JP7478399B2 - Method for producing scintillator and single crystal for scintillator - Google Patents

Description

The present disclosure relates to a scintillator used to measure radiation and a method for manufacturing a single crystal for the scintillator.

Scintillators are materials that emit light when exposed to radiation such as X-rays or gamma rays, and are used in combination with photodetectors to measure radiation. The main characteristics of scintillators are the amount of light emitted and the scintillation decay time constant. In particular, for medical imaging diagnostic applications such as computed tomography (CT) and positron emission tomography (PET), scintillators with high light emission and short scintillation decay time constants are required to improve image quality, reduce the amount of radiation exposure to subjects, and shorten examination times.

Garnet-based materials are attracting attention as candidates for scintillator materials. In Non-Patent Document 1, TAG (terbium aluminum garnet) and TAGG (terbium aluminum gallium garnet) doped with Ce are produced by the FZ (floating zone) method, and their properties are evaluated.

D. Nakauchi, G. Okada, N. Kawano, N. Kawaguchi, T. Yanagida, Effects of Ga substitution in Ce:Tb3GaxAl5-xO12 single crystals for scintillator applications, Jpn. J. Appl. Phys. 57, 02CB02 (2018). T. Yanagida, Y. Fujimoto, T. Ito, K. Uchiyama, K. Mori, Development of X-ray induced afterglow characterization system, Appl. Phys. Exp. 7, 062401 (2014).

The scintillator of the present disclosure includes a TAG (terbium aluminum garnet) single crystal doped with Ce. The single crystal is substantially free of Ga, and the atomic ratio Ce/(Tb+Ce+RE) of the constituent elements (Ce, Tb, RE (RE is a constituent element of the Tb site other than Tb and Ce)) is 0.4% or more and 0.7% or less.

The scintillator of the present disclosure includes a TAGG (terbium aluminum gallium garnet) single crystal doped with Ce. The atomic ratio Ga/(Ga+Al) of the constituent elements of the single crystal (Ce, Tb, RE (RE is a constituent element of the Tb site other than Tb and Ce), Al, and Ga) is 1% or more and 6% or less, and Ce/(Tb+Ce+RE) is 0.3% or more and 1.4% or less.

The manufacturing method of the scintillator single crystal disclosed herein produces a Ce-doped TAG (terbium aluminum garnet) single crystal using an oxide raw material that is substantially free of Ga and in which the atomic number ratio Ce/(Tb+Ce+RE) of the elements (Ce, Tb, RE (RE is the constituent element of the Tb site other than Tb and Ce)) in the raw material is 1.5% or more and 2.5% or less.

The manufacturing method of the scintillator single crystal disclosed herein produces a Ce-doped TAGG (terbium aluminum gallium garnet) single crystal using an oxide raw material in which the atomic ratio Ga/(Ga+Al) of the elements in the raw material (Ce, Tb, RE (RE is a constituent element of the Tb site other than Tb and Ce), Al and Ga), Al and Ga) is 2% or more and 10% or less, and Ce/(Tb+Ce+RE) is 1% or more and 5% or less.

1 is a graph showing the amount of light emitted and the scintillation decay time constant of a scintillator single crystal according to the present disclosure. 1 is a graph showing an example of a pulse height spectrum.

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

A scintillator according to an embodiment of the present disclosure includes a TAG (terbium aluminum garnet) single crystal to which Ce has been added. The TAG single crystal has a garnet crystal structure, contains Tb, Al, and O (oxygen) as main component elements, and does not substantially contain Ga. The TAG single crystal contains one or more elements capable of forming a solid solution in the crystal as a secondary component element, for example, an element RE capable of forming a solid solution by substitution with Tb. The element RE is a constituent element of the Tb site other than Tb and Ce, and examples thereof include rare earth elements such as Y, Gd, and Lu. The ratio of the number of atoms of the constituent elements (Ce, Tb, and RE) of the single crystal, Ce/(Tb+Ce+RE), is 0.4% or more and 0.7% or less. RE may not be contained in the crystal, or two or more types may be contained. When two or more types of RE are contained, the number of atoms of RE is the total number of atoms corresponding to RE.

The scintillator of another embodiment of the present disclosure includes a TAGG (terbium aluminum gallium garnet) single crystal to which Ce is added. The TAGG single crystal has a garnet crystal structure and contains Tb, Al, Ga, and O (oxygen) as main component elements. The TAGG single crystal contains one or more elements capable of forming a solid solution in the crystal as a secondary component element, for example, an element RE capable of forming a solid solution by substitution with Tb. The element RE is a constituent element of the Tb site other than Tb and Ce, and examples thereof include rare earth elements such as Y, Gd, and Lu. The atomic ratio of the constituent elements (Ce, Tb, RE, Al, and Ga) of the single crystal is Ga/(Ga+Al) 1% to 6%, and Ce/(Tb+Ce+RE) 0.3% to 1.4%. RE may not be contained in the crystal, or two or more types may be contained. When two or more kinds of RE are contained, the number of atoms of RE is the total number of atoms corresponding to RE.

The TAG-based crystals and TAGG-based crystals may contain at least one co-doped element other than Ce. Examples of such co-doped elements include Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Zr, Hf, Nb, Ta, Mo, W, Zn, Cd, B, In, C, Si, Ge, and Te.

The main characteristics of a scintillator are its light emission output and its scintillation decay time constant. In particular, scintillators used in medical imaging diagnostics such as computed tomography (CT) and positron emission tomography (PET) require a high light emission output and a short scintillation decay time constant in order to improve image quality, reduce the subject's exposure to radiation, and shorten examination times.

The light yield of a scintillator refers to the number of photons generated from the scintillator per unit energy of the irradiated radiation. The unit of emission may be photons/MeV. The light yield can be 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 refers to 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 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 curve can be measured, for example, by time-correlated single photon counting (TCSPC) using pulsed X-rays as an excitation source. The scintillation decay curve can be approximated by an exponential function of a single component or the sum of exponential functions of multiple components. The following formula can be used as an approximation formula.
Y(t) = Σ{A _i exp(-t/τ _i )} + C ... formula [1]

In the formula, Y(t) represents the luminescence intensity of the scintillator at time t. i is a positive integer that indicates 1, 2, 3, etc., and i is 1 in the case of a single-component exponential function, and i is 2 in the case of a two-component exponential function. Σ is a mathematical symbol that means calculating the sum of the functions in the brackets when i goes from 1 to n (the number of components). _{A i} is a value related to the proportion of the function of the i-th component to the entire decay curve, and C is a constant. τ _i is the scintillation decay time constant of the i-th component.

The scintillation decay time constant in this invention refers to the scintillation decay time constant of the faster component of the two scintillation decay time constants, fast and slow, obtained by approximating the portion of the scintillation decay curve obtained by time-correlated single photon counting using pulsed X-rays, excluding the component within 3 nanoseconds from the start of decay of the luminescence intensity, which corresponds to the instrument response function (IRF), with two components.

The scintillator crystal of the present disclosure is made of a single crystal. Single crystals can be manufactured by methods such as the FZ (floating zone) method and the CZ (Czochralski) method. In the present disclosure, single crystals were manufactured using the FZ method. In the FZ method, raw materials are formed into a rod shape, hung vertically, and a part of it is heated and melted to form a molten liquid portion. This method manufactures single crystals by precipitating a single crystal from the melt by moving the molten liquid portion in one direction.

In one embodiment of the present disclosure, a method for producing a scintillator single crystal is used to produce a Ce-doped TAG (terbium aluminum garnet) single crystal that is substantially free of Ga, using an oxide raw material that is substantially free of Ga and in which the element ratio Ce/(Tb+Ce+RE) in the raw material is 1.5% or more and 2.5% or less.

In another embodiment of the present disclosure, a method for producing a scintillator single crystal is provided for producing a Ce-doped TAGG (terbium aluminum gallium garnet) single crystal using an oxide raw material in which the element ratio Ga/(Ga+Al) in the raw material is 2% or more and 10% or less, and Ce/(Tb+Ce+RE) is 1% or more and 5% or less.

The primary raw materials _{are oxides of Ce, Tb, Al and Ga (e.g. CeO2, Tb4O7 , Al2O3 and Ga2O3} ), as well as oxides of auxiliary components and oxides of co-additive elements. A single crystal may be manufactured using a secondary raw material containing TAG- _based polycrystals or _TAGG -based polycrystals obtained by mixing and firing each element to a desired ratio. The raw material may contain oxides of constituent elements (e.g. rare earth elements such as Y, Gd and Lu) that can be substituted for Tb in the crystal and form a solid solution. Furthermore, it may contain oxides of co-additive elements other than Ce.

The above configuration makes it possible to provide a scintillator with a high light emission and a short scintillation decay time constant. That is, if the ratio of each element in the TAG crystal, Ce/(Tb+Ce+RE), is 0.4% or more and 0.7% or less, and particularly preferably 0.6% or more and 0.7% or less, the scintillator has a high light emission and a short scintillation decay time constant. If the ratio of each element in the TAG crystal, Ga/(Ga+Al), is 1% or more and 6% or less, and particularly preferably 1% or more and 3% or less, and Ce/(Tb+Ce+RE) is 0.3% or more and 1.4% or less, and particularly preferably 0.4% or more and 0.8% or less, the scintillator has a high light emission and a short scintillation decay time constant.

