JP2014016319A - Radiation discrimination detector and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation discrimination detector having improved detection sensitivity for α-rays and β-rays by optimizing the quantity of substance to pass through from radiation sources to each detection surface, and to provide a method of manufacturing the same.SOLUTION: A radiation discrimination detector 20 includes; a plastic scintillator 1 which generates first fluorescence when β-rays enter; a half mirror layer 2 which is formed on an incident surface of the plastic scintillator 1 by a dry plating method and reflects scattered light from the first fluorescence that is made incident on a surface opposite an incident surface thereof; a ZnS(Ag) layer 3 which is formed on the incident surface of the half mirror layer 2 by the dry plating method and which generates second fluorescence when α-rays enter and allows the second fluorescence to transmit through the half mirror layer 2; and a mirror layer 4 which is formed on an incident surface of the ZnS(Ag) layer 3 by the dry plating method and which reflects scattered light from the second fluorescence that is incident on a surface opposite an incident surface thereof and blocks external light.

Description

  The present invention relates to a radiation discrimination detector for measuring the concentration of artificial radiation radiated into a general environment from a management area such as a nuclear reactor facility or a radioisotope handling facility, and a method for manufacturing the same.

A method for quantifying radiation contained in dust in the air is disclosed in JIS Z4512-1995 “Method for Measuring Radiation Concentration of Airborne Particulate Matter”.
According to this method, the dust contained in the air is collected by allowing the air in the target environment area to pass through the HEPA filter at a predetermined flow rate for a necessary time using a dust sampler or the like.
And this alpha radiation or beta radiation is detected and a radioactive substance density | concentration is measured.

However, in the air, in addition to the artificial radionuclide to be measured, the rare radionuclide radon ( ²²² Rn) of the natural radionuclide and the thorium decay series of the thorium ( ²²⁰ Rn) isotope. And their decay products are always present.
When measurement is performed immediately after collection in a state where the natural radionuclide is collected by the HEPA filter, the measurement concentration increases due to the radiation emitted by the natural radionuclide when it decays, and an alarm is generated.

  In order to avoid such a situation, the measurement should be performed after providing an attenuation time of about 4 to 12 hours after collection, but there is a problem that the measurement of artificial radiation concentration is delayed.

  As a method of solving this problem, a Phoswich type radiation discrimination detector using two different types of scintillators is known.

The radioactivity of each of the natural radionuclides radon and thoron can be treated as equal.
Using the property that the ratio of α ray and β ray emitted by radon daughters is constant, α / β ratio is obtained by discriminating α ray and β ray simultaneously measured from HEPA filter that collects dust in the air. .
Thereby, the influence of the radiation emitted from the natural radionuclide and its decay product can be corrected, and the accuracy of the quantitative value of the artificial radiation concentration can be improved.

  For this reason, phoswich-type radiation discrimination detectors are widely used to determine the α / β ratio by measuring the concentration of artificial radiation released into the general environment from controlled areas such as nuclear reactor facilities and radioactive isotope handling facilities. Yes.

On the other hand, the structure of such a radiation discrimination detector is, for example, normally considered in view of the incident direction of radiation, in consideration of the passage of α-rays and a metallic protective lattice with an aperture ratio of about 90% for the purpose of protecting the radiation detection surface. Thin and expensive light-shielding film (for example, 2 to 4 light-shielding films with a passing substance amount of 0.25 mg / cm ² ) and ZnS (Ag) powder mixed in polymer or the like and coated with a uniform layer thickness The scintillator sheet for detecting α-rays, the plastic scintillator for detecting β-rays, and a light guide, etc. are juxtaposed, and a photomultiplier tube is connected directly below, and these are integrated into a light shielding case. (See, for example, Patent Document 1 to Patent Document 4).

  Moreover, what uses the aluminized mylar film as a light shielding film is also disclosed (for example, patent document 4).

JP 2009-294089 A JP 2007-218710 A Japanese Patent No. 4061367 JP 2012-26821 A

In the above conventional configuration, it is necessary to provide an extra metallic protective grid on the radiation incident surface of the radiation discrimination detector in order to compensate for the insufficient strength of the light shielding film.
For this reason, the distance from the α-ray generation source to the scintillator that detects the α-rays cannot be sufficiently reduced.
Furthermore, there is a limit to thinning some protective filters and light shielding films provided on the radiation incident surface of the scintillator that detects α rays.

In addition, since an adhesive layer is provided between various layers such as a light-shielding film provided on the radiation incident surface of the scintillator that detects α rays and β rays, the number of layers increases and α rays and β rays are increased. Absorption increases and detection sensitivity decreases.
Furthermore, when the fluorescence generated by the scintillator enters the adhesive layer, scattering and absorption occur, which causes a reduction in light collection.
Due to these various causes, the conventional radiation discriminating apparatus has several problems that lower the detection sensitivity of α rays and β rays.

For example, the light-shielding film of Patent Document 4 described above has a light-shielding function by forming Al on one or both surfaces of a colorless and transparent base film as its structure.
However, the base film itself does not have a light shielding function, and the density of the entire light shielding film is occupied by the base film.
Usually, 2 to 4 light-shielding films are stacked to form a light-shielding layer, which causes a significant reduction in detection sensitivity.

