JP2001177140A - Radioactive detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a very sensitive radioactive detector which has little generation of surface discharging even if the applied voltage is raised. SOLUTION: A lower electrode 2 is formed on a glass substrate 1. Then at least a selenium layer 4, an antimony trisulfide layer 8, and an upper electrode layer 5 are deposited in layers on the lower electrode 2. When applying DC voltage between the lower electrode 2 and the upper electrode 5, the voltage of the positive (+) polarity is applied to the upper electrode layer 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct conversion type radiation detector used in the medical, industrial, and nuclear fields.

[0002]

2. Description of the Related Art As a detector for radiation (for example, X-ray), an X-ray is converted into light and then converted into an electric signal.
There is a direct conversion type that immediately converts a wire into an electric signal. As a device of the latter direct conversion system, for example, as shown in FIG. 5, an ITO film is deposited on a glass substrate 1 to form a lower electrode 2, and a selenium layer 4 is formed thereon as an X-ray sensitive semiconductor film. A structure having a structure in which an upper electrode layer 5 is further provided thereon is known. Reference numeral 3 is a ground electrode. In the X-ray detector having such a structure, a DC voltage is applied from the power supply 6 between the lower electrode 2 and the upper electrode layer 5 and X-rays are irradiated from above the upper electrode layer 5 in that state. As a result, charges (electron-hole pairs) are generated in the selenium layer 4 and are collected on the lower electrode 2 before the X-ray irradiation current I ₁
Is read as The lower electrode 2 is obtained by doping indium oxide with tin oxide and obtaining the obtained ITO (Indium T
in Oxide) It was formed as an ITO film by evaporating powder.

Another example of the direct conversion method is a structure in which a large number of pixels of about 100 to 200 μm square are provided on a glass substrate to form a lower electrode, and a selenium layer and an upper electrode layer similar to those described above are provided thereon. Some are. Each pixel is provided with a function to collect charges generated when X-rays are irradiated, a function to accumulate the collected charges, and an electric switch function to read the accumulated charges. There are those read as X-ray emission current I ₁ through the switch mechanism.

By the way, the X-ray detector is required to have high sensitivity, but the sensitivity of the X-ray detector depends on the amount of electric charge collected at the lower electrode when the X-ray irradiation amount is constant, that is, at the lower electrode. determined by some of the X-ray emission current I ₁ flowing.
It is desirable that the S / N ratio of the X-ray detector (the ratio between the current value S when X-rays are irradiated and the dark current value N when X-rays are not irradiated) is large.

As described above, when a selenium layer is used as an X-ray sensitive semiconductor film, the transfer speed of charges generated in the selenium layer depends on the DC voltage applied between the lower electrode and the upper electrode layer. The higher the value, the faster the current, and as a result, the current flowing through the lower electrode increases. Therefore, in order for the X-ray detector to have high sensitivity, it is necessary to increase the DC voltage applied to the X-ray detector.

[0006]

[SUMMARY OF THE INVENTION However, as shown in FIG. 5, the higher the applied voltage surface discharge I ₂ along the surface of the selenium layer flows to the ground electrode 3, X-ray used as a detector irradiated It may adversely affect the image noise is contained in the current I _1. Therefore, as shown in FIG. 5, a surface discharge I ₂ hardly occurs structure by a distance Y is increased between the outer edge of the outer peripheral edge and the selenium layer 4 of the upper electrode layer 5, whereby the applied voltage However, there has been a problem in that the area of the upper electrode layer 5, which is the X-ray detection portion, decreases when the distance Y is increased.

It is an object of the present invention to provide a high-sensitivity radiation detector in which the occurrence of surface discharge is small even when the applied voltage is increased.

[0008]

In order to solve the above-mentioned problems, the present inventor has provided an antimony trisulfide layer between a selenium layer and an upper electrode layer, and selected the direction of the polarity of DC voltage application. It has been found that surface discharge can be suppressed. Specifically, the radiation detector according to claim 1 of the present invention has a structure in which a lower electrode is formed on an insulating substrate, and at least a selenium layer, an antimony trisulfide layer, and an upper electrode layer are stacked thereon, When a DC voltage is applied between the lower electrode and the upper electrode layer, a positive (+) polarity voltage is applied to the upper electrode layer.

