JPH022312B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a semiconductor radiation detector for gamma ray counting, and more particularly to a semiconductor radiation detector with improved relationship between gamma ray energy and gamma count in the same dose field. Conventionally, Geiger-Muller counters have been used as this type of detector, but they have a short lifespan and
The linearity of the γ count with respect to the dose rate was poor, and it also required a high-voltage power source. Therefore, in recent years, semiconductor radiation detectors that utilize the characteristics of semiconductors have been provided and are being put into practical use. This semiconductor radiation detector has a depletion layer sensitive to gamma rays formed by diffusing lithium (Li) into a wafer of germanium (Ge) or silicon (Si), for example. When γ-rays pass through this depletion layer, secondary electrons are generated through the photoelectric effect, Compton effect, or electron pair generation, and these secondary electrons further interact with lattice atoms to create electron-holes. By generating a pair of current pulses, detecting them as current pulses, and counting the number of pulses, γ
Dose can be counted. Although the original purpose of a gamma ray counter is to count the number of gamma rays as radiation, conventional radiation detectors cannot measure the energy of individual gamma rays even in the same dose field for the reasons explained later. The number of counting pulses differs depending on the level. That is, there is a problem that the counting pulse does not correctly indicate the gamma ray dose. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a radiation detector that eliminates the disadvantages of conventional radiation detectors and improves so-called radiation quality characteristics that indicate the relationship between γ-ray energy and the number of pulses in the same dose field. . Therefore, the semiconductor radiation detector of the present invention for achieving the above object includes forming a PN junction on a part of the main surface of a semiconductor substrate, and forming a depletion layer and a periphery of the depletion layer that spreads from the PN junction into the semiconductor substrate. In a semiconductor radiation detector that detects incident radiation, the area on the substrate surface of the PN junction for forming a depletion layer that is sensitive to radiation is πR ₁ ² or 4R ₁ R ₂ , and R ₁ , R ₂ ≦l _s (where, R ₁ and R ₂ are the radius when the shape of the PN junction is circular, and 1/2 of the length of one side when it is rectangular,
l _s indicates the average range of secondary electrons). An embodiment of the semiconductor radiation detector according to the present invention will be described below with reference to the drawings. First, the operating principle of the radiation detector will be explained in detail. As shown in Fig. 1, the depletion layer 1, which is sensitive to γ-rays, is sandwiched between electrodes 2, and a bias voltage _VB is applied thereto. , when the incident γ-ray 3 passes through the depletion layer 1, secondary electrons 4 are generated by one of the processes of photoelectric effect A, Compton effect B, and electron pair generation C, and these secondary electrons 4 further interact with lattice atoms. As a result, electron-hole pairs 5 are generated, which are detected as current pulses 6, which are counted by a counter 7 containing an amplifier. Note that some of the γ-rays pass through the depletion layer 1 as scattered γ-rays 8. At this time, the number of pulses per unit dose rate, that is, the γ count C, is expressed by the following equation (1). C=Kμsi・l/μair・E・S...(1) Shift K: Constant μsi: Absorption coefficient of radiation detector (Si) μair: Absorption coefficient of air l: Thickness of sensitive body for γ-rays S: Surface area of sensitive body for gamma rays E: Energy of gamma rays The structure of an actual radiation detector is shown in FIG. After forming the N-type diffusion layer 11 through the surface protection film 10,
Electrodes 2 are attached to one and two surfaces of the wafer 9 by vacuum evaporation. The depletion layer 1 is designed to expand when a reverse bias voltage is applied between the electrodes 2, 2. In this case, in addition to when the secondary electrons 4 are generated within the depletion layer 1, if secondary electrons generated within the silicon wafer 9 of the depletion layer also enter the depletion layer, the counter 7
It is counted by. In other words, it is thought that gamma ray detection has a sensitive region 12 outside the depletion layer 1, and when the gamma ray energy is small, the sensitive region 12 is located at the average range of secondary electrons. Since l _s1 is also short, it is small, as shown by dotted line A in Figure 2, and when the energy of γ-rays is high, the average range of secondary electrons is also long, so it becomes large, as shown by dotted line B. That is, the sensitive body surface area S in the above equation (1) is expressed as in the following equation (2). S=π(R+l+ls) ² ...(2) R: Radius of PN junction l: Thickness and lateral spread of depletion layer ls: Average range of secondary electrons Equations (1) and (2) Table 1 below shows the relationship between the energy E of γ rays, the average range ls of secondary electrons, the absorption coefficient μair of air, and the absorption coefficient μsi of silicon (Si) in the equation (2).