In addition, there is a limit to thinning the layer through which radiation passes by the conventional structure before reaching the scintillator that detects β rays.
This caused a problem that the β-ray detection sensitivity could not be sufficiently increased.

  The present invention has been made in consideration of such circumstances, and has optimized the amount of passing substances from the source of α-rays and β-rays to each detection surface to improve the detection sensitivity of these α-rays and β-rays. An object of the present invention is to provide a radiation discrimination detector and a manufacturing method thereof.

  The radiation discrimination detector of the present invention reflects a plastic scintillator that emits first fluorescence when β-rays are incident, and the scattered light of the first fluorescence that is formed on the incident surface of the plastic scintillator by the dry plating method and is reversely incident. A half mirror layer, a ZnS (Ag) layer that emits second fluorescence and transmits through the half mirror layer when α rays are incident on the incident surface of the half mirror layer by dry plating, and incidence of the ZnS (Ag) layer A mirror layer that reflects the scattered light of the second fluorescence that is formed on the surface by a dry plating method and that is reversely incident, and shields the external light.

  INDUSTRIAL APPLICABILITY According to the present invention, there are provided a radiation discrimination detector that optimizes the amount of passing substance from a radiation source to each detection surface and improves the detection sensitivity of α rays and β rays, and a method for manufacturing the same.

Sectional drawing of the radiation discrimination detector which concerns on embodiment of this invention. The Bragg curve graph which shows the ionization number per unit length of the air with respect to the passage distance in the air in the alpha ray of 5.49MeV. The flowchart which shows the manufacturing process of the radiation discriminator which concerns on embodiment of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
As shown in FIG. 1, the radiation discrimination detector 20 according to the embodiment includes a plastic scintillator 1 that emits first fluorescence when β-rays enter, and is formed on the incident surface of the plastic scintillator 1 by a dry plating method and is reversely incident. The half mirror layer 2 that reflects the scattered light of the first fluorescence, and ZnS that is formed by a dry plating method on the incident surface of the half mirror layer 2 and emits second fluorescence and transmits through the half mirror layer 2 when α rays enter. The (Ag) layer 3 and the mirror layer 4 that reflects the scattered light of the second fluorescence that is reversely incident on the incident surface of the ZnS (Ag) layer 3 and that shields the external light. Prepare.

  The plastic scintillator 1 is preferably composed of polyethylene naphthalate or a mixture containing polyethylene naphthalate that is arranged for the purpose of detecting β rays and emits first fluorescence upon incidence of radiation.

The plastic scintillator 1 is widely used in which polyvinyl toluene or polystyrene having a cyclic molecular structure is used as a base material, and an organic crystal such as an aromatic compound or a fluorescent material is dissolved therein.
For example, EJ-212 of Eljen Technology, BC-400 of Saint-Gobain, etc. are mentioned.
The thickness A of the plastic scintillator 1 is in the range of 0.1 mm to 1.0 mm for the purpose of suppressing the reaction of γ rays.

However, plastic scintillators such as EJ-212 and BC-400 are not implemented because there is no need for strict quality control of the surface properties in the manufacturing process, and even in the product, there are a plurality of fine scintillators on the surface. Unevenness and scratches may be mixed.
When the light-shielding layer is directly formed on the radiation incident surface in the state where the above-described unevenness and scratches are mixed on the surface of the plastic scintillator 1, the light-shielding layer has defects such as small holes due to the unevenness and scratches. Since it is generated, the light shielding function cannot be obtained.
In order to prevent such light shielding imperfections from being generated, it is necessary to planarize the surface by polishing or the like in advance.

In general, plastic has a property of starting to soften at a temperature lower than the glass transition point.
For this reason, conventionally, when various layers are directly formed on the incident surface of the plastic scintillator 1 by vacuum vapor deposition or coating, strict temperature control has been required to prevent deformation and deterioration of characteristics due to heating.

For example, the base material of the above-mentioned plastic scintillator EJ-212 is polyvinyl toluene, its glass transition point is about 70 ° C., and the actual softening start temperature is lower than that.
Therefore, it is desirable that the plastic scintillator 1 is made of a material based on any one of polyethylene naphthalate and a mixture containing polyethylene naphthalate.

Examples of such a scintillator material include Teijin Kasei's Teonex R: Q65 type.
In this embodiment, as a scintillator made of such a material, Teijin Chemical's scintillex TM, which suppresses crystallization in the material and improves luminous efficiency, is used.

The aforementioned Teonex R: Q65 type has a glass transition point of about 150 ° C.
In addition, when the scintillex TM is processed under manufacturing conditions that can obtain the same quality as the Teonex R: Q65 type described above, the surface flatness can be expected to be about 0.6 nm in terms of arithmetic average roughness (Ra).
Ra = 0.6 nm is equivalent to alkali-free glass for LCD (Liquid Crystal Display) applications that require high flatness.

For this reason, in the plastic scintillator 1 based on polyethylene naphthalate, polyethylene naphthalate or a mixture thereof, when the various layers are directly formed on the incident surface by vacuum vapor deposition or coating, the above-mentioned polyvinyl toluene or the like is used as the base. It is possible to eliminate the planarization process step required in some cases.
Furthermore, since the glass transition point is sufficiently higher than the temperature that is heated when various layers are directly formed, strict temperature control that has been implemented in the past is not required to suppress deformation and characteristic deterioration caused by heating. And can.

