV. Here, the sensitivity curve for each energy is called a response. The response of the effective dose equivalent has a sensitivity difference of more than 50 times between the thermal neutron region and the MeV region, and it is extremely difficult to realize this required response. In the following description, this required response is referred to as a dose equivalent response. In order to satisfy the dose equivalent response, it is important to roughly categorize the sensitivity curves and enhance the sensitivity for each energy.

[0005] First, from the standpoint of a personal neutron dosimeter. In the prior art 1, in order to match the sensitivity curves, in addition to the neutron detector described above, a neutron detector provided with only a polyethylene layer in a semiconductor detector is provided, and the responses of these two neutron detectors are added. . But,
As shown in FIG. 11 on page 155, the response performance is not satisfied in the energy range of 10 KeV to 1 MeV. Further, two neutron detectors are used, and the processing circuit becomes complicated.

The following is considered. The prior art 1 and 2 a film of boron 10 ⁽¹⁰ B) provided on the semiconductor detection element, the structure of the neutron in the film is detected by semiconductor detecting element charged particles generated a (alpha rays) when the incident It has become. Such a substance that generates charged particles when thermal neutrons are incident is called a converter. In the neutron detector of prior art 1, the boron 10 film was formed to a thickness of 1 μm by plasma doping. However, boron has a drawback that its melting point is as high as about 2300 ° C. and its workability is extremely poor. Also, the thermal expansion coefficient of the boron film and the silicon forming the semiconductor detection element is different by about 3.5 times. Therefore, in order to prevent the film from peeling due to a temperature change, the boron film cannot be made 1 μm or more. However, since the number of α-rays generated from boron increases in proportion to the film thickness, there is a problem in realizing a neutron detector having sufficient detection sensitivity. On the other hand, in the prior art 1, the method of injecting boron by diffusion is shown, but the boron concentration in the diffusion layer cannot be increased, and a neutron detector with sufficient detection sensitivity cannot be realized. Boron causes a nuclear reaction with a high probability for thermal neutrons and emits a large amount of α-rays, but does not react very much to fast neutrons (high-energy neutrons of several eV or more). Therefore, as described in Prior Art 1, a structure is provided in which a polyethylene layer is provided, and charged particles (protons) generated when fast neutrons are incident are detected by a semiconductor detection element. Such a substance that generates protons when fast neutrons are incident is called a proton radiator. However, there is a problem that the sensitivity cannot be improved for neutrons in the energy range of 10 KeV to 1 MeV.

Next, a neutron dose rate meter will be considered. In the prior art 3, the dose equivalent response cannot be satisfied. Further, the prior art 3 has a drawback in that the operating voltage for neutron detection is high, and the measurement circuit becomes complicated with this, and reliability is increased.

The prior arts 1 and 2 have no idea of using them as neutron dose rate meters, and no consideration is given to them.

It is an object of the present invention to provide a neutron dose rate meter that realizes a dose equivalent response.

[0010]

A feature of the present invention that achieves the object of the present invention is that a p-type silicon layer formed by impurity diffusion into silicon is provided.
An oxide film provided on the surface of the -n junction to insulate and protect the pn junction, a ground electrode drawn out from the side opposite to the oxide film by an ohmic contact, and a counter electrode of the ground electrode. a bonding electrode in the formed spot-on p-n junction, a converter in which the reacts with thermal neutrons provided on the surface side of the oxide film to generate a charged particle, a high speed is provided in a gap between the converter It has a proton radiator that generates charged particles by reacting with neutrons, and a processing unit that processes a signal obtained from the junction electrode.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the concept of the present invention will be described.

The response Sa (E) of the dose equivalent can be expressed by (Equation 1).

Sa (E) = Da (E) · fa (E) · Ia (E) (Equation 1) where Da (E) (see FIG. 2B) is a neutron sensitivity response and is a neutron sensitivity response. The sensitivity response to the neutron energy E of the detector itself is shown, and fa (E) (see FIG. 2 (d)) is the phantom sensitivity response and shows the response to the neutron detector of a neutron incident on a human body or a simulated human body. , Ia (E) (FIG. 2)
(C) shows the incident neutron spectrum. Therefore,
The effective dose equivalent response S (E) according to the ICRP recommendation is a desired response when the incident neutron spectrum Ia (E) is the unit spectrum I (E). That is, it is expressed by (Equation 2).

