JPH01227983A - Radiation dose measuring instrument - Google Patents

Abstract

PURPOSE:To make characteristic X-rays <=20KeV so that influences of the X-rays to a semiconductor element can be eliminated and a precise energy compensating design can become possible by providing a material composed principally of an element of <=44 in atomic number adjacent to the detecting element. CONSTITUTION:A semiconductor detecting element 1 is mounted on a flat base material (conductor) 2 and an impressing electrode is drawn out through a bonding wire 3 and hermetic seal 4. A shielding material 5 of <=44 in atomic number, for example, alumina or aluminum is provided on the outside of the detecting element 1 and supported by a supporting pole 7. The entire body is housed in a probe case 6 of iron or stainless steel and a signal line 8 for fetching radiant ray signals is drawn out. When alumina is used as the shielding material 5, the characteristic X-rays can be made <=20keV and, as a result, influence of the X-rays to the semiconductor detecting element can be eliminated and, at the same time, a precise energy compensating design can become easier because the absorption factor of alumina is continuous.

Description

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

[Conventional technology]

Irradiation dose sensitivity characteristics (hereinafter referred to as energy characteristics) corresponding to the incident photon energy of conventional semiconductor radiation detection elements
As described in JP-A-59-214787 i), a semiconductor detection element is surrounded by a button or a metal shielding material containing lead as a main component, and an aperture with a predetermined aperture ratio is formed in the metal shielding material. This is compensated for by the structure provided.

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. In addition, as described in Japanese Patent Application Laid-open No. 60-42672, a means is provided to perform discrimination using an external circuit such as a semiconductor radiation detector generator and provide a correction coefficient corresponding to each energy characteristic to compensate for the characteristic. There are some things. This correction coefficient applying means includes a method of calculating a logical product 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 conventional technology, the energy compensation method using a lead shield material and having an opening in a part takes into account 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 shielding effect of the lead shielding material and the aperture ratio with the actually measured characteristics. moreover.

The absorption coefficient of lead as a shield material, which is a parameter of 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 counter tubes that has been used for a long time, and a problem arises in that the aperture ratio and the material thickness must be selected on a trial basis based on actual measured values.

Furthermore, the method for discussing waveform discrimination means for radiation detection signals is as follows:
A correction coefficient applying means such as a microcomputer is required outside the detector, which creates a problem in 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 object is achieved in particular 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 the semiconductor detection element is 100 to 200
The amount of radiation absorbed by the photoelectric effect and Compton scattering varies greatly depending on the energy of the incident radiation. At low energy (200 KeV or less), 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, extremely low energy (20K s
(lower than V), it is absorbed by the electrode of the P-n junction of the detection element (n10 layer, ohmic contact AQ electrode) and becomes insensitive. Based on these phenomena, the energy characteristics of various semiconductor detection elements are determined.

In general, it becomes insensitive below 20KeV, and at 50 to 60KeV
The sensitivity increases with V. At higher energies, the sensitivity gradually decreases, and at 1 M e V or higher, Compton scattering is the main component and the sensitivity remains almost constant. In order to use a semiconductor radiation detection element with such energy characteristics in an irradiation dosimeter, the detection sensitivity corresponding to the energy of the incident radiation must be constant, according to the J and Is standard specifications for area monitors of irradiation dosimeters. Then '80 Ke
In the range of V to 3 MeV, the difference in sensitivity must be ±25% or less.
Energy-independent characteristics can be obtained by reducing the sensitivity in the high sensitivity region near 0 KeV.

In the detector structure of the present invention, a material whose main component is an element having an atomic number of 44 or less is provided with a predetermined thickness adjacent to the detection element.

Conventional detectors use lead (lead) because it is easy to process.
The current situation is that this energy compensation is attempted using materials mainly consisting of atomic number 82). 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 having 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 the incident radiation, and is within the energy characteristic specification range of the irradiation dosimeter.
Energy complementary M design (calculation of shielding effect) for KeV to 3M eV can be performed precisely.