The ZnS (Ag) layer 3 is formed on the front surface of the incident surface of the half mirror layer 2 by vacuum deposition, sputtering, or preferably plasma chemical vapor deposition for the purpose of detecting α-rays.
Conventionally, for forming a ZnS (Ag) layer, for example, a method of uniformly applying a mixture of a powder of ZnS (Ag) to a polyvinyl butyral resin adhesive on the incident surface of the plastic scintillator 1 is widely used. .
This ZnS (Ag) powder has an average particle diameter of several μm to several tens of μm.

  In addition, as another conventional example, a ZnS (Ag) mixture is uniformly applied to one surface of a colorless transparent film sheet such as polyethylene terephthalate to form a scintillator sheet, and is adhered to the incident surface of the plastic scintillator 1. The method has been taken.

However, in order to form a stable ZnS (Ag) layer, the ratio of the ZnS (Ag) powder to be mixed with the polyvinyl butyral resin adhesive described above is limited.
Due to this restriction, the relative density of the ZnS (Ag) powder in the conventional example is about 40% to 60%.

For example, the average particle size of ZnS (Ag) powder commercially available as a scintillator material is approximately 6 μm.
With a particle size of this size, the ZnS (Ag) layer 3 has a thickness of several tens of μm in order to sufficiently secure the α-ray detection sensitivity at the above-mentioned relative density, and the second fluorescence self There is a problem of increased absorption.

In addition, there is a problem in that the β-ray energy passing through the ZnS (Ag) layer 3 is greatly attenuated and the detection sensitivity of the β-ray in the plastic scintillator is lowered.
On the other hand, when the ZnS (Ag) layer 3 is formed by vacuum vapor deposition or sputtering, the relative density of the ZnS (Ag) layer 3 can be increased to about 70 to 95%.

The ZnS (Ag) layer 3 formed in this way can be stably reduced in thickness and less self-absorption of the second fluorescence compared to the case where the powder is applied or adhered as described above. .
Moreover, attenuation of β-ray energy passing through the ZnS (Ag) layer 3 can also be suppressed.
Furthermore, it becomes possible to adjust the layer thickness in increments of several nm, and it becomes possible to adjust the optimum layer thickness that minimizes α rays incident on the plastic scintillator 1 with high accuracy, as will be described later.

The layer thickness C of the ZnS (Ag) layer 3 is set within a range of 7.9 μm to 10.4 μm, for reasons described later.
The half mirror layer 2 is formed on the incident surface of the plastic scintillator 1, and transmits the second fluorescence generated when α rays are incident on the ZnS (Ag) layer 3 and attenuates the light.
Further, the half mirror layer 2 further has a function of reflecting the backward incidence of the scattered light of the first fluorescence generated by the β-rays entering the plastic scintillator 1.

In general, when the second fluorescence generated by the incidence of α rays and the first fluorescence generated by the incidence of β rays are compared in terms of the amount of emitted light, the second fluorescence is several times larger.
For this reason, the output signal obtained by detecting the second fluorescence and the first fluorescence with the photomultiplier tube 12 is formed with a large pulse height derived from the second fluorescence, and the pulse height derived from the first fluorescence is small. It is formed.
For this reason, when the pulse height of the second fluorescence exceeds the dynamic range allowed by the electronic circuit of the waveform discriminating unit 34, the discriminating operation cannot be performed.

However, by adopting the half mirror layer 2, the second fluorescence incident on the photomultiplier tube 12 can be reduced more than the first fluorescence, and the above-described problem of exceeding the dynamic range can be solved.
As a conventional method for solving this problem, it is widely adopted to provide a neutral density filter between the plastic scintillator 1 and the ZnS (Ag) layer 3 to diminish the second fluorescence amount detected by the photomultiplier tube 12. It had been.
Examples of such a neutral density filter include a Neutral Density Filter.

However, in this case, the neutral density filter adjusts the amount of light transmitted using a material that absorbs light, and does not take into account the first fluorescence that is reversely incident due to scattering in the plastic scintillator 1.
For this reason, the scattered light of the 1st fluorescence which is scattered among the 1st fluorescence and reversely enters into the neutral density filter is attenuate | damped there, and there existed a subject that the detection sensitivity of a beta ray fell as a result.

  In general, a half mirror refers to a reflection / transmission ratio of 1: 1, but the half mirror layer 2 described in the present invention is not limited to a reflection / transmission ratio, or a reflectance or transmittance.

On the other hand, since the emission of the fluorescence emitted by the excitation action by the radiation has the property of spreading isotropically, the second fluorescence incident angle to the half mirror layer 2 is not uniform.
For this reason, it is desirable that the half mirror layer 2 is a metal layer that can reduce the characteristic change due to the second fluorescence incident angle.

As a material of the metal layer of the half mirror, a metal containing any of Ag, Al, Cr, Au, Mg, Ni, or MgF2, or an alloy such as Inconel R can be used.
The layer thickness of the metal layer to be formed is determined according to the set light attenuation rate.

  The metal layer can be formed by using a dry plating method such as a vacuum deposition method or a sputtering method, and preferably by using a sputtering method or a plasma chemical vapor deposition method that does not require heating of the material. .

