JPH08295878A - Scintillator and scintillator for radiation detector and radiation detector - Google Patents

Abstract

PURPOSE: To obtain a scintillator (for a radiation detector) comprising a single crystal containing secium iodide as a main constituent and thallium, etc., as additives and capable of improving detective sensitivity for low energy radiations. CONSTITUTION: A scintillator comprises a single crystal containing cesium iodide as a main constituent and thallium and indium as additives. The scintillator for the radiation detector preferably contains the thallium in an amount of 0.10-1.0mol.% and the indium in an amount of 0.0050mmol.% to 0.30mol.%. A radiation detector preferably has the scintillator and a photoelectromotive force element provided with a light-receiving sensitivity characteristic wherein the strength of an outputted electric signal is changed in dependence to the wavelength of received light, and detects a radiation collided with the scintillator on the basis of an electric signal from the photoelectromotive force element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a luminescent substance (scintillator) used for detecting radiation and a radiation detector using this scintillator.

[0002]

2. Description of the Related Art Conventionally, in this type of radiation detector, there is an apparatus (hereinafter referred to as a first conventional apparatus) in which a scintillator and a light receiving element are combined. In this first conventional device,
For example, sodium iodide (NaI) as a main constituent component and a small amount of thallium (Tl) as an additive component are optically crystallized, that is, the amount of light emission per unit radiation dose is improved by single crystallizing them. A single crystal of thallium-activated sodium iodide [NaI (Tl)] was used as a scintillator, and a photomultiplier tube was used as a light receiving element. This scintillator generates a predetermined amount of light according to the amount of radiation and energy when the radiation collides, and a photomultiplier tube (photomultiplier tube, photomultiplier tube,
(Hereinafter referred to as PMT) receives light from the scintillator and generates an electric signal according to the amount of received light. Then, the radiation is detected based on the generated electric signal, and the amount and energy of the radiation are measured.

In such a first conventional device, the device becomes large in size, affected by the surrounding magnetic field, and it is necessary to apply a high voltage (400 to 1000 V) to the PMT, or There was a problem such as poor conversion efficiency to an electric signal. In order to improve such a problem, cesium iodide (CsI) is used as a main constituent component and a small amount of thallium (Tl) is used as an additive component. A device (hereinafter referred to as a second conventional device) in which a cesium oxide [CsI (Tl)] single crystal is used as a scintillator and a large-area photovoltaic element is used as a light receiving element has been proposed.

This second conventional device, as shown in FIG. 7, has a silicon photodiode having a spectral sensitivity characteristic indicated by a symbol C (dotted line) and an emission characteristic (wavelength-relative) indicated by a symbol B (dotted line). CsI (T with strength characteristics)
l) A scintillator. That is, the silicon photodiode has a wavelength of 400 nm and is 1 W
Light of 155 mA is output, the wavelength of 5
When 1 W of light is input at 00 nm, it outputs a current of 265 mA.

Further, as described above, the CsI (Tl) scintillator emits light having a central wavelength of 550 nm due to collision of radiation. Therefore, when the silicon photodiode and the CsI (Tl) scintillator are combined, the silicon photodiode is 310 when the CsI (Tl) scintillator emits 1 W of light.
It outputs a current of mA. When this silicon photodiode receives light of about 750 nm, its conversion efficiency, that is, detection sensitivity is maximized.

By the way, as the photovoltaic element having such characteristics, there are the types shown in Table 1 including the above-mentioned silicon photodiode. It can be seen that in the exemplified photovoltaic element, the maximum point of spectral sensitivity (corresponding to the center wavelength of the wavelength range) is generally on the longer wavelength side than the emission center wavelength of the CsI (Tl) scintillator.

In the exemplified photovoltaic element, the wavelength range, that is, the light receiving characteristic does not match the emission wavelength of the CsI (Tl) scintillator, the temperature characteristic is bad, or the frequency characteristic is bad and the scintillator emits light. He had various problems that he could not. Among them, the silicon photodiode (shown as Si pin PD in Table 1) is easy to obtain high sensitivity, that is, has a high detection rate,
It has advantages such as being less susceptible to the influence of a magnetic field, being applicable to a battery drive system without requiring a high voltage, and being inexpensive, and thus it is preferably used for such a radiation detector.

