JPS6291878A - Radiation material capturing type dosimeter - Google Patents

Abstract

PURPOSE:To improve the capturing efficiency of an alpha radioactive material and the detection sensitivity of a low level of alpha radiation dose by capturing pulverized particles by the electrostatic force of a dielectric high-polymer film (electret) which is polarized and electrified by the electric charge applied thereto under a high voltage. CONSTITUTION:For example, a tetrafluoroethylene/hexafluoropropylene copolymer resin is subjected to a prescribed treatment to obtain a high-polymer film 19 as the electret to forma thermoluminescence dosimeter (TLD) element. The electric charge is applied to such high-polymer film 19 under the high voltage to preliminarily hold the high surface charge; thereafter,the film is installed in a dust collecting chamber 32. The air contg. the suspended dust to be monitored is sucked by a block 34 and the dust is captured by the electrostatic force of the high-polymer film 19. The high- polymer film 19 with which the capturing ends is taken out after the prescribed time and is installed in a dark box 59 where the film is heated by a heater 52. The unstable crystal lattice by the dose to which the TLD element is subjected is thereby changed to the stable crystal lattice. Since the quantity of the fluorescence emitted in this stage is proportional to the instability of the crystal lattice, the measurement is made with a photomultiplier 55 and integration 57 is made, by which the radiation dose is obtd.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a radioactive substance collection type dosimeter, particularly when dust, gas molecules, or these fine particles in the air contaminated with alpha (α) radioactive substances are detected on the surface. The present invention relates to a radioactive substance collection type dosimeter suitable for measuring the radioactive concentration of substances attached to the body.

[Background of the invention]

Nuclear facilities release beta (β) radioactive materials and gamma (
γ) Along with radioactive substances, solids or gases contaminated with alpha (α) radioactive substances are generated. Among these, α-radioactive substances are ionic substances with helium (Hθ) as their core, and therefore have a short flight distance of about 20 to 30 cm even in the air.Therefore, the measurement of radiation dose is greatly influenced by the geometry at the time of measurement. At the same time, only the alpha radiation generated from the surface of the alpha-emitting substance is measured. For this reason, a flat radiation detector or a film-like radiation detection element with a large surface area is used. Mainly from a safety perspective, caution is required when α-radioactive substances are easily dispersed as floating dust, so monitoring of surface contamination and air contamination by α-radioactive substances is necessary to deal with this. .

Conventionally, when measuring the radioactivity of floating Jal (dust), the dust is collected by suctioning it to a filtration filter, or
Alternatively, a gas containing dust particles is introduced to a discharge electrode section to which a high DC electric field (30 KV or more) is applied, and the dust particles are charged, and then the dust is collected by adhering to a collection electrode plate having a polarity opposite to the charging polarity. (Static Electricity Handbook, Society of Polymer Science: p445~Jinjin Shoin (1966)),
Alternatively, the decay product nuclides of radon and thoron, which are already charged α-radioactive gases, can be directly 1 to 2
Ni Kuhan was collected on the electrode plate under KV load.
and C.R. Phillips: Health Physics, vol. 46, pp. 141-149.

1984 (A, Xhan and C, R, Ph1l
lips:HealthPhysics, VoI246
p141-149, 1984), L Karunochi.

The radioactivity of the collected dust or mist and gas molecules was measured using a radiation detector, and the radioactivity concentration in the collected air was determined. However, with the filtration filter method, it takes time to collect the dust particles, and the dust particles penetrate deep into the filter media, so even if the collection efficiency can be maintained at over 90%, when measuring radioactivity, the dust particles may penetrate deep into the filter media. Alpha radioactivity from penetrating dust particles cannot be measured with the same efficiency, and
When the amount of collected data is small and the radioactivity concentration is low, a long measurement time is required, so when the number of samples is large, a large number of radiation detectors are required. On the other hand, in the electrostatic precipitation method,
Since a device for generating high voltage is required, it is difficult to move it easily, and if it is fixed, it is necessary to use a large number of high voltage generating devices. Furthermore, this method also requires a radiation measuring instrument to measure the radioactivity of the dust particles collected on the collection electrode. Moreover, since there are many sampling points and the counting time is long in the case of low-level radioactivity concentration, multiple radiation detectors are required.

