JP7601330B2 - Scintillator and radiation measuring device - Google Patents

Description

The present invention relates to a scintillator and a radiation measuring device.

Neutron detectors (measuring devices) have a wide range of application areas, including crystal structure analysis of organic and inorganic substances, medical technologies such as boron neutron capture therapy using neutron beams, neutron beam detection at high-energy synchrotron radiation facilities, non-destructive testing, and security fields such as cargo inspection. Furthermore, with the development of neutron utilization technology, there is a demand for more sensitive neutron detectors.

For example, Patent Document 1 discloses a single crystal of a Ce-doped Cs ₂ LiYCl ₆ compound as a neutron scintillator that emits light at an amount of 73,000 photons per neutron.

For example, Patent Document 2 and Non-Patent Document 1 disclose a eutectic consisting of lithium fluoride crystals, calcium fluoride crystals, and strontium fluoride, which contains Ce or Eu as a luminescent center element, and in which calcium fluoride and lithium fluoride emit light when excited by radiation.

U.S. Pat. No. 8,969,824 JP 2011-232305 A

T. Yanagida et al., "Scintillation properties of LiF-SrF2 and LiF-CaF2 eutectic", Journal of Luminescence, vol. 144, pp. 212-216, 2013.

However, in the technology of Patent Document 1, although the transparency is high and the amount of light emitted is high because it is a single crystal, the chemical composition of the compound single crystal is limited, so that the Li content cannot be increased. Therefore, in the technology of Patent Document ₁ , the molar content of Li in _Cs2LiYCl6 is low at 10%, so that there is a problem that the sensitivity to neutron rays is low.

On the other hand, the technology disclosed in Patent Document 2 and Non-Patent Document 1 uses a eutectic scintillator in which layered lithium fluoride crystals, layered calcium fluoride crystals, and strontium fluoride crystals are alternately stacked, and the calcium fluoride crystals and strontium fluoride crystals emit light when excited by radiation. Here, the difference in refractive index between the lithium fluoride crystals and the layered calcium fluoride and strontium fluoride is large, at 0.04 or more at the emission wavelength. For this reason, this technology has a problem in that the light emitted by radiation excitation is scattered and absorbed between the crystal phases, reducing the transmittance, and the scintillation light is attenuated within the scintillator before reaching the light receiving element, reducing the amount of light emitted.

As mentioned above, conventional technology has the problem of being unable to achieve high sensitivity and light emission.

The present invention was made to solve the above problems, and aims to provide a scintillator that provides high sensitivity and light emission.

The scintillator according to the present invention is composed of a eutectic consisting of multiple crystalline phases, including a first crystalline phase containing Li and a second crystalline phase that emits light when excited by radiation, and each of the multiple crystalline phases has a refractive index difference of 0.04 or less.

In one example configuration of the above scintillator, the second crystal phase contains at least one element selected from the group consisting of Pr, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Dy, Er, Tm, Yb, Tl, Pb, Bi, Ag, Ti, and Cr as a luminescent center in an amount of 0.001 mol% or more relative to the total amount of substance of the second crystal phase.

In one configuration example of the scintillator described above, the first crystal phase includes a halide represented by a chemical formula A _x B _y X _x+2y (0 < x < 1, 0 < y ≦ 1), where A is an element including at least one of Li, Na, K, Rb, and Cs, B is an element including at least one of Be, Mg, Ca, Sr, and Ba, and X is an element including at least one of F, Cl, Br, and I.

In one example of the above scintillator configuration, the first crystal phase contains a halide represented by the chemical formula LiX, where X is an element containing at least one of F, Cl, Br, and I.

The radiation measuring device according to the present invention includes any one of the scintillators described above and a light receiving element that receives the scintillation light emitted by the scintillator.

As described above, according to the present invention, the first crystal phase of the eutectic that constitutes the scintillator contains Li, and the refractive index difference between each of the multiple crystal phases that constitute the eutectic is set to 0.04 or less, so that a scintillator that provides high sensitivity and light emission can be provided.

FIG. 1 is a diagram showing the structure of a scintillator according to the present invention. FIG. 2 is a diagram showing the configuration of the radiation measuring device 200.

The scintillator according to the embodiment of the present invention will be described below with reference to FIG. 1. This scintillator is composed of a eutectic consisting of multiple crystalline phases including at least a first crystalline phase 101 containing Li and a second crystalline phase 102 that emits light when excited by radiation. The first crystalline phase 101 can also be configured to emit light when excited by radiation. The eutectic constituting this scintillator can contain other crystalline phases containing Li in addition to the first crystalline phase 101 and the second crystalline phase 102. In addition to the first crystalline phase 101 and the second crystalline phase 102, it can contain other crystalline phases that emit light when excited by radiation. The refractive index difference between each of the multiple crystalline phases constituting the scintillator is 0.04 or less. The shape of the crystalline phase constituting the eutectic of this scintillator is not limited to a substantially cylindrical shape, an elliptical shape, or a matrix shape, as shown in FIG. 1, and can be composed of various shapes, for example, a three-dimensional lamellar structure including a polygon.

This scintillator functions as a neutron scintillator that generates scintillation light when neutrons are incident on it. When neutrons are incident on the first crystal phase 101 containing Li, the neutrons are captured by the ^6Li isotope in Li, and a capture reaction occurs to generate α particles and tritium, which are secondary particles. The Li contained in the first crystal phase has a superior linear absorption cross section.

Next, the secondary particles travel through the scintillator, which is a eutectic, and reach the second crystal phase 102, which emits light when exposed to radiation, and excite the second crystal phase 102. Finally, the excited second crystal phase 102 emits scintillation light. Thus, in the scintillator according to the embodiment, the first crystal phase 101 functions as a neutron capture material, and the second crystal phase 102 functions as a phosphor. As described above, the first crystal phase 101 constituting the scintillator contains Li, which has an excellent linear absorption cross section, and therefore this scintillator has high sensitivity to neutrons.

Furthermore, the scintillator according to the embodiment is characterized in that the difference in refractive index between each of the multiple crystal phases constituting the scintillator is 0.04 or less. With this configuration, light generated in the second crystal phase 102 that is incident at an angle that does not satisfy the total reflection condition at the interface with another crystal phase is transmitted without attenuation. As a result, the light transmits between the multiple crystal phases, spreads within the scintillator, and is ultimately emitted from the scintillator. As a result, for example, if a light receiving element is placed near the scintillator, the above-mentioned light will be received by the light receiving element.

The second crystal phase 102 and other crystal phases that emit light when excited by radiation preferably contain at least one element selected from Pr, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Dy, Er, Tm, Yb, Tl, Pb, Bi, Ag, Ti, and Cr as a luminescence center in an amount of 0.001 mol % or more relative to the total amount of substance of the second crystal phase 102.

When the second crystal phase 102 or another crystal phase that emits light upon excitation with radiation contains Nd, Pr, Ce, Eu, Tm, Tl, or Sm, it exhibits a high-speed fluorescence lifetime of several ns to several hundreds of ns due to f-d transitions, and when it contains Nd, Sm, Eu, Gd, Tb, Dy, Ho, Dy, Er, Tm, Yb, or Cr, it exhibits a fluorescence lifetime of several hundred ns to several μns due to f-f transitions. Also, when the second crystal phase 102 contains Yb, it may exhibit high-speed emission of approximately several ns due to charge transfer emission, depending on the composition of the host crystal.

The first crystal phase 101 contains a halide represented by a chemical formula A _x B _y X _x+2y (0 < x<1, 0<y≦1), where A is an element including at least one of Li, Na, K, Rb, and Cs, B is an element including at least one of Be, Mg, Ca, Sr, and Ba, and X is an element including at least one of F, Cl, Br, and I. The first crystal phase 101 also contains a halide represented by a chemical formula LiX, where X is an element including at least one of F, Cl, Br, and I.

In the first crystal phase 101, ^{the 6} Li isotope ratio of the Li element contained is preferably 20 to 99%. By making the ⁶ Li isotope ratio 20% or more, the probability of the above-mentioned capture reaction increases, and the neutron detection efficiency improves. On the other hand, in consideration of the cost related to isotope enrichment, the ⁶ Li isotope ratio is preferably 99% or less.

To further improve the above-mentioned neutron beam sensitivity and optical waveguide function of the scintillator according to the embodiment, the Li content is preferably 15 mol% or more, and more preferably 25 mol% or more, relative to the molar amount of each ion in the scintillator. The refractive index difference between each crystal phase is preferably 0.3 or less, and the light emission amount is preferably 15,000 photons per neutron, and more preferably 30,000 photons per neutron. The refractive index difference between each crystal phase is preferably less than 0.04 at the emission wavelength, more preferably 0.03 or less, and even more preferably 0.02 or less.

According to the embodiment, the difference in refractive index between the crystal phases of the eutectic that constitutes the scintillator is small, at 0.04 or less, so that the light emitted by radiation excitation diffuses to the surroundings without attenuation. As a result, according to the embodiment, a neutron scintillator with high transparency at the emission wavelength and high light emission can be obtained. In addition, by making the eutectic have a first crystal phase 101 that contains Li, the Li content can be increased, and a scintillator with high sensitivity to neutron rays can be obtained. In addition, a neutron measurement device using this scintillator can measure neutron rays with high sensitivity.

Incidentally, in order to obtain a scintillator having the eutectic structure described with reference to FIG. 1, it is preferable to manufacture the scintillator with a roughly eutectic composition. However, it is not necessary to deviate from the eutectic composition, and a range of ±20 mol% of the eutectic composition can be considered as an acceptable range for the eutectic composition ratio.

The reason for defining the above-mentioned allowable range of the composition is that a eutectic having the structure shown in Figure 1 can be obtained by solidifying the above materials near the eutectic composition. Outside the allowable range, that is, when the composition ratio deviates from the range of the eutectic composition ±15 mol%, one crystal phase will precipitate first, but if the difference in refractive index between the crystal phases is small, the scintillator light generated in the scintillator reaches the light receiving element with little attenuation, so there is no problem even if crystal phases of irregular shapes and sizes precipitate, and the structure of the eutectic and the shape of the crystal phases can be any shape.

[Production of scintillator eutectic]
The scintillator eutectic can be produced by any method that melts and solidifies a desired material system with an optimal composition. For example, it can be produced by a method that grows a crystal from a melt, such as the Czochralski method, the micro-pulling down method, the Bridgman method, the EFG (Edge-defined Film-fed Growth) method, or the Kyropoulos method. It can also be produced by the floating zone method.

In addition, the grain size of the crystal phase of the scintillator and the average period of the closest distance between crystal phases depend on the solidification rate, and in particular, the period of the closest distance is said to have the following correlation. In other words, if the period is λ and the solidification rate is v, then λ2·v = constant. Therefore, by controlling the solidification rate, it is possible to control the grain size of the crystal phase and the period of the closest distance between crystal phases.

[Use of scintillators]
A scintillator having a phase separation structure can be used as a radiation measuring device (detector) for neutrons and the like by combining with a light receiving element. In particular, the scintillator according to the embodiment has a light guide function, and therefore can be applied to a situation where light needs to be guided in a specific direction toward a light receiving element without providing a partition. For example, it can be combined with a light receiving element such as a position-sensitive photomultiplier tube, a semiconductor light receiving element array, an image intensifier, or a CCD camera. In these cases, it is possible to adjust the emission wavelength of the scintillator to match the light receiving sensitivity characteristics of the light receiving element by adding other materials or a luminescent center substance to the light emitting phase (second crystal phase 102).

For example, as shown in FIG. 2, by combining the scintillator 201 with a light receiving element 202 that can receive the scintillation light emitted by the scintillator 201, it can be used as a radiation measuring device 200.

The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. Furthermore, not all of the combinations of features described in the examples are necessarily essential to the solution of the present invention.

[Example]
First, the combinations of each crystal phase were weighed to obtain each composition ratio as shown in Examples 1 to 8 in Table 1. A sample in which europium halide was added at 1 mol% relative to the total amount of material was thoroughly mixed with this mixed powder to prepare a powder. For the lithium halide, a raw material with a ⁶ Li isotope ratio of 95% was used.

Next, the mixture of the above-mentioned raw materials was filled into a quartz tube having an inner diameter of 6 mm, and the inside of the quartz tube was evacuated to 2.0×10 ⁻⁴ Pa or less using a vacuum exhaust device, and then the quartz tube was sealed with a gas burner.

Next, the raw material mixture in the quartz tube was heated by a heater to melt and solidify. After this heating, the rate at which the temperature of the melt was lowered, i.e., the solidification rate, was 20 mm/hr. Through this temperature-lowering operation, the entire melt was solidified, and the eutectic in the example was obtained.

[Comparative Example]
First, the combinations of the crystal phases in Comparative Example 1 and Comparative Example 2 shown in Table 1 were weighed to obtain the respective composition ratios. A powder was prepared by thoroughly mixing this mixed powder with a sample in which 1 mol% of europium fluoride was added to the total substance amount. For lithium fluoride, a raw material with a ^6Li isotope ratio of 95% was used.

The raw material mixture was then filled into a carbon crucible having an inner diameter of 6 mm and placed in a vacuum chamber equipped with a heater. The inside of the vacuum chamber was evacuated to 2.0×10 ⁻⁴ Pa or less using a vacuum exhaust device, and argon gas was then introduced into the vacuum chamber.

After the gas replacement operation in the vacuum chamber was performed, the raw material mixture was heated by a heater to melt and solidify. After this heating, the rate at which the temperature of the melt was lowered, i.e., the solidification rate, was 20 mm/hr. Through this temperature lowering operation, the entire melt was solidified, and the eutectic of the comparative example was obtained.

Next, the eutectic bodies obtained in Examples 1 to 8 and Comparative Examples 1 and 2 were crushed into powders, and powder X-ray diffraction measurements were performed to identify the crystal phase. The obtained eutectic bodies were cut into pieces with a thickness of 0.1 mm using a wire saw equipped with a diamond wire, and the cut surfaces were mirror-polished to obtain wafers. The transmittance of the obtained wafers was measured. Furthermore, the obtained wafers were irradiated with X-rays, and an emission spectrum was obtained using a spectroscope, and the peak wavelength of the obtained spectrum was determined as the emission wavelength.

[Fabrication and Characterization of Neutron Detectors]
The response characteristics of the neutron scintillator to neutrons were evaluated by the following method. First, the neutron scintillator was attached to the photoelectric surface of a photomultiplier tube (R7600U, manufactured by Hamamatsu Photonics) on a surface perpendicular to the solidification direction to fabricate a neutron detector (measurement device). After covering the neutron detector with a light-shielding material made of a black vinyl sheet to prevent external light from entering the photoelectric surface of the photomultiplier tube, neutrons from ^252Cf with a radioactivity of 1MBq were moderated and irradiated through a neutron moderator made of a polyethylene block with a thickness of 40mm.

To measure the scintillation light emitted by the neutron scintillator, a high voltage of 800V was applied to the photomultiplier tube from the power supply line, and the scintillation light was converted into an electrical signal, which was output from the signal output line. Here, the electrical signal output from the photomultiplier tube was a pulsed signal reflecting the scintillation light, and the pulse height represented the luminous intensity of the scintillation light. The electrical signal output from the photomultiplier tube was shaped and amplified by a shaping amplifier, and then input to a multiple pulse height analyzer for analysis, creating a pulse height distribution spectrum, and the amount of light emitted was compared with that of Li-glass, the reference material for neutron scintillators, to calculate the amount of light emitted.

Table 1 shows the evaluation results of the eutectic bodies obtained in Examples 1 to 8 and Comparative Examples 1 and 2. In the scintillators of Comparative Examples 1 and 2, the difference in refractive index between the respective crystal phases is 0.04 or more, so the transmittance is low. Furthermore, the amount of light emitted is low, so performance is reduced when used as a neutron detector (measuring device). Generally, neutron detectors use large neutron scintillators with a diameter of over 1 inch and a thickness of over 1 inch, so in the scintillators of Comparative Examples 1 and 2, which have low transmittance, the larger the crystal size and thickness, the greater the attenuation of light within the scintillator, and the further the amount of light emitted is reduced.

The eutectic bodies shown in Examples 1 to 8 have a transmittance of 65% or more because the difference in refractive index between the crystal phases is 0.03 or less. Furthermore, the amount of light emitted is high at 35,000 photons or more per neutron, and the molar content of Li is also high at 14% or more, making it possible to realize a neutron detector with excellent sensitivity.

As described above, according to the present invention, the first crystal phase of the eutectic that constitutes the scintillator contains Li, and the refractive index difference between each of the multiple crystal phases that constitute the eutectic is set to 0.04 or less, making it possible to provide a scintillator that provides high sensitivity and a high amount of light emission.

The present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented by those with ordinary skill in the art within the technical concept of the present invention.

101...first crystal phase, 102...second crystal phase.

Claims (3)

The present invention is composed of a eutectic having a plurality of crystalline phases including a first crystalline phase containing Li and a second crystalline phase that emits light when excited by radiation;
The refractive index difference between each of the plurality of crystal phases is 0.04 or less ,
The second crystal phase contains at least one element selected from the group consisting of Pr, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Dy, Er, Tm, Yb, Tl, Pb, Bi, Ag, Ti, and Cr as a luminescence center in an amount of 0.001 mol% or more with respect to the total amount of substance of the second crystal phase;
The first crystalline phase contains a halide represented by a chemical formula A _x B _y X _x+2y (0<x<1, 0<y≦1),
A is Li;
B is an element including at least one of Be, Mg, Ca, Sr, and Ba;
X is an element containing at least one of Cl , Br and I. The present invention is composed of a eutectic having a plurality of crystalline phases including a first crystalline phase containing Li and a second crystalline phase that emits light when excited by radiation;
The refractive index difference between each of the plurality of crystal phases is 0.04 or less,
The second crystal phase contains at least one element selected from the group consisting of Pr, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Dy, Er, Tm, Yb, Tl, Pb, Bi, Ag, Ti, and Cr as a luminescence center in an amount of 0.001 mol% or more with respect to the total amount of substance of the second crystal phase;
The first crystal phase contains a halide represented by a chemical formula LiX, where X is an element containing at least one of Cl , Br, and I. A scintillator according to claim 1 or 2 ;
and a light receiving element that receives scintillation light emitted by the scintillator.

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