JP5205681B2 - Radiation detection element, radiation detector and radiation detection method - Google Patents

Description

The present invention relates to a radiation detection element for radiation measurement , a radiation detector, and a radiation detection method used in a wide range of scientific fields including science, engineering, and medicine.

  Generally, radiation that is charged particles such as α rays and β rays loses energy by ionizing, exciting, or dissociating atoms or molecules in the material when passing through the material. The energy transferred to the substance is further released as thermal kinetic energy or electromagnetic waves. When this substance is a substance that emits fluorescence, a large part of its energy is emitted as light in the visible region, this phenomenon is called scintillation, and the emitted light is called scintillation light.

  Furthermore, in the case of radiation that does not have a charge such as γ-rays or neutrons, the same phenomenon occurs due to secondary charged particles that are emitted when the radiation interacts with the substance. Radiation is detected by using it.

  A photomultiplier tube is used for measuring the scintillation light. The photomultiplier tube is a high-sensitivity photodetector to which a current amplification (= electron multiplication) function is added based on a photoelectric tube that converts light energy into electric energy by using a photoelectric effect.

  Substances that cause a scintillation phenomenon are generally called scintillators. In the field of radiation measurement, inorganic scintillators including inorganic crystals typified by NaI (Tl), organic crystals such as anthracene, fluoresce when irradiated with radiation such as terphenyl. Liquid scintillators in which a phosphor is dissolved in an organic solvent such as xylene and organic scintillators including a plastic scintillator in which the phosphor is dissolved and dispersed in a transparent resin such as polystyrene are used.

  In particular, plastic scintillators have been used very often for half a century because of their ease of handling and the ability to be molded into arbitrary and large shapes.

  A typical plastic scintillator is made of 2,5-diphenyloxazole (DPO) or 1,4-bis (5-phenyl-2-oxazole) on a styrol base resin such as polystyrene (PS) or polyvinyltoluene (PVT). Benzene (POPOP), p-tetraphenyl (P-TP), p-quarterphenyl (P-QP) and 2- (4-biphenyl) -5- (4-tert-butylphenyl) -1,3,4 A first phosphor such as oxadiazole (B-PBD) and a second phosphor such as bis (O-methylstil) benzene (bis-MSB), 9,10-diphenylanthracene, and 9,10-dimethylanthracene were added. Is.

  As a long-standing common sense in the technical field, plastic scintillators require the addition of phosphors. The main reason is that the wavelength of the electromagnetic wave emitted by the matrix resin excited by radiation irradiation is as short as 150 to 300 nm, so 1) It is suitable for the measurement of photomultiplier tubes used for measurement. 2) The amount of light reaching the detection part is not sufficient because self-absorption occurs due to the base resin. In this respect, by adding a phosphor, the energy of electromagnetic waves emitted from the base resin can be converted into light of ~ 350 nm by the first phosphor and ~ 420 nm by the second phosphor, which is suitable for measurement and has a transmittance. It is said that scintillation light having a wavelength that is high and difficult to absorb by itself can be obtained.

  Therefore, it is common technical knowledge in the technical field that the addition of a phosphor to the scintillator is essential, and studies on various combinations of a host resin and a phosphor have been conducted in order to improve the measurement technique.

  However, no attempt has been made so far to use only the base resin as a radiation detection element because of the common technical knowledge that it is essential to add a phosphor to the scintillator as described above.

The type and amount of phosphor added in the production of scintillators largely depends on the know-how of the manufacturer, and the number of manufacturers is limited, so the market is almost monopolized. Therefore, the scintillator is generally expensive, and the price of the radiation detector using the scintillator is also very expensive.
An object of the present invention is to provide a radiation detection element , a radiation detector, and a radiation detection method that can be manufactured at low cost, and to enable measurement of radiation at a lower cost.

