Scintillator and radiation detector

Abstract

A scintillator according to the present disclosure has a composition represented by formula (I), contains Ce and Al, and contains 0.001 mass%-0.01 mass% inclusive of Al:　(I): Lu(2-x-y)GdxYySiO5. In formula (I), x satisfies 0.05≤x≤0.4, and y satisfies 0.15≤y≤0.40.

Description

Scintillators and Radiation Detectors

[0001] The present invention relates to a scintillator and a radiation detector.

[0002] A scintillator is a substance that emits light when irradiated with radiation such as X-rays and gamma rays, and is used in combination with a photodetector to measure radiation. As such a scintillator material, for example, Patent Document 1 describes a scintillator single crystal containing a cerium-activated orthosilicate compound.

[0003] Japanese Patent Application Laid-Open No. 2009-7545

[0004] The scintillator according to the present disclosure has a composition represented by the following formula (I), and contains Ce and Al, with the Al content being 0.001% by mass or more and 0.01% by mass or less. （２－ｘ－ｙ） Gd ｘ Y ｙ SiO ５ In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.40.

[0005] A radiation detector according to the present disclosure includes the above-described scintillator and a light receiving element.

[0006] The main characteristics of scintillators are their light emission output and scintillation decay time constant. In particular, when scintillators are used for medical imaging diagnostics such as CT (computed tomography) and PET (positron emission tomography), improved image quality, reduced patient exposure, and shorter examination times are required. Therefore, scintillators with high light emission output and short scintillation decay time constants (short fluorescence lifetimes) are required.

[0007] The wiring board according to the present disclosure has a configuration as described in the section on means for solving the above problems, thereby enabling a large amount of light to be emitted and a short fluorescence lifetime.

[0008] The scintillator according to the present disclosure will be described below. As described above, the scintillator is a substance that emits light when irradiated with radiation such as X-rays and gamma rays. The scintillator is used in combination with a photodetector to measure radiation.

[0009] The main characteristics of scintillators are their light emission output and scintillation decay time constant. As mentioned above, when scintillators are used for medical imaging diagnosis, improved image quality, reduced exposure to subjects, and shorter examination times are required. To satisfy these requirements, scintillators must have a high light emission output and a short scintillation decay time constant (short fluorescence lifetime).

[0010] The light yield of a scintillator means the number of photons generated from the scintillator per unit energy of the irradiated radiation. The unit of emitted light is photons / MeV. The light yield is measured by evaluating the light emitted from the scintillator irradiated with radiation using a single photon counting method. In a scintillator according to one embodiment, the "light yield" is １３７ It refers to the amount of light emitted measured when a scintillator is irradiated with gamma rays of approximately 662 keV from a Cs source.

[0011] The scintillation decay time constant can be obtained from the fluorescence decay curve (scintillation decay curve) of the scintillator when irradiated with radiation. The luminescence intensity of the scintillator generally decays exponentially. The scintillation decay time is １３７ The scintillator is irradiated with gamma rays of about 662 keV from a Cs source, the resulting light emission is detected by a high-speed PMT, and the output is measured by an oscilloscope to calculate the value.

[0012] As described above, a scintillator according to an embodiment of the present disclosure has a composition represented by formula (I), and contains Ce and Al, with the Al content being 0.001% by mass or more and 0.01% by mass or less. （２－ｘ－ｙ） Gd ｘ Y ｙ SiO ５ In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.40.

[0013] The ratio of rare earth metals (Lu, Gd, and Y), Si, and oxygen does not have to match the stoichiometric ratio (2:1:5). For example, the amount of Si may be 0.8 or more and 1.1 or less when the amount of rare earth metal is 2. Like Si, oxygen does not have to match the stoichiometric ratio (5). The reality is that it is difficult to accurately evaluate the amount of oxygen.

