WO2025058071A1 - Scintillator and radiation detector - Google Patents

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)Gd_xY_ySiO₅. In formula (I), x satisfies 0.05≤x≤0.4, and y satisfies 0.15≤y≤0.40.

Description

Scintillators and Radiation Detectors

The present invention relates to a scintillator and a radiation detector.

Scintillators are substances that emit light when exposed to radiation such as X-rays and gamma rays, and are used to measure radiation in combination with a photodetector. For example, Patent Document 1 describes a single crystal for scintillators that contains a cerium-activated orthosilicate compound as one such scintillator material.

JP 2009-7545 A

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 mass % or more and 0.01 mass % or less.
Lu _(2-x-y) Gd _x Y _y SiO ₅ ...(I)
In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.40.

The radiation detector according to the present disclosure includes the above-mentioned scintillator and a light receiving element.

The main characteristics of a scintillator are its light emission output and its scintillation decay time constant. In particular, when scintillators are used for medical imaging diagnostics such as CT (computed tomography) and PET (positron emission tomography), there is a demand for improved image quality, reduced exposure to subjects, and shorter examination times. For this reason, there is a demand for scintillators that have a large light emission output and a short scintillation decay time constant (short fluorescence lifetime).

The wiring board according to the present disclosure has the configuration described in the section on means for solving the above problems, which allows for a large amount of light emission and a short fluorescence lifetime.

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

The main characteristics of a scintillator are its light emission output and its scintillation decay time constant. As mentioned above, when a scintillator is used for medical imaging diagnosis, it is required to improve image quality, reduce the subject's exposure to radiation, and shorten the examination time. To meet these demands, the scintillator must have a high light emission output and a short scintillation decay time constant (short fluorescence lifetime).

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 emission is photons/MeV. The light yield is measured by evaluating the light emitted from the scintillator irradiated with radiation by a single photon counting method. In the scintillator according to one embodiment, the "light yield" means the light yield measured by irradiating the scintillator with gamma rays of about 662 keV from a ¹³⁷ Cs source.

The scintillation decay time constant is obtained from the fluorescence decay curve (scintillation decay curve) of the scintillator when it is irradiated with radiation. The luminescence intensity of the scintillator generally decays exponentially. The scintillation decay time is calculated by detecting the luminescence obtained by irradiating the scintillator with gamma rays of about 662 keV from a ¹³⁷ Cs source with a high-speed PMT and measuring the output with an oscilloscope.

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

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 composition ratio (5). The reality is that it is difficult to accurately evaluate the amount of oxygen.

Normally, when trying to add a Group 13 element such as Al to a crystal that combines many different elements, lattice defects and crystal distortion increase. This makes crystal growth difficult and lattice defects can reduce the fluorescent output. By including Al in a proportion of 0.001% by mass or more and 0.01% by mass or less, the scintillator of one embodiment can increase the amount of light emitted and shorten the fluorescent lifetime. If the Al content exceeds 0.01% by mass, a crystal that can be used as a scintillator cannot be obtained.

Furthermore, in the composition of the scintillator according to one embodiment, represented by the above formula (I), 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 ratio of Y is higher than that of conventional scintillators.

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 speculated that Al is incorporated into the crystal mainly to substitute for Si (some may enter the rare earth metal sites or interstitial spaces). The ionic radius of Al is larger than that of Si. As mentioned above, the more Al is added, the greater the lattice defects and crystal distortion. Ce may be contained in the crystal in a ratio of, for example, 0.001% by mass or more and 0.1% by mass or less.

In one embodiment of the scintillator, the element ratio of the metal sites is adjusted by making the ratio of Y relatively high, lattice defects and crystal distortion are reduced, and Al is contained at the above-mentioned content. As a result, it is highly likely that characteristics are expressed due to nonlinear interactions between various constituent atoms, known as the multi-element superposition effect (cocktail effect).

Therefore, the scintillator according to one embodiment can emit a greater amount of light and have a shorter fluorescence lifetime than conventional scintillators. Specifically, the fluorescence lifetime of conventional scintillators is 40 nanoseconds or longer, whereas the fluorescence lifetime of the scintillator according to one embodiment can be approximately 30 to 35 nanoseconds.

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.

The element ratio (y/x) between Gd and Y is not limited as long as it satisfies the above range. The element ratio (y/x) between Gd and 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 greater.

The scintillator according to one embodiment may further contain a Group 2 element such as Ca. By including a Group 2 element, the fluorescence lifetime may be shortened and coloration of the resulting scintillator may be reduced. For example, in order 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.

The crystals forming the scintillator according to one embodiment are single crystals. Methods for producing such single crystals include, for example, the FZ (floating zone) method and the CZ (Czochralski) method. The FZ method is a method for producing single crystals by moving the molten liquid in one direction to precipitate a single crystal from the melt. The molten liquid is formed by forming the raw material into a rod shape, holding it vertically suspended, and heating and melting a portion of it.

The CZ method is a method for producing large bulk single crystals by pulling the raw materials molten in a crucible upward under a temperature gradient. The raw materials are prepared so that the ratio of each element falls within the range described above, and the crystals that form the scintillator of one embodiment are formed by employing the FZ method or the CZ method.