In this silicon photodiode, the wavelength at which the maximum detection sensitivity is obtained is 7 as described above.
It is around 50 nm and is on the long wavelength side of 200 nm with respect to the emission center wavelength of 550 nm of the CsI (Tl) scintillator.

[0009]

[Table 1] In the above table, the unit of detection rate is [10 ¹⁰ _cm · Hz ^1/2 / W] f is the abbreviation of frequency characteristic PD is the abbreviation of photodiode

[0010]

Such a second conventional device can be miniaturized by combining a silicon photodiode and a CsI (Tl) scintillator, and is not affected by an ambient magnetic field. In addition, it has features such as good conversion efficiency of radiation into electric signals.

However, when the silicon photodiode and the CsI (Tl) scintillator are combined, light having a wavelength of 550 nm is emitted from the CsI (Tl) scintillator as described above. Since this light has a wavelength much shorter than the wavelength (about 750 nm) at which the maximum detection sensitivity can be obtained from the silicon photodiode, the output peak value from the silicon photodiode is relatively low and the output electrical SN of signal
The ratio (signal noise ratio) becomes worse, for example, 1
There is a problem that it is difficult to obtain effective detection sensitivity for low-energy radiation (γ-rays) of 50 keV or less.

The present invention has been made in view of such circumstances, and a light-receiving element (silicon photodiode) obtained by light emission of a scintillator does not change when radiation is incident on the light-emitting element. An object of the present invention is to provide a scintillator and a radiation detector that can obtain effective detection sensitivity even for low energy radiation by increasing the output signal peak value.

[0013]

In order to solve this problem, a scintillator and a scintillator for a radiation detector made according to the present invention consist of a single crystal containing cesium iodide as a main constituent component and thallium and indium as an additive component. It is characterized by that.

The radiation detector scintillator contains thallium in an amount of 0.10 mol% to 1.0 mol%,
0.0050 mmol% to 0.30 molybdenum
It is characterized by containing 1%.

The radiation detector scintillator contains thallium in an amount of 0.10 mol% to 1.0 mol%,
0.010 mmol% to 0.10 mol of indium
It is characterized by containing%.

Further, the radiation detector made according to the present invention has a scintillator and a photovoltaic element having a photosensitivity characteristic in which the intensity of an output electric signal changes depending on the wavelength of the received light. In the radiation detector that detects the radiation that has collided with the scintillator based on the electric signal from the photovoltaic element, the scintillator is the scintillator for the radiation detector described above.

Further, a radiation detector scintillator configured as a single crystal and a photovoltaic element having a photosensitivity characteristic in which the intensity of an output electric signal changes depending on the wavelength of received light are provided. In a radiation detector that detects radiation that has collided with the scintillator based on an electrical signal from an electromotive force element, the radiation detector scintillator has cesium iodide as a main constituent component, and thallium and a wavelength variable substance as an additive component, The wavelength variable substance is characterized in that the wavelength of the light emitted from the scintillator is shifted to the high sensitivity side of the photovoltaic element.

[0018]

When a scintillator is produced by adding indium (In) as a wavelength tunable substance to a single crystal to prepare a scintillator, to a system containing cesium iodide (CsI) as a main constituent and thallium (Tl) as an additive. Emits light shifted to the long wavelength side. The reason why the light emission of this scintillator shifts to the long wavelength side is that the conventional cesium iodide-thallium system [CsI (Tl)] is changed to cesium iodide-thallium-indium system [CsI (Tl + In)], It is considered that the energy level changes to a level lower than that of the conventional system, and this change in the energy level shifts the wavelength of light to the long wavelength side. And, in general, the photovoltaic element is CsI (T
l) Since it has a sensitivity-wavelength characteristic that the detection sensitivity is good in a wavelength range longer than the wavelength of light emitted by the scintillator, the photovoltaic element can be made more efficient by shifting the wavelength emitted by the scintillator to the long wavelength side. It outputs a high output electrical signal. Therefore, the SN ratio of the radiation detector can be improved, and effective detection sensitivity can be obtained even for low-energy radiation.

