JP2564979B2 - Radiation detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to a radiation detector used in a medical radiation image receiving apparatus, a non-destructive inspection radiation inspection apparatus, and the like.

<Prior Art> In a semiconductor detector used for detecting medical radiation, a compound semiconductor such as CdTe having a high band gap is used. This type of semiconductor radiation detector generally has a structure in which a plurality of signal extraction electrodes are formed on one surface of a compound semiconductor substrate so as to correspond to each pixel, and a common bias electrode is formed on the back surface thereof. Are known. In such a radiation detector, a reverse bias is applied to the bias electrode, and when a radiation photon enters the semiconductor, carriers (electrons / holes) are generated according to the amount of the incident photon. Then, the electrons and holes move to the respective electrodes, so that a charge pulse is generated.

<Problems to be Solved by the Invention> By the way, crystal growth of a compound semiconductor such as CdTe is difficult, and lattice defects and residual impurities are inherent in the crystal.
These defects form an electrically deep level, and trap or emit carriers in the crystal. Therefore, when a large amount of photons are incident on the compound semiconductor crystal, the generated carriers are trapped by the trap in the crystal and re-emitted. As a result, for example, as shown in FIG. 4, the trapped charge e remains in the semiconductor crystal 1 and becomes a scattering center in the crystal or generates a space charge to hinder the movement of carriers. This causes a decrease in detection efficiency of the detector.

<Means for Solving the Problems> The present invention has been made to solve the above-mentioned conventional problems, and the configuration thereof will be described with reference to FIG. 1 corresponding to the embodiment. Electrodes 1b and 1c are formed on the surfaces of the compound semiconductor (CdTe) 1a facing each other,
A radiation sensor that generates a charge pulse upon incidence of radiation, and a light source for irradiating the compound semiconductor 1a with infrared light having an energy smaller than the band gap energy of the compound semiconductor 1a and larger than the trap level existing in the semiconductor crystal ( For example, an infrared laser diode) 4 is provided, and when the radiation sensor is operating, infrared rays are emitted from the light source 4 into the compound semiconductor 1a to excite the carriers trapped in the semiconductor crystal. Characterized.

<Action> When carriers generated in a crystal such as CdTe due to radiation incidence are trapped by crystal defects or the like, for example, as shown in FIG. It is not re-emitted at the temperature of. Therefore,
In the present invention, the crystal is irradiated with infrared rays having an energy higher than the trap level, and the trapped carriers are excited by the energy hν. As a result, the figure (b)
As shown in, the carrier C returns to the conduction band, moves in the CdTe crystal, and is collected in the electrode.

Here, when the energy of the infrared rays to be irradiated is larger than the band gap energy of the compound semiconductor such as CdTe, a leak current occurs in the crystal and the detection sensitivity is lowered,
The irradiation energy of infrared rays needs to be less than that.

<Examples> Examples of the present invention will be described below with reference to the drawings.

 FIG. 1 is a block diagram of an embodiment of the present invention.

A common bias electrode 1b is formed on one surface of the CdTe substrate 1a, and a signal extraction electrode 1c is formed on the opposite surface thereof, which constitutes the X-ray sensor 1 as a whole. In addition,
The radiation sensor 1 is a so-called array type X-ray sensor in which the signal extraction electrodes 1c are arranged in a one-dimensional form or a two-dimensional form to form a plurality of pixels.

The X-ray sensor 1 having the above structure is mounted on the wiring board 3 by a flip chip method or the like, and a reverse bias is applied to the bias electrode 1b via the wiring board 3.
Further, an IC chip 2 is also mounted on the wiring board 3 on the same surface as the sensor mounting surface by a flip chip method or the like, and for example, an amplifier or the like X-ray sensor 1 is mounted on the IC chip 2.
A signal processing circuit for amplifying the output signal from each pixel of FIG. The wiring board 3 on which the X-ray sensor 1 and the IC chip 2 are mounted is housed in the casing 5, and the X-ray sensor 1 is configured so that X-rays enter through the window of the casing 5.

Now, in the embodiment of the present invention, in the casing 5,
In addition, an infrared laser diode 4 is installed near the side of the X-ray sensor 1. The laser diode 4 outputs an infrared ray having an energy smaller than the band gap energy of CdTe used for the X-ray sensor 1 and larger than the trap level existing in the CdTe crystal, for example, an energy of about 0.6 to 1.5 eV. The output light is incident on the inside of the CdTe substrate 1a of the X-ray sensor 1 from the side.