As described above, the formation of the half mirror layer 2 not only attenuates the second fluorescence, but also has a function of reflecting the scattered light of the first fluorescence that reversely enters the half mirror layer 2. High absorption efficiency of one fluorescence is possible.
This makes it possible to improve the detection sensitivity for large β rays.

The mirror layer 4 is formed on the radiation incident surface of the ZnS (Ag) layer 3 by using a vacuum deposition method or a sputtering method, and reflects the scattered light of the second fluorescence that is reversely incident and blocks external light. Objective.
The layer thickness of the mirror layer 4 desirably has 70 nm to 200 nm.
As the mirror layer 4, Al, Mg, Ag, or the like is widely used.
In particular, since Al is easy to manufacture and has a high reflectance of light having a wavelength near ultraviolet light, it is preferably used in a radiation discrimination detector in which fluorescence generated in each scintillator is near ultraviolet light.
The plastic scintillator 1, the half mirror layer 2, the ZnS (Ag) layer 3 and the mirror layer 4 constitute a scintillator member 10.

  The mirror layer 4 includes a curable composition layer 5 that protects the mirror layer 4, and prevention of destruction of the surface by an external force that is formed on the incident surface of the curable composition layer 5 and is applied to the radiation incident surface of the scintillator member. And a protective layer 6 that prevents corrosion of the mirror layer 4 due to inflow of gas or the like is formed by a dry plating method.

The curable composition layer 5 protects the mirror layer 4 formed on the radiation incident surface of the mirror layer 4.
Since the layer thickness of the mirror layer 4 is very thin as described above, it is difficult to completely eliminate light-blocking imperfections due to defects such as small holes.
For example, even if one defect such as a small hole exists and external light enters from there, it will not be established as a radiation detector. Therefore, the mirror layer 4 is protected using the curable composition layer 5.

The curable composition layer 5 uses, for example, a UV curable resin mixed with carbon bragg or titanium bragg.
Examples of the UV curable resin mixed with carbon bragg include CFPR BK-4611 manufactured by Tokyo Ohka Kogyo Co., Ltd.
If a spin coater is used for application of the UV curable resin, it can be formed with a uniform layer thickness.

Further, the UV curable resin may contain an organic solvent or the like, and this may remain in a very small amount in a defect such as a small hole in the mirror layer 4 and deaerate.
In order to prevent such a situation, it is desirable to dry the scintillator member 10 at a temperature of 100 ° C. or higher before performing the curing process by UV irradiation.

The protective layer 6 is formed on the incident surface of the curable composition layer 5, and prevents the surface from being destroyed by an external force applied to the radiation incident surface of the scintillator member 10.
Furthermore, it also has a role to prevent the mirror layer 4 from corroding due to the inflow of various gases.
The protective layer 6 is preferably an amorphous layer made of carbon.
For example, tough carbon or diamond-like carbon manufactured by Nomura Metal Co., Ltd. can be used.

Since the dust itself contains a natural radionuclide, the protective layer 6 desirably has conductivity for preventing adhesion of dust in order to further increase the detection sensitivity.
For the same reason, the protective layer 6, the mirror layer 4, and the half mirror layer 2 are kept conductive and grounded.

By sufficiently increasing the strength of the protective layer 6 and the curable composition layer 5, it is possible to eliminate the need for a metal protective lattice or the like that increases the amount of passing material and further reduces the aperture ratio.
Further, by eliminating the need for the metal protective grid, the α-ray generation source 40 can be physically sufficiently close to the scintillator member 10.
By making the distance sufficiently close, it is possible to greatly improve the detection sensitivity of α rays that have a very short flight distance and a large attenuation even in the air.

The SiO ₂ layer 7 (7a, 7b, 7c) is formed when the half mirror layer 2 is formed on the plastic scintillator 1, the ZnS (Ag) layer 3 is formed on the half mirror layer 2 in order to reinforce the bonding between the layers. It is formed when the curable composition layer 5 is formed on the mirror layer 4.

In general, since Al has low wettability, adhesion with a UV curable resin may become a problem. To solve this problem, SiO ₂ having a layer thickness of about several tens of nanometers is formed on the surface of the mirror layer 4 on the radiation incident side. After the layer 7c is formed, the curable composition layer 5 is formed on the surface on the radiation incident side.
The other SiO ₂ layers 7 (7a, 7b) are also formed to solve the problem of adhesion between metal and nonmetal.

The layer thickness B of the SiO ₂ layer 7 (7a, 7b, 7c) is 10 nm to 200 nm.
Desirably, the thickness is 20 nm to 100 nm.
More desirably, the thickness is 20 nm to 60 nm.
The SiO ₂ layer 7 may be formed by using a vacuum evaporation method or a sputtering method, preferably a sputtering method.
By forming these SiO ₂ layers 7 (7a, 7b, 7c), the mechanical strength of the scintillator member 10 is increased.

On the exit surface of the scintillator member 10, a colorless and transparent flat light guide 11 that collects the emitted first fluorescence and second fluorescence, and the first fluorescence and second fluorescence collected by the light guide 11 are detected. Then, the photomultiplier tube 12 that converts the electric signal into an electric signal using the photoelectric effect is connected and integrated.
The photomultiplier tube 12 thus integrated is housed in a light shielding case 13 that has been subjected to a light shielding process and has a radiation inflow hole with its incident surface exposed to the outside.

