JPH06105302B2 - Radiation detector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a minute radiation detector for performing a radiation energy spectrum analysis, a diagnostic X-ray imaging apparatus, and a radiation detector used in a nondestructive inspection apparatus. .

Prior Art FIG. 13 shows the structure of a conventional full depletion type semiconductor radiation detector. That is, in the conventional radiation detector, the divided electrodes 2 and the common electrode 3 are provided on the two parallel planes of the semiconductor 1, and a charge generated in the semiconductor 1 by the radiation 4 is applied between the electrodes 2 and 3. Take it out. The current generated by the charge transfer is as shown in the first equation, which is known as Ramo's theorem.

i = q * dx / dt (1) q: Generated charge dx / dt: Charge transfer speed In order to perform energy analysis of radiation, the conventional method is (1)
The current i in the equation is integrated and the generated charge amount is measured. This relationship is shown in (2).

Q = idt (2) That is, the current due to the electrons and holes output by the detector is converted into the amount of electric charge and measured by using the integrating circuit of the external measuring device.

When an external measuring instrument using an integrating circuit is used, when the number of radiation photons incident on the detector is small, the output pulse height is proportional to the energy of the incident radiation photon, but the incident radiation photon The number of is increased, and counting occurs at an incident rate shorter than the time constant of the integrating circuit.

In order to eliminate this problem, if the high-speed current amplifier is used without using the integrating circuit, the following problems occur.
It is preferable if the mobility, that is, the speed of movement of charges in a constant electric field, is the same as in Si and Ge.However, in compound semiconductor materials such as GaAs, CdTe, and Hg12, the mobility of electrons and holes is different. If you try to measure the current due to any charge, the charge generated in the vicinity of the charge collection electrode will have a very short charge transit time, and the gain will be very small when passing through the amplifier as a current pulse. I will end up. That is, the pulse crest value of the current pulse that has passed through the amplifier depends on the charge generation position.

Furthermore, if both electrodes are arranged parallel to the radiation incident direction, the mounting of the sensor becomes difficult.

Means for Solving the Problems In order to solve the above problems, the electrodes should be arranged so that they travel for an appropriate time before reaching the electrodes, regardless of the position where the charges generated by the radiation incident on the detector are generated. Good. Therefore, in the radiation detector of the present invention, the divided electrodes are arranged on one plane of the semiconductor in parallel with the common electrode and at an appropriate distance, and the space between the common electrode and the divided electrodes is used as a radiation sensitive region.

Action With the above configuration, both electrodes are arranged in the same plane of the semiconductor, and by using the space between the electrodes as a sensitive portion, it is possible to perform measurement without impairing the energy information of the radiation even for incident radiation with a high counting rate. It will be possible. In addition, since both electrodes are provided on the same plane, it is very easy to mount as a multi-channel detector.

Furthermore, by arranging both electrodes symmetrically on the other parallel plane of the semiconductor, it becomes possible to obtain information in the incident depth direction of the incident radiation.

Embodiments of the radiation detector of the present invention will be described below with reference to the drawings.

Embodiment (Embodiment 1) FIG. 1 is a view showing the basic arrangement of a radiation detector according to an embodiment of the present invention. The common electrode 2 and the divided electrode 3 are provided on one plane of the semiconductor crystal 1. In order to use the semiconductor 1 as a total depletion layer, ohmic electrodes are formed as both electrodes 2 and 3. For example, when CdTe is used as the semiconductor 1, a Pt or Au electrode may be formed. Radiation 4 is incident on the plane on which this electrode is formed from the vertical direction.

A sectional view of the radiation detector shown in FIG. 1 is shown in FIG.
The broken line in FIG. 2 indicates the line of electric force when a voltage is applied between the electrodes 2 and 3. As shown in the figure, the lines of electric force run parallel to the radiation entrance plane. Radiation 4 incident between both electrodes 2 and 3
The electric charges generated by the electric field travel to both electrodes according to the lines of electric force. With such a structure, a signal that does not depend on the incident depth of radiation can be obtained.

FIG. 3 shows a plan view of the radiation detector of this embodiment as seen from the electrode side. Since the lines of electric force run in the direction perpendicular to the common electrode,
The separation between the channels is as shown by the broken line in the figure.

Furthermore, to make the distribution of the electric field between the electrodes 2 and 3 symmetrical,
This can be solved by making a notch in the common electrode as shown in FIG.

