JPH02678B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention belongs to the technical field of radiation detectors having a scintillator and a multichannel photodiode. [Technical background of the invention and its problems] Radiation tomography devices, for example, third-generation or fourth-generation X-ray CT devices, have an are doing. In recent years, solid-state scintillation detectors, which are a combination of a scintillator and a photodiode, have been widely used as X-ray detectors, instead of gas ionization chambers, which have been the mainstream in the past. This is because the photodiodes used in solid-state scintillation detectors can be mounted in high density, so in order to obtain high-resolution CT images, the arrangement pitch of the detection elements must be made as small as possible. This is because they can respond. As shown in FIG. 1, the X-ray detector 1 includes a plurality of light receiving elements (for example, p-type diffusion layers) 3A to 3E arranged one-dimensionally on a semiconductor substrate (for example, an n-type semiconductor substrate) 2. A multi-channel photodiode 3 and light receiving elements 3A to 3 formed as shown in FIG.
E is provided with scintillator elements 5A to 5E bonded with a transparent adhesive 4 such as a glass adhesive. In addition, in the figure, 6 is a coated light reflecting material. The reason why the light-receiving areas of the light-receiving elements 3A and 3E located at both ends of the semiconductor substrate 2 are smaller than the light-receiving areas of the light-receiving elements 3B to 4D located inside is due to manufacturing considerations of the multi-channel photodiode 3. This is because the light receiving element cannot be formed in a certain area of the semiconductor substrate 2 from the cut edge thereof.
Light receiving elements 3B~ other than the light receiving elements 3A and 3E at both ends
The 3D light receiving area is the corresponding scintillator element 5.
Conventionally, it was customary in manufacturing to make the area the same as the light output surface of B to 5D. This is due to the requirement that the light output from the scintillator element be received by the light receiving element as efficiently as possible. By the way, although the light receiving elements 3A to 3E have a capacitance, it is preferable that this capacitance is as small as possible in order to suppress noise generated when coupled with the first stage amplifier. As one means of reducing this capacitance, it is easy to obtain a semiconductor substrate 2 with a resistivity ρ of approximately several KΩcm. On the other hand, it is better to make the distance d ₁ between the light receiving elements as narrow as possible in order to increase the X-ray incident area of the corresponding scintillator element. Usually, the distance _d1 between the light receiving elements is about 0.1 to 0.2 mm. However, when the resistivity ρ of the semiconductor substrate 2 is increased and the distance d ₁ between the light receiving elements is decreased, the following problem occurs. The amount of crosstalk increases. It is a well-known fact that if the resistivity ρ of the semiconductor substrate is increased, the carrier diffusion length L also increases. If this diffusion length L increases and the distance d ₁ between the light-receiving elements becomes narrower, carriers naturally leak to adjacent channels, causing crosstalk. Electrical isolation between channels deteriorates. Ideally, the channels should be completely electrically isolated. However, when a multi-channel light receiving element is formed on a single semiconductor substrate, the channels are not completely insulated. In particular, reducing the distance d ₁ between the light-receiving elements deteriorates the electrical insulation between channels, which has an undesirable effect on coupling with subsequent circuits. [Object of the Invention] The present invention has been made in view of the above circumstances, and it reduces the amount of crosstalk of a multi-channel photodiode, provides good electrical insulation between channels, and further reduces the capacitance of the channels. The purpose is to provide a radiation detector that can reduce noise and reduce noise. [Summary of the Invention] The outline of the present invention for achieving the above object is to provide scintillator elements arranged at the same pitch to output light according to the amount of incident radiation, and arranged at predetermined intervals in the longitudinal direction on a semiconductor substrate. , a radiation detector formed by joining the formed light receiving elements, characterized in that the light receiving area of each light receiving element is smaller than the light output area of the corresponding scintillator element. [Embodiments of the Invention] Hereinafter, the radiation detector of the present invention will be specifically described according to embodiments shown in the drawings. FIG. 2 is a schematic sectional view showing an embodiment of the present invention. As shown in FIG. 2, a radiation detector, for example, an X-ray detector 1, which is an embodiment of the present invention, differs from the X-ray detector 1 shown in FIG.
The light receiving area of ~3E is made smaller than the light output area of the corresponding scintillator elements 5A~5E. However, it goes without saying that the edge sides of the light receiving elements located at both ends of the semiconductor substrate 2 are the same as in the case of FIG. 1, and the scintillator elements 5A to 5E are
All surfaces other than the light output surface of the scintillator elements 5A to 5E are coated with a light reflecting material 6, and the light output surfaces of the scintillator elements 5A to 5E and the light receiving elements 3A to 3E are coated with a light reflecting material 6.
The conventional example is that the light-receiving surface is bonded with a transparent adhesive 4, such as glass adhesive (as shown in Figure 1).
is the same as By the way, there is a concern that the sensitivity will decrease as the light-receiving area of the light-receiving element becomes smaller. distance △d
If is set to an appropriate value, there is almost no concern about the decrease in sensitivity. This will be specifically explained by taking as an example a case where a pn junction photodiode is used as a light receiving element. A thin p-layer is typically formed on an n-type substrate to form a photodiode. Third
As shown in the figure, the distance between the light-receiving surface of the light-receiving element
Carrier K ₁ that occurred in the area of d ₂ (hereinafter referred to as d ₂ )
Of course, it contributes as a signal current. The carrier _K2 generated in the area of distance △d (hereinafter referred to as △d) from the end of the photodetector to the end of the corresponding scintillator element is determined by the diffusion length L of the minority carriers of the semiconductor substrate (hereinafter referred to as L). ), it can still contribute as a signal current. In other words, △
If d<L is satisfied, there will be almost no deterioration in sensitivity compared to the case where Δd=0, that is, the light receiving area of the light receiving element is made the same as the light output area of the corresponding scintillator element. According to experimental confirmation, for example, △d=200μm, L=500μm
In the case of m (Δd<L is satisfied), no deterioration in sensitivity was required at all. Next, the effect of reducing crosstalk will be described.
Carrier K ₂ generated in the △d region in Fig. 3
In order for the light to reach the p-n junction of the adjacent channel, it must travel the minimum distance (d ₁ - △d) by diffusion in relation to the distance d ₁ (hereinafter referred to as d ₁ ) between each light receiving element. There must be. Therefore, if the condition d ₁ -Δd>L is satisfied, no crosstalk will occur. Next, since the electrical insulation between the channels increases as d ₁ increases, it is clear that the embodiment of the present invention is better than the conventional example. The fact that this was experimentally confirmed by measuring two values of d ₁ is shown below.

