JP3151487B2 - Radiation detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation detecting method using a CdTe radiation detector. CdTe
The radiation detector measures a photocurrent generated in the CdTe semiconductor due to radiation by using an electrode provided on the surface thereof. CdTe can operate at room temperature because of its wide band gap, and has a large absorption coefficient of radiation such as X-rays and γ-rays because of the large atomic number of the constituent elements, so that high sensitivity is obtained. Such a detector is used for a monitor of a radiation use facility, a spectrum survey meter, and the like. In addition, detectors can be miniaturized and arrayed, and arrayed detectors have begun to be applied to medical diagnostic equipment, industrial nondestructive testing devices, and the like.

[0002]

2. Description of the Related Art In a measurement using a CdTe radiation detector,
A relatively high bias voltage is applied between the electrodes, and a current pulse resulting from the incidence of radiation is read. The energy of the incident radiation is measured as the height of the current pulse, and the intensity is measured as the frequency of the current pulse.

[0003] In such a radiation measurement method, it is known that continuous measurement for a relatively long time of about 15 minutes causes a temporal change such that the height and frequency of the read current pulse are reduced even when the same radiation is input. I was By suspending the application of the bias voltage for several seconds, such deterioration can be recovered. (ROBell et al .; NUCLEA
R INSTRUMENTS AND METHODS, vol.117 (1974) pp.267)

[0004]

However, when the bias voltage is stopped, compared to the initial state where the bias is applied continuously, the resolution of the current pulse is reduced due to the variation in the height, and the pulse is not changed. In some cases, the frequency decreased and the detection efficiency decreased. Further, during the period in which the bias voltage is stopped, the detection efficiency is reduced because radiation cannot be detected.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a radiation detection method in which resolution and detection efficiency do not decrease even when a bias voltage is stopped.

[0006]

SUMMARY OF THE INVENTION The present invention provides a high-resistance CdT
e, a semiconductor comprising: an anode electrode formed on one surface of the semiconductor and having a barrier for preventing injection of holes; and a cathode electrode formed on another surface of the semiconductor and having a barrier for preventing injection of electrons. In the method of detecting radiation using a detector, (a) start detection of radiation by applying a DC bias voltage between the anode electrode and the cathode electrode,
(b) When the voltage value of the bias voltage is E (V),
The semiconductor thickness between the cathode electrode upon the d (cm), t = ( E / d) 2/10 6 at time defined t (seconds)
The detection of radiation is continued for the following time, and (c) thereafter, the application of the bias voltage is stopped.

As the anode electrode, an electrode made of a metal such as indium or tin formed by a vacuum evaporation method and having a relatively high barrier (0.5 eV or more) against holes is used.
Further, as the cathode electrode, an electrode made of a metal such as platinum or gold formed by electroless plating and having a relatively high barrier against electrons (0.5 eV or more) can be used.

[0008] It is desirable that the time for continuous radiation detection be as long as 1.0 t or less. If the continuous time is less than 0.1 t, the time during which the measurement cannot be performed due to the suspension of the bias voltage becomes longer as compared with the measurement time, so that the actual detection efficiency decreases. Further, in order to avoid instability of the operation immediately after the application of the bias voltage, it is desirable that the time is 0.5 seconds or longer. If the time for stopping the application of the bias voltage is about 0.1 second or more, the state before the change with time can be returned.

[0009]

According to the operation and effect of the present invention, since the time to continue the application of the bias voltage ^{t = (E / d) 2} /10 6 at time defined t (in seconds) following times, Ya resolution degradation Continuous measurement is possible without lowering the detection efficiency, and the bias voltage is reduced because an anode electrode having a barrier to prevent hole injection and a cathode electrode having a barrier to prevent electron injection are used as electrodes. Even if it increases, the leak current does not increase. Therefore, in radiation detection, high detection efficiency and high resolution can be obtained.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As an embodiment of the present invention, a gamma ray detection method will be described in detail below. The semiconductor detector is one in which electrodes are formed on opposing main surfaces of a high-resistance chlorine-doped CdTe semiconductor single crystal of 2 mm square and 1 mm thick. The cathode electrode is a Pt (platinum) electrode formed by electroless plating, and the anode electrode exhibiting ohmic characteristics to electrons is an In (indium) electrode formed by vacuum evaporation. The height of the barrier against holes of the cathode and anode electrodes is 0.6 eV and 1.4 eV, respectively. The heights of the barriers to the electrons of the cathode and the anode are 0.9 eV and 0.1 eV, respectively.