Further, in the structure of the present invention, the surface state between the electrodes is important, especially when a semiconductor is used as the total depletion layer. moisture,
To protect the semiconductor surface from contamination,
As shown in FIG. 5, a passivation film 5 is formed between the electrodes formed on the surface of the sensitive element such as SiO ₂ .
What is important here is that the electric field is applied even on a plane not formed on the electrode, so that it is important to form a passivation film symmetrical to the electrode side.

(Embodiment 2) FIG. 6 shows another embodiment of the present invention. The divided electrodes are formed on both sides of the common electrode 2 centered on the same plane of the semiconductor. Further, as shown in FIG. 7, by forming the divided electrodes formed on both sides so as to be shifted from each other by a half pitch, it is possible to enhance the spatial separation ability without lowering the sensitivity which is the performance of the detector.

Further, when a material having a small τ (average life of electric charge) of the semiconductor material is used, the electric charge disappears while the electric charge travels, so that the electric charge traveling distance, that is, the distance between the electrodes cannot be increased. Therefore, as shown in FIG. 8, the divided electrodes 2 are formed on the same plane of the semiconductor 1 and the common electrodes are formed on both sides. With this configuration, the inter-electrode distance can be reduced without reducing the detection sensitivity of each channel.

(Embodiment 3) FIG. 9 shows another embodiment of the present invention. The common electrode 21 and the divided electrode 31 are formed on one of the two parallel planes of the semiconductor 1, and the common electrode 22 of the same shape is symmetrically formed on the other plane.
And the divided electrode 32 is formed. When voltages are applied to the common electrode and the divided electrodes respectively as shown in the figure, the lines of electric force run as shown by the broken lines in the figure, and the electric field that is vertically symmetrical with respect to the center line (dashed line) of the semiconductor 1 in the figure is a boundary. Distribution.

When the radiation enters from one surface, a large amount of the low energy radiation 41 is absorbed in the vicinity of the entrance surface. Then, the generated charges move along the lines of electric force. Therefore, most of the low energy is detected as a current pulse from the split electrode 31. Further, since the high-energy radiation 42 is absorbed throughout the semiconductor 1, it is detected as a current pulse from both the divided electrodes 31 and 32.

In this way, by considering the absorption coefficient and the thickness of the semiconductor material, it is possible to obtain a detector having an energy discriminating mechanism that is divided by specific energy.
Example 4 Next, a method for further improving energy resolution will be described. A schematic diagram of the incident position of radiation and the travel of generated charges is shown in FIG. When the electrode interval is W, a negative voltage is applied to the common electrode, and a positive voltage is applied to the divided electrodes, and the electrode interval is W and the radiation incident position from the common electrode is X, the generated electrons and holes travel as shown To do. When the semiconductor material is a compound semiconductor, the mobilities of electrons and holes are different. The current waveform is as follows by modifying the equation (1).

i = q * dx / dt = q * μ * E (3) μ: Mobility E = electric field strength The figure showing this current waveform in relation to the charge generation position is
11 is a diagram. Since the mobility of electrons is generally higher than that of holes, the current value of electrons is larger than that of holes. The waveforms for X to W are shown in FIG. 7A, the waveform for X = 1/2 W is shown in FIG. 7B, and the waveforms for X to 0 are shown in FIG.
As can be seen from FIG. 11, the current waveform is dominated by the electron waveform. The transit time Te of the electron is as shown in equation (4).

TE = (W−X) / μ * E (4) When Te is small, the current pulse observation is limited by the frequency characteristic of the current amplifier connected to the outside of the detector. 12th
The figure shows the frequency characteristics of the current amplifier and the output current waveform. First
In FIG. 12, (a) shows the frequency characteristic of the current amplifier,
fc is the cutoff frequency of the current amplifier. When Te is very small and Te <1 <fc, the output pulse waveform from the amplifier becomes smaller than the gain of the amplifier as shown in (b) of FIG. However, when Te is larger and Te ≧ 1 / fc, the output waveform from the amplifier has a constant pulse peak value as shown in (c) and (d). If this condition is rewritten using the equation (4), it becomes the equation (5).