As mentioned above, the conventional method for measuring the radioactive concentration of dust particles requires a long time for monitoring and a large number of devices are required when sampling points are changed arbitrarily. The disadvantage of
there were.

On the other hand, in order to measure the surface contamination of an object due to dust particles attached to the surface of the object, conventionally, based on the smear method, a predetermined surface area (101 square) is wiped using filter paper and cloth, and the radioactivity of the adhering material on the surface of the filter paper is measured. was measured. In this case as well, it took time and effort to measure the radioactivity after each wipe. Further, when wiping, it is not necessarily easy to always wipe a constant area with a constant force.

As an alternative to the above-mentioned method, polymer membranes can be coated with high DC voltage (
An electret is created by subjecting it to electrical discharge under approximately 10 KV) to internally polarize and charge the surface.The electret and a radioactive substance-contaminated specimen are brought into close contact or close together, and the contamination level of the specimen is determined from the attenuation rate of the surface charge density of the electret. There is a way to measure it. However, when this method is applied to areas where particles such as dust or mist are suspended, these particles adhere to the electret material and similarly attenuate the surface charge, making it difficult to distinguish between radiation and particle adhesion. There was a problem that it became difficult to distinguish.

[Purpose of the invention]

An object of the present invention is to provide a radioactive material collection type dosimeter that has a large ability to collect alpha radioactive materials and can measure low-level alpha radiation doses with high sensitivity.

[Summary of the invention]

The features of the present invention include a transparent substance that can hold radioactive substances attached to its surface, and a crystal lattice that is distorted and becomes unstable by irradiation with radiation mixed into the transparent substance and emitted from the radioactive substance. Moreover, it is a radioactive material collection type dosimeter that serves as a dose detection element that emits light when the crystal lattice is stabilized by supply of external energy.

The inventors focused their attention on a method of collecting α-radioactive substances using electrostatic force or the like.The present invention uses a dielectric polymer material, which has been polarized and charged by electrical discharge under high voltage, as a collector.
That is, the function of amping particles such as dust or mist using the electrostatic force of the electret is utilized. Further, the present invention is a device in which a dosimeter element capable of outputting radiation and an integrated dose after irradiation is mixed and integrated in a dust collection material (for example, electret). A typical dosimeter element is a thermoluminescent element. Also, the material suitable for the electret material is fluoropolymer.

Examples include tetrafluoroethylene (TFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), and polyvinylidene fluoride (PVDF). In addition to fluorine-based materials, for example, polypropylene (PP) can also be used. It is. In addition, dielectric materials having charging ability are applicable.

[Embodiments of the invention]

A preferred embodiment of the present invention will be described in detail based on the drawings. Figure 1 shows the thermoluminescence dosimeter (hereinafter referred to as
An example of manufacturing an element (referred to as TLD) is shown in which a fluororesin (for example, tetrafluoroethylene-hexafluoropropylene copolymer resin) is used as the polymer material.

Hereinafter referred to as FEP, ) powder or pellets is supplied by the feeding device! ! 11 is filled. Calcium sulfate (CaSOa(Dy)) is supplied to the supply device 12 as powder of TLD element material.
filled inside. FEP powder in the supply device 111 and calcium sulfate powder in the supply device 12.

It is fed into a mixing tank 13, where the two substances are mixed. The mixture obtained in the mixing tank 13 is passed through the screw 16
The rotation of the molding material causes the molding material to be pushed into an extrusion molding machine 15 having a heater 14. The mixture pushed in by the screw 16 is heated by the heater 14. This heating causes F in the mixture to
EP melts.