S (E) = D (E) · f (E) (Equation 2) Accordingly, in the neutron personal dosimeter mounted and used for the human body, the human body itself necessarily has a desired phantom sensitivity response f.
(E), it is necessary that the neutron sensitivity response Da (E) shows a desired response D (E). On the other hand, the neutron dose rate meter has an effective dose equivalent response S at least as a response of the product of both the neutron detector and the phantom simulating the human body.
It is necessary to satisfy (E). That is, if the neutron detector itself satisfies the effective dose equivalent response D (E),
The phantom may be configured to simulate the human body. Conversely, if the neutron detector itself cannot meet the effective dose equivalent response, overall sensitivity may be reduced, but the phantom is configured to complement it,
What is necessary is to satisfy the effective dose equivalent response S (E) as a whole. Based on the above concept, the means for achieving the purpose and the operation thereof will be described below.

First, a personal neutron dosimeter will be described. In the case of a personal neutron dosimeter, it is desired that the response of the neutron detector be as shown in FIG. In the present invention, the neutron sensitivity response D (E) has roughly two curves, and the first curve depends on a converter in which D ₁ (E) emits low-energy neutrons mainly composed of thermal neutrons and α rays. The second curve is based on the finding that D ₂ (E) depends on a proton radiator that generates protons by high-energy neutrons mainly composed of fast neutrons.
Therefore, the neutron moderator installed in front of the neutral detector as shown in the prior art 1 is unnecessary. However, if the converter and the proton radiator are simply provided separately in layers, the converter and the proton radiator are provided on the upper side, for example, on the semiconductor detecting element, so that the proton generated by the proton radiator is blocked by the converter and the semiconductor detection is performed. Does not reach the element. Therefore, in the embodiment of the present invention, a layer is formed by mixing the converter and the proton radiator, and is mounted on the surface of the semiconductor detection element.
As a result, the neutron sensitivity response D (E) can be satisfied by adjusting the respective amounts of the converter and the proton radiator and the overall layer thickness. The response of a semiconductor neutron detection element equipped with a converter and a proton radiator greatly changes depending on the mounting structure of the converter and the proton radiator. That is, if the amount of the converter mounted on the surface of the detecting element is large, the sensitivity to the thermal neutron component increases, and if the amount of the proton radiator is large, the sensitivity to the fast neutron component increases. Therefore, by providing a power supply and a processing circuit in such a neutron detector, a personal neutron dosimeter can be realized. The converter only needs to react with thermal neutrons to generate charged particles.
Lithium and the like. Examples of the proton radiator include paraffin of a hydrogen compound, polyethylene, and other organic substances or resins composed thereof. Hereinafter, in general, boron is often used as a converter and a hydrogen compound is often used as a proton radiator.

Next, means for increasing the sensitivity to thermal neutrons will be described. In order to increase the sensitivity to thermal neutrons, it is necessary to increase the thickness of the layer of the converter, that is, the boron layer. However, the melting point of boron is as high as about 2300 ° C., and there is a drawback that the workability is extremely poor. Also, the thermal expansion coefficient of the boron film and the silicon forming the semiconductor detecting element is different from about 3.5 times, and 1 μm I couldn't do more. In the embodiment of the present invention, the effective layer thickness can be increased by forming boron into a granular or powdery state and mixing it with another substance. Other substances include:
A material whose coefficient of thermal expansion does not change much from that of silicon or a material whose elasticity matches and absorbs the thermal expansion of boron are preferred. In the case where the boron is made into a granular form, the processing can be simplified if the granular boron is fired to form a boron layer. Also, in this case, the boron layer is made of granular boron,
Because of the difference in thermal expansion coefficient from silicon, there is no fear of peeling due to temperature change. As described above, it is possible to form a boron layer sufficiently thicker than before, and it is possible to increase sensitivity to thermal neutrons. When the hydrogen compound used as the proton radiator is another substance, the amount of the hydrogen compound adhered to the silicon surface can be controlled by arbitrarily setting the particle size (average particle size) of boron. This also changes the sensitivity of fast neutrons, so that the response characteristics of the detector to the total neutron energy can be variably set, which is also desirable from the viewpoint of realizing an effective dose equivalent response.
In addition, the hydrogen compound has elasticity and is desirable also from a peeling point. Further, in the above-described structure in which the gap between the granular converters is filled with the proton radiator, the particle size of the granular converter determines the response. The smaller the particle size, the higher the sensitivity to thermal neutron components. When using boron ⁽¹⁰ B) to the converter, to generate α rays 1.47MeV a nuclear reaction with thermal neutrons. Similarly, proton radiators interact with fast neutrons to produce protons. These charged particles (α-rays and proton rays) enter the detection element, and generate an electron-hole pair charge by the ionization action. α range is 10μm
The following shows that even if the converter is provided with a thickness larger than that, no difference occurs in the sensitivity response. On the other hand, the range of the proton beam generated from the proton radiator is about 1 mm at 10 MeV. Therefore, in a detection element having this structure, a thin layer (several tens of μm) of a converter is generally provided on the element surface, a proton radiator is buried in a gap between the converters, and a proton radiator is stacked thereon by about 1 to 2 mm.

