JPH0943357A - Driving method for radiation detector - Google Patents

Abstract

(57) Abstract: A semiconductor radiation detector such as CdTe has a high X
Provided is a driving method which can be operated even in a line photon amount region. SOLUTION: When a high dose of X-rays enters a detector, charges are trapped inside the crystal, and the trapped charges form space charges, which cause the crystals to bypass. However, by operating the detector with a space charge limited current, the charge is injected from the electrode into the crystal and recombines with the trapped charge. The charge space of is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a radiation detector used in a medical X-ray imaging apparatus.

[0002]

2. Description of the Related Art A compound semiconductor detector is known as an X-ray detector, and among these semiconductor detectors, particularly Cd.
Since the Te radiation detector can be used at room temperature and has advantages such as high energy detection, various developments have been made.

In this type of semiconductor detector, generally, electrodes are formed on both sides of a compound semiconductor substrate, a crystal is depleted by applying a bias between these electrodes, and X-ray photons are incident inside the crystal in this state. The charge generated at that time is taken out to the outside through the electrode, and the amount of the incident X-ray photons can be known by counting the charge pulse.

[0004]

By the way, in order to use a semiconductor detector such as CdTe in the field of medical X-ray imaging, it is necessary that the detector be operable even in a high X-ray photon amount region. .

However, since CdTe radiation detectors and the like are conventionally used exclusively for energy decomposition in the region of low X-ray photon amount, that is, spectrum measurement, the operating conditions (bias application conditions) of the detector are also crystallized. Is set to a depletion layer, that is, the ohmic region in terms of current-voltage characteristics.

Therefore, although the operation in the high X-ray photon amount region is unknown, it is clear that the count rate decreases when the incident X-ray becomes a high dose due to the generation of space charge, and in fact, Moreover, when the operating condition was set to the ohmic region and the detection was performed in the high X-ray dose range, it was found that the count rate decreased.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a driving method capable of operating a semiconductor radiation detector such as CdTe even in a high X-ray photon amount region.

[0008]

In order to achieve the above object, the present invention has a structure in which electrodes are formed on both front and back surfaces of a compound semiconductor substrate such as CdTe, and a bias is applied between the front and back electrodes. In a radiation detector that generates a charge pulse when radiation enters in a state, it is characterized by setting a bias application condition to a space charge limited current region.

Here, the space charge limited current region referred to in the present invention refers to a region of a current that changes in proportion to the square of the voltage above the ohmic current in the current-voltage characteristics of this type of detector ( See FIG. 2).

By operating the detector in such a space charge limited current region, the detector operates normally even when a high dose of X-rays enters the detector. The reason is described below.
First, in this type of radiation detector, charges generated by X-ray incidence are trapped in the crystal, and the trapped charges form space charges. If the incident X-rays have a low dose and the time interval of the incident X-ray photons is sufficiently long, this space charge is resolved within the incident intervals, so there is no problem, but when the incident X-rays have a high dose, Since the next X-ray photon is incident before the space charge is eliminated, the crystal is not biased, which causes a decrease in the counting rate and a decrease in the counting stability.

Therefore, in order to enable normal detection in the high X-ray dose range, it is sufficient to promptly eliminate the space charges formed by the charge traps. In the invention, the space charge limited current region (Fig. 2)
Under these operating conditions, charges are injected from the electrodes into the crystal and recombine with the trapped charges.As a result, the space charge inside the crystal is instantly eliminated and Be biased.

[0012]

FIG. 1 is a diagram showing an embodiment of the present invention. In the radiation detector shown in FIG. 1, a common electrode 2 formed by uniformly depositing Au is formed on one surface of a compound semiconductor substrate 1, and a plurality of split electrodes (Ni plating) are formed on the opposite surface. 3 ... 3 is a detector having a structure formed in a matrix, and a reverse bias (-V) is applied to the common electrode 2 thereof, and in this state, a charge pulse is generated when radiation is incident. Is configured.