The light guide 11 is disposed at the fluorescence emission of the scintillator member 10.
It is colorless and transparent and has a flat plate shape for condensing both fluorescence emitted from the scintillator member 10.
Since the first fluorescence wavelength center value is about 425 nm and the second fluorescence wavelength center value is about 435 nm, the light guide 11 uses a material that absorbs light at a wavelength near 430 nm.

Desirably, a resin material such as PMMA or polycarbonate that has been subjected to transparent processing is desirable as a material that is not easily broken by an impact or the like.
Although not shown, a fluorescent reflecting material, an aluminum vapor deposition film, or the like is preferably applied to the entire surface except the optical coupling surface of the light guide 11.

Thus, by apply | coating a fluorescent reflector, the effect | action with which fluorescence fills the inside of the light guide 11 is acquired.
For this reason, the effect which raises the probability of making both fluorescence inject into the light-incidence surface of the photomultiplier tube 12 is acquired.
At the same time, since both fluorescent lights are not emitted from the inside of the light guide 11 to the outside, it is not necessary to apply a fluorescent reflecting material to the inner surface of the light shielding case 13 described later.

However, the linear expansion coefficient of these resin materials is about 10 times larger than that of glass.
For this reason, the thermal expansion and contraction of the light guide 11 due to the ambient temperature change is repeatedly transmitted to the reflective material deposited on the surface thereof, so that the reflective material may be cracked.
Further, the reflective material may peel off from the light guide 11.

In order to prevent these, it is desirable to deposit the entire surface of the reflective material with, for example, a black acrylic silicon resin having a light shielding property.
By applying this resin, the reflective material is protected against stress, so that the fluorescent reflective material is not easily cracked.
Moreover, even when a crack occurs, the light collecting power is not impaired because an effect of suppressing defects such as peeling off from the light guide 11 can be obtained.

The photomultiplier tube 12 is made of a cylindrical glass tube, detects both fluorescence collected by the light guide 11 and converts it into an electrical signal using the photoelectric effect.
For example, in the case of the head-on type, the tip end surface of the cylinder is a light incident surface.
The light incident surface of the photomultiplier tube 12 may be smaller than the coupling surface with the light guide 11.

In this case, it is desirable to provide the light guide 11 with a taper or the like so that the incident surface size of the photomultiplier tube 12 and the coupling surface size of the light guide 11 are equal.
By such a measure, the photomultiplier tube 12 can obtain an effect of efficiently detecting both fluorescence emitted from the scintillator member 10.

Furthermore, although not shown, when the photomultiplier tube 12 and the light guide 11 and between the light guide 11 and the plastic scintillator 1 are bonded, it is desirable to bond them using optical grease or optical cement.
By such measures, it is possible to suppress fluorescence loss due to Fresnel reflection or the like that occurs at the coupling portion.
Examples of the optical grease include OKEN 6262A manufactured by Applied Koken Kogyo.
Examples of the optical cement include EJ-500 of Eljen Technology, USA.

The wavelength center value of the first fluorescence is approximately 425 nm, and the wavelength center value of the second fluorescence is approximately 435 nm.
It is desirable to use the photomultiplier tube 12 having a high quantum efficiency in the light wavelength band of 430 nm.

On the other hand, when the photomultiplier tube 12 cannot be arranged on the straight line in the radiation incident direction, for example, the photomultiplier tube 12 is laid 90 ° and coupled to the side end face of the light guide 11.
In this case, preferably, the light can be condensed more efficiently if the light guide 11 is made of a fluorescence conversion material.
As the fluorescence conversion material, Wavelength Shifting Material or the like is preferably used.
As a result, the fluorescent light incident on the light guide 11 and re-emitted again repeats total reflection in the light guide 11, so that it easily gathers on the side end face of the light guide 11.

Moreover, since the wavelength center value emitted from the scintillator member 10 is about 430 nm as described above, a fluorescent conversion material to be applied is one having a fluorescence wavelength center value of around 430 nm.
For example, US-based Eljen Technology EJ-280 or Kuraray Y-11 is suitable.

The light shielding case 13 has a radiation inflow hole, and a fluorescent reflecting material is attached to the entire inner surface of the light shielding case 13, and the connection portion and the like are light-shielded so as not to take in external light.
The radiation entrance hole is accommodated with the entrance surface exposed to the outside so that radiation can enter the entrance surface of the scintillator member 10.
Examples of the fluorescent reflecting material include NE-560 of Eljen Technology, USA.

  The photomultiplier tube 12 includes a high voltage power supply unit 31 that supplies a high voltage, a signal amplifying unit 32 that amplifies the output signals of the first fluorescence and the second fluorescence detected by the photomultiplier tube 12, and signal amplification. A discrete unit 33 for removing noise from the output signal amplified by the unit 32; an output pulse waveform of β-rays from the first fluorescence out of the output signals from which noise has been removed by the discrete unit 33; and α-rays from the second fluorescence A waveform discriminating unit 34 for discriminating the output pulse waveform and a counting unit 35 for measuring the counting rates of the α ray and the β ray discriminated by the waveform discriminating unit 34 are connected.