Since the personal neutron dosimeter is portable, it is not practical if it is surrounded by a neutron moderator having a large size like a neutron dosimeter. However, since the personal dosimeter is fixed to the chest of the human body, the human body itself acts as a neutron moderator. That is, fast neutrons are scattered (decelerated) in the human body, and the scattered components are incident (this neutrons are generally called albedo neutrons). Under these conditions, by adjusting the particle size of boron and the thickness of the proton radiator, a personal neutron dosimeter having a dose equivalent response can be realized.

Next, a method for increasing sensitivity to at least one of thermal neutrons and fast neutrons will be described. In the prior art, charged particles generated by a converter or a proton radiator are blocked by an electrode or an oxide film on a surface of a semiconductor detection element, and sensitivity is reduced. In the prior art 2, the electrodes are gathered and provided on the side opposite to the surface on which the converter and the proton radiator are provided. However, this type is difficult to manufacture. In the present invention, the electrode on the incident side of the charged particles is formed so as to cover a part of the depletion layer. By doing so, the area of the electrode can be reduced, and the inhibition of charged particles by the electrode can be reduced. As a result, the number of charged particles incident on the semiconductor detection element increases, and the detection sensitivity can be improved.

Finally, a neutron dose rate meter will be described.
As described above, if the neutron detector itself satisfies the effective dose equivalent response D (E) as described above, the phantom may be configured to simulate a human body. Conversely, if the neutron detector itself cannot satisfy the effective dose equivalent response, the overall sensitivity may decrease, but the phantom is configured to compensate for it, and the effective dose equivalent response as a whole is S (E) should be satisfied.

In the former case, if the neutron personal dosimeter described above is installed on a phantom that simulates a human body,
An exposure dose rate meter that satisfies the effective dose equivalent response S (E) can be provided. As such a phantom, there is water or acrylic having a certain size. In the embodiment of the present invention, a neutron moderator or a thermal neutron absorber is used to replace the phantom. For example, the semiconductor detection element is surrounded by a neutron moderator or a thermal neutron absorber. By increasing the thickness of the neutron moderator, the sensitivity of thermal neutrons can be reduced and the rate of increase in sensitivity of fast neutrons can be adjusted. In addition, the thickness of the thermal neutron absorber and the width of the opening are adjusted. An apertured thermal neutron absorber completely transmits fast neutrons, and sensitivity to thermal neutrons can be adjusted by the aperture ratio. By doing so, the sensitivity to each energy is adjusted,
A phantom that simulates the human body can be provided.

In the latter case, a detector composed of a semiconductor detecting element other than silicon may be used as the neutron detector.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 shows an embodiment of the semiconductor neutron personal dosimeter according to the present invention. The semiconductor detection element 1, a radiation measurement circuit (hybrid circuit) 33 such as a preamplifier, a discriminator, and a reverse bias application circuit, an operation display unit 34, and a power supply 3
5 is stored in the portable case 32. The portable dosimeter is fixed to the chest of the human body with the clip 36. FIG. 3 shows a state in which a semiconductor neutron personal dosimeter 32 is fixed to a human body 31.

FIG. 4 is a block diagram showing the structure of the semiconductor detecting element and the measuring circuit of the personal neutron dosimeter. The structure of the semiconductor detection element 1 will be described using an example using n-type silicon. n
A p-type impurity (boron) is diffused into the surface 1 of the type silicon to form a pn junction (P layer) 8. The surface of this P layer 8 is insulated and protected by a silicon oxide film (SiO ₂ ) 9. A bonding electrode 10 for extracting a point-like signal is provided, and a ground electrode 11 is drawn out from the opposite side of the bonding surface by an ohmic contact. When a reverse bias 12 is applied to both electrode portions 10 and 11, P
The depletion layer 7 extends below the layer 8. Granular (powder) boron 5 is baked in close contact with the surface of the semiconductor detection element 1 with an oxide film 9 interposed therebetween, and the gap and the outer surface are filled with a proton radiator 6 (hydrogen compound). As the hydrogen compound, for example, paraffin, epoxy, polyethylene or the like is used. Although not shown, the entire detection element is sealed in a can, and the lead electrodes of the P layer 8 and the N layer 4 are drawn out of the can. The electrode 10 of the P layer 8 has α
It is dotted to prevent attenuation of lines and protons. The oxide film 9 serving as an entrance window for α rays or the like can be easily formed to a thickness of about 100 °. By providing the above-described converter and proton radiator layers in the entrance window, the attenuation of charged particles due to the electrodes 10 and the oxide film 9 can be suppressed. A neutron moderator (not shown) may be used outside or inside the can as necessary. Such thermal neutrons (Nth) incident from the outside of the dosimeter cause a nuclear reaction with the thermal neutron converter 5 to generate α rays. Fast neutrons (Nf) generate recoil protons (protons) by a scattering action with a proton radiator. These α-rays and proton rays are depleted in the depletion layer 7
Generates a charge. Thermal neutrons (Nth) and fast neutrons (N
The electric charge generated based on the incident of f) is amplified by the preamplifier 14 and the linear amplifier 15 via the AC coupling capacitor 13. The amplified signal is subjected to wave height discrimination by the discriminator 16 and pulse counting by the counting circuit 17. Counting circuit 1
Based on the count value counted in step 7, the calculation display section 18 calculates and displays the exposure dose.