FEP melts at about 270°C. After the molten FEP mixed with calcium sulfate powder is extruded, a cooler 17 is installed on the exit side of the extruder 15.
When the film is cooled and further stretched by rolls 18, an FEP polymer film 19 containing calcium sulfate is formed. The thickness of this film 19 is controlled by the extrusion rate of the melt from the extruder 15 and the tension stretching rate by the rolls 18. FIG. 1(B) shows the powder of the TLDi child material produced in this way, that is, the calcium sulfate powder 10
2 shows a longitudinal section of a polymer film 19 of FEP in which FEP is uniformly dispersed. When measuring α radiation, the thickness of the polymer membrane 19 is set to about 50 μm according to the range of α radiation in the membrane material. However, considering the strength and handling of the membrane material, the appropriate thickness of the polymer membrane 19 is 50 to 100 μm. The content rate of the TLD element material 10 in the obtained polymer film 19 is 5% to 30% (weight ratio), and the content rate is increased when sensitivity is to be improved.

Next, the surface of this polymer film 19 is charged using a discharge device shown in FIG. The polymer film 19 is inserted within the container 25 of the discharge device and between the discharge electrode 22 and the return electrode plate 23. A heater 24 is provided below the counter electrode plate 23. The polarity of the discharge electrode 22 can be either positive or negative. Assume that the polarity of the discharge electrode 22 is set to positive and discharge is performed in air at a voltage of 20 KV/m or less. In this case, the surface of the polymer 11 [19 is positively charged, and the counter electrode plate 23 of the discharge electrode 22 is negatively charged. However, when this discharge operation is performed at room temperature, although charges are placed on the surface of the polymer film 19, a stable and high charge density cannot be obtained.
Therefore, after the polymer film 19 is heated to the entrainment temperature by the heater 24, it is gradually cooled under electric discharge. The polymer 1119 subjected to such treatment has internal polarization. This state is generally called electret.

Furthermore, in order to prevent the surface charge of the polymer film 19 from disappearing over time, if the polymer film 19 is covered with conductive foil such as aluminum foil, the polymer film 19 will maintain a high surface charge over a long period of time. can be held.

Measurement of radiation dose using the polymer film 19 will be explained below. A polymer film 19 holding a high surface charge is installed in a dust collection ta 32 as shown in FIG. Air containing floating dust to be monitored is sucked in by driving the blower 34 and guided into the dust collection box 32. The dust particles in the sucked air become high while flowing along the electret wavy polymer membrane 19 (the polymer membrane 19 is attached to both sides of the wavy thin plate support) in the dust collection box 32. Dust is collected on the surface of the polymer film 19 due to the electrostatic force on the surface of the molecular film 19 . The polymer membrane 19 can also be arranged in a wave pattern by being hooked alternately to a plurality of thin rods arranged vertically within the dust collecting box 32. By arranging the polymer membrane 19 in a wavy manner, the surface area of the polymer M419 increases, and the residence time of air increases. After collecting dust at a predetermined suction flow rate for a predetermined time, the polymer membrane 19 is taken out from the dust collection box 32 and left for a predetermined time in a radiation #I amount field comparable to that in nature or in a low background room. The reason why the polymer film 19 is left for a predetermined period of time is that when the polymer film 19 is removed from the dust collection box 32, the TLD element contained in the polymer film 19 remains in the polymer film 1.
This is because the TLD element is not sufficiently affected by the alpha radionuclides contained in all the dust particles attached to the surface of the TLD element, and the TLD element is sufficiently affected by all the alpha radionuclides. The crystal lattice of the TLD element included in the polymer film 19 becomes unstable due to α radiation emitted from the α radionuclide. The reason for the above-mentioned neglect is that the unstable state of the crystal lattice of the TLD element reaches a state corresponding to the intensity of alpha radiation from all alpha radionuclides. The integrated dose (instability of the crystal lattice of the TLD element) received by the polymer film 19 during the standing time due to the α radiation of the attached dust is measured by the TLD reader shown in FIG. First, the polymer film 19 taken out from the dust collection box 32 and left for a predetermined time as described above is placed in a dark box 59 that is shielded from light. Thereafter, the polymer film 19 is heated by the heater 52. By heating the polymer film 19, the unstable crystal lattice (distorted crystal lattice) of the TLD element contained therein changes into a stable crystal lattice. Fluorescence is generated during this change.1The amount of fluorescence generated is:
It is proportional to the instability (skewness) of the crystal lattice of the TLD element, and increases when the instability is large. During the heating process, the amount of fluorescence generated from the TLD element present in the polymer film 19 is measured with a photomultiplier tube 55, and the temperature during the heating process is measured with a thermocouple 54. The amount of fluorescence measured by the photomultiplier tube 55 is integrated by an integrator 57 in the temperature rising section, and the amount of the integrated device is outputted to a display or recorder 58. The temperature increase section is determined by the output of the thermocouple 54.The relationship between the exposure time and the integrated dose when the α radioactivity of the dust obtained by this method is 10-8 μCi is shown. The collection speed Dv is 10', cj/min, and the collection time t is 10
It was 0 minutes. In this case, the α radioactivity concentration C of the dust is determined by the following equation.