When this CsI (Tl + In) scintillator is used as a scintillator for a radiation detector, Tl is added at 0.10 mol% to 1.0 mol% and In is added at 0.0050 mmol% to 0.30 mol.
%, The detection sensitivity, that is, the SN ratio, is better than that of the prior art, and Tl is 0.10 mol% to 1.0.
mol%, In 0.010 mmol% to 0.10 m
By adding ol%, effective detection sensitivity can be obtained even for low energy radiation.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The scintillator and radiation detector of the present invention will be described below with reference to the drawings. First, a method of creating a scintillator will be described. First, each constituent substance, that is, cesium iodide (CsI), thallium iodide (TlI) and indium iodide (InI ₃ ) is weighed in a predetermined amount, and each weighed substance is pulverized and mixed in an agate mortar or ball mill. In this example, in order to confirm the effect of indium addition, the ratio of TlI was fixed at 0.10 mol% and the ratio of InI ₃ was 0.0050 mmol% to 0.3.
A plurality of samples varied in the range of 0 mol% were prepared. In addition, in order to confirm the effect of the addition of thallium, in addition to the above sample, in the system consisting of two constituents of CsI and TlI, the addition ratio of TlI was 1.0 mmol.
% (0.0010 mol%) to 10 mol%, and a plurality of samples that were varied in the range were prepared.

Then, the crushed and mixed samples (constituent substances) are transferred to a quartz crucible, and the lower end of the quartz crucible is melted by a heating means such as a high frequency coil. Then, the fused portion is gradually moved upward by lowering the quartz crucible at a slow speed to crystallize from the lower portion of the quartz crucible. Note that this crystallization can also be performed by using a so-called pulling method in which a seed crystal is brought into contact with a liquid obtained by melting each sample and the seed crystal is pulled while rotating to generate a cylindrical crystal. Then, the crystal thus produced is cut using a microtome or a string saw to obtain a CsI (Tl + In) scintillator.

Next, the CsI (T
The characteristics of the (l + In) scintillator will be described. This CsI (Tl + In) scintillator has a wavelength of 590 nm as indicated by the solid line A in the spectral characteristic diagram of FIG.
It emits light in a band extending from 430 nm to 780 nm centered on. The CsI (Tl +
In) scintillator has an In addition ratio of 0.10
It is mol%. The addition ratio of In will be described later. The light emitted by the CsI (Tl + In) scintillator is compared with the light emitted by the conventional CsI (Tl) scintillator indicated by the one-dot chain line B in the figure, that is, the light having a wavelength of 550 nm as the emission center. About 40n from the light from CsI (Tl) scintillator
The light is shifted to the long wavelength side.

In this way, the center wavelength of the light emission shifts to the long wavelength side and becomes light having the light emission center at the wavelength of 590 nm, so that the output from the silicon photodiode is the conventional CsI as shown in FIG. It is improved as compared with the case of using the (Tl) scintillator. That is, the light from the conventional CsI (Tl) scintillator indicated by the one-dot chain line B in the figure has a wavelength of its emission center of 550 nm, and the light from the CsI (Tl + In) scintillator indicated by the solid line A in the figure is the same. The wavelength of the emission center is 590 nm.

In the figure, the spectral sensitivities of the photovoltaic element, that is, the silicon photodiode, at these central wavelengths are CsI (Tl + In) scintillator, that is, point a for light having a wavelength of 590 nm, and conventional CsI.
The (Tl) scintillator, that is, light b having a wavelength of 550 nm is point b. These points a and b are, as described above,
The current value output from the photovoltaic element (silicon photodiode) when 1 W of light of that wavelength is input is shown. Therefore, when the conventional CsI (Tl) scintillator emits 1 W of light, the silicon photodiode outputs a current of 310 mA, and CsI (Tl) scintillator
+ In) 3 if the scintillator emits 1 W of light
It outputs a current of 50 mA.

In short, the CsI (Tl + In) scintillator emits light shifted to the longer wavelength side than the conventional CsI (Tl) scintillator. If the same amount of light is emitted from both scintillators, the silicon photodiode outputs more current when it receives the light from the CsI (Tl + In) scintillator. As described above, the CsI (Tl + In) scintillator has a conventional C
When compared with the sI (Tl) scintillator, it emits light suitable for its characteristics to a photovoltaic element whose maximum spectral sensitivity is on the longer wavelength side than the emission center wavelength of the scintillator, such as a silicon photodiode. To do.