FIG. 1 shows the configuration of a γ-ray detector used in the present embodiment. The cathode electrode of the semiconductor detector 1 is grounded,
The anode electrode is input to the relay 2. The relay 2 composed of a three-contact reed relay switches the anode electrode of the semiconductor detector 1 to a bias circuit or a ground state. On the bias circuit side, +300 V from bias power supply 3
And a signal is input to the preamplifier 4 via a capacitor. The output signal of the preamplifier 4 is analyzed by the pulse height analyzer 5. Relay 2 is connected to the bias circuit for 9.8 seconds, then
It is controlled by the relay drive circuit 6 to be grounded for two seconds. The switching state of the relay 2 is also input to the pulse height analyzer 5 as a measurement synchronization signal.

The measurement procedure according to the present embodiment is as follows. (A) The relay is switched, connected to a bias circuit, a DC bias voltage of +300 V is applied, and detection of a current pulse caused by radiation is started, and (b) 9.8. The application of the bias voltage is stopped for 0.2 seconds by continuously detecting the current pulse for 2 seconds and switching the relay (c) to ground the anode electrode.
Thereafter, the relay is switched again to bring the state of (a) above.

FIG. 2 shows the measurement results according to the present embodiment. This measurement is based on the emission of 60% from the radioisotope ²⁴¹ Am.
The height distribution of the current pulse corresponding to the energy spectrum of the KeV γ-ray is shown. The channel on the horizontal axis indicates the height of the current pulse, and the count on the vertical axis indicates the frequency of the current pulse. As is clear from these results, a good result was obtained with an energy resolution of 2.8 KeV in half width.

[0014]

COMPARATIVE EXAMPLE 1 As Comparative Example 1, the anode and cathode electrodes of a semiconductor detector were formed by electroless plating.
The measurement was carried out in the same manner as in the above example, except that a t (platinum) electrode was used. FIG. 3 shows the measurement results of Comparative Example 1. In this case, when the bias voltage was increased, the dark current due to hole injection increased, so that the bias voltage could not be applied exceeding 50 V, and the bias voltage was set to 50 V. In this case, the energy resolution is 4.0 Ke in half width.
V, and the response time, which is the duration of the current pulse, also deteriorated about 10 times.

[0015]

COMPARATIVE EXAMPLE 2 As Comparative Example 2, the measurement was carried out continuously by applying a DC bias voltage for 10 seconds or more, and otherwise the measurement was carried out in the same manner as in the above example. FIG. 4 shows a temporal change after the application of the DC bias voltage (300 V), with the relative value of the peak count in the height distribution of the current pulse as the vertical axis. It can be seen that the relative value of the count peak number decreases by 1% after 10 seconds, and the decrease continues thereafter. The time required for the relative value of the count peak number to decrease by 1% was measured as the stable time by changing the DC bias voltage, and the results are summarized in FIG. As is apparent from this drawing, the stabilization time t is proportional to the square of the average electric field strength E / d, and t
= Defined in ^{(E / d) 2/10} 6. (However, the voltage value of the bias voltage is E (V), and the thickness of the semiconductor between the anode electrode and the cathode electrode is d (cm).)

Therefore, if the measurement is performed while the application of the DC bias voltage is continued only for the time equal to or less than the stabilization time t,
The count number of the current pulse corresponding to the detection sensitivity is not reduced. The radiation can be detected by counting the number of current pulses or by integrating the current pulses and measuring the average current value.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram for explaining a configuration of a γ-ray detection device used in the present embodiment.

FIG. 2 is a diagram illustrating a height distribution of a current pulse corresponding to an energy spectrum of a γ-ray according to the present embodiment.

FIG. 3 is a diagram showing a height distribution of a current pulse corresponding to an energy spectrum of a γ-ray according to Comparative Example 1.

FIG. 4 is a diagram showing a change over time of a relative value of a count peak number in a height distribution of a current pulse.

FIG. 5 is a diagram illustrating a relationship between an average electric field intensity and a stabilization time in a semiconductor detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor detector 2 Relay 3 Bias power supply 4 Preamplifier 5 Pulse height analyzer 6 Relay drive circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01T 1/24 H01L 31/09

Claims (1)

(57) [Claims] 1. A high-resistance CdTe semiconductor, an anode electrode formed on one surface of the semiconductor and having a barrier for preventing injection of holes, and a barrier formed on another surface of the semiconductor for preventing injection of electrons. A method for detecting radiation using a semiconductor detector comprising a cathode electrode having: (a) applying a DC bias voltage between an anode electrode and a cathode electrode to start detecting radiation; When the voltage value is E (V), the anode electrode
The semiconductor thickness between the cathode electrode upon the d (cm), t = ( E / d) 2/10 6 at time defined t (seconds)
(C) after that, the application of the bias voltage is stopped.