JPH0716025B2 - Semiconductor 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 radiation energy spectrum analysis, a diagnostic X-ray imaging apparatus, and a semiconductor radiation detector used in a nondestructive inspection apparatus. is there.

Prior Art FIG. 17 shows the structure of a conventional full depletion layer type semiconductor radiation detector. That is, in the conventional radiation detector, the divided electrodes 12 and the common electrode 13 are provided on the two parallel planes of the semiconductor 1, and the charge generated in the semiconductor 1 by the radiation 4 is applied between the electrodes 12 and 13. 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 amount, dx / dt: charge transfer speed) Conventionally, (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 that the mobility, that is, the charge transfer rate in a constant electric field, is the same as in Si and Ge.However, in the case of compound semiconductor materials such as GaAs, CdTe, and Hg12, the mobility of electrons and holes should be different. If one tries 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. 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-mentioned problems, the electrodes are arranged so that they travel for an appropriate time before reaching the electrodes, regardless of the position where the electric charges generated by the radiation incident on the detector are generated. do it. Therefore, in the radiation detector of the present invention, the divided electrode and the common electrode are arranged at diagonal positions on two parallel planes of the semiconductor, and the divided electrode and the common electrode are arranged in a region sensitive to the radiation between the common electrode and the divided electrode. To use as.

Action With the above configuration, by disposing both electrodes at diagonal positions on two parallel planes of the semiconductor and using the space between the electrodes as a sensitive portion, even if the incident radiation with a high count rate is used, It is possible to measure without damaging the energy information. In addition, since both electrodes are provided on parallel planes, it is very easy to mount as a multi-channel detector.

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

Embodiment (First Embodiment) FIGS. 1 to 5 are views showing a radiation detector in a first embodiment of the present invention.

FIG. 1 shows the basic configuration. Semiconductor crystal 1 of 2
The common electrode 2 and the divided electrode 3 are attached on one plane. In order to use the semiconductor 1 as a total depletion layer, ohmic electrodes are formed as both electrodes 2 and 3. For example, as semiconductor 1
When using CdTe, a Pt or Au electrode may be formed. Radiation 4 is incident on the plane on which the electrodes 2 and 3 are formed in a direction perpendicular to the plane.

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 between the electrodes 2 and 3 according to the lines of electric force. With such a structure, a signal that does not depend on the incident depth of the radiation 4 can be obtained.

FIG. 3 shows a plan view of the radiation detector of this embodiment as seen from the side of the split electrode 3. Since the lines of electric force run in the direction perpendicular to the common electrode 2, 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 2 as shown in FIG.

Further, in the structure of this embodiment, particularly when the semiconductor 1 is used as the total depletion layer, the surface condition between the electrodes 2 and 3 is important. In order to protect the surface of the semiconductor 1 from moisture and contamination, a passivation film 5 such as SiO ₂ is formed between the electrodes 2 and 3 as shown in FIG. What is important here is that the electric field is applied to the planes on which the electrodes 2 and 3 are not formed, so that it is important to form the passivation film 5 symmetrical to each other on both planes.

(Second Embodiment) FIGS. 6 to 8 show a second embodiment of the present invention. FIGS. 6A and 6B show the common electrode 2 at the center of one plane of the semiconductor 1.
And the divided electrodes 3 are formed on both sides of the other plane. Further, as shown in FIGS. 7 (a) and 7 (b), the split electrodes formed on both sides are formed so as to be offset from each other by a half pitch, thereby increasing the spatial resolution without lowering the sensitivity which is the performance of the detector. be able to.

Further, when a material having a small τ (average life of electric charge) is used for the semiconductor 1, the electric charge disappears during traveling of the electric charge,
It is not possible to increase the distance traveled by the charges, that is, the distance between the electrodes 2 and 3. Therefore, as shown in FIG. 8, the divided electrode 3 is formed in the center on one plane of the semiconductor 1 and the common electrodes 2 are formed on both sides of the other plane. With this configuration, the distance between the electrodes 2 and 3 can be reduced without lowering the detection sensitivity of each channel.