Contrary to many years of common technical knowledge in the field of radiation measurement, the present invention provides a radiation detection element comprising only a matrix resin containing no phosphor, that is, the radiation detection element shown below.
1. A state in which a fluorescent substance that converts a fluorescence emitted from the matrix resin excited by radiation irradiation into light having a wavelength longer than that is isolated in the matrix resin, the main resin being a PET bottle recycled resin. and ray detection element release you characterized by not including even in a state bound to the matrix resin.
2 . 2. The radiation detecting element according to 1 above , further comprising polystyrene, polyvinyltoluene, polyethylene terephthalate which is not a recycled raw material, and copolymers or mixtures thereof.
3 . A radiation detection element comprising only a plastic bottle recycle resin .
4). 4. The radiation detecting element according to any one of 1 to 3 above, which is for a photomultiplier tube having a sensitive wavelength peak of 300 nm to 500 nm .
5. 5. A radiation detector comprising: the radiation detection element according to any one of 1 to 4; and a detection unit that detects fluorescence emitted from the radiation detection element.
6). 6. A radiation detection method for detecting radiation using the radiation detector according to claim 5, wherein the radiation is incident on the radiation detection element and fluorescence is emitted by an interaction between the radiation and the plastic bottle regeneration resin. The radiation detection method, wherein the emitted fluorescence is detected by the detection unit.

According to the present invention, it is possible to detect radiation by using a base resin used in a conventional plastic scintillator as a radiation detection element without adding a phosphor.
Further, according to the present invention, superior resolution can be obtained as compared with a plastic scintillator including a conventional phosphor.
Furthermore, according to the present invention, a cheaper material can be used in the manufacture of the scintillator, and the cost of the radiation detector can be reduced.

²⁰⁷ Combined energy distribution of 11 groups of radiation emitted from Bi source. Three gamma rays (569.7 keV, 1063.6 keV, 1770.2 keV) and eight internal conversion electrons (K shell 975.6 keV, L1 shell 1047.8 keV, L2 shell 1048.4 keV, L3 with different energy levels. Shell 1050.6 keV, K shell 481.6 keV, L1 shell 553.8 keV, L2 shell 554.5 keV, L3 shell 556.6 keV). ²⁰⁷ Energy distribution of only 976 keV K-shell internal conversion electrons emitted from Bi source. Due to the energy loss in the source, an asymmetric energy distribution is shown. The composite energy distribution of the radiation group (total 6) emitted from the ¹³⁷ Cs radiation source. Separate one gamma ray, one beta ray, and four internal conversion electrons (k shell 625.6 keV, L1 shell 655.9 keV, L2 shell 656.3 keV, L3 shell 656.6 keV) with different energies Yes. Comparison of energy resolution between a matrix resin (polyvinyltoluene) not containing a phosphor and a plastic scintillator containing a conventional phosphor (model number: BC-408 manufactured by Saint-Gobain). The data are 976 keV and 482 keV K-shell internal conversion electrons from a ²⁰⁷ Bi source and 626 keV K-shell internal conversion electrons from a ¹³⁷ Cs source. ²⁰⁷ Combined energy distribution of 11 groups of radiation emitted from Bi source. As can be seen from comparison with FIG. 1, the peak is a 976 keV K-shell internal conversion electron. Emission spectrum from polyvinyltoluene resin, PET bottle recycled resin, conventional plastic scintillator (BC-408). The horizontal axis represents the fluorescence wavelength (in nm), and the vertical axis represents the light amount / quantum efficiency.

Examples of the base resin used in the present invention include polymers used as a base resin for known plastic scintillators. In the present invention, the main component of the base resin preferably means that the base resin is contained in the scintillator at 80 wt% or more, preferably 90 wt% or more, more preferably 95 wt% or more. It does not exclude becoming.
In particular, an aromatic polymer containing an aromatic ring in the main chain or side chain is desirable, and examples include aromatic polyimide, aromatic polyamide, aromatic polyester, aromatic polycarbonate, and aromatic polyether.

For example, commercially available polystyrene can be used. Aromatic polyesters such as polyethylene toluene and polyethylene terephthalate, which are polymers having a phenyl group substituted with a lower alkyl group as a side chain, can also be used, but are not limited thereto. Two or more of these aromatic polymers may be used in combination, and a copolymer or a mixture may be used.
Furthermore, the plastic bottle reproduction | regeneration resin manufactured using the used plastic bottle as a raw material can be used. The PET bottle used here is a resin bottle mainly made of PET (polyethylene terephthalate) used as a container for filling foods (soy sauce, dairy products, etc.), soft drinks, alcoholic beverages, etc. Examples of production methods and compositions are disclosed in, for example, US Pat. No. 3,733,309. As long as it is a recyclable PET bottle currently distributed, it is not particularly limited, such as a PET bottle, a pressure-resistant PET bottle, a pressure PET bottle, and a room-temperature aseptic filling PET bottle. PET bottle reclaimed resin is a resin manufactured using recycled raw materials that have been processed in the usual reclaiming processes such as washing PET bottles, removing foreign substances, and crushing. Plasticizers, stabilizers, lubricants, and electrification as necessary. An additive usually used such as an inhibitor and an antioxidant, and polyethylene terephthalate which is not a recycled material may be included.

In this specification, “phosphor” means 2,5-diphenyloxazole (DPO), 1,4-bis (5-phenyl-2-oxazole) benzene (POPOP), p-tetraphenyl (P-TP), p-quarterphenyl (P-QP) and 2- (4-biphenyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (B-PBD), 2- (4-tert -Butylphenyl) -2- (4-biphenyl) -1,3,4-oxadiazole (butyl PBD) or bis (O-methylstil) benzene (bis-MSB), 9,10-diphenylanthracene, 9,10 -Fluorescent substances such as dimethylanthracene, etc., or 4,4'-bis- (2,5-dimethylstyryl) biphenyl (BDB), fluorescent molecules, or scintillates Is a molecule called a chemical substance.
In the present specification, “does not contain a phosphor” means that the molecule is not contained even in a state where the above-described molecule is isolated or in a state where the molecule is bound (including a covalent bond, an ionic bond, etc.) to the base resin.

(Material form)
In the present invention, the shape of the radiation detection element is not particularly limited, and examples thereof include a rectangular parallelepiped of about 5 × 5 × 10 mm, a cylinder, and the like. It can be further increased in a range that does not cause an optical distortion that hinders detection by the detection unit, or in a range in which no light loss occurs.

(Material processing)
In the embodiment of the present invention, processing may be performed to improve the measurement efficiency by effectively guiding the light generated inside the base resin of the interaction unit to the detection unit. For example, the surface of the detection element may be polished and mirror-finished for the purpose of utilizing total internal reflection. The processing surface is preferably at least one surface. Further, a process of providing a light reflection layer on the surface of the detection element may be performed.
The reflecting layer is not particularly limited as long as it can reflect light generated inside the base resin, but for example, aluminum or Teflon (registered trademark) is desirable.

(Type of radiation detector)
A radiation detector capable of measuring radiation using the present invention includes an interaction unit that emits light by interacting with radiation and a detection unit that detects the light emission. As the light emitting part, a photomultiplier tube or a semiconductor detector can be used. For example, photomultiplier tubes R8900-00-M16, H5773, H6780, H7195, H7732, and H9305 manufactured by Hamamatsu Photonics can be used, but are not limited thereto. When using polyvinyl toluene resin, PET bottle regenerated resin or the like as the base resin, it is preferable to use a photomultiplier tube having a sensitive wavelength peak of 300 nm to 500 nm, more preferably 300 to 400 nm.
It is also possible to use an ultraviolet sensor (semiconductor detector) made based on a zinc oxide (ZnO) single crystal having high sensitivity to ultraviolet rays of 380 nm or less.
The interaction unit is composed of the radiation detection element of the present invention described above.
The radiation detector of the present invention may have any shape and configuration as long as it includes the above-described interaction unit and detection unit.

(Measurable line types)
In the present invention, since fluorescence in the short wavelength region from the base resin can be sufficiently detected, spectroscopic measurement of various types of radiation (β rays, γ rays, internal conversion electrons, etc.) is possible.

  Examples of radiation measurement using a radiation detection element made of a base resin to which no phosphor is added according to the present invention will be described below.

Example 1 A detection element having a size of 62 mm × 62 mm × 10 mm made of polyvinyl toluene (PVT) having a measurement purity of 100% of a radiation source was produced. In order to efficiently detect fluorescence from the detection element, photomultiplier tubes (model number H7195, Hamamatsu Photonics) were respectively arranged on four sides of 62 mm × 10 mm of the detection element. The upper surface and the lower surface 62 mm × 62 mm of the detection element were mirror-finished. Lead collimator (size 30mm x 30mm x 8mm, hole with a diameter of 10mm in the center) is used to reduce incidental events caused by different types of energy and radiation emitted from radiation sources. Centered. As a radiation source, a ²⁰⁷ Bi radiation source and a ¹³⁷ Cs radiation source purchased from the Japan Isotope Association were used.

The analysis method of the measurement data is H.264. Nakamura, et. al. , Rad. Res. 170, 811 (2008). In other words, in the past, when the measured values were calibrated assuming that the theoretical values were the same as the measured values that lost energy due to the radiation source itself and the surrounding protective film, it was noted that there was actually an error. Then, calibration was performed to separate the error component from the energy distribution measured by the radiation measuring instrument.
Since this method dramatically improves the energy resolution in the radiation measuring instrument, it is particularly effective in measuring a radiation source that emits a plurality of radiations.

(Performance evaluation using ²⁰⁷ Bi radiation source)
^{207 From the} Bi-ray source, 11 gamma rays, mainly 3 gamma rays with different energies and 8 internal conversion electrons with different energies (2 sets of internal conversion electrons of K-shell, L1, L2, and L3 shells) Are emitted from a radiation source.
FIG. 1 shows the energy spectrum obtained by the detecting element, and FIG. 2 shows the result of extracting only K-shell internal conversion electrons from the energy spectrum.
Here, the reason why the K-shell internal conversion electron having a single energy of 976 KeV theoretically shows an asymmetric energy distribution without showing a symmetric normal distribution is due to energy loss in the radiation source.
From this measurement result, it was found that the energy resolution of the detection element was σ = 3.58 ± 0.02% with respect to the 976 KeV region. As a result of using a normal plastic scintillator, a higher performance was exhibited than σ = 3.7 to 4.1%.

(Performance evaluation using ¹³⁷ Cs radiation source)
^{The 137} Cs source mainly emits six radiations, including gamma rays and beta rays, and four internal conversion electrons of different energies (one set of K shell, L1, L2, and L3 shell internal conversion electrons). The The energy spectrum is shown in FIG. As a result of measurement by the detection element, it was found that the energy resolution was σ = 4.57 ± 0.01 with respect to the 625 KeV region as shown in FIG.

(Summary of source results)
Summarizing the energy resolution results obtained from ²⁰⁷ Bi and ¹³⁷ Cs sources, 100% pure polyvinyltoluene has σ = 3.5 / E (MeV) ¹ / ² %, and the same size plastic scintillator The result obtained when the same experiment and analysis were performed in (Saint-Gobain crystals, BC408) was a value exceeding σ = 3.7 / E (MeV) ¹ / ² %. The result is shown in FIG.
In other words, it was shown that a detection element made only of a base resin not containing a phosphor can sufficiently detect radiation, and has a resolution superior to that of a plastic scintillator, instead of a conventional plastic scintillator containing a phosphor.
In addition, even with a normal photomultiplier tube, fluorescence in the short wavelength region from the detector element can be sufficiently detected, allowing spectroscopic measurement of various types of radiation (β rays, γ rays, internal conversion electrons, etc.). It has been proven. Furthermore, it was shown that measurement with higher sensitivity is possible by using a detection unit such as a photomultiplier tube having high sensitivity in the short wavelength region.