Next, the operation of the embodiment of the present invention will be described. FIG. 2 is an explanatory view of its operation.

First, when X-rays enter the X-ray sensor 1, the CdTe substrate 1a
Carriers (electrons / holes) corresponding to the amount of incident photons are generated inside. This generated carrier is the substrate surface
A pulsed charge is generated by moving toward the electrode 1b or 1c formed on 1a. The charge pulse is
It is guided to the IC chip 2 and undergoes predetermined signal processing there. By counting the number of processed pulses, the X-ray dose incident on each pixel of the X-ray sensor 1 can be known from the counted value.

By the way, carriers generated in the CdTe substrate 1a by X-ray incidence are trapped by crystal defects and the like existing in the CdTe crystal and remain in the crystal, which has conventionally been a cause of reduction in detection efficiency. Therefore, in the embodiment of the present invention, when the X-ray is detected, that is, when the X-ray sensor 1 is operating, the CdTe
By irradiating the substrate 1a with infrared rays from the laser diode 4, carriers trapped by defects and the like are released and quickly collected in the electrode 1b or 1c. That is, when the carriers generated in the crystal are trapped by crystal defects or the like, they fall into a deep level as shown in FIG. 2 (a), and since the level is deep, they are not re-emitted at a temperature around room temperature. However, when the crystal is irradiated with infrared rays having an energy higher than the trap level, the trapped carriers are excited by the light energy hν, and the carrier C returns to the conduction band as shown in FIG. It moves through the CdTe crystal and is collected at the electrode. As described above, by irradiating the CdTe substrate 1a with infrared rays, all the generated carriers are collected in the electrodes, and as a result, the detection efficiency of the X-ray sensor is improved.

Here, using the same X-ray sensor 1, the CdTe substrate
The sensor output was measured with and without irradiation of infrared rays in 1a. As shown in Fig. 3, when infrared rays were not irradiated (b), the detection output was detected in the region where the intensity of the incident X-ray was high. In contrast, in the case of irradiation with infrared rays (a), no decrease in the detected output was observed in that area, and it was confirmed that the decrease in output in the high photon area was recovered by infrared irradiation. It was

In the above-described embodiments of the present invention, another infrared source such as an infrared laser may be used instead of the laser diode.

Further, the present invention is applicable to radiation detectors using other compound semiconductors in addition to CdTe.

<Effects of the Invention> As described above, according to the present invention, the compound semiconductor of the radiation sensor is irradiated with infrared rays having an energy higher than the trap level existing in the crystal, so that the carriers are crystal defects. Trapped by
The carrier is promptly released and collected on the electrode. Therefore, all the carriers generated by the incident radiation are collected, and the detection efficiency is improved. In particular, in the prior art, the output reduction was remarkable in the radiation detection in the high photon region, but in the present invention, the output in that region can be recovered.

Since the distribution of crystal defects existing in the compound semiconductor such as CdTe is not uniform, the probability that generated carriers are trapped in each pixel of the radiation sensor is different. For this reason, in the past, variations in output occurred between the pixels, but the present invention also has the effect of reducing the variations.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is an explanatory view of its operation. FIG. 3 is a graph showing the output of the X-ray sensor with respect to the intensity of X-rays. (A) shows the case where the compound semiconductor of the sensor is irradiated with infrared rays, and (b) shows the case where it is not irradiated. FIG. 4 is a diagram for explaining the problems of the conventional radiation detector. 1 ... X-ray sensor 1a ... CdTe substrate 1b ... Bias electrode 1c ... Signal extraction electrode 2 ... IC chip 3 ... Wiring substrate 4 ... Infrared laser diode

Claims (1)

(57) [Claims] 1. A radiation sensor in which electrodes are formed on surfaces of a compound semiconductor facing each other, and a charge pulse is generated by incidence of radiation, and a trap having a band gap energy smaller than that of the compound semiconductor and existing in the semiconductor crystal. A light source for irradiating the inside of the compound semiconductor with an infrared ray having an energy larger than the level is irradiated, and the infrared ray is radiated from the above-mentioned light source to the inside of the compound semiconductor during the operation of the radiation sensor and trapped in the semiconductor crystal. A radiation detector configured to excite a carrier.