The scintillator according to one embodiment is used, for example, in a radiation detector. The radiation detector according to one embodiment of the present disclosure includes the scintillator according to the above-described embodiment and a light receiving element. In the radiation detector according to one embodiment, light emitted from the scintillator according to one embodiment is converted into an electrical signal by the light receiving element. As a result, the radiation detector according to one 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.

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

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 less light is scattered without reaching the light receiving surface. A light reflective 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 a light reflective film can better prevent the light generated by the scintillator from scattering.

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 the light from the scintillator with high sensitivity. An example of such a light receiving element is the MPPC (Multi-Pixel Photon Counter, manufactured by Hamamatsu Photonics K.K.). The MPPC is also called a SiPM (Si-Photo-Multiplier), and is a Geiger mode APD that has been made into a multi-pixel.

The photodetector can be used with high sensitivity by applying a voltage, and the output electrical signal can be observed to confirm the presence of radiation such as gamma rays or X-rays. The electrical signal output from the photodetector can be input to a preamplifier, waveform shaping amplifier, or multiple pulse height analyzer, and measured by single photon counting. By connecting it to any current measuring device (e.g., a picoammeter) and examining the change in the current value, the change in the amount of light received can also be confirmed by the change in the current value.

In one embodiment of the radiation detector, the light receiving element may be covered with any light-blocking material that is difficult for light to pass through, in order to prevent the incidence of light from the environment.

Below, the scintillator according to the present disclosure will be specifically described with reference to examples. However, the scintillator according to the present disclosure is not limited in any way to these examples.

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 performed in the following procedure. First, high-purity raw material (Gd ₂ O ₃ , Lu ₂ O ₃ , Y ₂ O ₃ , CeO ₂ , SiO ₂ and Al ₂ O ₃ ) powders were prepared so that the composition of the resulting single crystal would be the composition shown in Table 1. After mixing the raw material powders, they were filled into an iridium crucible and heated to melt. After immersing the seed crystal in the melt, it was pulled up at a predetermined pulling speed and rotation speed to grow a single crystal having a cylindrical straight body. By appropriately selecting the crystal orientation of the seed crystal, a single crystal with a desired crystal orientation can be produced.

The obtained single crystal ingot was cut and polished to be processed 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 the scintillators according to the present disclosure, and samples No. 7 and No. 8 correspond to comparative examples. The amount of light emitted and the scintillation decay time constant were evaluated for samples No. 1 to No. 8.

<Light output>
The amount of light emitted was measured by a single photon counting method. Specifically, the obtained sample was attached to the optical window of a photomultiplier tube using optical grease. The sample was then irradiated with gamma rays from a ¹³⁷ Cs source to emit 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 position of the photoelectric absorption peak observed in the obtained pulse height spectrum with the peak position of the photoelectric absorption peak of sample No. 7.

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

<Scintillation decay time constant>
The scintillation decay time was measured by the following procedure. First, a gamma ray of about 662 keV from a ¹³⁷ Cs source was irradiated onto a scintillator, and the resulting light emission was detected by a high-speed PMT. The output was then measured by an oscilloscope. The data was plotted on a graph, and the decay time was calculated by fitting using software.

As shown in Table 1, samples No. 1 to No. 6, which correspond to the scintillators according to the present disclosure, have stronger luminescence intensity (larger amount of light emitted) and shorter fluorescence lifetimes than the reference conventional scintillator (sample No. 7), all of which have a fluorescence lifetime of 35 nanoseconds or less. On the other hand, the conventional scintillators (samples No. 7 and No. 8) all have a longer fluorescence lifetime, exceeding 35 nanoseconds.

The above describes the embodiments of the present disclosure. However, the invention of this 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.

(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 mass % or more and 0.01 mass % or less.
Lu _(2-x-y) Gd _x Y _y SiO ₅ ...(I)
In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.40.
(2) In the scintillator according to (1) above, the element ratio of Gd to Y (y/x) is 0.5 or more and 3 or less.
(3) In the scintillator according to (2) above, an element ratio (y/x) is 1 or more and 3 or less.
(4) The scintillator according to any one of (1) to (3) above, further comprising a Group 2 element.
(5) In the scintillator according to any one of (4) above, the Group 2 element is Ca.
(6) In the scintillator according to (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 according to any one of (1) to (6) above and a light receiving element.

Claims (7)

A scintillator having a composition represented by the following formula (I), containing Ce and Al, the Al being contained in an amount of 0.001 mass % or more and 0.01 mass % or less.
Lu _(2-x-y) Gd _x Y _y SiO ₅ ...(I)
In formula (I), x satisfies 0.05≦x≦0.4, and y satisfies 0.15≦y≦0.40. The scintillator of claim 1, in which the element ratio (y/x) of Gd to Y is 0.5 or more and 3 or less. The scintillator according to claim 2, wherein the element ratio (y/x) is 1 or more and 3 or less. The scintillator according to any one of claims 1 to 3, further comprising a Group 2 element. The scintillator according to claim 4, wherein the Group 2 element is Ca. The scintillator according to claim 4 or 5, wherein the content of the Group 2 element is less than the content of Al. A radiation detector comprising a scintillator according to any one of claims 1 to 6 and a light receiving element.