c = i ・Q/ov-t --(1
) Here, E is the collection efficiency and Q is the total α radioactivity dose (μCi) converted from the TLD measurement value.

In addition, when calculating the total amount of alpha radioactivity Q, the conversion coefficient differs depending on the energy of the alpha radioactive substance.

It is necessary to determine the type and proportion of alpha radionuclides in advance. However, in general, the types of nuclear materials handled at nuclear facilities are fixed, and variations in nuclides are thought to be slight, so even if the conversion factor is constant, it can be applied with an error of less than 10%1. Looking at alpha radionuclides.

When 10-BμCi/a1α radioactive material adheres to the surface of the polymer membrane 19, the membrane absorbed dose per minute is 10-”m
It becomes rad/cxl. The concentration of α radionuclide in the dust is IQ-12 μCi/cd, and the collection efficiency is 90%.
When the air suction flow rate is 10 Q/min and the surface area of the polymer membrane 19 is 100 d, the total amount of α radioactive substances collected is 10-"
X O, 9X 104X 10”= 0.9×10-a
In μCi, the α radioactivity density on the surface of the polymer membrane 19 is 0.9 Xl0−δμCi/al.

When left in this state for 10 hours, the integrated dose of alpha radiation will be approximately 1 mrad. This radioactivity concentration in the air (1
Since 0-"μCi/cxl) corresponds to the permissible concentration of uranium in the air of 1710, the target sensitivity can be obtained.

Furthermore, in order to increase the sensitivity for measuring low concentration dust, (1) lengthen the dust collection time; (2) Enlarging the surface area of the polymer membrane 19. (3) Polymer membrane 19 after dust collection
One example of this is to lengthen the standing time (accumulation time). Further, as described above, when the polymer film 19 is left to stand after being taken out from the dust collection box 32, the background should be low.

As described above, the radiation dose measuring method and device according to this embodiment utilizes a relatively inexpensive polymer membrane material and a highly sensitive TLD.
By mixing and integrating the elements, it is particularly suitable for monitoring the radioactivity concentration of particles containing α-emitting substances, such as dust, mist, and layered substances. In particular, the feature of this book and the examples is that particles are collected on the membrane surface by the dust collection ability of electret. By changing the exposure time depending on the level of contamination, it is possible to measure even low-level contamination. In addition, even if the gamma radiation dose rate at the site to be monitored is relatively high, after collecting the contaminants, the detection membrane can be immediately transferred to the low-background room, and the dose accumulation due to the contaminants collected in the low-background room can be carried out. It is possible to measure the Therefore, there is an advantage that monitoring can be performed without being influenced by the background of the sampling field. Furthermore, once the thermal product dose has been measured by TLD, if re-measurement is required because the integrated dose is insufficient, it is also possible to leave the measuring device alone and calculate the integrated dose.

Furthermore, in this embodiment, even if the dust particles adhere to the polymer film 19, the crystal lattice of the TLD element becomes unstable due to the dose emitted from the α-radioactive nuclide contained in the dust particles. This method does not pose the same problems as radiation dose measurement using electrets.