The scintillator according to the present disclosure can be combined with any light receiving element such as a silicon photodiode to form a gamma ray detector. In other words, the light emitted from the scintillator can be converted into an electrical signal by the light receiving element, thereby capturing the presence and amount of gamma rays as an electrical signal.

The scintillator according to the present disclosure can be used after being 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 can be used without any restrictions. There are no particular restrictions on the shape. It has a light exit surface facing the light receiving element, and it is desirable that the light exit surface is flat, and may be optically polished. By having a light exit surface, light generated from the scintillator can be efficiently incident on the light receiving element.

The shape of the light exit surface is not limited, and can be selected from shapes appropriate for the application, such as a square with sides measuring several mm to several hundred mm, or a circle with a diameter of several mm to several hundred mm. A smaller size than the light receiving surface of the silicon photodiode is preferable, since less light is emitted without reaching the light receiving surface and dissipates. By applying a light reflective film made of aluminum, barium sulfate, polytetrafluoroethylene, etc. to the surface not facing the light receiving element, it is possible to prevent the light generated by the scintillator from dissipating.

Any light receiving element can be used. By using a Geiger mode APD (Avalanche Photodiode) that can achieve a large gain, the light from the scintillator can be received with high sensitivity. One example is the MPPC (Multi-Pixel Photon Counter) made by Hamamatsu Photonics. The MPPC is an element also known as a SiPM (Si-Photo-Multiplier), and is a Geiger mode APD made into a multi-pixel.

The scintillator of the present disclosure can be bonded to the light receiving surface of a light receiving element with any optical adhesive or engineering grease and used as a gamma ray detector. The light receiving element to which the scintillator is bonded may be covered with any light-shielding material that is difficult for light to pass through in order to prevent the incidence of light from the environment.

The photodetector can be used with high sensitivity by applying a voltage, and the detection of gamma rays can be confirmed by observing the electrical signal that is output. The electrical signal output from the photodetector can be input to a preamplifier, waveform shaping amplifier, multiple pulse height analyzer, etc., and measured by single photon counting. It can also be connected to any current measuring device (e.g. a picoammeter) to examine changes in the current value, and changes in the amount of light received can be confirmed by the change in the current value.

The present invention will be explained in detail below with reference to examples, but the present invention is not limited in any way to these examples.

Scintillator single crystals under conditions 1 to 13 were produced by the FZ method with the raw material compositions shown in Table 1. Of these, conditions 1, 5, and 14 are comparative examples. Condition 5 is a TAGG single crystal described in Non-Patent Document 1, and condition 14 is a commercially available Ce-doped LYSO (lutetium yttrium silicate) single crystal.

The scintillator single crystal was produced by the FZ method in the following procedure. The raw material powders with the composition shown in Table 1 were mixed in a mortar, then molded into a rod shape and pressed with a hydrostatic press at a pressure of 20 kN for 10 minutes. The raw material rod was then sintered in an electric furnace at 1400°C for 8 hours to obtain a raw material rod. The raw material rod was single crystallized using an FZ method tabletop single crystal growth device (Canon Machinery, Desktop IR Furnace) equipped with a heating mechanism using a halogen lamp. The raw material rod was melted while checking the camera image, and single crystal growth was performed while setting the lamp output to a level where the solid solution interface could be confirmed. The rotation speed of the raw material rod was 3 rpm, and the pulling speed was 3 mm/h. The obtained single crystal rod was cut, polished, and processed into a thin plate with a thickness of 2 mm to be used as an evaluation sample.

The light emission and scintillation decay time constant of each single crystal, the analysis results of the crystal composition under condition 8 by ICP-OES (IPC optical emission analysis), and the crystal composition predicted from the results under each condition are shown in Table 1. As a comparative example, a commercially available Ce-doped LYSO single crystal (condition 14) was also measured.

The amount of light emitted was measured by single photon counting. The prototype evaluation sample was attached to the optical window of a photomultiplier tube (Hamamatsu Photonics, R7600U-200) using optical grease (Applied Photonics, TSK5353). The sample was 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 (ORTEC, Model 113 Scintillation Preamplifier) and a waveform shaping amplifier (ORTEC, Model 570 Spectroscopy Amplifier), and a pulse height spectrum was obtained through a multichannel analyzer (AMPTEK, Pocket MCA8000A). 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 the TAG single crystal under condition 5.