Next, the relationship between the added amount of In and the relative intensity of the light emitted from the scintillator will be described. As described above, in this example, a plurality of samples in which the addition ratio of In was varied from 0.0050 mmol% to 0.30 mol% were prepared, and the characteristics of each sample were measured. FIG. 2 is a view showing the spectral characteristics of each sample, and is an enlarged view of the main part.

The addition ratio of In is 0.0050 mmol%
In this case, as indicated by the chain double-dashed line A2 in the figure, the first peak centered at a wavelength of 550 nm and the second peak centered at a wavelength of 590 nm (reference A21).
(Shown with) and exhibit a spectral characteristic. That is, when compared with the spectral characteristics of the conventional CsI (Tl) scintillator indicated by the one-dot chain line of symbol B, In is 0.0050 mmol.
With the addition, a second peak indicated by reference sign A21 appears. In the scintillator containing 0.0050 mol% of In, the wavelength of emitted light is 59
Since the proportion of 0 nm light is increased by the amount of the second peak A21, when this scintillator and the above-mentioned photovoltaic element are combined, better characteristics than before can be obtained.

Further, the addition ratio of In is 0.010 mmo.
When it is set to 1% to 0.10 mol%, as shown by the solid line of reference numeral A1, the peak centered at the wavelength of 550 nm disappears, and the spectral characteristics of the peak centered at the wavelength of 590 nm are exhibited. The characteristics at the peak centered on the wavelength of 590 nm are as described in FIG. And
In addition ratio is 0.0050 mmol% to 0.01
Up to 0 mmol%, the first peak at the wavelength of 550 nm decreases and the second peak at the wavelength of 590 nm increases as the proportion of In added increases. When the proportion of In added is further increased, the central wavelength of the peak becomes 5
The relative intensity decreases due to the self-absorption of the scintillator at 90 nm, and the addition ratio of In is 0.30 mol%, for example.
Then, the spectral characteristic indicated by the dotted line A3 in the figure is exhibited.

Next, the relationship between the addition ratio of In in the scintillator and the output signal of the photovoltaic element will be described with reference to FIG. The figure shows the relative output (vertical axis) of the electric signal output from the photovoltaic element (silicon photodiode) and the In addition ratio (horizontal axis) when a predetermined amount of light is emitted from the CsI (Tl + In) scintillator. Shows the relationship. Further, in the same figure, the symbol X indicates the level of the relative output corresponding to the In addition amount of 0.0 mol%, that is, the conventional CsI (Tl) scintillator.

The output from the photovoltaic element is indicated by the symbol E in FIG.
As shown by the solid line in Fig. 3, the amount of In starts to rise with the addition of a trace amount of In less than 0.010 mmol%, and the addition ratio is 0.010 mm.
At ol%, the maximum value becomes E1. This output value (maximum value) is maintained up to the value E2 of the In addition ratio of 0.10 mol%, and gradually decreases when the In addition ratio exceeds 0.10 mol% and reaches the value E3 of 0.30 mol%. It is almost the same as the conventional one. That is, in this example, the In addition ratio was 0.010 mmol% to 0.10.
The best results were obtained in the mol% range, and better characteristics than in the past were obtained in the In addition ratio up to 0.30 mol%.

Next, the relationship between the amount of Tl added and the relative intensity of the light emitted from the scintillator will be described with reference to FIG. The figure shows that for the CsI (Tl) scintillator, the addition ratio of Tl was 0.0010 mol% (1.0 m
mol%), 0.010 mol%, 0.10 mol% and 1.0 mol%, and 10 mol% of the adjusted spectral characteristics of the five samples, as in FIG. 2 described above. It is the figure which expanded and showed.

When the addition ratio of Tl is set to 0.0010 mol%, as shown by the chain double-dashed line of B4 in the figure, a spectral characteristic of a relatively sharp peak centered at a wavelength of 435 nm is exhibited, and Tl Addition ratio of 0.010
When it is set to mol%, the central wavelength shifts to the long wavelength side, and as shown by the dotted line B3, the wavelength is 510 nm.
It exhibits a broad peak spectral characteristic centered on. T
When the addition ratio of l is further increased to 0.10 mol%, the shift of the central wavelength advances, and as shown by the symbol B1, a spectral characteristic of a relatively sharp peak centered on a wavelength of 550 nm is exhibited. . This addition rate 0.10m
From ol% to 1.0 mol%, no change in properties is observed even if the addition ratio is increased. That is, when the addition ratio of Tl is set to 1.0 mol%, a similar peak spectral characteristic centering on a wavelength of 550 nm is exhibited.