In the above, a film-like electret, that is, a polymer film 1
An example has been described in which the polymer film 19 is arranged in a wavy manner in order to increase the dust collection efficiency. Figure 5 shows a dust collection electret devised to roughly increase collection efficiency. This uses an electret membrane having micron-order pores 42, that is, a filter-like electret 44 made by stacking one or more polymer membranes. In this case, a dust particle collection efficiency of 99% or more is obtained, and the collected dust particles are held on the surface of the filter-like electret 44 by electrostatic force. After collecting dust particles for a predetermined period of time, the filter-like electret 44 is removed from the container 41 and left for a predetermined period of time as described above. Thereafter, using the apparatus shown in FIG. 4, the integrated dose accumulated in the TLDJ element of the filter-like electret 44 is determined in the same manner as in the previous embodiment. In this case, the filter-like electret alone can retain the collected dust particles to a certain extent, so the purpose can be effectively achieved even without the use of electret.

Next, we will describe the material conditions when combining + TLD element material and polymer material. Since the TLD element material needs to be heated in order to generate fluorescence during measurement, it is not preferable that the polymer material melts or significantly deforms at the heating temperature. Furthermore, in order to efficiently measure the fluorescence generated from the TLD element material, it is preferable to use a transparent polymer material with high light transmittance. On the other hand, the TLD element material is
It is preferable to use a material whose fluorescence generation temperature is as low as possible. Specifically, among the fluororesins that have a high melting point and are easily converted into electrets, the polymer materials include tetrafluoroethylene (+CF2・CFz+, abbreviated as TFE), tetrafluoroethylene and hexafluoropropylene (+CF- CF8・
Suitable are a copolymer of CFz-3-T (abbreviated as FEP), or a fluoropolymer material in which TFE and the side chain of TFE are substituted with an organic fluorinated alkyl group. These substances are suitable because they have a melting point of 250°C or higher. TLD cord material is Ca5Oa(Dy)-CaSO, which is calcium sulfate crystal doped with dysprosium (Dy) and manganese (Mn).
a (Mn), calcium fluoride-based CaFz (Dy
) *CaFz (Mn), and lithium fluoride (LiF
) can be given. Of these, except for CaSO4 (Mn), 2
This is suitable because it emits fluorescence at temperatures below 50°C.

Up to now, examples have been given in which an electret material is used as a dust particle collection material and TLD powder is dispersed therein as an integrated dosimeter element. However, if the electret film is a polymer film of 10 μm or less, T
The same effect can be obtained by stacking two thin film TLDs obtained by depositing or coating an LD element on an electret film in a flat plate shape.

Next, the dust is absorbed by the dust collecting power other than electret,
Similarly, an example will be described in which integrating dosimeter elements such as TLDs are dispersed in the absorbent material. Dispersing the TLD element material in the transparent polymer film is the same as in the previous example, but 1
This method involves applying a sticky substance to the surface, sucking in air containing dust particles, and blowing it onto the surface of the sticky polymer film.

After suctioning for a certain period of time and leaving it for a certain period of time, α in the dust attached to the surface
The membrane absorption cumulative dose due to radioactive substances is measured by TLD.

In the above embodiments, the main focus was on measuring the radioactivity of dust present in the air, but below, an embodiment will be described in which the concentration of surface contamination due to layered substances deposited on the surface is measured. The legal control standard value for surface contamination density due to alpha radioactive substances is 1
0-'μCi/d, and the classification boundary value for determining the presence or absence of contamination is 10-"μCi/d, which is 10-6 for operational purposes.
~10-7 μCi/aJ is often adopted. Now, when the aforementioned electret polymer membrane is pressed onto the surface of a substance contaminated with 10-7 μCi/d, floating dust is instantly attracted to the membrane surface. The electret membrane shape at this time was 10 cm x 1. Appropriately, it has a constant area of about 0 cm square and a thickness of 50 to 11001 L. Further, as for the polymer membrane material, a fluorine-based polymer material is preferable as described in the example of dust collection, but polypropylene, which is another high-melting point polymer material, is also applicable.

The TLD element material is dispersed in the electret film, and after collecting the surface contaminant particles and leaving them for a predetermined time as in the dust collection example, the integrated dose to the TLD element is measured using the apparatus shown in Figure 4. Measure. In this case as well, when the surface contamination level by alpha radionuclides is 10-7 μCi/cd and the collection efficiency is 90%, if left for 1 hour, 1 mr
It can be measured as the absorbed dose of ad.

When measuring surface contamination, it can also be used instead of electret.
Affix the surface contaminants with adhesive tape and calculate the integrated dose using the same method.

Until now, alpha radioactive substances have been the target radioactivity, but
This can also be applied to β-radioactive substances. However, since there are differences in sensitivity depending on the radiation quality, comparative calibration using standard materials is required for each individual.

Furthermore, in the method and apparatus according to each of the embodiments described above, -
The used detection material is cleaned and heat treated (annealed).
It has the advantage of being economical because it can be reused by applying

In the radiation dose measuring method and device according to the present invention, if the integrated dosimeter can output without heating during measurement like a TLD, there will be no restrictions on the collection material such as dust, which will widen the range of uses and make it possible to select cheaper materials. Light can be considered as an example of deexcitation using a method other than heat. The integrated radiation dose can be measured by generating fluorescence through optical excitation and spectroscopically measuring the fluorescence wavelength. At present, a glass gravimeter is used as an element that generates fluorescence through optical excitation and measures the radiation dose. An example in which this is applied is shown in FIG. 7. In FIG. 7, elements that emit light upon optical excitation are dispersed in an electret polymer film 71. Excitation light of a certain wavelength is generated from, for example, an ultraviolet emitter 73, and then is irradiated through the light guide 72 onto the polymer film 71 containing the detection element irradiated with the α-emitting substance. Only the fluorescence of a certain wavelength emitted when the metastable electrons excited by ultraviolet light return to their reference state is separated by a spectrometer 75, and the intensity of the light is received by a photometer 76 and then displayed as an output. It is outputted by the device 77.

The measurement method described above does not require the addition of heat;
It can be used for polycarbonate-based polymer materials, which are materials with a melting point of 00°C or lower.

〔Effect of the invention〕

According to the present invention, radioactive substances can be collected efficiently, and the detection sensitivity of low-level alpha radiation doses can be significantly improved.

[Brief explanation of drawings]

Figure 1(a) is a block diagram of a device for creating a radioactive material trapping dosimeter in which TLD element powder is dispersed.
b) is a vertical cross-sectional view of a radioactive substance trapping dosimeter that is a preferred embodiment of the present invention, FIG. 2 is a configuration diagram of a polymer membrane electretization device, and FIG. 3 is a diagram of FIG. 1(b). A configuration diagram of a dust collection device using a radioactive material collection dosimeter shown in Figure 4.
The figure is a configuration diagram of a TLD measuring device of a radioactive material trapping dosimeter taken out from the collection device of FIG. 3, and FIG. 5 is another example of a dust collecting device using a radioactive material trapping dosimeter. Fig. 6 is a characteristic diagram showing the relationship between the integrated dose value and integration time due to α radioactivity of the TLD dispersed electret film, Fig. 71
ii! I is a block diagram of another embodiment of the apparatus shown in FIG. 4; DESCRIPTION OF SYMBOLS 10... TLD element powder, 1.1.12... Feeding device, 14... Heater, 15... Extrusion molding machine, 16...
... Screw, 17 ... Cooler, 18 ... Roll, 19 ... Polymer membrane, 22 ... Discharge electrode, 23 ...
... Counter electrode, 31 ... Suction duct, 32 ... Dust collection tube,
34...Blower-152...Heater, 54...Thermometer sensor, 55...Photoelectron amplifier tube, 57...Integrator, 58...Display device.

Claims (1)

[Claims] 1. A transparent substance that can hold a radioactive substance attached to its surface, and a state in which the crystal lattice is distorted and unstable due to irradiation with radiation mixed into the transparent substance and emitted from the radioactive substance. and a dose detection element that emits light when the crystal lattice reaches a stable state by supplying external energy. 2. The radioactive substance trapping dosimeter according to claim 1, wherein the transparent substance is an electret.