JP2012127705A - Collimator for radiation, radiation detector, and radiation detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a collimator for radiation, a radiation detector including the collimator, and a method for manufacturing the collimator for radiation that improve the detection efficiency by eliminating a problem such that detection efficiency is reduced since radiation which is shielded by the collimator for radiation and is not used for detection is generated.SOLUTION: In a radiation detector including a collimator for radiation, a scintillator material is used for the collimator and radiation shielded by the collimator and not used for detection is detected to improve efficiency of detecting radiation. The collimator for radiation is manufactured by a micro pull-down method and a Czochralski method.

Description

  The present invention relates to a novel radiation detector and detection method having a radiation collimator made of a scintillator material. It can be suitably used for a gamma ray detector for medical diagnosis such as SPECT (single photon emission tomography) or security for baggage inspection. Application to other radiation detectors such as neutron detectors is also possible.

  In the field of nuclear medicine, radiopharmaceuticals are administered into the body, and the radiation (mainly gamma rays) emitted from them is measured with a radiation detector placed outside the body, and the spatial distribution is obtained, so that the degree of radiopharmaceutical accumulation is observed. Diagnose the presence or absence of lesions. In the gamma camera used at this time, it was necessary to use a collimator in order to identify the direction of gamma ray flight. However, conventionally, heavy metals such as lead have been used for the gamma ray collimator, which is shielded. Since gamma rays that are not used for detection are generated, there is a problem that the detection efficiency decreases (FIG. 1). Similarly, for other radiation, for example, a neutron beam, a material having a high absorption efficiency with respect to a neutron beam such as LiF ceramics is used as a collimator.

  The inventors of the present invention have tried to avoid the problem that the detection efficiency is lowered by the gamma ray collimator by attaching an imaging plate to the collimator wall surface (Non-patent Document 1). However, it is not a practical method in consideration of application to SPECT or the like, such as the necessity of disassembling the detector for each measurement or the time and labor required for image reading.

There is a scintillator as a substance that absorbs radiation such as α rays, β rays, γ rays, X rays, neutron rays and the like. A scintillator is a substance that absorbs radiation and emits light, and is used for radiation detection in combination with a photodetector such as a photomultiplier tube. There are various types of scintillator functions (scintillator materials) depending on the type of radiation and the purpose of use, such as inorganic crystals such as Bi ₄ Ge ₃ O ₁₂ and Ce: Gd ₂ SiO ₅ , anthracene, etc. And organic polymers such as polystyrene and polyvinyltoluene containing an organic phosphor. Although these are not as much as materials for conventional collimators, they can be said to have a relatively high shielding ability against radiation. However, as described above, conventionally, lead is mainly used for gamma rays and LiF ceramic is used as a collimator for neutron rays, and a collimator made of a scintillator material has not been used.

Junichi Sugiyama et al., “Development of Gamma Camera Using Functional Collimator”, Proceedings of the 69th JSAP Conference, 126 (2008)

  In conventional radiation collimators, a certain amount of radiation that is shielded and not used for detection is generated, so there is a problem that the detection efficiency decreases when incorporated in a radiation detector, and the detection efficiency of a radiation detector equipped with a radiation collimator is low. There was a need for improvement.

  As a result of intensive studies on a radiation collimator that does not lower the detection efficiency when incorporated in a radiation detector, the present inventors have used a collimator part for radiation by using a radiation collimator made of a scintillator material as a radiation collimator. The present inventors have found that radiation detection efficiency can be improved by using, as detection data, radiation that is shielded and not used for detection, and has completed the present invention.

  That is, the present invention is a radiation collimator comprising a scintillator material, a radiation detector equipped with the collimator, a radiation detection method for identifying the direction in which radiation has traveled using the collimator, and radiation It is a manufacturing method of the hollow columnar single crystal for collimators.

  According to the present invention, in a conventional radiation collimator, radiation that has been shielded and not used can be used for detection, so that the detection efficiency is increased, and radiation with a radiation collimator can be obtained even with a smaller amount of radiation than in the past. It can be used as a detector. Thereby, when applied to a gamma camera for medical diagnosis, for example, the dose of the radiopharmaceutical can be reduced.

This figure is a schematic diagram of a gamma camera provided with a collimator. This figure is a schematic diagram of the principle of specifying the gamma ray incident position by a radiation detector equipped with a scintillator having a collimator function. This figure is a conceptual diagram of a radiation detector provided with a scintillator having a collimator function. This figure is a schematic view of a crystal manufacturing apparatus by a general micro pulling-down method. This figure is a schematic view of a crucible, a crucible lid, and a crucible hole shape in a crystal manufacturing apparatus using the micro pull-down method of the present invention. This figure is the schematic of the shape of the metal wire in the crystal manufacturing apparatus by the micro pulling-down method of this invention. This figure is a photograph of the obtained hollow prism scintillator. This figure is a diagram of an experimental system by simulation. This figure is a figure of the radiation imaging result by simulation. This figure is a diagram of cosine similarity of radiation imaging results by simulation. This figure is a figure of the radiation imaging result by simulation. This figure is a diagram of an experimental system for a specific test of a radiation irradiation position. This figure is a diagram of an experimental result of a specific test of a radiation irradiation position.

  The radiation collimator of the present invention is made of a scintillator material.

The scintillator material is not particularly limited, and a known scintillator material can be used, and may be selected according to the type of radiation. For gamma _{rays, BaF 2, CeF 3, Bi} 4 Ge 3 O 12, Ce: Lu 2 SiO 5, Ce: Gd 2 SiO 5, CsI, Tl: NaI, those selected from such as CdWO 4, PbWO ₄ It can be used suitably. For neutron beams, one selected from lithium glass, Li compounds such as Eu: LiI, Ce: LiCaAlF ₆ , Eu: LiCaAlF ₆ , B compounds such as Ce: CaB ₂ O ₄ , Eu: CaB ₂ O ₄ , etc. Can be suitably used.

  Examples of the shape of the radiation collimator of the present invention include a bundle of optical fibers made of a single crystal or a scintillator material whose shape is controlled in a hollow column shape, a combination of small scintillator blocks, and the like.

  The radiation collimator of the present invention can be a radiation detector with improved radiation detection efficiency by constituting a radiation detector together with a scintillator and a photodetector.

  In the radiation detector equipped with the radiation collimator of the present invention, a scintillator material that has not been conventionally used as a material for the radiation collimator is used as a collimator and is used for detection by being shielded by the collimator portion. Radiation detection efficiency can be improved by using the missing radiation as detection data. A radiation collimator generally has a hollow columnar structure in which a hole is made in a material having a high radiation shielding ability. However, the radiation collimator is composed of a scintillator material like the radiation collimator of the present invention. Both direction limitation and detection can be performed simultaneously. In addition, the detection signal can be sequentially read out.

  The direction in which the radiation came is based on the fact that when the distance from the photomultiplier tube is far from the photomultiplier tube, the amount of light emitted from the radiation collimator made of scintillator material decreases and the amount of light increases. By measuring the amount of light emitted from the collimator, the vertical position (distance from the photomultiplier tube) of the light emitting portion of the detection surface (side surface of the radiation collimator) of the plurality of radiation collimators is specified. (FIG. 2), the position data is specified from the vertical position data. The amount of light is greatest when the direction in which the radiation has come is perpendicular to the photomultiplier tube. Further, by arranging collimators for radiation made of adjacent scintillator materials in a checkered pattern with different emission decay time constants, it is possible to identify them by the emission decay time (FIG. 3). The hollow portion at the center of the collimator need not be installed as shown in FIG. 3, but a scintillator may be installed. When used as a gamma ray detector, a plastic scintillator having a low effective atomic number may be installed.

  For the production of hollow spherical single crystals for radiation collimators, single crystal growth by the micro pull-down method or Czochralski method is applicable, and an annular hole is provided near the interface between the melted raw material and the crystallized part. In addition, two or more seed crystals or metal wires are used at the same time for the purpose of stabilizing the growth shape using a non-melting member for shape control.

  Although the production method is not particularly limited, it can be produced by a type of melt growth method in which a seed crystal represented by a micro pull-down method or a Czochralski method is moved to pull down or pull up the crystal from the melt.

  Hereinafter, a specific method for producing a scintillator whose shape is controlled to be a hollow prismatic shape when the micro-pulling-down method is used will be described taking growth of a fluoride single crystal as an example.

  The micro pull-down method is a method for producing a crystal by drawing a raw material melt from a hole provided in the bottom of the crucible 5 using an apparatus as shown in FIG. FIG. 4 is a schematic diagram of crystal growth using a generally used hole shape and one metal wire. When growing a fluoride single crystal, carbon can be used as the material for the crucible and the heater, and an induction heating type growth furnace using a high frequency coil can be used.

  As a method, first, a predetermined amount of raw material is filled into a crucible 5 having a hole at the bottom. The shape of the hole provided in the bottom of the crucible is a feature of the present invention, and is an annular hole as shown in FIG.

In the present invention, the raw materials are not particularly limited, but it is preferable to use those having a purity of 99.99% or more. By using such a high-purity mixed raw material, the purity of the crystal can be increased and characteristics such as emission intensity are improved. In the case of producing a neutron scintillator, a ⁶ Li-enriched raw material may be used as the Li compound raw material for the purpose of improving the detection efficiency with respect to the neutron beam. Similarly, a ¹⁰ B concentrated raw material may be used for the B compound. The mixed raw material may be used after being sintered or melted and solidified after mixing.

  Next, the crucible 5 filled with the above raw material, the after heater 1, the heater 2, the heat insulating material 3, and the stage 4 are set as shown in FIG.

Further, after evacuating the inside of the chamber 6 to 1.0 × 10 ⁻³ Pa or less using a vacuum evacuation device, a gas (for example, argon) used for an atmosphere during growth is introduced into the chamber 6 to perform gas replacement. Do. The pressure in the chamber after gas replacement is not particularly limited, but atmospheric pressure is common.

By the gas replacement operation, moisture attached to the raw material or the chamber can be removed, and deterioration of crystals derived from the moisture can be prevented. In order to avoid the influence of moisture that cannot be removed even by the gas replacement operation, it is preferable to use a scavenger such as CF ₄ gas. When using a solid scavenger, a method of mixing in the raw material in advance is preferable, and when using a gas scavenger, a method of mixing with the above inert gas and introducing it into the chamber is preferable.

  After performing the gas replacement operation, the raw material is heated and melted by the high-frequency coil 7, and the melted raw material melt is drawn out from the hole at the bottom of the crucible to start crystal growth.

  Here, a metal wire or seed crystal is provided at the tip of the pulling rod, inserted into the crucible from the hole at the bottom of the crucible, and after the raw material melt is attached to the metal wire or seed crystal, the raw material melt is pulled down together. This makes it possible to grow crystals. A refractory metal such as Pt or W—Re alloy can be used for the metal wire, and it is preferable to use the same crystal as the crystal to be grown for the seed crystal.

  At this time, the metal wire or seed crystal used for pulling down has a shape that can contact the raw material at two or more points (for example, the metal wire shown in FIG. 6). This is a feature of the present invention. When a metal wire or seed crystal that comes into contact with the raw material at one point is used, the growth of a single crystal whose shape is controlled to be a hollow prism is stable because the growth direction is bent. Therefore, single crystal growth is difficult.

  In such an apparatus configuration, while adjusting the output of the high frequency and gradually raising the temperature of the raw material, the metal wire is inserted into the hole at the bottom of the crucible and pulled out. This operation is repeated until the raw material melt is drawn together with the metal wire, and crystal growth is started.

  After pulling out the raw material melt with the metal wire, the crystal can be obtained by continuously pulling it down at a constant pulling rate.

  The pulling speed is not particularly limited, but if it is too fast, the crystallinity tends to be poor, and if it is too slow, the crystallinity is improved, but the time required for crystal growth becomes enormous. / Hr is preferable.

  In the production of the fluoride crystal of the present invention, an annealing operation may be performed after the production of the crystal for the purpose of removing crystal defects caused by thermal strain.

  Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.

Example (Production of hollow columnar single crystal for material collimator for radiation)
A hollow prismatic BaF ₂ single crystal was manufactured using the crystal manufacturing apparatus shown in FIG.

As a raw material, BaF ₂ having a purity of 99.99% was used. The after heater 1, the heater 2, the heat insulating material 3, the stage 4, and the crucible 5 were made of high-purity carbon, and the shape of the hole provided in the bottom of the crucible was as shown in FIG.

First, the crucible 5 was filled with BaF ₂ raw material. The crucible 5 filled with the raw material was set on the upper part of the after heater 1, and the heater 2 and the heat insulating material 3 were sequentially set around it. Next, the inside of the chamber 6 is evacuated to 1.0 × 10 ⁻⁴ Pa using an evacuation apparatus including an oil rotary pump and an oil diffusion pump, and then a 90% argon-tetrafluoromethane 10% mixed gas is added to the chamber. The gas was replaced by introducing the gas into 6.

  After the gas replacement, the pressure in the chamber 6 was changed to atmospheric pressure, and then the raw material was heated to about 400 ° C. with the high-frequency coil 7, but no leaching of the raw material melt from the bottom of the crucible 5 was observed. Then, while adjusting the high frequency output and gradually raising the temperature of the raw material melt, the W-Re wire provided at the tip of the pull-down rod 8 was inserted into the hole and pulled down repeatedly. The liquid could be drawn out from the hole.

  The shape of the W-Re wire used at this time was branched into two shapes as shown in FIG. 6. The W-Re wire was brought into contact with the raw material melt at two points through the hole in the crucible shown in FIG. Crystal pulling could be started.

The high frequency output is fixed so that the temperature at this point is maintained, and the raw material melt is continuously reduced at a speed of 6 mm / hr for 12 hours, and finally the outer diameter is 3 mm square, the inner diameter is 1 mm square, and the length is 19 mm. A hollow prismatic BaF ₂ single crystal (FIG. 7) was obtained.

(Evaluation of detector)
In order to demonstrate the usefulness of a functional collimator-type gamma camera composed of a radiation collimator made of scintillator material, a one-dimensional functional collimator is simulated by Monte Carlo simulation, and the reconstructed image is obtained from the obtained detector output. Acquired. A simulation was performed when a BGO scintillator (outer diameter 3 mm square, inner diameter 1 mm square, length 17 mm hollow prism) was used as a collimator, and 140 keV monochromatic gamma rays were used as a radiation source.

FIG. 8 shows a schematic diagram of the experimental system in which simulation was performed. The position specifying accuracy in the collimator is 2.5 mm. In addition, the gamma rays detected in the region of the upper end portion of 7 mm are not used because the direction-limiting characteristics are not good. An example of the result of image reconstruction by the ml-em method (statistical image reconstruction method) from the projection data obtained by the simulation is shown in FIG. Even if the data of the collimator part is used, the deterioration of the spatial resolution is not confirmed. FIG. 10 shows a change in the cosine similarity between the true image and the reconstructed image when the number of gamma rays emitted from the radiation source is changed. If the cosine similarity is 1, it is equal to the true image (similarity). It can be seen that an image close to the true image can be acquired by taking into account the information in the collimator section even if the number of gamma rays emitted is small. As an example, FIG. 11 shows a reconstructed image obtained from the result of emitting 1.25 × 10 ⁵ gamma rays. As the number of gamma rays emitted decreases, it can be seen that the vibration of the pixel value increases only with the bottom surface information, and the image quality is disturbed. In this way, by adding information on the collimator unit, the number of gamma rays required to obtain sufficient image quality can be reduced, and the dose of radiopharmaceutical can be reduced, leading to lower exposure. I can say that.

Next, an experiment was conducted to confirm whether or not the gamma ray incident position can be specified with a radiation collimator made of a scintillator material. A collimator (outer diameter 3 mm square, inner diameter 1 mm square, length 19 mm) (FIG. 7) (FIG. 7) made of a BaF ₂ hollow prismatic single crystal manufactured as described above was incorporated into the experimental system shown in FIG. 12. FIG. 13 shows the received light amount spectrum obtained by the photomultiplier tube when the height of the 60 KeV gamma irradiation position is changed by ²⁴¹ Am of the α ray sealed radiation source. It can be seen that the amount of received light varies depending on the irradiation position height, and that the position information in the direction perpendicular to the detection surface has been successfully obtained.

DESCRIPTION OF SYMBOLS 1 After heater 2 Heater 3 Heat insulation material 4 Stage 5 Crucible 6 Chamber 7 High frequency coil 8 Pull-down rod

Claims (5)

A radiation collimator comprising a scintillator material. The radiation collimator according to claim 1, comprising a hollow columnar single crystal. A radiation detector comprising a collimator, a scintillator and a photodetector, wherein the collimator is the collimator according to claim 1. The collimator according to claim 1, wherein the amount of light emitted from the collimator is measured to determine the vertical position of the light emitting portion due to the radiation of the collimator, and the direction in which the radiation has come from the vertical position of the light emitting portion. The radiation detection method characterized by specifying. In single crystal growth by the micro pull-down method or Czochralski method, the purpose of stabilizing the growth shape by using a non-melting member for shape control with an annular hole near the interface between the melted raw material and the crystallized part A process for producing a hollow columnar single crystal for a collimator for radiation, wherein two or more seed crystals or metal wires are brought into contact with the crystal and grown simultaneously.

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