Example 2 Evaluation of radiation measurement possibility using PET bottle recycled resin ( ²⁰⁷ Performance evaluation using Bi source)
Next, an example is shown in which a detection element made of a base resin containing impurities and not containing a phosphor is used. Here, the possibility of radiation measurement was examined using a block made of a plastic bottle recycled resin (hereinafter also referred to as recycled PET resin) using a plastic bottle made in Japan. This block has a complicated shape (step-like shape, and the outer dimensions of the portion used for detection are 50 mm × 110 mm × 5 mm), and since the surface processing for measurement is not performed, the bottom surface (50 mm × A single photomultiplier tube (model number H7195, manufactured by Hamamatsu Photonics) was installed on the 110 mm surface, and light excited by radiation incident from the opposite surface was measured using a ²⁰⁷ Bi ray source. As a result, as shown in FIG. 5, the peak of K-shell internal conversion electrons at 976 keV could be sufficiently observed, like polyvinyltoluene.
As a result, it was clarified that it is possible to use as a radiation detecting element even if a base resin having a purity of not 100% is used, and that radiation can be sufficiently measured even without special processing. .

The properties of the PVT resins and recycled PET resins of Examples 1 and 2 are shown in the following table.
table

(Emission spectrum from polyvinyl toluene resin and recycled plastic bottle)
The emission spectrum of the fluorescence from the plastic bottle regenerated resin using the polyvinyl toluene resin of Example 1 and the PET bottle of Example 2 was measured with a Hitachi spectrofluorometer (F-2700 type). Further, a scattering peak removal filter (manufactured by Hitachi High-Technologies) was inserted into the Hitachi spectrofluorometer to remove the influence of the scattering peak due to the resin itself. Light having a wavelength of 330 nm was incident on the recycled plastic bottle, and fluorescence emitted from the recycled plastic bottle was measured. In FIG. 6, the emission spectrum of the fluorescence emitted from the recycled plastic bottle of Example 2 is indicated by a broken line, the emission spectrum from 100% purity polyvinyltoluene used in Example 1 is indicated by a dotted line, and a conventional plastic scintillator ( The emission spectrum from BC-408) is shown by a one-dot chain line.
Here, the fluorescence peaks 330 nm and 380 nm of the polyvinyltoluene resin and the PET bottle regenerated resin and the peak of the sensitive wavelength of the photomultiplier tube indicated by the line are almost matched. That is, it was shown that the polyvinyl toluene resin and the PET bottle regenerated resin emit light having the highest sensitivity of the photomultiplier tube and are extremely suitable for radiation measurement.

The wavelength range of the fluorescence emission spectrum emitted from the polyvinyl toluene resin and the PET bottle regenerated resin is a conventional plastic scintillator, for example, plastic scintillators manufactured by Saint-Gobain crystals (model numbers BC-418, BC-420, etc.) (355 nm to Emission peak at 500 nm, see product data sheet of Saint-Gobain crystals, URL: http: //www.detectors. 422-Data-Sheet.pdf).
From the above, the fluorescence emitted from the polyvinyltoluene resin and the plastic bottle recycling resin emits light in the same wavelength region as the conventional plastic scintillator, and the polyvinyl toluene resin and the plastic bottle recycling resin are the same as the conventional plastic scintillator. It was shown that even if a conventional photomultiplier tube that cannot detect light in a short wavelength region of less than 300 nm can be used as a radiation detection element.

Claims (6)

The main component is PET bottle recycled resin as the main resin .
Included in the isolated state in the matrix resin and in the state bonded to the matrix resin, the phosphor that converts the fluorescence emitted by the matrix resin excited by radiation irradiation into light having a longer wavelength than this. There is no element for radiation detection characterized by not having. The radiation detection element according to claim 1, further comprising polystyrene, polyvinyl toluene, polyethylene terephthalate which is not a recycled material, and a copolymer or a mixture thereof. A radiation detection element comprising only a plastic bottle recycle resin. Radiation detection element according to any one of claims 1 to 3, the sensitive wavelength peak is equal to or is for photomultiplier tubes in 300 nm to 500 nm. The radiation detection element according to any one of claims 1 to 4,
A radiation detector comprising: a detection unit that detects fluorescence emitted by the radiation detection element. A radiation detection method for detecting radiation using the radiation detector according to claim 5,
Radiation is incident on the radiation detection element, and fluorescence is emitted by the interaction between the radiation and the plastic bottle regeneration resin,
The detected fluorescence is detected by the detection unit.
The radiation detection method characterized by the above-mentioned.

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