JPH03238387A - Radiation detector - Google Patents

Abstract

PURPOSE:To detect radiation high in the efficiency of transmission to a photomultiplier tube by finishing the surface opposed to the emitting surface of scintillation light among the surfaces of a Bi4Ge3O12 crystal into a rough surface and subjecting the emitting surface and residual surfaces to specular finish. CONSTITUTION:The surface 2a opposed to the emitting surface of scintillation light among the surfaces of a Bi4Ge3O12(BGO) crystal 2 is finished into a rough surface and the emitting surface and the residual surfaces are subjected to specular finish. The surface roughness Ra of the rough surface 2a of the BGO crystal 2 is desirably at least 0.8 mum. When the surface roughness Ra is below 0.8 mum, the quantity of emitted light as a scintillator becomes little. A reflecting layer 3 is desirably provided to the surfaces other than the emitting surfaces of scintillation light of the BGO crystal. Because of this constitution, even when scintillation light is incident to the surface 2a vertically, the light is reflected toward a photomultiplier tube 1 without being transmitted through the surface 2a. The scintillation light incident to side surfaces is totally reflected to reach the photomultiplier tube 1.

Description

[Detailed description of the invention] [Industrial application field]

TECHNICAL FIELD The present invention relates to a radiation detector used in radiation medical diagnostic equipment and high-energy physics, such as an X-ray tomography device (XMCTI) or positron emission tomography device (positron CT).

[Conventional technology]

One type of radiation detector is one in which a photomultiplier tube 1 and a scintillator 2 are joined together. These radiation detectors are
As shown in the cross section in FIGS. 3 and 4, the photomultiplier tube 1 has a rectangular Bi4GesO+m crystal (hereinafter referred to as rBGo crystal).
) are optically joined together. BGO crystal 2
A reflective layer 3 made of a reflective material such as BaSO4 powder is provided on the exposed surface. The gamma rays 4 incident on the BGO crystal 2 are converted into photons, and the scintillation light is diffusely reflected by the reflective layer 3 and guided to the photomultiplier tube l. The BGO crystal 2 is required to have the ability to convert the incident γ-rays 4 into as many photons as possible and to transmit the scintillation light to the photomultiplier tube 1 to the maximum extent, that is, to have a large amount of light emission. The amount of luminescence is BGO crystal 2
It changes depending on the surface condition of the surface, but the relationship between them has not been elucidated at this stage. The surface state of BGO crystal 2 is the third
As shown in the figure, only the one in which the scintillation light output surface is mirror-finished and the other surfaces are roughened, and the one in which all the surfaces are mirror-finished as in FIG. 4 are known.

[Problem to be solved by the invention]

For example, when positron annihilation gamma rays having an energy of 551 keV are incident on the BGO crystal 2, the gamma rays are converted into photons at a relatively shallow portion of the BGO crystal. However, when the side surface of the BGO crystal 2 is rough as in the detector shown in FIG. 3, scintillation light diffusely reflected by the rough surface leaks through the reflective layer 3. When the surface of the BGO crystal 2 facing the scintillation light output surface, that is, the main radiation incidence surface 2a, is a mirror surface as in the detector shown in FIG. There is a high probability that
The light passes through the reflective layer 3 and escapes without being reflected. The present invention has been made to solve the above problems, and an object of the present invention is to provide a radiation detector that emits a large amount of scintillation light and has high transmission efficiency to a photomultiplier tube.

[Means to solve the problem]

In order to solve the above problems, the present inventors used a BGO crystal for positron CT and conducted research on the relationship between the surface state of the crystal and the amount of light emitted. As a result, they discovered that a BGO crystal whose surface facing the scintillation light emission surface is rough and whose other surfaces are mirror-finished has the highest amount of light emission, and has completed the present invention. That is, the radiation detector of the present invention is a radiation detector in which a BGO crystal 2 is bonded to the entrance window of a photomultiplier tube 1, as shown in FIG. 1 corresponding to the embodiment. Among the surfaces of the BGO crystal 2, the surface 2a facing the scintillation light emission surface is a rough surface, and the emission surface and the remaining surfaces are mirror-finished. The surface roughness Ra of the rough surface 2a of the BGO crystal 2 is at least
Desirably, it is 0.8I1m. Surface roughness Ra is 0
．． If it is less than 8 um, the amount of light emitted as a scintillator will be small. It is desirable to provide a reflective layer 3 on a surface of the BGO crystal 2 other than the scintillation light output surface.