According to the present invention, antimony trisulfide (S
b ₂ S ₃ ) layer has a small charge accumulated on its surface,
Discharge does not easily occur along the surface of the antimony trisulfide layer. In addition, the dark current when the X-ray is not irradiated in the dark is smaller than when only the selenium layer is used, and the S / N value is increased. In addition, as the insulating substrate used in the present invention,
There are glass and plastic.

According to a second aspect of the present invention, in the radiation detector according to the first aspect, the lower electrode formed on the insulating substrate is made of an ITO film.

According to a third aspect of the present invention, in the radiation detector according to the first aspect, the lower electrode formed on the insulating substrate collects electric charges generated in the selenium layer by the incidence of radiation. It is characterized by comprising a plurality of pixels having a collecting function, a charge storing function of storing the collected charges, and an electric switch function of reading the stored charges.

According to a fourth aspect of the present invention, in the radiation detector according to the first aspect, the lower electrodes formed on the insulating substrate are strip-shaped conductors arranged at predetermined intervals. And

As described above, the selection range of the lower electrode formed on the insulating substrate is increased, and it is possible to form an optimum lower electrode according to the application and the like.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a radiation detector according to the present invention will be described with reference to the accompanying drawings. Figure 1 shows the first
It is a figure showing the basic composition of the X-ray detector concerning an embodiment. In this figure, 1 is a glass substrate, 2 is a lower electrode using an ITO film, 3 is a ground electrode, 4 is a selenium layer,
5 is an upper electrode layer, 6 is a power supply for applying a DC voltage,
Reference numeral 8 denotes an antimony trisulfide layer provided between the selenium layer 4 and the upper electrode layer 5.

The above-mentioned antimony trisulfide layer 8 has a function of suppressing surface discharge generated in the upper electrode layer 5. This is because the resistivity of the antimony trisulfide layer 8 is about 10 ⁹ Ωcm.
Because the resistivity of the selenium layer 4 is smaller than about 10 ¹⁴ Ωcm, the electric charge accumulated on the surface of the antimony trisulfide layer 8 is small, and it is difficult to generate discharge along the surface of the antimony trisulfide layer 8. . Further, when the antimony trisulfide layer is provided, the dark current when the X-ray is not irradiated in the dark is smaller than when only the selenium layer is used, and the S / N value is increased. This is because, as shown in FIG. 1, near the upper electrode layer, charge injection from the upper electrode layer into the selenium layer, which is one of the causes of dark current, is suppressed.
While the current in the seven directions decreases and the dark current decreases, X
This is because the electron generated in the selenium layer does not act as a barrier to move in the direction of arrow 18 at the time of X-ray irradiation, so that the current at the time of X-ray irradiation does not change.

As described above, the ITO film constituting the lower electrode 2 can be formed by doping indium oxide with tin oxide and depositing the obtained ITO (Indium Tin Oxide) powder, or by sputtering. It can also be formed by a method or the like. Also, the lower electrode 2
For example, a conductor such as aluminum, chromium, and nickel can be used.

The thickness of the selenium layer 4 formed on the upper surface of the lower electrode 2 is preferably in the range of about 200 to 1500 μm. If the thickness is less than 200 μm, the quantum efficiency with respect to X-rays decreases, and sufficient sensitivity cannot be obtained.
If the thickness is larger than 00 μm, the charge trapped in the selenium layer 4 increases, and the response to X-rays deteriorates.

The selenium layer 4 includes not only a case where the selenium is formed alone but also a case where the selenium layer 4 is formed of an alloy of selenium and arsenic or an alloy of selenium and tellurium. The selenium / arsenic alloy layer has an effect of preventing the selenium layer 4 from being crystallized, and preferably has a thickness of 30 μm or less. Further, as shown in FIG. 2, an intermediate layer 15 can be provided between the glass substrate 1 and the selenium layer 4. When the intermediate layer 15 is formed of an organic film of a polycarbonate resin, the adhesiveness of the selenium layer 4 to the glass substrate 1 is improved. In the case where the intermediate layer 15 is formed of an inorganic film such as cadmium telluride (CdTe), it plays a role of preventing charge injection from the glass substrate 1 to the selenium layer 4.