[0014] Typically, adding a Group 13 element such as Al to a crystal made up of a combination of multiple elements increases lattice defects and crystal distortion. This makes crystal growth difficult, and the lattice defects can reduce fluorescent output. In one embodiment, the scintillator contains Al in an amount of 0.001% by mass or more and 0.01% by mass or less, thereby increasing the amount of light emitted and shortening the fluorescent lifetime. If the Al content exceeds 0.01% by mass, a crystal suitable for use as a scintillator cannot be obtained.

[0015] Furthermore, in the scintillator according to one embodiment, in the composition represented by formula (I) above, x satisfies 0.05≦x≦0.4 and y satisfies 0.15≦y≦0.40. In particular, since y satisfies 0.15≦y≦0.40, the proportion of Y is higher than in conventional scintillators.

[0016] The crystal forming the scintillator according to one embodiment is composed of rare earth metal sites of Gd, Lu, Y, and Ce, Si sites, and O sites. It is believed that Al is incorporated into the crystal to primarily substitute for Si (with some of it potentially entering rare earth metal sites or interstitial sites). The ionic radius of Al is larger than that of Si. As mentioned above, the greater the amount of Al added, the greater the lattice defects and crystal distortion. Ce may be contained in the crystal at a ratio of, for example, 0.001% by mass or more and 0.1% by mass or less.

[0017] In one embodiment of the scintillator, the element ratio of the metal site is adjusted by relatively increasing the ratio of Y, thereby reducing lattice defects and crystal distortion, and the Al content is as described above. As a result, it is highly likely that characteristics are exhibited due to nonlinear interactions between various constituent atoms, known as the multi-element superposition effect (cocktail effect).

[0018] Therefore, the scintillator according to an embodiment can emit a larger amount of light and have a shorter fluorescence lifetime than conventional scintillators. Specifically, the fluorescence lifetime of conventional scintillators is 40 nanoseconds or more, whereas the fluorescence lifetime of the scintillator according to an embodiment can be approximately 30 to 35 nanoseconds.

[0019] When the value of y is 0.15 or more and 0.40 or less, it is presumed that the effect of reducing lattice defects and crystal distortion due to the addition of Al is obtained. Therefore, a scintillator crystal with a large amount of light emission and a short fluorescence lifetime is obtained. The value of y may be 0.17 or more and 0.36 or less.

[0020] The element ratio (y / x) of Gd to Y is not limited as long as it satisfies the above range. The element ratio (y / x) of Gd to Y may be, for example, 0.5 or more and 3 or less. When the element ratio (y / x) is 0.5 or more and 3 or less, lattice defects and crystal distortion can be further reduced. As a result, the amount of Al added can be increased. Furthermore, when the element ratio (y / x) is 1 or more and 3 or less, the amount of light emitted is increased.

[0021] The scintillator according to one embodiment may further contain a Group 2 element such as Ca. The inclusion of a Group 2 element may shorten the fluorescence lifetime or reduce coloration of the resulting scintillator. For example, to shorten the fluorescence lifetime while reducing coloration of the scintillator, the content of the Group 2 element may be less than the content of Al.

[0022] The crystal forming the scintillator according to one embodiment is a single crystal. Examples of methods for producing such single crystals include the FZ (floating zone) method and the CZ (Czochralski) method. The FZ method is a method for producing single crystals by moving a melt in one direction to precipitate a single crystal from the melt. The melt is formed by forming a raw material into a rod shape, hanging it vertically, and heating and melting a portion of it.

[0023] The CZ method is a method for producing large bulk single crystals by pulling raw materials molten in a crucible upward under a temperature gradient. Raw materials are prepared so that the ratios of each element fall within the above-mentioned ranges, and by employing the FZ method or the CZ method, crystals forming the scintillator according to one embodiment are formed.

[0024] A scintillator according to an embodiment is used in, for example, a radiation detector. A radiation detector according to an embodiment of the present disclosure includes the scintillator according to the embodiment described above and a light receiving element. In the radiation detector according to an embodiment, light emitted from the scintillator according to an embodiment is converted into an electrical signal by the light receiving element. As a result, the radiation detector according to an embodiment detects the presence or absence of radiation and the amount of radiation as an electrical signal. Examples of radiation include, but are not limited to, gamma rays and X-rays.