The principle of core detection in this embodiment is divided into the following two interactions. One is that low-energy thermal neutrons cause a nuclear reaction shown in (Equation 3) with boron 5 and α
By generating lines.

¹⁰ B (n, α) ⁷ Li (Equation 3) The α ray has 1.47 MeV energy, and generates a charge of a pair of electron e and hole h in the depletion layer 7 of the detection element 1. This changes the detector current. The other is that fast neutrons interact (scatter) with the proton radiator 6,
The recoil protons (protons) are generated and generate charges of the electron e and the hole h pairs in the depletion layer 7, which change the detector current. The other is that fast neutrons interact (scatter) with the proton radiator 6 and produce recoil protons (protons).
Is generated in the depletion layer 7 to generate charges of the pair of electrons e and holes h. This embodiment enables the detection sensitivity to be improved and the detection in a wide energy range to be possible by improving the probability of occurrence of these two interactions and the charge collection efficiency of the generated charged particles (α rays, protons). The operation will be described in detail below.

Nuclear reaction number N of boron 5 as a converter
(l) can be expressed by the following equation.

N (l) = φ · n · σ · l (Equation 4) where φ: the number of neutrons incident (n / cm ² · S) n: the atomic density of the converter (l / cm ³ ) σ: Nuclear reaction cross section (barn) l: thickness of the converter (cm). As is apparent from this equation, the number of nuclear reactions N (l) depends on the thickness l of the converter, and a numerical example is shown in FIG. On the other hand, the range of the α-rays generated from this converter in boron and silicon is up to 7 μm, and the range of silicon in the case where the thickness of the boron converter is 7 μm or more is 7 μm.
With a configuration in which the light passes through m or more, the generated α-rays decrease, and do not contribute to the improvement in the sensitivity of the detector. That is, the contribution of the α-ray is saturated at a converter thickness of 7 μm or more.

On the other hand, the fast neutrons
The probability of generation σp of protons generated by scattering at is given by the following equation, where the incident energy of neutrons is En (MeV).

That is, -1 of the incident energy En of the neutron
Although the occurrence probability σp changes in proportion to the / 2 power, this occurrence probability does not depend on the scattering angle of protons. On the other hand, since the energy of the generated proton depends on the scattering angle, the energy distribution of the proton generated from the monochromatic neutron is changed from the energy of 0 eV to the incident energy E of the neutron.
It has a continuous distribution up to n. The range of this proton is about 1 mm at 10 MeV, but the low-energy proton rapidly attenuates when passing through boron.

The embodiment shown in FIG. 4 has been devised in consideration of the above points, and is easy to work with granular boron (average particle diameter of 10).
The layer thickness 1c of 5 μm) is fired as thick as possible within a range not exceeding the range of the α-ray passing therethrough, and a large number of nuclear reactions N
In order to obtain (l), the thickness was set to 30 μm (a porosity of 75%) corresponding to a layer thickness of 7 μm with 100% boron. The gap between the granular boron 5 is filled with a proton radiator 6, and furthermore,
On the outside, a proton radiator 6 having a thickness (lp =) of 2 mm
Coating with With this structure, most of the low-energy components of the protons generated from the proton radiator 6 do not pass through the boron, and thus do not attenuate.
It is possible to get inside. In addition, granular boron 5
Has an energy of 1.47 MeV, and is hardly shielded even when it passes through a proton radiator composed of a hydrogen compound having a small atomic number by several μm. Further, on the side of the semiconductor detecting element 1, the oxide film 9 and the p layer 8 which are insensitive to the charged particles only attenuate α-rays or the like, so that the thickness is 0.3 μm or less to prevent a decrease in sensitivity. .

As described above, the present embodiment has the following effects even if it is a mere neutron personal dosimeter.
By making boron into a granular shape, the effective thickness of boron can be increased, so that the thermal neutron detection sensitivity can be improved about 10 times as compared with the conventional technique. In addition, since the shielding effect of boron against fast neutrons, particularly fast neutrons on the low energy side can be reduced, the detection sensitivity of fast neutrons can be improved as a whole. In addition, there is no difficulty in firing (evaporating to dryness) the granular boron in terms of manufacturing technology, and by permeating and coating with a permeable hydrogen compound,
It is possible to prevent the peeling off due to the temperature change of the boron layer. Note that there may be a forming method other than firing. For example, a mixed layer of boron and a proton radiator may be fixed to a semiconductor detection element with a thin fixed film made of plastic. Finally, by making the electrode on the conversion element side into charged particles such as boron into a point shape, it is possible to eliminate the electrode in the portion where the converted charged particles pass to the semiconductor detection element side, and furthermore, the oxide film 9 is formed. Since the thickness can be reduced, the transmittance of charged particles to the semiconductor detection element side can be improved, and the detection sensitivity can be increased over a wide range of neutron energy.

Next, a neutron personal dosimeter capable of evaluating an effective dose equivalent in accordance with the ICRP recommendation will be described. According to today's radiation hazard prevention regulations, a neutron detector with an energy response characteristic capable of evaluating the effective dose equivalent in accordance with the ICRP recommendation is required, and the detector in the above-mentioned embodiment meets such a purpose. Is what you do. That is, the detection sensitivity to fast neutrons can be adjusted by changing the thickness lp of the coating layer of the proton radiator 6 in FIG. 4, the particle size of the particulate boron 5, or the ratio of the amount of the proton radiator to the particulate boron. The solid line curve in FIG. 6 shows the relationship between the neutron energy and the effective dose equivalent (indicating the degree of influence on the human body) according to the ICRP recommendation. 3 shows the characteristics of the detector of FIG. FIG.
Is limited to thermal neutrons and the range from several 100 KeV to 15 MeV, but the range from several 100 KeV to thermal neutrons was interpolated by Monte Carlo calculation. This is because the data of this intermediate area cannot be experimentally collected with the current technology.

The neutron personal dosimeter is used by being mounted on the chest of a human body as shown in FIG. Therefore, the calibration test of the personal neutron dosimeter needs to be performed in consideration of neutrons scattered from the human body. FIG. 6 shows an example in which a dosimeter is fixed to the center of a phantom (acrylic or water, dimensions 40 × 40 × 15 cm) instead of fixing the dosimeter to the chest of a human body. FIG. 7 shows the behavior of neutrons in a phantom simulating a human body. Some of the fast neutrons interact with the proton radiator in the dosimeter, but most reach the phantom 30 and scatter. Some of the scattered rays enter the neutron detection element 1. Naturally, the energy of the neutrons incident on the dosimeter reaches the thermal neutrons with the energy of the incident neutrons being maximized. The response of the effective dose equivalent can be easily obtained by adjusting the type of converter, the particle size, the layer thickness, and the type and layer thickness of the proton radiator in the dosimeter.

The energy response characteristics of the neutron personal dosimeter according to the present embodiment described above match well in the high energy region as compared with the conventional example. This is in good agreement with the response of the effective dose equivalent, indicating that a practical neutron personal dosimeter can be provided.

In the embodiment shown in FIG. 4, the charged particles are generated by firing the granular boron and filling the gaps and the above with a proton radiator. Etc. may be replaced.

FIG. 8 shows a second embodiment of the neutron detector of the present invention. In this embodiment, a structure is adopted in which a boron plate 50 having an opening is provided in the semiconductor detection element 1 and a proton radiator 51 is embedded in the opening. In this embodiment, the response of the detector can be adjusted by adjusting the area of the opening of the boron plate (the diameter of the opening when the opening is a circle). According to the method of manufacturing a boron plate having an opening, a boron layer is formed on the front surface of a metal mask by sputtering or the like, and then the metal mask is removed to form boron of an arbitrary shape. The boron plate thus formed is connected to the semiconductor detecting element 1
Attach in close contact with the surface of. The attachment method can function with an adhesive or simple means of mechanically securing the ends.

In this embodiment, the same effects as in the first embodiment can be achieved.

FIG. 9 shows a third embodiment of the present invention. In the present embodiment, a proton radiator 5
Only 3 are provided, and only the fast neutrons are used as detectors. The proton radiator 53 can be easily manufactured by applying paraffin or the like as in the case of FIG. In the present embodiment, it is possible to provide a neutron personal dosimeter with high detection sensitivity for fast neutrons.

FIG. 10 shows a fourth embodiment of the present invention. In the present embodiment, the granular boron layer 55 is formed on the surface of the semiconductor detecting element 1.
Is formed, and its upper surface is protected by a substance containing no hydrogen, for example, an aluminum vapor-deposited film 56. This detector detects only thermal neutrons, and by using granular boron, the thickness of the layer is sufficiently increased without fear of separation due to a temperature change, thereby improving the thermal neutron detection sensitivity. In the present embodiment, a personal neutron dosimeter with high detection sensitivity to thermal neutrons can be provided.