(Third Embodiment) FIG. 9 shows a third embodiment of the present invention. The common electrode 21 and the split electrode 31 are formed on one plane of two parallel planes of the semiconductor 1, and the common electrode 22 and the split electrode of the same shape are symmetrically formed on the other plane.
Forming 32. When a voltage is applied between the common electrodes 21 and 22 and the divided electrodes 31 and 32, electric lines of force run as shown by the broken lines in the figure, and the center line of the semiconductor 1 (dotted line) in the figure. The electric field distribution is vertically symmetrical with respect to.

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 taking into consideration the absorption coefficient and the thickness of the semiconductor material, it is possible to obtain a detector having an energy discriminating function, which is divided by specific energy.

(Fourth Embodiment) Next, as a fourth embodiment of the present invention, in the radiation detectors of the first to third embodiments, a technical method for further improving the energy resolution will be described with reference to FIGS. Will be described with reference to.

Figure 10 shows a schematic diagram of the incident position of radiation and the travel of generated charges.
Shown in the figure. When a negative voltage is applied to the common electrode 2 and a positive voltage is applied to the divided electrodes 3, the distance between the electrodes 2 and 3 is W, and the distance from the common electrode 2 to the radiation incident position is X, the generated electrons and holes are Run like. 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 waveform at the time of X-W is shown in the same figure (a), the waveform at X = 1/2 W is shown in the same figure (b), and the waveform at the time of X-O is shown in the same figure (c).
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.

Figure 12 shows the frequency characteristics of the current amplifier and the output current waveform. In FIG. 12, (a) shows the frequency characteristic of the current amplifier, and 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, Te is larger and Te ≧ 1 /
In the case of 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 is provided on the divided electrode 3 side over the distance W-X from the divided electrode 3.
Should be provided. Furthermore, by providing a radiation shield also on the part of the common electrode 2 where the lines of electric force are distorted,
The effect can be increased.

(Fifth Embodiment) As a fifth embodiment of the present invention, a technique for improving the energy characteristics of output pulses by utilizing the shape effects of the first to third radiation detectors will be described with reference to FIGS. It will be described with reference to FIG.

As shown in FIG. 14 (a), when the distance between the electrodes 31 and 32 becomes narrower than the thickness of the radiation detector, the electric field strength distribution (potential distribution) is distorted and the electric field is concentrated on the end faces of the electrodes 31 and 32. To do.
FIG. 14B is a diagram showing a locus of charges generated by radiation in the detector having such a shape. Radiation 41
3 shows the trajectories of the electrons of the charges generated at the three generation depths (1), (2), and (3). The velocity of the electric charges moving according to this locus becomes a current pulse generated in the electrode 31. Figure 15 shows the current waveform. (1),
(2) and (3) correspond to the charge generation locations shown in FIG. 14 (b). As can be seen from FIG. 15, the current waveforms near the electrode 31 are greatly different.

Fig. 16 shows the current waveform observed when such a pulse current waveform is connected to the externally connected current amplifier. First
In FIG. 16, (a) shows the frequency characteristic of the current amplifier. fc is the cutoff frequency. (B), (c),
(D) shows the current waveforms shown in (1), (2), and (3) of FIG. 15 in correspondence with the current waveforms through the current amplifier. By properly combining the frequency characteristic of the amplifier, that is, the cutoff frequency, and the pulse width of the output current from the radiation detector, that is, the frequency of the pulse, the height of the output pulse can be made uniform. That is,
The frequency of fc is selected for the frequencies corresponding to (1), (2), and (3). As shown in (a), (1) is set to a frequency higher than fc, (2) is set near fc, and 3) may be selected so that the frequency is lower than fc. Therefore, by properly selecting fc, the heights of Ve can be made uniform as shown in the figure. That is, it is possible to obtain the pulse height that does not depend on the charge generation location.