[Table] It can be seen that increasing the value of d ₁ improves electrical insulation. Furthermore, since the capacitance of a channel is proportional to the light-receiving area of the light-receiving element, the capacitance can be reduced according to the embodiments of the present invention.
The fact that this was experimentally confirmed for two values of d ₂ is shown below.

[Table] It can be seen that the smaller the value of d ₂ (that is, the smaller the light-receiving area of the light-receiving element), the smaller the capacitance of the channel. Finally, in FIG. 2, we will discuss the effect of reducing noise generated by X-rays that are not absorbed by each of the scintillator elements 5A to 5E but are directly absorbed by the semiconductor substrate 2 (X-rays that have passed through the X ₁ path). . Carriers are also generated when X-rays that have passed through _one pass are absorbed by the semiconductor substrate. It can be proven that when this carrier mixes into the signal current, the ratio of quantum noise to signal current increases due to the difference in X-ray energy conversion efficiency between the scintillator and the semiconductor. The X-ray energy conversion efficiency here is an index that shows what percentage of the absorbed X-ray energy contributes to signal current generation.For example, if cadmium tungstate (cdW0 ₄ ) is used as a scintillator, the scintillator's energy The conversion efficiency is about 2%, but in a semiconductor substrate with a large diffusion length L, it is about 100%. However,
According to the embodiment of the present invention, carriers generated by being absorbed by the semiconductor substrate through the _X1 path are mixed into the signal current as the distance between the carrier generation point and the pn junction increases. This makes it possible to make the ratio smaller than in the conventional example, and it is possible to reduce quantum noise due to X-rays in the _X1 path. [Effects of the Invention] As explained above, the present invention changes the value of △d to △d
By appropriately determining the relationship such that <L<d ₁ -Δd holds, the following effects can be achieved with almost no deterioration in sensitivity. That is, the present invention can reduce the amount of crosstalk in a multi-channel photodiode, improve the electrical insulation between channels, reduce the capacitance of the channels, and reduce quantum noise caused by X-rays in the _X1 path. It is possible to provide a radiation detector that exhibits effects such as reducing the amount of radiation.

[Brief explanation of drawings]

FIG. 1 is a schematic sectional view showing a conventional radiation detector, FIG. 2 is a schematic sectional view showing an embodiment of the present invention, and FIG. 3 is an explanatory diagram showing the effects of the present invention. 1... Radiation detector (X-ray detector), 2... Semiconductor substrate, 3A to 3E... Light receiving element, 5A to 5E
...Scintillator element.

Claims (1)

[Claims] 1. In a radiation detector formed by joining scintillator elements arranged at the same pitch that output light according to the amount of incident radiation and respective light receiving elements arranged and formed at predetermined intervals in the longitudinal direction on a semiconductor substrate,
A radiation detector characterized in that the light receiving area of each light receiving element is smaller than the light output area of the corresponding scintillator element.