JPS6135518B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a gamma ray detector with particularly high resolution and high S/N.
The present invention relates to a gamma ray detector applied to positron CT.

Here, the detector refers to a scintillator crystal, a photocoupling film, a reflective film, and a photomultiplier tube (PMT).
Refers to an integrated element that has the function of converting gamma rays into electrical signals.

In order to configure a ring-type positron CT with high resolution, the scintillator crystal must be made as small as possible.

In addition, in order to reduce the influence of scattered gamma rays, which are noise components, and obtain positron CT with a high S/N ratio, the fluorescence generated within the scintillator crystal is transmitted to the photomultiplier tube light receiving surface in as large a quantity as possible. Must be.

The conventional γ-ray detector for positron CT was arranged as shown in FIG. In the figure, 1
2 is a scintillator single crystal (Bi ₄ Ge ₃ O ₁₂ ), and 2 is a photomultiplier tube, which has a round shape.

In order to improve the positional resolution of positron CT, the width a of the scintillator crystal is selected to be small enough to prevent excessive crystal transmission in the width direction of the scintillator single crystal (for example, 511 KeV). Note that if the thickness C is selected to be small, the resolution in the body axis direction will improve, but the gamma ray detection area (a x C) will be reduced and the sensitivity will be reduced.
Therefore, it is common that only the width a is reduced. Therefore, the crystal shape must necessarily be rectangular.

On the other hand, photomultiplier tubes are circular, and it is extremely difficult to miniaturize them due to manufacturing technology (13 mm in diameter is the current limit). Therefore, the PMT diameter b is usually much larger than the crystal width a, and as shown in FIG. 1, the PMT only covers a portion of the crystal. When such a configuration is adopted, the amount of fluorescent light generated within the crystal that is transmitted to the PMT is reduced, even if all sides (excluding the PMT optical coupling surface) are covered with a reflective film, and the energy of the detector is reduced. Resolution deteriorates. Therefore, even if a detector is suitable for high resolution, it cannot be expected to have a high S/N ratio.

If it is assumed here that there is a rectangular PMT with a light-receiving surface of exactly the same size as the rectangular crystal size, the fluorescence collection efficiency can be maximized.

However, due to technical problems in PMT production, it is extremely difficult to realize a rectangular PMT in which the lengths of the long and short sides are too different. Furthermore, even with a square light-receiving surface, it is extremely difficult to realize a compact PMT, just like a circular PMT.

The present invention eliminates the above-mentioned conventional technology and provides a γ
The purpose of this paper is to propose a line detector.

The configuration of one embodiment of the present invention is shown in FIG. 1′
and 1" is a Bi ₄ Ge ₃ O ₁₂ single crystal, and it is preferable that only the side opposite to the PMT side, that is, the γ-ray arrival side, is roughly polished, and the other entire surface (5 surfaces) is mirror polished. However, A crystal whose entire surface is roughly polished may also be used.It has been experimentally proven that the fluorescent light collection efficiency is generally higher when the entire upper surface is roughly polished.

3 is a reflective film such as BaSO ₄ , as described later.
The entire surface of the crystal, including the outer wall of the PMT light-receiving surface,
(Excluding PMT side). 4 is an adhesive for optically coupling the crystal and PMT, and any transparent adhesive having a refractive index greater than that of the PMT face plate and smaller than that of Bi ₄ Ge ₃ O ₁₂ may be used. 5 shows a rectangular PMT having a light-receiving surface that is approximately the same as the external dimensions of two crystals whose long sides are connected. In this embodiment, since C=2a is selected, a substantially square shape is adopted. Furthermore, in the present invention, the rectangular PMT has a position discrimination function so that both crystals 1' and 1'' can be distinguished positionally, and the amount of fluorescence from 1' is transferred to the anode terminal 7 and from 1''. The amount of fluorescent light is outputted to the anode terminal 8 as an electrical signal. It also has a position discrimination function.
For the specific structure of PMT, see DEPersyk,
etal: IEEE, NS-26, No. 1p364/367 1979. 6 is a photocathode deposited on the inner wall of the face plate of the rectangular PMT 5, and bialkali is usually used. However, this type of photocathode has a refractive index of 2 or more, which necessitates the use of a material whose refractive index far exceeds that of the face plate (approximately 1.5). In this case, part of the light transmitted to the PMT propagates to the outer wall of the PMT due to surface reflection on the photocathode and total reflection on the photocathode vacuum surface.
Causes loss of light intensity. The situation during this time will be explained with reference to FIG. The scintillation light (fluorescence) generated within the crystal 1' has almost no directivity and reaches the face plate 9 as diffused light. However, since the refractive index of the photocathode 6 is large, about 7% of the light that reaches the photocathode is reflected from the surface. Furthermore, even if the light reaches the photocathode, if it is not converted into photoelectrons, it is totally reflected at the boundary between the photocathode and the vacuum. These lights propagate to the outer wall using the face plate as a light guide, are further reflected and diffused at the boundary with air, and are emitted to the outside.