[Table] Note that the radiation quality characteristics are usually 2MeV to 0.1MeV or
Since we evaluate in the range 3MeV to 0.1MeV, ls is the maximum energy in that range (2MeV or 3MeV)
All you have to do is choose the value of . According to Table 1, ls is 1170 μm or 1900 μm. As is clear from equation (2) above, when R≫ls, the surface area S of the sensitive body is almost constant even if the energy E of the γ-ray changes, and as is clear from equation (1),
The γ count C depends on the energy E of the γ rays. In other words, the γ count C counted by the counter 7 changes not only depending on the actual number of γ rays but also on the energy of the γ rays, and the difference between the energy of the γ rays and the number of measured pulses in the same dose field. The so-called radiation quality characteristics that indicate the relationship deteriorate. On the other hand, when R≦ls, the sensitive body surface area S changes depending on the average range of the secondary electrons. As is clear from Table 1, the value of ls is the energy of γ-ray E
Since the value of S changes greatly depending on E, the value of S also changes greatly depending on E. In this case, the value of S increases greatly as E increases, and changes in a direction that cancels out the fractional part μsil/μairE in equation (1). Therefore, the energy dependence of the γ coefficient C becomes smaller, and the radiation quality characteristics are improved. The present invention was made based on the recognition of the characteristics of various elements related to the above-mentioned γ coefficient C, and is characterized by the radius R of the PN junction constituting the depletion layer 1.
is characterized by being approximately equal to or smaller than the average range of secondary electrons. FIG. 3 is a logarithmic graph showing the relationship between γ-ray energy E (unit: MeV) and relative γ-count ratio, where the relative γ-count ratio does not change with respect to changes in γ-ray energy E. In other words, the closer the curve is to the horizontal, the better the radiation quality characteristics. In the figure, a curve 11 shows the characteristics of a conventional radiation detector. This is a case where the area of the PN junction is 154 mm ² (R = 7 mm), and on the high energy level side, curve 1 shows the counted value according to equation (1).
It almost matches 2. Curve 1 on the low energy side
The reason why 1 and the curve 12 do not match is because the discrimination level of the counter 7 (see FIG. 1) is set to 100 KeV so that the pulse 6 having a level of 100 KeV or less is not counted. In contrast, the characteristics of the radiation detector according to the present invention, as shown by curves 13 and 14 in FIG. 3, are closer to horizontal than curve 11, and it can be seen that the radiation quality characteristics are significantly improved. In addition, curve 13 has a PN junction area of 3.14mm ²
(R=1mm), curve 14 has the area of the PN junction.
This is the characteristic of a radiation detector of 0.79mm ² (R = 0.5mm), and the applied bias voltage is common in Figure 3.
It is 100V. FIG. 4 shows a modified embodiment of the present invention, in which a small radius PN is placed on one silicon wafer 9.
A plurality of depletion layers 1, 1, 1 are formed at the junction, and by doing so, the number of γ-ray sensitive bodies with good radiation quality characteristics increases, so the sensitivity as a radiation detector increases. The embodiment shown in FIG. 5 also includes a radiation detector chip 15 having a single PN junction.
This is an example in which a plurality of transistors are bonded onto a single base plate 16 of a hermetic case for a transistor, and the same effects as in the previous embodiment can be obtained in such an embodiment as well. As is clear from the above explanation, in the present invention, the radius of the PN junction forming the depletion layer is made equal to or smaller than the average range of secondary electrons, so the radiation quality characteristics are significantly improved, making it suitable for gamma dosimeters. An applicable semiconductor radiation detector can be obtained. In the above explanation, it is assumed that the shape of the PN junction is circular, and its size is expressed by the radius R. For example, if it is a rectangle, its width and length are expressed, and if it is a polygon, it is expressed by the area equivalent to this. The radius of the circle can be replaced with the above PN junction. Furthermore, it goes without saying that the present invention can be similarly applied to a semiconductor having a surface barrier type structure.

[Brief explanation of drawings]

Figure 1 is a diagram showing the operating principle of a semiconductor radiation detector, Figure 2 is a cross-sectional view of the semiconductor radiation detector showing the relationship among the PN junction, depletion layer thickness, and average range of secondary electrons, and Figure 3 is a diagram showing the operating principle of a semiconductor radiation detector. Graphs showing differences in radiation quality characteristics between a conventional semiconductor radiation detector and a radiation detector according to the present invention, and FIGS. 4 and 5 are schematic diagrams showing radiation detectors according to other embodiments of the present invention. It is. 1... Depletion layer, 2... Electrode, 3... γ ray, 4... Secondary electron.

Claims (1)

[Scope of Claims] 1. In a semiconductor radiation detector that forms a PN junction on a part of the main surface of a semiconductor substrate and detects radiation incident on a depletion layer spreading from the PN junction into the semiconductor substrate and around the depletion layer. , the area of the PN junction on the substrate surface for forming a depletion layer that is sensitive to radiation is πR ₁ ² or 4R ₁ R ₂ , and R ₁ , R ₂ ≦l _s (however, R ₁ , R ₂ is the radius when the shape of the PN junction is a circle, and 1/2 of the length of one side when it is a rectangle.
l _s indicates the average range of secondary electrons) A semiconductor radiation detector characterized in that: