JPWO2015146424A1 - Radiation detection material - Google Patents

Abstract

A radiation detection material that can be used in a practical radiation detector such as PET is proposed with respect to a radiation detection material containing a single crystal that can be represented by Tl4 + 2xSxI4. A radiation detection material containing a single crystal that can be represented by the formula: Tl4 + 2xSxI4 (wherein x = 0.77 to 1.10). A radiation detection material characterized by the above is proposed.

Description

  The present invention relates to a radiation detection material capable of detecting radiation such as alpha rays, beta rays, and gamma rays. In particular, the present invention relates to a direct conversion type radiation detection material that can absorb radiation, particularly gamma rays, and convert it directly into an electrical signal.

As a radiation detector, a direct conversion system that converts radiation directly into electric charge and stores the charge, and indirect conversion that converts the radiation into light once with a phosphor, and converts the light into electric charge and stores it with a photoconductive layer. There is a method.
Since the indirect conversion type radiation detection apparatus has problems such as difficulty in miniaturization, in recent years, the direct conversion type radiation detection apparatus has attracted attention.

For example, CdTe and CdZnTe are known as materials that can absorb radiation and convert it directly into an electrical signal. Further, TlBr has been studied as a gamma ray detection material.
More recently, it has been disclosed that Tl ₆ SeI ₄ can directly convert X-rays into electrical signals (Patent Documents 1 and 2), and Tl ₆ SI ₄ has also been reported to be able to directly convert X-rays into electrical signals. (Non-Patent Document 1).

US8519347B2 WO20112021519

Sandy L. Nguyen.``Photoconductivity in TI6SI4: A Novel Semiconductor for Hard Radiation Detection ''. CHEMISTRY OF MATERIALS, 2013.25, P2868-2877

The material that can be represented by Tl _{4 + 2x} S _x I ₄ has a higher density than CdTe and CdZnTe, so it can not only increase the detection sensitivity of radiation, but also has a large band cap and high specific resistance. Since the energy resolution can be increased and a high-resolution image can be obtained, it is one of the remarkable radiation detection materials.

However, Tl _{4 + 2x} S _x I ₄ that has been disclosed in the past is not limited to the numerical value of specific resistance, but PET (positron emission tomography) or SPECT (single photon emission computed tomography) It was not practical for use in radiation detectors such as radiation tomography)).

The present invention relates to a radiation detecting material containing a single crystal body which can be represented by _{Tl 4 + 2x S x I 4} , it can be sufficiently used in a practical radiation detector, such as a PET, a new radiation It is intended to propose a detection material.

The present invention relates to a radiation detection material containing a single crystal that can be represented by the formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.77 to 1.10). A radiation detection material having a specific resistance of 1 × 10 ¹¹ Ω · cm or more is proposed.

  The radiation detection material proposed by the present invention not only absorbs radiation, particularly gamma rays, and can be directly converted into an electric signal, but also has a sufficiently high specific resistance, so that it can be used sufficiently for practical radiation detectors such as PET. Can do.

3 is an X-ray diffraction profile of a single crystal produced in Example 3. FIG. 4 is a transmission image (photograph) of a detection element produced using the single crystal produced in Example 3. FIG. It is a wave height distribution spectrum before and after irradiating the single crystal produced in Example 3 with gamma rays from a ¹³⁷ Cs ray source. Similarly, it is a wave height distribution spectrum when the single crystal body of Example 3 is irradiated with gamma rays from a ¹⁰⁹ Cd ray source. Similarly, it is a wave height distribution spectrum when the gamma ray from a ²⁴¹ Am ray source is irradiated to the single crystal body of Example 3. FIG. It is the IV curve of the single crystal body of Example 3 similarly. 5 is an IV curve of a single crystal produced in Comparative Example 3. For a single crystal body produced in Examples 1 to 4 and Comparative Examples 1 to 3, wherein: the x value of _{Tl 4 + 2x S x I 4} , a diagram plotting the relationship between the specific resistance.

  Next, the present invention will be described based on an embodiment. However, the present invention is not limited to the embodiment described below.

<Radiation detection material of the present invention>
The radiation detection material according to the present invention (hereinafter referred to as “the present radiation detection material”) can be represented by the formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.77 to 1.10). A radiation detection material containing a single crystal (referred to as “the present single crystal”).

The density of the single crystal that can be represented by Tl _{4 + 2x} S _x I ₄ (wherein x = 0.77 to 1.10) is 7.25 g / cm ³ , and CdTe (6.2 g / cm ³ ) And CdZnTe (6.0 g / cm ³ ), the radiation detection sensitivity can be increased. In addition, since the probability of causing a photoelectric effect important for detection of gamma γ rays is proportional to the fifth power of the atomic number Z of the substance, Tl _{4 + 2x Sx} I ₄ (including Tl with a large Z (Z = 81)) In the formula, a single crystal that can be represented by x = 0.77 to 1.10) has higher radiation detection sensitivity than CdTe and CdZnTe containing Cd (Z = 48) and Te (Z = 52). can do.
Furthermore, since the band cap is large and the specific resistance is high, the energy resolution can be increased and a high-resolution image can be obtained.
In addition, a single crystal that can be represented by Tl _{4 + 2x} S _x I ₄ (wherein x = 0.77 to 1.10) has an appropriate hardness in device formation and does not have deliquescence. Therefore, it is excellent in that there is no problem of workability.
Furthermore, the melting point of the single crystal that can be represented by Tl _{4 + 2x} S _x I ₄ (wherein x = 0.77 to 1.10) is as low as 440 ° C. at the maximum, and CdTe (melting point 819 ° C.) Compared with a high melting point material such as CdZnTe (melting point: 1092 to 1295 ° C.), the power cost at the time of crystal production is low and the productivity is excellent.

(Composition of this single crystal)
In the composition formula Tl _{4 + 2x} S _x I ₄ of this single crystal, it is important that x is 0.77 to 1.10, and it is particularly preferably 0.83 or more or 1.07 or less.

  From the viewpoint of increasing the specific resistance, it is important that the composition of this single crystal is within a predetermined range from x = 1.0 which is a stoichiometric composition, that is, x = 0.77 to 1.10. It is. However, it is not easy to control x = 0.77 to 1.10. As will be described later, in zone melt refining (zone refining), TlI (thallium iodide) tends to become rich by TlI (thallium iodide) evaporation and remixed, so special measures as described later are required. It is. For example, in zone melting and refining, the inside of the ampoule is made an inert gas atmosphere, and the heating temperature is made as low as possible, specifically 440 to 450 ° C., while suppressing evaporation of TlI (thallium iodide) After performing the purification at least 50 times or more, it is necessary to devise such that only the high-purity tip is taken out and placed in another ampoule and further purified 50 times or more.

(Resistivity)
The radiation detection material of the present invention preferably has a specific resistance of 1 × 10 ¹¹ Ω · cm or more, more preferably 1 × 10 ¹² Ω · cm or more, and more preferably 1 × 10 ¹³ Ω · cm or more. Preferably there is.
Since the specific resistance of Tl ₆ SI ₄ that has been conventionally disclosed was about 5.7 × 10 ⁹ Ω · cm to 2.6 × 10 ¹⁰ Ω · cm, the specific resistance of the radiation detection material of the present invention is Remarkably high. As described above, the reason why the specific resistance of the radiation detection material of the present invention is high is that the composition of the single crystal is controlled within a predetermined range, that is, x = 0.77 to 1.10, and the impurities are extremely small. It can be inferred that there is.

(purity)
The single crystal body preferably has a purity of 4N or higher, more preferably 6N or higher, and particularly preferably 8N or higher.
As described above, the composition of the present single crystal body is controlled within a predetermined range, that is, x = 0.77 to 1.10. The resistance can be significantly increased.

  In order to increase the purity of the single crystal as described above and reduce the impurity concentration, for example, as described later, in the zone melt purification, the ampoule is made an inert gas atmosphere and the heating temperature is set as low as possible. Specifically, by suppressing the evaporation of thallium iodide at a temperature of 440 to 450 ° C., after performing the number of purifications at least 50 times or more, only the high-purity tip is taken out and put in another ampule again, It is preferable to carry out purification 50 times or more. However, it is not limited to this method.

<Characteristics of the radiation detection material of the present invention>
The radiation detection material of the present invention is a direct conversion type radiation detection material capable of absorbing radiation, particularly gamma rays, and converting it directly into an electrical signal.
Examples of radiation include electromagnetic radiation such as gamma rays and X-rays, and particle radiation such as alpha rays, beta rays, electron beams, proton beams, neutron beams, and heavy particle beams.

This single crystal, that is, a single crystal that can be represented by Tl _{4 + 2x} S _x I ₄ (wherein x = 0.77 to 1.10) has a high density and is used as a material for detecting radiation, particularly gamma rays. Detection sensitivity can be increased.

When a bias voltage is applied to a detection element fabricated using this single crystal, charge is generated by the interaction that occurs when gamma rays from a ¹³⁷ Cs radiation source enter the detection element, and the response of gamma rays as a current peak is generated. Can be confirmed.
In addition, when a bias voltage is applied to a detection element manufactured using this single crystal body, signals with respect to gamma rays of ¹⁰⁹ Cd ray source and ²⁴¹ Am ray source can be confirmed, and a photoelectric peak can be measured.
Therefore, the radiation detection material of the present invention can be effectively used as a radiation detection material for a radiation medical apparatus such as PET or SPECT.

<Manufacturing method>
As an example of the method for producing the single crystal, for example, a predetermined amount of TlI (thallium iodide) powder and a predetermined amount of Tl ₂ S powder are mixed, and the mixture is sealed in a glass tube and heated to Tl. A method of synthesizing a —SI compound, subjecting the resultant compound to a predetermined purification, then crystal growth to obtain a single crystal, and polishing as necessary to obtain the present single crystal Can be mentioned.

As raw materials, single-phase TlI (thallium iodide) and single-phase Tl ₂ S are preferably manufactured or purchased and prepared, and these are preferably used as raw materials.
At this time, if either TlI (thallium iodide) or Tl ₂ S has a heterogeneous phase, there is a high possibility that the produced single crystal will also have a heterogeneous phase. Like a material, it is difficult to make the specific resistance 1 × 10 ¹¹ Ω · cm or more.

  The atmosphere in the sealed tube when synthesizing the Tl-SI compound is preferably an inert gas atmosphere such as argon, and the heating temperature at that time is preferably 500 to 700 ° C, and more preferably 550 ° C or more or 650 ° C or less. More preferably. The heating time is preferably several minutes or longer, preferably 6 hours or longer, and appropriately adjusted depending on the heating temperature.

The purification method is extremely important in producing this single crystal.
For example, the Tl-S-I compound synthesized as described above is put in an ampule, sealed as an inert atmosphere such as argon, and the ampule is heated from the surroundings with a moving heater (zone melt purification). Is preferably performed repeatedly.
At this time, after performing the purification at least 50 times while suppressing evaporation of TlI (thallium iodide) by setting the temperature during the zone purification as low as possible, specifically 440 to 450 ° C., It is preferable to take out only the high-purity tip and place it again in another ampoule, and further purify it 50 times or more.
Thereby, evaporation of TlI (thallium iodide) can be suppressed, and furthermore, evaporation of TlI (thallium iodide) can be suppressed from being mixed again, so that it is 1 × 10 ¹¹ Ω · cm or more. A high-resistance single crystal can be obtained.

  The crystal growth method is arbitrary as long as it can grow a single crystal. For example, the Czochralski method (CZ method), the slow cooling method, the horizontal Bridgman method (HB method), the vertical Bridgman method (VB method), the traveling heater method (TH method) and the like can be mentioned.

  The polishing method is also arbitrary. For example, polishing with abrasive paper or wet polishing may be employed as appropriate.

<Explanation of words>
In the present specification, when expressed as “X to Y” (X and Y are arbitrary numbers), unless otherwise specified, “X is preferably greater than X” or “preferably Y”. It also includes the meaning of “smaller”.
In addition, when expressed as “X or more” (X is an arbitrary number) or “Y or less” (Y is an arbitrary number), it is “preferably greater than X” or “preferably less than Y”. Includes intentions.

  Hereinafter, the present invention will be further described in detail based on the following examples and comparative examples.

<Production of single crystal>
The amounts of single-phase TlI (thallium iodide) (4N) and single-phase Tl ₂ S (4N) were mixed for each of the Examples and Comparative Examples, and the mixture was sealed in a glass tube (Argon 0.5 atm). ) And heated at 600 ° C. for 6 hours to synthesize a Tl-SI compound. The obtained compound was subjected to repeated zone purification at 440 ° C. 50 times, then only the high-purity tip was taken out and sealed in a glass ampoule (Argon 0.5 atm), and the zone purification was repeated again at 440 ° C. The purified product was obtained 50 times.
The purified product thus obtained was crystal-grown by a traveling heater method (TH method) at a heating temperature of 440 ° C. and a growth rate of 5 mm / hour to obtain a single crystal, which was obtained using a 0.3 μm alumina abrasive. A single crystal was obtained by buffing.

<Composition measurement>
The single crystal was weighed as described above, dissolved in a mixture of hydrochloric acid and nitric acid, diluted as appropriate, and then Tl, S, and I in the liquid were quantified using an ICP emission spectrometer. The Tl, S, and I composition (% by weight) of the single crystal was calculated from the weight and dilution rate of the single crystal. From the respective Tl, S, and I composition values, _{x in the} formula: Tl _{4 + 2x} S _x I ₄ was calculated and shown in Table 1.

<XRD measurement>
The single crystal obtained as described above was pulverized in an agate mortar, and powder X-ray diffraction measurement (XRD measurement) was performed. As a measuring apparatus, a sample horizontal strong X-ray diffractometer RINT-TTR III (manufactured by Rigaku Corporation) was used. CuKα rays were used for the measurement, the acceleration voltage was 50 kV, and the applied current was 300 mA. An X-ray profile of the single crystal produced in Example 3 is shown in FIG.
As a result of XRD measurement, it was found that all the single crystals produced in Examples 1 to 4 and Comparative Examples 1 to 3 were single crystals made of single-phase Tl ₆ SI ₄ .

<Preparation of detection element>
As described above, the single crystal was buffed using a 0.3 μm alumina abrasive to obtain a flat single crystal having a thickness of 0.3 to 1.5 mm. This was ultrasonically washed in acetone, then dried at room temperature, and set in a vacuum deposition machine SVC-700 (manufactured by Sanyu Electronics Co., Ltd.). A gold wire is set on a tungsten electrode, vacuumed to 5 × 10 ⁻³ Pa, heated at a current of about 35 mA to deposit gold on the single crystal, and an electrode having a thickness of about 100 nm and a diameter of 3 mm is formed. The detection element was obtained.
FIG. 2 shows a transmission image (photograph) of the detection element produced using the single crystal produced in Example 3.

<Radiation evaluation: gamma-ray response measurement>
The detection element was placed in an Al guard box, and the gold electrode and the external circuit were connected by a spring contact. The output of the detection element was connected to a charge sensitive preamplifier 581K type (manufactured by Clear Pulse Co., Ltd.), and the bias voltage was applied by a bias power source 6661P type (manufactured by Clear Pulse Co., Ltd.). The output from the preamplifier is amplified by a spectroscopic amplifier 4417 type (manufactured by Clear Pulse Co., Ltd.), and the current peak (signal) generated by the interaction with gamma rays using a digital phosphor oscilloscope TDS5052B (manufactured by Tektronix) is used. Observed. For measurement of the pulse height distribution spectrum, the output of the spectroscopic amplifier was processed by an analog / digital converter 1125 type (manufactured by Clear Pulse Co., Ltd.), and a signal for 600 seconds was recorded by a PC. As the gamma ray source, 1 MBq of ¹³⁷ Cs, ¹⁰⁹ Cd, and ²⁴¹ Am was used, and the gamma ray source was installed at a distance of 5 mm from the detection element.

The gamma-ray response was evaluated according to the following criteria, and the evaluation results are shown in Table 1.
○ (good): A signal could be detected with gamma rays from a ¹³⁷ Cs radiation source, and photoelectric peaks of gamma rays from a ¹⁰⁹ Cd radiation source and ²⁴¹ Am radiation source could be confirmed.
Δ (fair): Signals could be detected with gamma rays from a ¹³⁷ Cs source, but no photopeaks of gamma rays from a ¹⁰⁹ Cd source and ²⁴¹ Am source could be confirmed.
X (poor): No signal was detected with gamma rays from a ¹³⁷ Cs source.

<Specific resistance measurement>
The detection element was placed in an Al guard box, and the gold electrode and the external circuit were connected by a spring contact. The output of the detection element was connected to a digital electrometer R8252 (manufactured by ADC Corporation) with a BNC coaxial cable, and IV measurement was performed. The resistance R (Ω) of the detection element was determined from the slope of the IV measurement. The specific resistance ρ (Ω · cm) of the single crystal was calculated from the following equation. However, S is an area (cm < ² >) of an electrode, d is the thickness (cm) of a detection element.
ρ = R · S / d
FIG. 6 shows the IV curve of Example 3 and FIG. 7 shows the comparative example 3.
Table 1 shows the results of x, specific resistance, and gamma ray measurement for Examples 1 to 4 and Comparative Examples 1 to 3.

(Discussion)
As for the specific resistance of the example and comparative example, the specific resistance of the detection element using the example 3 is as high as 1.1 × 10 ¹³ Ω · cm, so the noise (dark current) at the time of gamma ray detection is low. I was able to suppress it. Similarly, in other examples, noise (dark current) at the time of gamma ray detection could be suppressed to a low level.
On the other hand, since the resistance of the detection element using Comparative Example 3 is as low as 7.7 × 10 ⁵ Ω · cm, noise (dark current) at the time of gamma ray detection has increased. Similarly, the noise (dark current) at the time of gamma ray detection was large for the other comparative examples.

When a bias voltage of 800 V was applied to the detection element of Example 3 and a ¹³⁷ Cs radiation source was brought closer, a signal could be confirmed. The wave height distribution spectrum of Example 3 before and after gamma ray irradiation is shown in FIG. ¹³⁷ Cs gamma rays could be detected because the wave height distribution spectrum clearly differed with and without gamma ray irradiation. In addition, FIG. 4 and FIG. 5 show the wave height distribution spectra of Example 3 when gamma rays from a ¹⁰⁹ Cd, ²⁴¹ Am radiation source are irradiated under the same conditions.
^The photoelectric peaks due to gamma rays with ¹⁰⁹ Cd of 22 keV, 88 keV, and ²⁴¹ Am of 59.5 keV can be clearly distinguished. From this, it is considered that Example 3 has an ability to measure gamma dose by discriminating gamma ray energy.

When a bias voltage of 100 V was applied to the detection element of Example 1 and a ¹³⁷ Cs radiation source was brought closer, a signal was confirmed, and it was confirmed that a response to gamma rays was possible. However, in the wave height distribution spectrum when gamma rays from a ¹⁰⁹ Cd, ²⁴¹ Am radiation source were irradiated, the photoelectric peak could not be confirmed, so the ability as a gamma ray detection element was inferior to that of Example 3. This is considered to be because x obtained from the composition analysis result is small and the specific resistance is low.
On the other hand, in the case of the detection element using a single crystal having a low specific resistance as in Comparative Examples 1 to 3, no signal could be confirmed even when a ¹³⁷ Cs radiation source was brought close to the detection element. This is presumably because current due to interaction with gamma rays does not flow or the signal is weak and buried in noise.

In Examples 2, 3, and 4 having a large specific resistance, ¹³⁷ Cs gamma rays could be detected, and photoelectric peaks of ¹⁰⁹ Cd and ²⁴¹ Am ray sources were observed. In Example 1, gamma rays of ¹³⁷ Cs could be detected, but no photoelectric peak of gamma rays from ¹⁰⁹ Cd, ²⁴¹ Am source was observed.
On the other hand, since Comparative Examples 1 to 3 have low specific resistance, ¹³⁷ Cs gamma rays could not be detected.

FIG. 8 plots the relationship between the x value of the formula: Tl _{4 + 2x Sx} I ₄ and the specific resistance for Examples 1 to 4 and Comparative Examples 1 to 3.
From this figure, in order to detect gamma rays, it is necessary that the specific resistance of the radiation detection material is 10 ¹¹ Ω · cm or more, and x expressed in the Tl _{4 + 2x} S _x I ₄ equation at that time. Was found to be 0.77 to 1.10. Desirably, the specific resistance of the single crystal is 10 ¹² Ω · cm or more, and the range of x at that time is considered to be 0.83 or more or 1.07 or less.

Based on the above examples and the results of tests conducted by the inventors so far, a single crystal body represented by the formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.77 to 1.10) is obtained. The radiation detection material contained, in particular, the radiation detection material having a specific resistance of the single crystal of 1 × 10 ¹¹ Ω · cm or more can confirm ON / OFF of the ³⁷ Cs source with respect to gamma rays. In addition, the response signal of ¹⁰⁹ Cd, ²⁴¹ Am radiation source to gamma rays could be confirmed, and the photopeak could be measured. Therefore, it turned out that it can fully be used for practical radiation detectors, such as PET.

Claims (4)

A radiation detection material containing a single crystal that can be represented by the formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.77 to 1.10). 2. The radiation detection material according to claim 1, wherein the specific resistance of the single crystal body is 1 × 10 ¹¹ Ω · cm or more. The radiation detection material according to claim 1, wherein the single crystal is a single crystal manufactured using single-phase TlI (thallium iodide) and single-phase Tl ₂ S as raw materials.   The radiation detection material according to claim 1, which is a gamma ray detection material.