JPH0574029B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a radiation dosimeter using a semiconductor radiation detection element, and particularly relates to radiation dose sensitivity and absorption dose corresponding to photon energy incident on the detection element. The present invention relates to a detector structure suitable for highly accurate compensation of sensitivity characteristics.

[Conventional technology]

The irradiation dose sensitivity characteristics (hereinafter referred to as energy characteristics) corresponding to the incident photon energy of conventional semiconductor radiation detection elements are as described in Japanese Patent Application Laid-Open No. 59-214787. surrounded by metal shielding material, and
This is compensated by a structure in which openings with a predetermined aperture ratio are provided in the metal shield material.

In the case of an irradiation dosimeter, compensation of this energy characteristic refers to a procedure for making the sensitivity corresponding to incident photon energy constant. Also, JP-A-60-
As in Publication No. 42672, a means is provided to discriminate the output waveform (pulse width and peak value) of a semiconductor radiation detector using an external circuit such as a discriminator, and provide a correction coefficient corresponding to each energy characteristic. Some compensate properties. This correction coefficient applying means includes a method of calculating logic with a predetermined reference oscillation signal for each pulse, and a method of performing calculation processing with a microcomputer.

[Problem to be solved by the invention]

In the above-mentioned conventional technology, the energy compensation method using a lead shield material and providing an opening in a part takes into account the fact that the shield material itself emits characteristic X-rays when it interacts with incident radiation. Therefore, it is difficult to compensate various semiconductor detection elements having different energy characteristics to an arbitrary characteristic. In other words, it is difficult to match the energy characteristics designed and calculated from the lead shielding material's shearing effect and aperture ratio with the actually measured characteristics. Furthermore, the absorption coefficient of the lead shielding material, which is a parameter for the shielding effect, changes discontinuously in the energy range of 90 KeV, making design calculations for energy compensation extremely difficult. The energy compensation method described above is the same as the energy compensation method for GM coefficient pipes that has been used for a long time, and the problem arises that the aperture ratio and the material thickness must be selected on a trial basis based on actual measured values.

Furthermore, the method of discussing the waveform discrimination means of the radiation detection signal requires correction coefficient applying means such as a microcomputer outside the detector, resulting in the problem that hardware and software must be provided.

An object of the present invention is to compensate the energy characteristics of a semiconductor detection element with a very simple principle and with high precision, and to significantly reduce the manufacturing and adjustment cost of the detector.

[Means to solve the problem]

The above objects can be achieved all at once by using a material with an atomic number of 44 or less for the energy compensation shield material provided adjacent to the semiconductor detection element.

[Effect]

The operating principle of a semiconductor radiation detection element is that radiation enters a semiconductor material and causes interactions such as the photoelectric effect and Compton scattering to generate secondary electrons. These secondary electrons create charges of electron-hole pairs in a depletion layer within the detection element, which becomes a radiation detection signal. The depletion layer thickness of a semiconductor detection element is approximately 100 to 200 μm, and the amount of radiation absorbed by the photoelectric effect and Compton scattering varies greatly depending on the energy of the incident radiation. At low energies (below 200 KeV), a high proportion of photoelectric effects occur, resulting in high sensitivity. At high energies, the proportion of photoelectric effects occurring is extremely small, resulting in low sensitivity. In addition, at extremely low energy (20 KeV or less), the electrode of the detection element P/n junction (n ⁺ layer,
It becomes insensitive because it is absorbed by the Al electrode of the ohmic contact. Based on these phenomena, the energy characteristics of various semiconductor detection elements are determined. in general
It becomes insensitive below 20 KeV, and increases when it becomes sensitive at 50 to 60 KeV. At energies above this, the sensitivity gradually decreases, and above 1 MeV, Compton scattering is the main component and the sensitivity remains almost constant. In order to use a semiconductor radiation detection element having such energy characteristics in an irradiation dosimeter, detection sensitivity corresponding to the energy of incident radiation must be made constant. According to the JIS standard specifications for area monitors of irradiation dosimeters, the sensitivity difference must be within ±25% within the range of 80KeV to 3MeV. In order to set this characteristic to a semiconductor detection element, it is possible to obtain energy-independent characteristics by providing a shield in front of the detection element and reducing the sensitivity in the high sensitivity region around 50 to 60 KeV. can.

The detector structure of the present invention includes, adjacent to the detection element,
A material whose main component is an element with an atomic number of 44 or less is provided to a predetermined thickness. In conventional detectors, this energy compensation is actually achieved using a material mainly composed of lead (atomic number 82) because it is easy to process. In this conventional energy compensation method, the characteristic X-rays emitted from the compensation material itself have a large influence on the energy range in which the semiconductor detection element is sensitive.