[0025] The scintillator according to one embodiment may be processed into a shape suitable for combination with a light receiving element. For processing, a known cutting machine such as a blade saw or wire saw, a grinding machine, or a polishing disk may be used. The shape is not particularly limited. A light emitting surface facing the light receiving element is located, and the light emitting surface may be flat or may be optically polished. The presence of the light emitting surface allows light generated from the scintillator to efficiently enter the light receiving element.

[0026] The shape of the light exit surface is not limited. Examples of the shape of the light exit surface include a rectangle with a side length of several mm to several hundred mm, or a circle with a diameter of several mm to several hundred mm, and the shape may be appropriately selected depending on the application. The size of the light exit surface may be smaller than the light receiving surface of the light receiving element, since this reduces the amount of emitted light that dissipates without reaching the light receiving surface. A light-reflecting film containing aluminum, barium sulfate, polytetrafluoroethylene, or the like may be located on the surface that does not face the light receiving element. The presence of the light-reflecting film can better prevent the dissipation of light generated by the scintillator.

[0027] The light receiving element is not limited, and any light receiving element can be used. For example, a Geiger-mode APD (avalanche photodiode), which can achieve a large gain, can receive light from a scintillator with high sensitivity. An example of such a light receiving element is an MPPC (Multi-Pixel Photon Counter, manufactured by Hamamatsu Photonics K.K.). An MPPC, also known as a SiPM (Si-Photo-Multiplier), is a Geiger-mode APD with multi-pixel configuration.

[0028] The photodetector can be used with high sensitivity by applying a voltage, and radiation such as gamma rays or X-rays can be confirmed by observing the output electrical signal. The electrical signal output from the photodetector may be input to a preamplifier, a waveform shaping amplifier, or a multi-pulse-height analyzer, and measured by a single-photon counting method. By connecting the photodetector to an arbitrary current measuring device (e.g., a picoammeter) and examining changes in the current value, changes in the amount of received light can also be confirmed by the change in the current value.

[0029] In a radiation detector according to an embodiment, the light receiving element may be covered with a light-shielding material made of any material that is difficult for light to pass through, in order to prevent the incidence of light from the environment.

[0030] The scintillator according to the present disclosure will be specifically described below with reference to examples, but the scintillator according to the present disclosure is not limited to these examples.

[0031] A single crystal for a scintillator was produced by the CZ method so as to have the composition shown in Table 1. The production of a single crystal for a scintillator by the CZ method was carried out in the following procedure. First, a high-purity raw material (Gd ２ O ３ , Lu ２ O ３ , Y ２ O ３ , CeO2, SiO ２ and Al ２ O ３ ) powder was prepared. After mixing the raw material powders, they were filled into an iridium crucible and heated to melt. A seed crystal was immersed in the melt and then pulled up at a specified pulling speed and rotation speed to grow a single crystal with a cylindrical straight body. By appropriately selecting the crystal orientation of the seed crystal, a single crystal with the desired crystal orientation can be produced.

[0032] The obtained single crystal ingot was cut and polished into a thin plate having a thickness of 1.5 mm to obtain Samples No. 1 to No. 8 (scintillators) for evaluation. Samples No. 1 to No. 6 correspond to scintillators according to the present disclosure, and Samples No. 7 and No. 8 correspond to scintillators for comparative examples. The light emission amount and scintillation decay time constant were evaluated for Samples No. 1 to No. 8.