In each of the above embodiments, boron is used as a material for a nuclear reaction with thermal neutrons, but this may be a nuclear reaction of lithium ⁶ Li (n, α) ³ H, Uranium may be used. In this case, since the energy of the generated α-ray is different from that of boron, it is necessary to set the converter thickness according to its range. Also, a proton radiator such as a hydrogen compound is described as an example of a fast neutron sensitizer, but other materials may be used as long as they interact with fast neutrons to generate heavy charged particles. Further, although an example of a semiconductor detection element using a silicon semiconductor is described, a detection element using a compound semiconductor such as cadmium telluride or mercury iodide can also be used.

Next, the neutron dose rate meter will be described.

FIG. 11 shows an embodiment of a semiconductor neutron dose rate meter. In this embodiment, a spherical neutron moderator 2 and its outer shell constitute a phantom that simulates a human body with a thermal neutron absorber 3 having an opening, and the above-described semiconductor neutron detector 40 is provided therein. . The neutron detection signal is sent to an external radiation measurement circuit 60 via a signal cable. The system using the spherical neutron moderator 2 can realize a dose rate meter that maintains the most omnidirectionality. FIG. 12 is a schematic view of the neutron detector of the present invention. The semiconductor detector shown in FIG. 4 is provided at the center of a neutron moderator 2 such as paraffin or polyethylene and a thermal neutron absorber 3 such as a cadmium plate having an opening. In FIG. 12, a metal case (can) for enclosing the semiconductor detection element 1 and a signal cable for signal extraction are not shown. Thermal neutrons (Nth) incident from the outside of the present dosimeter are partially absorbed by the thermal neutron absorber 3 and the neutron moderator 2 and partially reach the detection element 1. Here, a nuclear reaction occurs with the thermal neutron converter 5 to generate α rays. The fast neutrons (Nf) reach the neutron moderator 2 without being absorbed by the thermal neutron absorber 3, where they are partially decelerated by thermal neutrons, but the other portions reach the proton radiator 2. The proton radiator generates recoil protons (protons) by the scattering action with fast neutrons. The charges of these generated α-rays and proton rays are collected in the depletion layer 7 in the semiconductor detection element 1.

FIG. 13 is a block diagram of the structure of the neutron detector and the radiation measuring circuit 60. The structure of the neutron detector is the same as that shown in FIG. The radiation measurement circuit 60 is basically the same as that shown in FIG. 4 except that two types of discriminators 16 and counting circuits 17 are provided, a subscript “a” indicates the total counting of the energy dependency, and a subscript “b” will be described later. This is a circuit for high-speed neutron discrimination having a certain energy or more.

FIG. 14 shows an example of the result of examining the energy response of the neutron dose rate meter of the present invention. This figure shows the result of investigation using the neutron moderator thickness as a parameter. At this time, the main structural specifications of the neutron dosimeter are as follows.

Boron particle size: 20 μm Proton radiator thickness: 2 mm Thermal neutron absorber opening ratio: 70% (opening ratio) (thickness: 0.2 mm)
(5mm cadmium) Thus, by changing the neutron moderator thickness,
The dose equivalent response can be varied.

FIG. 15 shows the result of comparing the response of the neutron dose rate meter of the present invention with the response of the dose equivalent. The neutron moderator thickness is 80 mm (data of moderator thickness 80 mm in FIG. 14). In this case, the data is complemented in the same manner as in FIG. As is clear from these results, the response of the neutron dose rate meter of the present invention agrees with the dose equivalent response with high accuracy of ± 30% from the thermal neutron over the energy range of 15 MeV. At the same time, a response example of a conventional dose rate meter is shown. The sensitivity of the conventional dose rate meter decreases at several MeV or more.

As described above, an effective dose equivalent response can be realized by adjusting the neutron moderator thickness. In the above description, the thickness of the neutron moderator is adjusted, but the thickness of the thermal neutron absorber and the aperture ratio thereof may be adjusted. The neutron dose rate meter of the present invention can be used for an area monitor, a surrounding environment monitor, a survey meter, and the like of a radiation handling facility such as a nuclear power plant, and can provide a practical new measuring instrument.

FIG. 16 shows an embodiment of the second neutron dose rate meter according to the present invention. This is a neutron moderator close to a sphere in the shape of a right circular cylinder having the same diameter and height in order to maintain omnidirectionality. A semiconductor neutron detector 40 is provided at the center,
A thermal neutron absorber 3 having an opening along the outer shape of the neutron moderator 2 is provided. The neutron detection signal is sent to the measuring instrument 60 and the indicating instrument 20 including the data processing unit via the signal cable. When the neutron moderator 2 has a spherical outer shape, the manufacturing cost increases. By making the shape of the neutron moderator 2 a right circular body close to a sphere, significant cost reduction can be achieved. When the shape of the neutron moderator 2 is not spherical, directivity may be generated. FIG. 17 shows the result of examining the directivity in the vertical direction of the present embodiment. As is clear from this result, the directivity can be ignored within ± 10%. This is because neutrons are scattered many times in the moderator, so that directivity is reduced.