  Next, referring to the Bragg curve graph showing the number of ionizations per unit length of air with respect to the distance from the source 40 of the α ray of the α ray having the energy of 5.49 MeV in FIG. A method for determining the layer thickness will be described.

Radon and thoron, which exist in the air as natural radionuclides, consist of decay series of uranium and thorium.
The uranium decay series refers to the decay process from uranium 238 to lead 206, and the thorium decay series refers to the decay process from thorium 232 to lead 208.

Simplified decay series of radon daughter nuclide is ²¹⁸ Po → ²¹⁴ Pb → ²¹⁴ Bi → ²¹⁴ Po.
When the decay sequence of the TRON daughter nuclide is simplified, it is ²¹⁶ Po → ²¹² Pb → ²¹² Bi → ²¹² Po.

The half-life is as short as about 55.6 seconds for TRON compared to about 3.8 days for radon.
Therefore, the radon daughter nuclide (half-life is about 3.1 minutes) is measured after a short time for attenuating the Thoron daughter nuclide (half-life is about 0.15 seconds).
As a result of such measurement, the energy of α rays emitted by α decay of the natural radionuclide is about 5.5 MeV to 6.8 MeV.

The α-ray has a characteristic of proceeding linearly until it stops, and in the process, the speed decreases while losing energy due to the interaction with the substance.
Meanwhile, the rate of interaction increases, and eventually the electrons in the material are captured and become neutral helium atoms, losing ionization.
In general, all α-rays having the same energy stop at a point that has traveled substantially the same distance in the air.

A relationship between the α-ray passage distance in air and the number of ionizations per unit length is called a Bragg curve, and an α-ray energy of 5.49 MeV is shown in FIG. 2 as an example.
The air passage distance F shown in FIG. 2 indicates the air passage distance band F from about half of the range R (= R / 2) to the air passage distance where the ionization number per unit length is a peak. ing.
Focusing on the properties of the α-rays indicated by the Bragg curve, the layer thickness C through which the α-rays pass and the amount D of the passing material are adjusted so that the ZnS (Ag) layer 3 is positioned in the air passage distance zone F. .
Thus, by adjusting the amount of each passing substance, the detection sensitivity of α rays can be increased.

  Then, since the α ray incident on the plastic scintillator 1 ahead of it loses most of the energy at that time and emits light weaker than the β ray, the discrete portion 33 removes it similarly to the noise component. Can do.

Specifically, the layer thickness C of the ZnS (Ag) layer 3 and the passing substance amount D from the α-ray generation source 40 to the ZnS (Ag) layer 3 are calculated from the calculation.
In the Bragg curve of the α ray having the energy of about 5.5 MeV to 6.8 MeV emitted from the natural radionuclide, which is the measurement result described above, this in-air passing distance band F is positioned just in the ZnS (Ag) layer 3. Want to.

For this purpose, in addition to the layer thickness C of the ZnS (Ag) layer 3, it is necessary to determine a passing substance amount D corresponding to the distance from the incident surface of the scintillator member 10 to the incident surface of the ZnS (Ag) layer 3. It is.
The range R of α rays in standard air is given by Equation (1) as an empirical formula for the energy E of α rays.
R = 0.318E ^3/2 Formula (1)

From equation (1), the range R in the air of α rays having the energy of 5.5 MeV to 6.8 MeV described above corresponds to approximately 4.10 cm to 5.64 cm.
Further, when converted from the range R in the air to the amount of passing material, the amount of passing material of α-rays having this energy is about 4.85 mg / cm with an air density of 1.184 kg / m ³ (25 ° C., 760 Torr). ^{2 to} 6.69 mg / cm ² .

When the air layer from the α-ray generation source 40 to the incident surface of the scintillator member 10 is set to 5 mm, for example, the amount of passing substance corresponding to the air layer of 5 mm is 0.59 mg / cm ² .
Further, as described above, the passing substance amount corresponding to the range R in the air of 5.5 MeV is 4.85 mg / cm ² .
Therefore, in order for the α-ray having the minimum energy emitted by the natural radionuclide to reach the ZnS (Ag) layer 3, the amount D of passing material is (4.85 × 0.5) −0.59 = 1. Desirably about 25 mg / cm ² .
Hereinafter, when the layer thickness C of the ZnS (Ag) layer 3 is calculated, the passing substance amount D is calculated as 1.25 mg / cm ² .

In addition, since the passing substance amount corresponding to the above-described range 6.8 in the air of the 6.8 MeV α-ray is 6.69 mg / cm ² , the Bragg curve peak in the air has a distance of 80% of the range R. Assuming that the position corresponds to the vicinity, the amount of the substance passing through the ZnS (Ag) layer 3 is ((6.69 × 0.8) −0.59) −1.25 = 3.51 mg / cm ^2. Is desirable.

When the relative density of the ZnS (Ag) layer 3 is 3.70 g / cm ³ corresponding to about 90% of the true density, the layer thickness C of the ZnS (Ag) layer 3 is approximately 9.5 μm.

When a nuclear fuel material that is an artificial radionuclide is a target of measurement, the target α-ray energy is about 4 MeV to 6 MeV.
In this case, the range R of α rays corresponds to approximately 2.54 cm to 4.67 cm.