The amount of light emitted from the TAG single crystal under condition 5 used as a reference 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 ^59Fe source using the same silicon APD. Since the photon energy required to create one electron-hole pair in Si is 3.6 eV, irradiation with 5.9 keV gamma rays from a ^55Fe source generates 5900/3.6 = 1640 electron-hole pairs. 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 scintillation decay time constant was obtained by approximating the scintillation decay curve obtained by a time-correlated single photon counting device (Hamamatsu Photonics, non-patent document 2) using pulsed X-rays as an excitation source to the exponential function of formula [1]. The scintillation decay time constant obtained here refers to the scintillation decay time constant of the faster component out of the two scintillation decay time constants, fast and slow, obtained by approximating the part of the scintillation decay curve excluding the component within 3 nanoseconds from the start of decay of the emission intensity corresponding to the instrument response function in the approximation to the exponential function.

Figure 1 shows the amount of light emitted and the scintillation decay time constant for each single crystal. It can be seen that conditions 2 to 4 and 6 to 13 are superior in at least one of the scintillation decay time constant and amount of light emitted to the single crystal under condition 1 and the well-known comparative examples, condition 5 (Non-Patent Document 1) and condition 14 (commercially available LYSO single crystal). In particular, conditions 2 to 4, 8 to 10 and 12 have high amounts of light emitted (50,000 or more), while conditions 7 to 12 have small scintillation decay time constants (29 or less). It can therefore be seen that conditions 8 to 10 and 12 are particularly superior in both scintillation decay time constant and amount of light emitted.

Next, an example of the gamma ray detector disclosed herein will be described.

A gamma ray detector according to the present disclosure was fabricated by combining the scintillator single crystal of condition 9 in Table 1 with a silicon light receiving element. For the silicon light receiving element, an MPPC (manufactured by Hamamatsu Photonics, S13360-6075CS) consisting of multiple Geiger mode APDs was used. The polished surface of the scintillator single crystal of condition 9 was bonded to the light receiving surface of the MPPC using optical grease (Applied Koken, TSK5353), and the surface not facing the light receiving surface was covered with a light reflective film made of polytetrafluoroethylene to form the detection section.

The MPPC of the detection section was connected to a commercially available circuit system (manufactured by ANSeeN) capable of applying voltage and reading out signals, and the entire system was shielded from light. The gain (voltage gain) of the MPPC varies with temperature changes. This circuit system has the function of controlling the gain to be constant by slightly changing the voltage applied to the MPPC in response to temperature changes around room temperature. The signal line from this circuit system was connected in the following order to a preamplifier (Model 113 Scintillation Preamplifier, manufactured by ORTEC), a waveform shaping amplifier (Model 570 Spectroscopy Amplifier, manufactured by ORTEC), and a multichannel analyzer (Pocket MCA8000A, manufactured by AMPTEK), forming an example of a gamma ray detector according to the present disclosure.

The gamma ray detector thus fabricated was irradiated with gamma rays having an energy of 662 keV from a ¹³⁷ Cs source and gamma rays having an energy of 59.5 keV from ²⁴¹ Am while applying a voltage of about 53 V, and pulse-height distribution spectra were created from the obtained signals. At that time, the amplification factor of the waveform shaping amplifier was appropriately adjusted so that the high-intensity signal fell within the drawing range of the pulse-height distribution spectrum. As a result, a clear photoelectric absorption peak was confirmed from each of the obtained pulse-height distribution spectra. In the case of gamma rays of any energy, it was confirmed that the peak area increased in proportion to the irradiation time (exposure dose). From these facts, it can be seen that the gamma ray detector of the present disclosure can be suitably used for measuring gamma rays of relatively high energy such as 662 keV and relatively low energy such as 59.5 keV.

Claims (5)

A scintillator comprising a TAGG (terbium aluminum gallium garnet) single crystal doped with Ce, in which the atomic ratio of the constituent elements of the single crystal (Ce, Tb, RE (RE is a constituent element of the Tb site other than Tb and Ce), Al and Ga) Ga/(Ga+Al) is 1% or more and 3 % or less, and Ce/(Tb+Ce+RE) is 0.6 % or more and 0.8 % or less. A gamma ray detector combining the scintillator according to claim 1 with a light receiving element. 3. The gamma ray detector according to claim 2 , wherein the light receiving element is a Geiger mode APD. A method for producing a Ce-doped TAGG (terbium aluminum gallium garnet) single crystal using an oxide raw material in which the atomic ratio Ga/(Ga+Al) of the elements in the raw material (Ce, Tb, RE (RE is an element constituting the Tb site other than Tb and Ce), Al and Ga) is 2% or more and 5 % or less, and Ce/(Tb+Ce+RE) is 2 % or more and 3 % or less. 5. The method for producing a scintillator single crystal according to claim 4 , wherein the TAG type single crystal or the TAGG type single crystal is produced by a FZ method.