Based on the verification result, the following modified examples can be considered. FIG. 18 shows a shape obtained by cutting all the corners of the neutron moderator 2 of a rectangular solid. FIG. 19 shows a front view thereof. The neutron detectors 40 are all provided at the center. FIG.
Shows a modified example in which the corner of the neutron moderator 2 is cut. All of these modifications can sufficiently maintain the performance of the present invention. 18, 19 and 20, the illustration of the thermal neutron absorber 3 is omitted.

FIG. 21 shows a modification of the installation of the thermal neutron absorber 3. In FIG. 21, a thermal neutron absorber 3 having an opening is provided inside the neutron moderator 2. FIG. 22 shows a modification in which a thermal neutron absorber 3 having an opening is provided between the neutron moderator 2 and the neutron detector 40. Providing the thermal neutron absorber 3 inside the neutron moderator 2 has the effect of reducing the installation amount of the thermal neutron absorber 3.

FIG. 23 shows an example of a signal peak distribution (spectrum) measured by the present invention. The areas indicated by the circles α indicated by the peak values A and B are peak areas of α rays generated by a nuclear reaction with thermal neutrons. The peak value B or more is a peak region of protons generated by interacting with fast neutrons of several MeV.
The peak value B or less is the peak value of the γ-ray. By providing multiple levels of discrimination levels above A, B or B,
Simultaneous discrimination of fast neutron energy components from each output signal can be easily performed (see the measurement circuit block diagram in FIG. 13). Naturally, the calibration coefficients of the neutron energy and the peak value and the sensitivity coefficient of neutron detection in each peak region are obtained in advance, and the data conversion into the absolute value is required. There are many needs for simultaneous high-speed neutron discrimination measurement in facilities using high-energy accelerators, and the applicable range of the present invention is extremely wide.

In the thermal neutron absorber, boron,
Lithium can be used in place of cadmium. In the above-described embodiment, the detection sensitivity of the thermal neutron component is adjusted by the aperture ratio of the thermal neutron absorber. However, the sensitivity can also be adjusted by changing the thickness of the thermal neutron absorber.

Although a semiconductor detecting element is used as the detecting element in the same manner as the personal dosimeter, a compound semiconductor such as cadmium tellurium and mercury iodide may be used.

As a phantom simulating the human body, an effective dose response can be realized on water or acrylic having a certain size as described in the personal dosimeter using a neutron moderator or a thermal neutron absorber. By mounting a neutron detector, it is possible to realize a neutron dose rate meter capable of realizing an effective dose equivalent response.

In the above description, the case where the neutron detector realizes the response of the effective dose equivalent has been described. However, the response of the dose equivalent can be changed by changing the thickness of the neutron moderator or the thermal neutron absorber. Therefore, even if the neutron detector itself cannot realize the response of the effective dose equivalent, the response of the effective dose equivalent can be achieved by changing the thickness of the neutron moderator and thermal neutron absorber surrounding them, as a whole, with the neutron dose rate meter. It is also possible to realize. In this case, from the standpoint of providing a neutron dose rate meter realizing the response of the effective dose equivalent, the neutron detector is BF
_{A 3} counter tube or a ³ He counter tube may be used.

Finally, in the above description, the neutron moderator and the thermal neutron absorber are installed so as to surround them. However, when the neutron dose rate meter is installed along a wall, the direction in which the neutrons enter is Because it is limited, a neutron moderator or a thermal neutron absorber may be provided only in that direction.

[0057]

As described above, according to the present invention,
The electrode area can be reduced and the charged particles
Load on the semiconductor detector
The number of electric particles increases, the detection sensitivity can be increased, and a dose equivalent response can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of a personal neutron dosimeter according to the present invention.

FIG. 2 shows a relationship between an effective dose equivalent response, a neutron detector response, a phantom response, and an incident energy spectrum.

FIG. 3 is a diagram showing a human body fixed state of the neutron personal exposure dosimeter of the present invention.

FIG. 4 shows a structure of a semiconductor neutron detector of a personal neutron dosimeter and a measurement circuit block diagram.

FIG. 5 is a diagram showing the relationship between the number of nuclear reactions N (1) and the thickness of a converter.

FIG. 6 is a diagram showing the results of a comparison between the response of the neutron personal dosimeter of the present invention and the response of the effective dose equivalent.