The thickness of the antimony trisulfide layer 8 is set at 0.1 mm.
The range of 01 to 10.0 μm is desirable. The film thickness is 0.01
When the thickness is smaller than μm, the effect of suppressing surface discharge decreases, and the effect of suppressing dark current also decreases. On the other hand, when the film thickness is more than 10.0 μm, although the effect of suppressing surface discharge is obtained, the X-ray irradiation current decreases. The antimony trisulfide layer 8 can be formed by a vapor deposition method.

The material of the upper electrode layer 5 provided on the antimony trisulfide layer 8 is a conductive material such as gold, silver, aluminum, and nickel, and can be formed by a vapor deposition method, a sputtering method, or the like. The thickness of the upper electrode layer 5 is
The range of 0.01 to 3.0 μm is desirable.

3 and 4 show another embodiment of the lower electrode 2 formed on the glass substrate 1. FIG. The lower electrode 2 shown in FIG.
Pixels 10 of 200 μm square are arranged in a matrix. Each pixel 10 has a charge collecting electrode 11 for collecting charges generated in the selenium layer 4 by X-ray incidence, and a charge storage for accumulating the collected charges. A capacitor 12 and a thin film transistor 13 as an electrical switch mechanism for reading the stored electric charge are formed. Further, the lower electrode 2 shown in FIG. 4 is formed by arranging strip electrodes 2a on the upper surface of the glass substrate 1 at regular intervals. Aluminum, chromium, ITO, or the like is used as a material of the strip electrode 2a.

The X-ray irradiation current I ₁ in the lower electrode 2
In the case where the lower electrode 2 is an ITO film or a strip-shaped electrode 2a, the size is read by an ammeter 7 connected to the lower electrode 2 as shown in FIG. In addition, as shown in FIG. 3, when the lower electrode 2 is formed by a large number of pixels 10, the current collected by the charge collecting electrode 11 is charged in the charge storage capacitor 12, and is supplied from the gate driver through the switching line 19. When a voltage is applied to the gate terminal of the thin film transistor 13, the thin film transistor 13 is turned on. Then, the charge charged in the charge storage capacitor 12 is read through the read line 20 connected to the source terminal of the thin film transistor 13.

[0023]

Embodiments of the present invention will be described below. (Example 1) Size 75 mm x 75 mm, thickness 0.7 mm
An ITO film was formed on a glass substrate 1 (product number 7059, manufactured by Corning Incorporated) to form a lower electrode 2. Lower electrode 2
Is 28 mm × 28 mm. A selenium layer 4 having a size of 40 mm × 40 mm was formed thereon by vacuum evaporation to a thickness of 500 μm.
m, and an antimony trisulfide layer 8 having a size of 40 mm × 40 mm was formed on the selenium layer 4 by vacuum evaporation to a thickness of 0.1 μm. Further, on the antimony trisulfide layer 8, a size of 28 mm × 28 mm is formed by vacuum evaporation.
Was formed to a thickness of 0.1 μm to form the upper electrode layer 5, and an X-ray detector as shown in FIG. 1 was produced. In addition,
The upper electrode layer 5 is located at a position facing the lower electrode 2 with the selenium layer 4 interposed therebetween.

A voltage of 10 V is applied between the upper electrode layer 5 and the lower electrode 2 per 10 μm of the selenium layer 4 to the X-ray detector produced above. irradiating the line (X dose 60MR / min) was investigated whether the surface discharge from the read X-ray emission current I ₁ of the waveform. Table 1 shows the results when a positive (+) polarity voltage was applied to the upper electrode layer 5 when a voltage was applied. Table 2 shows the results when the S / N ratio was determined by measuring the dark current and the X-ray irradiation current.

Example 2 An X-ray detector was manufactured in the same manner as in Example 1 except that the thickness of the antimony trisulfide layer 8 was changed to 0.01 μm. Further, the presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results.
Shown in

(Example 3) An X-ray detector was manufactured in the same manner as in Example 1 except that the thickness of the antimony trisulfide layer 8 was changed to 10.0 µm. In addition, the first embodiment
The presence / absence of surface discharge was investigated in the same manner as described above. Table 1 shows the results.

Example 4 An X-ray detector was manufactured in the same manner as in Example 1 except that the selenium layer 4 was formed of a selenium-arsenic alloy layer (concentration: 0.1% by weight). Further, the presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results.