The characteristic X-ray emitted from the material with an atomic number of 44 or less used in the present invention is 20 KeV or less, and can have no effect on the semiconductor detection element. In addition, the absorption coefficient of this low atomic number material has no discontinuity with respect to the energy of incident radiation, making it possible to design energy compensation (calculation of the Shiyahei effect) for the energy characteristic specification range of 80 KeV to 3 KeV of the irradiation dosimeter. Can be executed with precision.

As described above, by using the present invention,
Energy characteristics essential to irradiation dosimeters and absorption dosimeters can be designed and realized as desired.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. A semiconductor detection element 1 is mounted on a flat base substrate (conductor) 2, and a reverse bias application electrode is led out via a bonding wire 3 and a hermetic seal 4. Atomic number 44 on the outside of detection element 1
The following hard materials 5 For example, alumina (Al ₂
O ₃ ) and aluminum (Al), and the entire probe case 6 is made of iron (Fe) or stainless steel (SUS).
The detector is configured by storing the detector in the detector. Fig. 2 shows its three-dimensional view. A detecting element 1 and alumina 5 made of a ceramic material are supported by a support pole 7, and a signal line 8 for extracting a radiation detection signal is drawn out of the probe case 6. FIG. 3 shows a sectional view of the semiconductor detection element 1. This semiconductor detection element has an element structure using p-type silicon. When a reverse bias is applied to the positive electrode (aluminum) 10, a depletion layer 12, which becomes a radiation-sensitive portion, expands below the n ⁺ layer 11. The positive electrode aluminum 10, the n ⁺ layer 11, and the surface protective film (SiO ₂ etc.) of the element serve as radiation insensitive layers. These dead areas reduce the radiation sensitivity of the detection element.
Sensitivity deteriorates rapidly below 20KeV. If the energy of the characteristic X-rays emitted from the shielding material used for energy compensation is less than 20 KeV, its influence can be completely ignored.

Next, the generation mechanism of characteristic X-rays will be described. When an atom's orbital electrons are displaced from their normal positions by interaction with incident radiation, the atom becomes excited. Then a single time (nsec or less time)
The vacancies of the orbital electrons are filled by the outer orbital electrons and free electrons, and the atom returns to the ground state. At this time, characteristic X-rays are emitted. The maximum amount of this energy is released from the orbit of the K shell. This energy depends on the atomic number of the element and increases regularly.
Figure 4 shows the relationship between the characteristic X-ray energy and atomic number of the K shell. From this relationship, it is clear that the atomic number at which the characteristic X-ray energy is 20 KeV or less is 44.

FIG. 5 shows the energy characteristics of an irradiation dosimeter using the present invention in a silicon detector. The solid line indicated by A in this figure is the data before energy compensation in the case where the stiffening material (alumina) 5 and the probe case 6 shown in FIG. 1 are not present. The experimental point indicated by B in the figure is the energy characteristic when the thickness of the probe case 6 is 1.5 mm and the thickness of the alumina material 5 is 12 mm. The energy compensation performance of this characteristic is ±15%, which fully satisfies the JIS standard for irradiation dosimeters. This energy compensation method can be arbitrarily designed according to the thickness of the probe case 6 intended to protect the detector from mechanical impact and the characteristics of the detection element itself. When the thickness of the probe case 6 becomes thinner than 1.5 mm, the same characteristics can be obtained by increasing the thickness of the alumina material by the same amount as the reduction rate of the absorption layer. At this time, the design standard is based on the linear absorption coefficient μ(E) of radiation energy (E) = 80 KeV, and the calculation results in good agreement.
In other words, the material of the probe case 6 is iron (Fe).
If the reduced thickness is t(Fe), the rate of decrease in radiation absorption is e ^- 〓 ^(Fe

Claims (1)

[Claims] 1. In a radiation dosimeter using a semiconductor radiation detection element, at a position closest to the detection element,
A radiation dosimeter equipped with a material whose main component is an element with an atomic number of 44 or less. 2. The radiation dosimeter according to claim 1, wherein the dose detection sensitivity depending on incident photon energy is relatively equal to the energy dependence of absorbed dose equivalent in a living body. 3. A radiation dosimeter that uses a semiconductor radiation detection element and uses a material whose main component is an element with an atomic number of 44 or less as a shielding material for energy compensation.