In the structure of FIG. 1, the split electrode 3
..3 is placed on the ground side, and each split electrode 3 ...
Preamplifiers 3a ... 3a are connected to each. Further, in this embodiment, the operating condition, that is, the bias applying condition is set to a space charge limited current region in which the ohmic current is equal to or higher than the ohmic current in the voltage-current characteristics shown in FIG. Is characterized by using a chlorine-doped CdTe single crystal produced by natural solidification, such as the method or temperature gradient method.

When the radiation detector is operated in the space charge limited current region in this manner, as described above, a high dose of X
The counting rate when a ray is incident does not decrease, and the counting stability in the high X-ray dose range is improved.

Further, chlorine-doped C is added to the compound semiconductor substrate 1.
When the dTe single crystal is used, in the current-voltage characteristics shown in FIG. 2, the point Cp at which the ohmic region changes to the space charge limited current region can be set to a lower value than when other dopants are used. As a result, it is possible to reduce the adverse effects and effects caused by applying a high bias to the crystal.

When the compound semiconductor substrate 1 is made of CdTe single crystal as in the above-mentioned embodiments, other elements such as In and Ge may be used instead of chlorine as the dopant. Alternatively, a non-doped single crystal may be used. However, as described above, considering that the bias condition Cp for operating in the space charge limited current region can be set low, it is preferable to use chlorine-doped CdTe.

Further, in the above-mentioned embodiments, the example in which the present invention is applied to the detector using the CdTe single crystal as the compound semiconductor substrate 1 is shown, but the present invention is not limited to this.
The present invention can be applied to a radiation detector using other compound semiconductor such as GaAs or HgI ₂ crystal.

Here, in the present invention, the condition for operating the detector with the space charge limited current, that is, the point Cp shown in FIG. 2 is the kind of compound semiconductor used for the substrate, the kind of dopant element, the material of the electrode. (For example, Au, Ni, P
t, Ir, Se, In, Cu or Al, etc., and the contact method between the substrate and the electrode, etc., so that appropriate bias application conditions, that is, as shown in FIG. The condition that the point Cp becomes the minimum value can be determined.

[0019]

EXAMPLES CdTe (chlorine-doped) having the structure shown in FIG.
In the radiation detector, the incident X-ray dose was changed and the count rate was measured when the bias application conditions were the space charge limited current region and the ohmic region, respectively. Counting characteristics were obtained.

As is apparent from FIG. 3, under the operating conditions in the ohmic region, the count rate decreases at high dose X-ray incidence, whereas in the space charge limited current region, the count rate decreases. It was confirmed that did not appear.

Similarly, when the bias application condition is set to the space charge limited current region and the ohmic region, the count rate time is set in two regions, a high X-ray dose region and a low X-ray dose region, respectively. When the stability was measured, the results shown in FIGS. 4 (A) and 4 (B) were obtained.

As is clear from the measurement results, it was confirmed that the counting stability was improved even in the high X-ray dose region by operating the radiation detector in the space charge limited current region.

[0023]

As described above, according to the method of the present invention,
Since the bias condition applied to the radiation detector is the space charge limited current region, the problem of space charge formed inside the crystal is solved, which allows the detector to operate normally even when a high dose of X-rays is incident. To work. As a result, stable count measurement is possible even in a high X-ray photon amount region, and for example, the CdTe radiation detector can be used in the field of medical X-ray imaging.

[Brief description of drawings]

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

FIG. 2 is a graph showing a current-voltage characteristic of a radiation detector.

FIG. 3 is a graph showing measurement results of count rate characteristics in an explanatory diagram of an example of the present invention.

FIG. 4 is a graph showing the measurement results of the time stability of counting rate in the high X-ray region (A) and the low X-ray region (B), which are also explanatory views of the embodiment.

[Explanation of symbols]

 1 Compound semiconductor substrate (chlorine-doped CdTe single crystal) 2 Common electrode 3 Split electrode

Claims (1)

[Claims] 1. A radiation detector, which has a structure in which electrodes are formed on both front and back surfaces of a compound semiconductor crystal substrate, and which generates a charge pulse when radiation is incident with a bias applied between the front and back electrodes, A method of driving a radiation detector, characterized in that a bias application condition is set in a space charge limited current region.