As described above, by using the present invention, energy characteristics essential for irradiation dosimeters and absorption dose correction can be arbitrarily designed and realized.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. A semiconductor detection element 1 is attached to 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. Detector 7-
On the outside of 1, put a shielding material 5 with a number 44 or less, such as alumina (AQ 203) or aluminum (AQ).
is provided, and the whole is housed in a probe case 6 made of iron (Fe) or stainless steel (STJS) to constitute a detector. Fig. 2 shows its three-dimensional view. The detection element 1 and the alumina 5 made of aluminum are supported by support poles 7, and a signal line 8 for radiation detection (No. 4) is drawn out to the outside of the probe case 6.

FIG. 3 shows a cross-sectional view of the semiconductor detection element 1. This semiconductor detection element is made of a bare metal structure using type silicon. When a reverse bias is applied to the positive electrode (aluminum) 10, a depletion layer 12, which becomes a radiation sensitive part, expands below the n+ electrode 1511. The positive electrode aluminum 10 and ni&11 and the element surface protective film (S'02, etc.) serve as radiation insensitive layers. Due to these insensitive areas, the radiation sensitivity of the detection element rapidly deteriorates below 20 KeV.

If the energy of the characteristic X-rays emitted from the barrier material for energy compensation is 20 K e V or less, its influence can be completely ignored.

Next, the generation mechanism of characteristic X-rays will be described. When the orbital charge of an atom is displaced from its normal position by interaction with the incident radiation, the atom becomes excited. Thereafter, in a single time (nspc or longer r time), the vacancies of the orbital electrons are filled by the outer orbitals Fl p and free 'a atoms, 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 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 of the shell and the atomic number. 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 when the shielding material (alumina) 5 and the probe case 6 shown in FIG. 1 are not provided. At the experimental point indicated by B in the figure, the thickness of the probe case 6 was 1.5 mm.
This is the energy characteristic when the thickness of the aluminum alumina 5 is 1211 nm. 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 m, 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. The design standard at this time is radiation energy (E) = 80K
Shielding calculations using the eV linear absorption coefficient μ(E) are in good agreement. ] The material of the probe case 6 is iron (
If the reduced thickness is t (Fe), then the increment t(,'M)) in the alumina material thickness when the radiation absorption decreases is -μ(A,
N・E)・t, (AQ,) u(I'e−E)・t
(F'e)e
= found from e. The effective absorption coefficients of alumina and aluminum are comparable. Although the characteristics of the detection element itself are slightly different, when the probe case thickness is 1 to 2 m, the energy compensation performance shown in FIG. 5 can be obtained when the alumina material has a thickness of 5 to 15 IIn.

The reason why highly accurate energy compensation as described above can be arbitrarily designed [1] In addition to the above-mentioned considerations for characteristic This is due to the fact that high design calculations are possible. Figure 6 shows the relationship between photon energy and mass absorption coefficient. As can be seen from this figure, P b + S
A material with a high atomic number such as r+ has a voltage of 100 KeV (0
, 1 MeV), there is a point where it becomes extremely discontinuous.

For this reason, it becomes extremely difficult to calculate energy compensation to make it uniform over a wide energy range. Low atomic number materials do not suffer from this problem and allow for accurate energy compensation designs.

In the above example of energy compensation, even if there is no probe case 6 at all, it is possible to compensate the energy characteristics based on the same idea. If there is no probe case at all, the same energy compensation performance can be obtained with alumina materials ranging from 1011II to 35m+.