When the addition ratio of Tl is set to 10 mol%, the CsI (Tl) scintillator is colored and cannot be measured. Further investigation of this coloring phenomenon revealed that the addition ratio of Tl was 1.0 m.
It was found that this coloring phenomenon occurs at the time when it exceeds ol%. From the above, the addition ratio of Tl is 0.10 mol%
It can be seen that it is desirable to set the content to ˜1.0 mol%.
Then, by adding In to Tl added at this ratio, it becomes possible to shift the emission wavelength to 590 nm.

Next, a radiation detector constructed using the scintillator produced as described above will be described.
This radiation detector has a device configuration shown in FIG. 5A, in which 1 is a detection unit for detecting radiation, 2 is an amplification unit for amplifying a signal from the detection unit 1, and 3 is a signal component. And a noise removing unit for separating noise components, 4 a timer for generating a timing signal at predetermined time intervals, 5 a control unit for controlling the apparatus, 6 a counter for temporarily holding count information,
Reference numeral 7 is a display unit including a display and the like.

As shown in FIG. 5B, the detection section 1 includes a scintillator 11 and a silicon photodiode 1.
2, a reflector 13, a silicon rubber 14, and a package 15. The silicon photodiode 12 is adhered to the bottom surface (right end in the figure) of the scintillator 11 by optical cement. Further, the peripheral surface portion and the upper surface portion of the scintillator 11 are covered with a reflector 13 configured as a white reflecting member such as MgO (magnesium oxide) powder, CaO (calcium oxide) powder, or Teflon tape, whereby the scintillator 11 emits light. The light is guided to the silicon photodiode 12. Further, the scintillator 11, the silicon photodiode 12, and the reflector 13 are housed in a package 15 as a protective member made of a metal such as aluminum or copper or ABS resin, and these members and the package 15 together. The gap is filled with silicon rubber 14 which functions as a shock absorbing member and a moistureproof member.

In such a detector 1, when the scintillator 11 is hit by radiation, the scintillator 11 emits light in accordance with the amount of the hit radiation. Then, this light is guided to the photodiode 12. At the time of this light emission, the light emitted by the reflector 13 covering the scintillator 11 is emitted from the silicon photodiode 1 as described above.
It is structured to be guided by 2. Then, the photodiode 12 generates a current signal according to the amount of received light.

The current signal from the photodiode 12, that is, the detector 1 is converted into a voltage signal by the amplifier 2 and is amplified. The amplifying unit 2 is composed of a charge amplifier acting as a preamplifier and a linear amplifier acting as a main amplifier (neither is shown). Then, the amplification unit 2 outputs the waveform signal indicated by the symbol E1 in FIG. Then, this waveform signal is sent to the noise removing unit 3. The noise removing unit 3 has a comparator 31, a resistor 32, and a variable resistor 33. And the positive input terminal (+ terminal) of the comparator 31
The waveform signal from the amplifier 2 is input to the input terminal, and the comparison voltage divided by the resistor 32 and the variable resistor 33 is input to the negative input terminal (-terminal).

In the noise removing section 3 as described above, FIG.
As shown in (a), the waveform signal of the reference voltage level F or less is removed as a noise component to obtain the rectangular wave signal E2 shown in FIG. 6 (b). Then, this rectangular wave signal E
2 is sent to the counter 6. The control unit 5 includes a CPU 51 as a central control device, a ROM 52 in which an operation program for operating the CPU 51 is stored, and a CP.
It is composed of a RAM 53 that temporarily holds information required when the U 51 operates.

Then, the CPU 51 fetches the count value from the counter 6 based on the timing signal from the timer 4, and sequentially stores the fetched count value in the RAM 53 as radiation information. Then, based on the accumulated radiation information and the timing signal from the timer 4, the radiation dose per unit time, for example, the dose equivalent rate is calculated. Further, the control unit 5 generates a display signal based on the calculated dose equivalent rate and sends it to the display unit 7. Then, the display unit 7 displays a temporal change of the dose equivalent rate as shown in FIG. 6C, for example. In this way, this radiation detector detects radiation dose and the like.