FIG. 7 is a diagram showing a state at the time of calibration of the personal exposure dosimeter of the present invention.

FIG. 8 is a diagram showing a second embodiment of the personal neutron dosimeter according to the present invention.

FIG. 9 is a view showing a third embodiment of the neutron personal dosimeter according to the present invention.

FIG. 10 is a diagram showing a fourth embodiment of the personal neutron dosimeter according to the present invention.

FIG. 11 is a diagram showing a first embodiment of the neutron dose rate meter of the present invention.

FIG. 12 is a diagram showing a schematic diagram of a neutron detector of the first embodiment.

FIG. 13 is a diagram showing a structure of a neutron detector and a block diagram of a radiation measurement circuit 60.

FIG. 14 is a diagram showing an energy response when the neutron moderator thickness of the neutron dose rate meter of the present invention is changed.

FIG. 15 is a diagram showing a result of comparing a response of the neutron dose rate meter of the present invention with a response of an effective dose equivalent.

FIG. 16 is a view showing a second embodiment of the neutron dose rate meter of the present invention.

FIG. 17 is a view showing the directivity in the vertical direction of the second embodiment of the neutron dose rate meter of the present invention.

FIG. 18 is a view showing another embodiment of the neutron dose rate meter of the present invention.

FIG. 19 is a sectional view of another embodiment shown in FIG. 18;

FIG. 20 is a view showing another embodiment of the neutron dose rate meter of the present invention.

FIG. 21 is a diagram showing an embodiment of a neutron dose rate meter in which a thermal neutron absorber is provided inside a neutron moderator.

FIG. 22 is a diagram showing an embodiment of a neutron dose rate meter in which a thermal neutron absorber is provided inside a neutron moderator.

FIG. 23 is a diagram showing an example of a wave height distribution of a signal measured by the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Semiconductor detection element, 4, 10 ... Electrode, 5 ... Boron, 6
... Proton radiator, 7 ... depletion layer, 8 ... P layer, 9 ... oxide film, 11 ... ground electrode, 12 ... reverse bias, 13 ... capacitor, 14 ... preamplifier, 15 ... linear amplifier, 1
6 discriminator, 17 counting circuit, 18 calculation display section.

Continuation of the front page (56) References JP-A-1-253971 (JP, A) JP-A-59-55075 (JP, A) JP-A-60-233866 (JP, A) Nakamura. M. Hori guchi, T .; Suzuki and T.S. Yamamoto, A REAL TIME WIDE ENERGY RANGE PERSONAL NEU TRON DOSEMETER WITH H TWO SILICON DETE CTORS, Radiation Protection Technology, National Railways, Germany. 27 No. 3, p. 149-156 (58) Field surveyed (Int. Cl. ⁷ , DB name) G01T 3/08 G01T 1/02

Claims (6)

(57) [Claims] A pn formed by impurity diffusion in silicon.
An oxide film provided on the surface of the junction to insulate and protect the pn junction; a ground electrode drawn out from the side opposite to the oxide film by an ohmic contact; and the pn junction being a counter electrode of the ground electrode. And a converter that generates charged particles by reacting with thermal neutrons provided on the surface side of the oxide film, and a charged electrode that reacts with fast neutrons provided in the gap between the converters. A neutron dose rate meter comprising: a proton radiator that generates particles; and a processing unit that processes a signal obtained from the bonding electrode. 2. A pn formed by impurity diffusion in silicon.
An oxide film provided on the surface of the junction to insulate and protect the pn junction; a ground electrode drawn out from the side opposite to the oxide film by an ohmic contact; and the pn junction being a counter electrode of the ground electrode. And a converter that generates charged particles by reacting with thermal neutrons provided on the surface side of the oxide film, and a charged electrode that reacts with fast neutrons provided in the gap between the converters. A semiconductor detection element including a proton radiator that generates particles, a radiation measurement circuit connected to the bonding electrode and the ground electrode, an operation display unit that performs operation and display based on a measurement result of the radiation measurement circuit, A neutron dose rate meter comprising a portable case containing a semiconductor detection element, a radiation measurement circuit, an operation display section, and a power supply. 3. The neutron dose rate meter according to claim 2, further comprising a neutron moderator or a thermal neutron absorber covering the neutron input side of the semiconductor detecting element. 4. The neutron dose rate meter according to claim 1, further comprising a reverse bias application circuit for applying a reverse bias to the junction electrode and the ground electrode. 5. The neutron dose rate meter according to claim 1, wherein said oxide film has a thickness of about 100 °. 6. A radiation measuring circuit comprising: an amplifier for amplifying a signal; a discriminator for discriminating a height of the amplified signal; and a count for pulse-measuring the pulse discriminated by the discriminator. 3. The neutron dose rate meter according to claim 2, which is constituted by a circuit.