Next, we will discuss the energy characteristics of absorption dosimeters. Irradiation dosimeters are based on roentgen (R) units, and absorption dosimeters are based on rem (rem) or sievert (Sv) units.
) is based on units. The absorbed dose varies depending on the material being absorbed, and is generally calculated by multiplying by an absorbed dose conversion coefficient that is energy dependent. This is an important conversion factor when accurately assessing individual exposure doses. Recently, this conversion factor was established by an international committee (ICRU-39, ICRU: International Co+m+aissi
on Radio logical units
and measurement+1ents) *If an energy characteristic with the same profile as this conversion factor is realized in the detector, it becomes possible to directly measure the absorbed dose to the living body without conversion.

In the present invention, we have designed energy characteristics that match the absorbed dose characteristics to the living body, and the results of demonstrating its performance are shown in Figure 7.For the same detection element as in Figure 5, the probe case thickness was 1.5 mm. This is the result of designing the alumina thickness to 7 mm.

FIG. 8 shows a modification of the present invention. In this modification, a stiffening material 20 is also provided on the back surface of the detection element 1 to achieve non-directivity. Naturally, considering the radiation absorption of the flat base substrate 2, the shielding material 20 on the back side of the detection element 1 is designed to be thinner than the shielding material 5 on the front side.

As another modification, a structure in which a shielding material with an atomic number of 44 or higher is provided outside of a shielding material with an atomic number of 44 or lower provided adjacent to the detection element may also prevent characteristic X-rays emitted from the material with a high atomic number. Since the material having an atomic number of 44 or less provided adjacent to the detection element has a blocking effect, it is possible to realize a dosimeter similar to that described above.

FIG. 9 shows another modification in which the detection element 1 is completely surrounded by a shielding material 15 of a thickness necessary for energy compensation, and the detection signal line 8 connected to the detection element 1 is shielded. Pull out the material 5 to the outside. Alumina 5 is an insulating material, and contact with detection element 1 will not cause any problems. By adopting this structure, it is possible to realize a very compact and non-directional dosimeter of the present invention.

This has high practical value as a portable detector such as a personal exposure dosimeter.

〔Effect of the invention〕

According to the present invention, a semiconductor dosimeter with arbitrary energy characteristics is possible, and a highly accurate irradiation dosimeter or absorption dosimeter can be realized.The energy compensation performance of the detector of the present invention is as follows:
For the energy range of 80 KeV to 3 M e V, it is possible to improve by 40% from the conventional ±25% to ±15%. In addition, it is possible to eliminate the need for complex processes such as repeating conventional trials to adjust the energy characteristics and creating openings in the compensation material, reducing the manufacturing and adjustment cost of the detector by more than 50%. Can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a three-dimensional view of the detector of the present invention, FIG. 3 is a cross-sectional view of a semiconductor detection element, and FIG.
The figure shows the relationship between characteristic X-rays and atomic number, Figure 5 shows the energy characteristics of the irradiation dosimeter, Figure 6 shows the relationship between photon energy and mass absorption coefficient of various materials, and Figure 7 8 is a diagram showing energy characteristics of a living body absorption dosimeter, FIG. 8 is a diagram showing a modification of the present invention, and FIG. 9 is a diagram showing another modification. DESCRIPTION OF SYMBOLS 1... Semiconductor detection element, 2... Flat base base material, 3... Bonding wire, 4°... Seametic seal, 5... Shrinkage material, 6... Probe case, 7 ...Detector support pole, 8...Detection signal line,
10... positive electrode, 11-n ten layer, 12 depletion layer, 2
0 Shiyahei wood. Ki 1st day '2 - Figure 3 Figure 1] 10 Graduation map) Nagou No. S map and V are Engineering 3.1 Shigi (Mev) High school 6th ward Yoshiko Furuya - (MeV) No. 9 figure book 8 figure

Claims (1)

[Scope of Claims] 1. In a radiation dosimeter using a semiconductor radiation detection element, at a position closest to the detection element, atomic number 44
A radiation dosimeter equipped with materials containing the following elements as main components. 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. In radiation dosimeters using semiconductor radiation detection elements, atomic number 4 is used as a shielding material for energy compensation.
Radiation dosimeter using materials whose main components are 4 or less elements.