EFFECTS OF THE INVENTION According to the present invention, when a radiation sensor using materials having different charge mobilities such as a compound semiconductor is used and a high-speed measurement of a high pulse rate radiation photon is performed by using a current amplifier, the charge It is possible to obtain a current pulse wave height that does not depend on the charge generation position by increasing the moving distance of. 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. Furthermore, even when the shape effect of the radiation detector is produced, the height of the output pulse can be made uniform by selecting the frequency band of the current amplifier according to the shape.

[Brief description of drawings]

1 is a perspective view of a semiconductor radiation detector according to a first embodiment of the present invention, FIG. 2 is a sectional view of the semiconductor radiation detector, FIG. 3 is a plan view of the semiconductor radiation detector, and FIG. Is a plan view showing an example of the shape of a common electrode of the semiconductor radiation detector, FIG. 5 is a sectional view of the semiconductor radiation detector showing a protective film provided between the electrodes, FIG. 6, FIG. 7, and FIG. The figure which shows the structure of the semiconductor radiation detector in the 2nd Example of this invention, FIG. 9 is sectional drawing of the semiconductor radiation detector in the 3rd Example of this invention which provided the electrode symmetrically on both surfaces of the semiconductor, 10 and 11 are sectional views and waveform diagrams for explaining the relationship between the incident position of radiation and the output current, and FIG. 12 is a waveform diagram showing frequency characteristics and current pulse waveforms of the current amplifier,
FIG. 13 is a sectional view of a semiconductor radiation detector provided with a radiation shield in a fourth embodiment of the present invention, FIG. 14 is a sectional view of a radiation detector in a fifth embodiment of the present invention, and FIG. FIG. 16 is a diagram showing an incident position of radiation and an output current, FIG. 16 is a diagram showing a frequency characteristic and an output pulse waveform of a current amplifier, and FIG. 17 is a diagram showing a conventional semiconductor radiation detector. 1 ... Semiconductor, 2, 21, 22 ... Common electrode, 3, 31, 32 ... Divided electrode, 4, 41 ... Radiation, 5 ... Passivation film.

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

Claims (7)

[Claims] 1. A semiconductor radiation detector for outputting an electric pulse signal corresponding to the energy of a radiation photon, which is divided into a diagonal position and a common electrode on two parallel planes of a semiconductor sensitive to an incident radiation photon. A semiconductor radiation detector comprising electrodes. 2. The semiconductor radiation detector according to claim 1, wherein the semiconductor radiation detector is of 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 semiconductor 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 semiconductor device, wherein split electrodes are arranged on both sides of another plane with respect to a common electrode arranged substantially in the center of one plane of the semiconductor, and the split electrodes arranged on both sides are arranged in parallel or shifted by a half pitch. The semiconductor radiation detector according to claim 1, 2 or 3, wherein 5. A radiation incident on a radiation-sensitive portion between a common electrode and one of the divided electrodes, wherein divided electrodes are arranged on both sides on the other plane with respect to a common electrode arranged substantially at the center of one plane of the semiconductor. 5. The semiconductor radiation detector according to claim 4, wherein the radiation incident side is covered with a material having a high atomic number in order to limit a part of the radiation. 6. A semiconductor surface between a common electrode formed on one plane of a semiconductor sensitive to radiation and a polarized electrode formed on the other plane is covered with a passivation film such as SiO ₂ , SiON, or an organic thin film. Semiconductor radiation detector. 7. A radiation detection element having a common electrode and a divided electrode at diagonal positions on two parallel planes of a semiconductor sensitive to radiation, and a high-speed current amplifier, and electrons or holes in the charge generated by the radiation. The semiconductor radiation detector is characterized in that a window is provided on the radiation incident side to limit the radiation-sensitive portion so that the traveling time of the above becomes longer than the reciprocal time of the cutoff frequency of the high-speed current amplifier.