Further, as in the case of the above-mentioned natural radionuclide, the passing substance amount corresponding to an air layer having a layer thickness of 5 mm is 0.59 mg / cm ² .
And the amount of passing substances corresponding to the range R in the air of 4 MeV is 3.01 mg / cm ² .

Therefore, in order to prevent α-rays of 4 MeV or less from entering the plastic scintillator 1, the passing substance amount D should be about (3.01 × 0.5) −0.59 = 0.92 mg / cm ^2. Is desirable.
However, considering the assumption of air density or the fact that the protective grid can be removed, this value can actually have a range.
If 0.7mg ^/ cm ² ~2.1mg ^/ cm 2 can be said to be appropriate as a range.

Comparing the energy of alpha rays emitted from natural radionuclides with that of artificial radionuclides, it can be seen that the energy of alpha rays emitted from natural radionuclides is higher.
Therefore, the maximum range R among the α rays to be measured is realized by the one having the energy 6.8 MeV emitted from the natural radionuclide.
It is necessary to prevent the peak of the α-ray Bragg curve having 6.8 MeV from being absorbed by the ZnS (Ag) layer 3 and reaching the plastic scintillator 1.

Assuming that the air passage distance at the peak of the Bragg curve is a position corresponding to about 80% of the α ray range R, the amount of passing substance corresponding to the α ray range R of energy 6.8 MeV is 6 69 mg / cm ² .
((6.69 × 0.8) −0.59) −0.92 = 3.84 mg / cm ² , and the amount of substances passing through the ZnS (Ag) layer 3 is about 3.84 mg / cm ^2. desirable.

When the relative density of the ZnS (Ag) layer 3 is 3.70 g / cm ³ corresponding to about 90% of the true density, the layer thickness C of the ZnS (Ag) layer 3 is approximately 10.4 μm.
Since the density in the air is assumed, the layer thickness C actually has a width around 10.4 μm.
From the above, the layer thickness C of the ZnS (Ag) layer 3 is preferably set in the range of 6.0 μm to 16.0 μm.
Desirably, it is 8.0 micrometers-12.0 micrometers.
More desirably, it is 9.5 μm to 10.4 μm.

Further, passing substance amount D from the incident surface to the entrance surface of ZnS (Ag) layer 3 of the scintillator member 10, from the outlet of the ^above, to set in the range of 0.7mg / cm ² ~2.1mg / cm 2 Is good.
^Desirably, a ^{0.8mg / cm 2 ~2.0mg / cm 2} .
More ^preferably, it is ^{0.9mg / cm 2 ~1.9mg / cm 2} .

Next, the manufacturing process of the radiation discrimination detector according to the embodiment of the present invention will be described with reference to FIG.
As the plastic scintillator 1, a polyethylene naphthalate having a thickness A of 0.1 mm to 1.0 mm or a mixture containing polyethylene naphthalate is used.
The upper forming an SiO ₂ layer 7a using either dry plating method of vacuum deposition or sputtering method (S1).
Desirably, the adhesion strength can be further increased by using a sputtering method.

Next, the half mirror layer 2 is formed on the incident surface of the SiO ₂ layer 7a by using a dry plating method of either a vacuum deposition method or a sputtering method (S2).
Next, the SiO ₂ layer 7b is formed on the incident surface of the half mirror layer 2 using a dry plating method of either a vacuum vapor deposition method or a sputtering method (S3).
Desirably, the adhesion strength can be further increased by using a sputtering method.

Next, a ZnS (Ag) layer 3 is formed on the incident surface of the SiO ₂ layer 7b by using a dry plating method of any one of a sputtering method, a plasma chemical vapor deposition method and a vacuum vapor deposition method (S4).
Next, the mirror layer 4 is formed on the incident surface of the ZnS (Ag) layer 3 by using a dry plating method of either a vacuum deposition method or a sputtering method (S5).

Next, the SiO ₂ layer 7c is formed on the incident surface of the mirror layer 4 using a dry plating method of either a vacuum deposition method or a sputtering method (S6).
Desirably, the adhesion strength can be further increased by using a sputtering method.

Next, the scintillator member 10 thus formed is dried at a temperature of 100 ° C. or higher (S7).
By drying at such a high temperature, it is possible to prevent deaeration from remaining in a defect such as a small hole in the mirror layer 4 caused by an organic solvent or the like contained in the curable composition layer 5 to be applied next. It becomes possible.

Next, a curable composition is uniformly applied to the incident surface of the dried SiO ₂ layer 7c using a spin coater (S8), and a curing process is performed to form a curable composition layer 5 (S9).
Next, the protective layer 6 is formed on the incident surface of the curable composition layer 5 using a dry plating method (S10).
In this case, a plasma chemical vapor deposition method is preferably used as the dry plating method.

Next, the half mirror layer 2, the mirror layer 4, and the protective layer 6 are electrically connected and grounded (S11).
Since the dust itself contains a natural radionuclide, it is possible to prevent the adhesion of dust and improve the detection sensitivity by grounding in this way.