In order to prevent such external propagation loss of light, in the present invention, the reflective film 3 does not cover only the crystal plane, but covers the side surface of the PMT sufficiently below the level of the photocathode as shown in FIG. Configure. At this time, the light propagating toward the outer wall is completely diffusely reflected by BaSO ₄ coated on the outer wall and propagates toward the photocathode side again. If necessary, the outer wall reflective coating can be further modified by septa such as tungsten lead to effectively reduce the fluorescence of crystals 1' and 1".
A reflective film is also formed between 1' and 1" to reach the side, and the optical coupling film 4 is thickened to reduce the crosstalk of light from both crystals. It is also possible to form a wedge-shaped reflective film on the lower part of the connecting surface to reflect the light in the diagonal direction.

As described above, in the present invention, (1) rectangular crystals with narrow widths are connected to achieve high resolution, forming a large crystal outer shape close to a square, and optically coupling with a rectangular PMT that matches the outer shape of the crystal. As a result, PMT can be manufactured in large size, and the technology is
Difficult points are easily removed. And rectangular
Since it is possible to manufacture a PMT that is close to a square instead of a PMT, it is possible to significantly reduce manufacturing costs in terms of manufacturing problems or yield.

Furthermore, since the outer shape of the crystal group and the shape of the light-receiving surface are rectangular and exactly match, the fluorescence from the crystal is transmitted to the PMT side without wasting any waste.

Furthermore, since the outer wall of the PMT photocathode is sufficiently covered with the reflective film, the amount of light lost from the face plate portion is significantly reduced.

Thus, the gamma ray conversion efficiency of the detector is improved.

(2) When the conversion efficiency of the detector improves, the energy resolution of the detector system improves.

On the other hand, the chance coincidence rate, which is a noise component of positron CT, is reduced as the tom window becomes narrower, that is, the detector has better time resolution, which helps improve the S/N ratio.

There is a corresponding relationship between the time resolution and the energy resolution, and the detector obtained by the present invention can naturally be applied as a high time resolution detector.

Thus, the detector according to the present invention is suitable for positron CT with high resolution and high S/N ratio.
It can be understood as a line detector.

In the embodiment, two Bi ₄ Ge ₃ O ₁₂ scintillator crystals are illustrated, but it goes without saying that two or more crystals may be connected. However, in that case, the rectangular PMT must have a number of position detection functions corresponding to the number of crystals.

[Brief explanation of the drawing]

FIG. 1 shows an example of a conventional gamma ray detector for positron CT and its arrangement configuration, FIG. 2 shows the configuration of an embodiment of the detector according to the present invention, and FIG. It is a figure explaining the propagation state of light. 〓〓〓〓〓

Claims (1)

[Claims] 1 A connected crystal consisting of a plurality of Bi ₄ Ge ₃ O ₁₂ scintillator single crystals having a rectangular light emitting surface and whose peripheral parts are connected, and a photoelectron whose light receiving surface is connected to the light emitting surface of the connected crystal. In a γ-ray detector for positron CT equipped with a multiplier tube, the light-receiving surface of the photomultiplier tube has approximately the same external dimensions as the light-emitting surface of the connected crystal, and the light is received from which single crystal. It is a rectangular photomultiplier tube having a function of identifying the photomultiplier tube, and is characterized in that a continuous reflective film is coated on the side wall surface of the connecting crystal and the connecting portion of the connecting crystal and the photomultiplier tube. γ for positron CT
line detector.