[0040]

As described above, according to the scintillator of the present invention, a system containing cesium iodide (CsI) as a main constituent and thallium (Tl) as an additive is
Since indium (In) as a frequency variable substance is added to form a single crystal, a scintillator and a scintillator for a radiation detector which emit light shifted to a longer wavelength side than conventional can be prepared.

Since the scintillator for radiation detector is combined with a photovoltaic element whose maximum spectral sensitivity is on the longer wavelength side than the emission center wavelength of the scintillator like a silicon photodiode, the radiation detector is constructed. , A radiation detector having higher efficiency, that is, better detection sensitivity than the conventional CsI (Tl) scintillator can be constructed.

In the above radiation detector scintillator, Tl is 0.10 mol% to 1.0 mol%,
Since it is configured to add In in an amount of 0.0050 mmol% to 0.30 mol%, it is possible to configure a radiation detector that can obtain better detection sensitivity than before.

In the radiation detector scintillator, Tl is 0.10 mol% to 1.0 mol%,
Since it is configured to add In in an amount of 0.010 mmol% to 0.10 mol%, it is possible to configure a radiation detector having the best detection sensitivity.

[Brief description of drawings]

FIG. 1 is a diagram comparing spectral characteristics of a scintillator of the present invention and a conventional scintillator.

FIG. 2 is a diagram for explaining changes in spectral characteristics depending on the In addition ratio in the scintillator of the present invention.

FIG. 3 is a diagram illustrating a change in spectral characteristics depending on a Tl addition ratio in a CsI (Tl) scintillator.

FIG. 4 is a diagram illustrating a change in output depending on the In addition ratio when the scintillator of the present invention and a photovoltaic element are combined.

FIG. 5 is a diagram illustrating a configuration of a radiation detector according to the present invention.

FIG. 6 is a diagram illustrating an output signal of the radiation detector according to the present invention.

FIG. 7 is a diagram illustrating the relationship between the emission wavelength and detection sensitivity (spectral sensitivity) when a scintillator and a photovoltaic element are combined.

[Explanation of symbols]

1 Detection Section 11 Scintillator 12 Silicon Photodiode (Photovoltaic Element) 3 Noise Removal Section 5 Control Section 7 Display Section A, A1, A2, A3 CsI (Tl + In) Scintillator Characteristics B, B1, B2, B3, B4 CsI (Tl) ) Scintillator characteristics C Photovoltaic device characteristics

Claims (6)

[Claims] 1. A cesium iodide as a main constituent,
A scintillator comprising a single crystal containing thallium and indium as additive components. 2. A cesium iodide as a main constituent,
A scintillator for a radiation detector, comprising a single crystal containing thallium and indium as additional components. 3. The radiation detector according to claim 2, wherein the scintillator for a radiation detector contains 0.10 mol% to 1.0 mol% thallium and 0.0050 mmol% to 0.30 mol% indium. Dexterous scintillator. 4. The radiation detector according to claim 2, wherein the scintillator for a radiation detector contains 0.10 mol% to 1.0 mol% thallium and 0.010 mmol% to 0.10 mol% indium. Dexterous scintillator. 5. A scintillator for a radiation detector configured as a single crystal, and a photovoltaic element having a photosensitivity characteristic in which the intensity of an output electric signal changes depending on the wavelength of the received light. A radiation detector that detects radiation that has collided with the scintillator based on an electrical signal from an electromotive force element, wherein the radiation detector scintillator is the radiation detector scintillator according to claim 2, 3, or 4. Radiation detector. 6. A scintillator for a radiation detector configured as a single crystal, and a photovoltaic element having a photosensitivity characteristic in which the intensity of an output electric signal changes depending on the wavelength of the received light. In a radiation detector that detects radiation that has collided with the scintillator based on an electrical signal from an electromotive force element, the radiation detector scintillator has cesium iodide as a main constituent component, and thallium and a wavelength variable substance as an additive component, A radiation detector characterized in that the wavelength tunable substance shifts the wavelength of light emitted from the scintillator to the high sensitivity side of the photovoltaic element.