[Effect]

In the radiation detector of the present invention, the surface 2a of the BGO crystal 2 facing the scintillation light output surface is a rough surface, and the other surfaces are mirror surfaces, so that the scintillation light is transmitted to the surface 2a.
Even when the light is incident perpendicularly to the photomultiplier tube 1, the light is reflected toward the photomultiplier tube 1 without being transmitted. Further, the scintillation light incident on the side surface is totally reflected and reaches the photomultiplier tube 1.

【Example】

Examples of the present invention will be described in detail below. FIG. 1 is a perspective view showing an embodiment of a radiation detector to which the present invention is applied. In the figure, l is a photomultiplier tube, and 2 is a rectangular parallelepiped BGO crystal optically joined to the entrance window of the photomultiplier tube 1. A reflective layer 3 made of Teflon tape as a reflective material is provided on five surfaces of the BGO crystal 2 excluding the scintillation light output surface. The surface 2a of the BGO crystal 2 facing the scintillation light output surface is a rough surface, and the other surfaces are mirror-finished. Experimental examples of the present invention are as follows. 4 x 12 x 3 from BGO crystal using inner circumferential cutter
Cut out 27 blocks of 0mm size. All surfaces of the cut BGO crystal are polished with GC #600 abrasive grains to obtain a rough surface with a surface roughness Ra = 0.8μ. The roughened BGO crystals are divided into three groups of nine each, and each group of crystals is processed into a different scintillator. A first group of BGO crystals is processed as an example of the present invention. Finish the page surface except for one side 2a of the 4 x 12++m side to a mirror surface, wrap Teflon tape on each side except for the 4 x 12m- surface that has been mirror finished, and apply a reflective layer 3 with a thickness of 400u.
was provided, and scintillator 2 was created. When the surface of the produced scintillator 2 without the reflective layer 3 is joined to the photomultiplier tube 1, the radiation detector shown in FIG. 1 is obtained. The second group of BGO crystals has a mirror finish on only one side of 4 x 12 mm, and a reflective layer 3 is applied to the other surfaces in the same manner as above.
will be established. When the mirror surface is connected to the photomultiplier tube 1, the third
The radiation detector shown in the figure (Comparative Example 1) is obtained. The third group of BGO crystals has all surfaces mirror-finished. 4
A reflective layer 3 is provided except for one side of 4 x
When the 12m5+ surface is joined to the photomultiplier tube l, a radiation detector (comparative example 2) shown in FIG. 4 is obtained. The gamma spectrum of the radiation detector prepared above was measured using the measurement system shown in FIG. 2, and the amount of luminescence was determined from it. This measurement system uses γ! The light generated by increasing I4 to 4.1 is received by a photomultiplier tube 1, amplified by a preamplifier 5 and an amplifier 6, and then a counter 7 counts the number of photons generated. Note that the reference numeral 8 in the figure is a power source. Table 1 shows the measurement results of Examples and Comparative Examples. (The following is a blank space) Table 1 According to these measurement results, it can be seen that the radiation detector of the present invention has the highest amount of light emission. Also, the external dimensions are 3 x 12 x 30+am, 5 x 1
When a 2×30 mm scintillator was made and measured, the same results as above were obtained. In the above embodiment, Teflon tape was used as the reflective material of the reflective layer 3, but for example, Ba5O<, Ti0i
, Al*Ox, and MgO can also be used. Further, in the embodiment, an example in which a single BGO crystal is bonded to a photomultiplier tube is described, but a scintillator in which a plurality of GO crystals are connected via a reflective layer as described above may also be used.

【Effect of the invention】

As explained in detail above, the radiation detector of the present invention has B
The transmission efficiency of scintillation light within the GO crystal is high and the amount of light emitted is large. Therefore, radiation can be detected with high sensitivity and accuracy.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an embodiment of the radiation detector of the present invention,
FIG. 2 is a block diagram showing a measurement system, and FIGS. 3 and 4 are sectional views showing a conventional radiation detector. l...Photomultiplier tube 2...BGO crystal 2a...
-Incidence surface 3...Reflection layer 4...γ fringe

Claims (3)

[Claims] 1. Bi_4Ge_3O_1_ in the entrance window of the photomultiplier tube
In a radiation detector in which two crystals are bonded, Bi_4G
A radiation detector characterized in that the surface of the e_3O_1_2 crystal that faces the emission surface of scintillation light is a rough surface, and the emission surface and the remaining surfaces are mirror-finished. 2. The radiation detector according to claim 1, wherein the rough surface of the Bi_4Ge_2O_1_2 crystal has a surface roughness Ra of at least 0.8 nm. 3. 2. The radiation detector according to claim 1, wherein a reflective layer is provided on a surface of the Bi_4Ge_2O_1_2 crystal other than the scintillation light output surface.