According to the radiation discrimination detector of the embodiment described above, it is possible to eliminate unnecessary layers that lower the detection sensitivity, and to extremely reduce the thickness of each layer.
Moreover, since the metal grating used conventionally can be made unnecessary by sufficiently increasing the strength of the protective layer 6 formed on the incident surface of the scintillator member 10, the incident surface of the scintillator member 10 is extremely used as the α ray generation source 40. You can get closer.

As a result, the detection sensitivity of α rays having a high attenuation rate could be improved.
And it became possible to implement | achieve the high detection sensitivity of both the alpha ray and beta ray radiation.
A radiation discrimination detector that optimizes the layer thickness C of the ZnS (Ag) layer 3 and the amount D of passing substance from the incident surface of the scintillator member 10 to the incident surface of the ZnS (Ag) layer 3 to achieve higher detection sensitivity. Obtained.

  In addition, the mechanical strength of the radiation discrimination detector is improved and the purchase and maintenance costs can be reduced by adopting a structure that does not require several thin and fragile light-shielding films that have been widely used in the past. .

Although the embodiment of the present invention has been described, this embodiment is presented as an example and is not intended to limit the scope of the invention.
This embodiment can be implemented in various other forms, and various omissions, replacements, changes, and combinations can be made without departing from the spirit of the invention.
This embodiment and its modifications are included in the scope of the present invention and the gist thereof, and are also included in the invention described in the claims and the equivalent scope thereof.

1 ... plastic scintillator, 2 ... half mirror layer, 3 ... ZnS (Ag) layer, 4 ... mirror layer, 5 ... curable composition layer, 6 ... protective layer, 7 (7a, 7b, 7c ) ... SiO 2 layer, DESCRIPTION OF SYMBOLS 10 ... Scintillator member, 11 ... Light guide, 12 ... Photomultiplier tube, 13 ... Light shielding case, 20 ... Radiation discrimination detector, 31 ... High voltage power supply part, 32 ... Signal amplification part, 33 ... Discrete part, 34 ... Waveform discrimination , 35 ... counting unit, 40 ... alpha ray generation source, A ... plastic scintillator thickness, B ... SiO ₂ layer thickness, C ... ZnS (Ag) layer thickness, D ... radiation discrimination detector Amount of passing substance from the incident surface to the ZnS (Ag) layer, F: a passing distance band in air, R: range.

Claims (11)

a plastic scintillator that emits first fluorescence when β-rays are incident;
A half mirror layer that reflects the scattered light of the first fluorescence that is reversely incident on the incident surface of the plastic scintillator by a dry plating method;
A ZnS (Ag) layer that emits second fluorescence and transmits through the half mirror layer when alpha rays are incident on the incident surface of the half mirror layer by dry plating;
Radiation comprising: a mirror layer which reflects the scattered light of the second fluorescence incident on the incident surface of the ZnS (Ag) layer by a reverse plating method and which is reversely incident and shields external light. Discrimination detector. The radiation discrimination detector according to claim 1, further comprising a curable composition layer that is formed on an incident surface of the mirror layer and improves imperfection of light shielding caused by a defect of the mirror layer. Formed on the incident surface of the curable composition layer by a dry plating method,
The radiation discrimination detector according to claim 2, further comprising a protective layer that reduces a physical action or a chemical action applied to the incident surface. The half mirror layer and the mirror layer are
The radiation discrimination detector according to any one of claims 1 to 3, wherein each of the radiation discrimination detectors is a metal layer containing any one of Al, Mg, Ag, Ni, and Cr. The plastic scintillator is
The radiation discrimination detector according to any one of claims 1 to 4, comprising polyethylene naphthalate or a mixture containing polyethylene naphthalate. The amount of passing substance from the incident surface of the radiation discrimination detector to the ZnS (Ag) layer is
Radiation discriminating detector according to any one of claims 1 to 5, characterized in that it is set in a range of ^{0.7mg / cm 2 ~2.1mg / cm 2} . The layer thickness of the ZnS (Ag) layer is:
The radiation discrimination detector according to claim 1, wherein the radiation discrimination detector is set in a range of 6.0 μm to 16.0 μm. An interface between the plastic scintillator and the half mirror layer;
An interface between the half mirror layer and the ZnS (Ag) layer;
8. The SiO ₂ layer having a layer thickness of 10 nm to 200 nm for improving the adhesion strength between the layers is provided at the interface between the mirror layer and the curable composition layer. The radiation discrimination detector according to any one of the above. forming a half mirror layer that reflects scattered light, which is reversely incident among the first fluorescence, on an incident surface of a plastic scintillator that emits first fluorescence when β rays are incident;
Forming a ZnS (Ag) layer that emits second fluorescence and transmits the half mirror layer when α rays are incident on the incident surface of the half mirror layer by a dry plating method;
Forming a mirror layer that reflects scattered light that is reversely incident on the incident surface of the ZnS (Ag) layer and shields external light by a dry plating method. Manufacturing method of radiation discrimination detector. Forming a curable composition layer on the incident surface of the mirror layer to improve imperfection of light shielding caused by defects in the mirror layer by a spin coater;
Forming a protective layer for reducing a physical action or a chemical action imparted to the incident surface of the curable composition layer by a dry plating method. Manufacturing method. The dry plating method is
The method for manufacturing a radiation discrimination detector according to claim 9 or 10, wherein the method is one selected from vacuum deposition, sputtering, and plasma chemical vapor deposition.

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