W−X ≧ μ * E / fc (5) When calculated by substituting specific numerical values, it is regarded as a semiconductor material.
Using CdTe, electron mobility μ = 1000 (Cm ² / VSec),
The electric field strength is E = 1000 (V / cm), 1 / fc = 10 ^-7 Sec (5)
Substituting into the equation, W−X = 0.1 cm. That is, the incidence of the radiation may be limited so that the electrons generated by the radiation travel at least W−X = 0.1 cm or more.

Figure 13 shows the method of limiting the incidence of radiation. The radiation shield 6 may be provided on the split electrode side at a distance W-X from the split electrode. In order to further enhance the effect, the effect can be further enhanced by providing the radiation shield on the common electrode side as well as on the portion where the lines of electric force are distorted.

EFFECTS OF THE INVENTION According to the present invention, when a radiation sensor using materials having different charge mobilities such as compound semiconductors is used, high-speed measurement of a high pulse rate radiation photon is performed using a current amplifier, By increasing the moving distance of the electric charge, it is possible to obtain a current pulse wave height that does not depend on the position where the electric charge is generated. That is, high-speed pulse measurement can be performed without losing energy information of incident radiation. Further, the effect can be enhanced by disposing the radiation window on the detector in consideration of the frequency band of the current amplifier.

[Brief description of drawings]

1 is a perspective view of the radiation detector according to the first embodiment of the present invention, FIG. 2 is a sectional view of the radiation detector, FIG. 3 is a plan view of the radiation detector, and FIG. 4 is the radiation. FIG. 5 is a plan view showing an example of the shape of a common electrode of the detector, FIG. 5 is a sectional view of a radiation detector showing a protective film provided between the electrodes, and FIGS. 6, 7, and 8 are second embodiments. Diagram showing the configuration of the radiation detector of
FIG. 9 is a sectional view of a radiation detector of the third embodiment in which electrodes are symmetrically provided on both sides of a semiconductor, and FIGS. 10 and 11 are sectional views for explaining the relationship between the incident position of radiation and the output current. 12 and FIG. 12. FIG. 12 is a waveform diagram showing the frequency characteristics and current pulse waveforms of the current amplifier. FIG. 13 is a sectional view of the radiation detector of the fourth embodiment in which the radiation entrance window is provided in the detector. FIG. 14 is a sectional view of a conventional radiation detector. 1 ... semiconductor, 2 ... common electrode, 3 ... divided electrode, 4 ...
…radiation.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sueki Baba 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Hiromasa Funakoshi 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. (72) Inventor Toshiyuki Kawahara 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor ▲ Yoshikazu Sumitomo 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (8)

[Claims] 1. A semiconductor radiation detector for outputting an electric pulse signal corresponding to the energy of a radiation photon, wherein a common electrode and a split electrode are provided in the same plane of a semiconductor sensitive to an incident radiation photon. Radiation detector. 2. The radiation detector according to claim 1, wherein the semiconductor radiation detector is a fully depleted layer type, and a voltage is applied to the common electrode to extract an electric signal from the divided electrodes. 3. The radiation detector according to claim 1, wherein the common electrode and the divided electrodes are arranged linearly while keeping the distance between the electrodes constant. 4. A divided electrode is arranged on both sides of a common electrode arranged substantially in the center of a plane of a semiconductor, and the divided electrodes arranged on both sides are arranged in parallel or shifted by a half pitch. The radiation detector according to claim (1), (2), or (3). 5. The divided electrodes are arranged on both sides of the common electrode, and the radiation incidence side has a high atomic number in order to limit a part of the radiation incident on the radiation sensitive portion between the common electrode and one of the divided electrodes. The radiation detector according to claim 4, wherein the radiation detector is covered with a material. 6. The radiation detector according to claim 1, wherein a common electrode and a split electrode are symmetrically provided on two parallel planes of the semiconductor. 7. A radiation detector characterized in that a semiconductor surface between a common electrode and a split electrode formed on one plane of a semiconductor sensitive to radiation and a semiconductor surface on the other side are covered with a passivation film. 8. A radiation detection element having a common electrode and a split electrode on one surface of a semiconductor sensitive to radiation, and a high-speed current amplifier, wherein the transit time of electrons or holes in the charge generated by the radiation of the high-speed current amplifier. A radiation detector characterized in that a window is provided on the radiation incident side to limit the radiation-sensitive portion so that the reciprocal time of the cutoff frequency becomes longer than a few hours.