[0033] <Amount of luminescence> The amount of luminescence was measured by the single photon counting method. Specifically, the obtained sample was adhered to the optical window of a photomultiplier tube using optical grease. １３７ The sample was irradiated with gamma rays from a Cs source to generate light. The signal from the photomultiplier tube was amplified and shaped by a preamplifier and a waveform shaping amplifier, and a pulse height spectrum was obtained through a multichannel analyzer. The amount of light emitted was measured by comparing the peak positions of the photoelectric absorption peaks observed in the obtained pulse height spectrum with the peak positions of the photoelectric absorption peaks of Sample No. 7.

[0034] The amount of light emitted by the reference sample No. 7 was calculated by measuring the photoelectric absorption peak obtained by pulse height spectrum measurement using a silicon APD maintained at 20°C. ５９This was calculated by comparing the peaks obtained by directly detecting gamma rays from an Fe source with the same silicon APD. The photon energy required to create one electron-hole pair in Si is 3.6 eV. Therefore, ５５ When 5.9 keV gamma rays from an Fe source were irradiated, 5900 / 3.6 = 1640 electron-hole pairs were generated. The number of electron-hole pairs generated was determined by comparing with this directly detected peak value, and the amount of light emitted was measured based on the wavelength sensitivity characteristics of the silicon APD used. The amount of light emitted by each sample was shown relative to the amount of light emitted by Sample No. 7, which was set to 100%, and is shown in Table 1 as light emission intensity.

[0035] <Scintillation Decay Time Constant> The scintillation decay time was measured by the following procedure. １３７ The scintillator was irradiated with approximately 662 keV gamma rays from a Cs source, and the resulting luminescence was detected with a high-speed PMT. The output was then measured with an oscilloscope. The data was plotted on a graph and the decay time was calculated by fitting using software.

[0036]

[0037] As shown in Table 1, Samples 1 to 6, which correspond to the scintillators according to the present disclosure, have stronger luminescence intensities (larger amounts of light emitted) and shorter fluorescence lifetimes, all of which are 35 nanoseconds or less, compared to the reference conventional scintillator (Sample 7).On the other hand, the conventional scintillators (Samples 7 and 8) all have longer fluorescence lifetimes, exceeding 35 nanoseconds.

[0038] The embodiments of the present disclosure have been described above. However, the invention according to the present disclosure is not limited to the above-described embodiments, and various modifications and improvements are possible within the scope of the present disclosure as shown in (1) to (7) below.

[0039] (1) A scintillator according to the present disclosure has a composition represented by the following formula (I), and contains Ce and Al, with the Al content being 0.001% by mass or more and 0.01% by mass or less. （２－ｘ－ｙ） Gd ｘ Y ｙ SiO５ ... (I) In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.40. (2) In the scintillator described in (1) above, the element ratio (y / x) of Gd to Y is 0.5 or more and 3 or less. (3) In the scintillator described in (2) above, the element ratio (y / x) is 1 or more and 3 or less. (4) In the scintillator described in any of (1) to (3) above, the scintillator further contains a Group 2 element. (5) In the scintillator described in any of (4) above, the Group 2 element is Ca. (6) In the scintillator described in (4) or (5) above, the content of the Group 2 element is less than the content of Al. (7) A radiation detector according to the present disclosure includes the scintillator described in any of (1) to (6) above and a light-receiving element.

Claims

1. A scintillator having a composition represented by the following formula (I), containing Ce and Al, and the Al is contained in an amount of 0.001% by mass or more and 0.01% by mass or less. （２－ｘ－ｙ） G.D. ｘ Y ｙ SiO ５ In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.

40.

2. The scintillator according to claim 1, wherein the element ratio (y / x) of said Gd to said Y is 0.5 or more and 3 or less.

3. The scintillator according to claim 2, wherein the element ratio (y / x) is 1 or more and 3 or less.

4. The scintillator according to any one of claims 1 to 3, further comprising a Group 2 element.

5. The scintillator of claim 4, wherein said Group 2 element is Ca.

6. A scintillator according to claim 4 or 5, wherein the content of said Group 2 element is less than the content of said Al.

7. A radiation detector comprising the scintillator according to any one of claims 1 to 6 and a light receiving element.

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