(Embodiment 5) In the first embodiment, the IT
Instead of the O film, the electrode width is 0.4 mm on the glass substrate 1,
An X-ray detector was manufactured by the same method except that the strip electrodes 2a made of chromium having a pitch width of 0.6 mm were arranged and an electrode substrate as shown in FIG. 4 was used. Further, the presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results. The current was measured by connecting a connector to the strip electrode 2a.

(Embodiment 6) In the first embodiment, the IT
Instead of the electrode substrate provided with the O film, the size of the glass is 7
An X-ray detector was manufactured in the same manner except that a 5 mm × 75 mm TFT matrix substrate (pixel size 150 μm square) was used. Each pixel includes a charge collection electrode 11 for collecting charges generated in the selenium layer 4 by the incident X-rays,
It has a charge storage capacitor 12 for storing collected charges and a thin film transistor 13 which is an electric switch for reading the stored charges. The presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results.

Example 7 An X-ray detector was manufactured in the same manner as in Example 6, except that the size of the upper electrode layer 5 was changed to 32 mm × 32 mm. In addition, the first embodiment
The presence / absence of surface discharge was investigated in the same manner as described above. Table 1 shows the results.

(Eighth Embodiment) In the first embodiment, the selenium layer having a thickness of about 20 μm is provided between the selenium layer 4 and the lower electrode 2.
An X-ray detector was manufactured in the same manner except that an arsenic alloy layer (arsenic concentration 0.5% by weight) was provided as the intermediate layer 15.
Further, the presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results.

Comparative Example 1 An X-ray detector was manufactured in the same manner as in Example 1 except that the antimony trisulfide layer was omitted. Table 1 shows the results when the presence or absence of surface discharge was measured in the same manner as in Example 1. Table 2 shows the results when the S / N ratio was determined by measuring the dark current and the X-ray irradiation current.

Comparative Example 2 An X-ray detector was manufactured in the same manner as in Example 5 except that the antimony trisulfide layer was omitted. Further, the presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results.

Comparative Example 3 An X-ray detector was manufactured in the same manner as in Example 6 except that the antimony trisulfide layer was omitted. Further, the presence or absence of surface discharge was examined in the same manner as in Example 1 above. Table 1 shows the results.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

As described above, according to the radiation detector of the present invention, even if the applied voltage is increased, the occurrence of surface discharge is small and the X-ray with high sensitivity which can increase the S / N ratio can be obtained. A line detector was obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing one embodiment of an X-ray detector according to the present invention.

FIG. 2 is a conceptual diagram showing another embodiment of the X-ray detector according to the present invention.

FIG. 3 is an explanatory diagram in a case where a lower electrode is formed of an aggregate of pixels.

FIG. 4 is an explanatory diagram in the case where a lower electrode is constituted by a strip electrode.

FIG. 5 is a conceptual diagram showing an example of a conventional X-ray detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass substrate (insulating substrate) 2 Lower electrode 2a Strip electrode 4 Selenium layer 5 Upper electrode layer 8 Antimony trisulfide layer 10 Pixel 11 Charge collection electrode 12 Charge storage capacitor 13 Thin film transistor 15 Intermediate layer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2G088 FF02 GG21 JJ05 JJ09 JJ31 JJ37 LL11 5F088 AB01 BA04 BB06 BB07 FA04 FA05 FA09 FA14 LA07

Claims (4)

[Claims] 1. A structure in which a lower electrode is formed on an insulating substrate, and at least a selenium layer, an antimony trisulfide layer and an upper electrode layer are laminated thereon, and a direct current is provided between the lower electrode and the upper electrode layer. A radiation detector characterized in that a positive (+) polarity voltage is applied to the upper electrode layer when a voltage is applied. 2. The radiation detector according to claim 1, wherein the lower electrode formed on the insulating substrate is made of an ITO film. 3. A charge collecting function for collecting a charge generated in the selenium layer by the incidence of radiation, a charge storing function for storing the collected charge, and a lower electrode formed on the insulating substrate. 2. The radiation detector according to claim 1, comprising a plurality of pixels having an electric switch function for reading electric charges. 4. The radiation detector according to claim 1, wherein the lower electrode formed on the insulating substrate is a band-shaped conductor arranged at predetermined intervals.