JPS6111474B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a semiconductor radiation detector that identifies and detects types of radiation. Semiconductor radiation detectors use silicon or germanium to form a pn junction or surface barrier, and apply a reverse bias to these to detect electron-hole pairs generated when radiation enters the depletion layer of the semiconductor. There is a known method that extracts and detects current pulses from electrodes on both sides of a plate. For example, as shown in FIG.
An aluminum electrode layer 2 is deposited on one side, and a gold electrode layer 3 is deposited on the other side. In this case, silicon plate 1
In contrast, electrode layer 2 forms a surface barrier and electrode layer 3 forms an ohmic contact. A voltage that becomes a reverse bias to this surface barrier is applied to both electrodes 2 and 3 by a power source 4.
When the voltage is applied between
spreads. When radiation 6 enters this depletion layer 5 from the electrode 2 side, electron-hole pairs are generated, and due to the bias from the electrode 4, the holes are carried to the electrode 3 side and the electrons are carried to the electrode 2 side, creating a pulse current. . This pulse current is passed through an amplifier 7 and counted by a detector (not shown) to determine the intensity of the incident radiation. Although such a detector can detect radiation such as α-rays and β-rays, it is necessary to thicken the depletion layer to increase counting efficiency for highly penetrating radiation such as X-rays and γ-rays. A lithium drift type detector is also used in which lithium is diffused into p-type silicon or germanium and an intrinsic region is formed by impurity compensation to widen the depletion layer. However, lithium drift type detectors have the inconvenience of having to be constantly cooled and stored, so detectors using ultra-high purity silicon, which has recently become commercially available, are being manufactured. Moreover, it is still not possible to discern from the output pulses of these conventional detectors what radiation they are based on. Furthermore, in order to detect neutron beams that do not have an electric charge, a dedicated detector must be prepared separately. An object of the present invention is to eliminate such inconveniences and provide a semiconductor radiation detector that can identify and detect multiple types of radiation with a single detector as described at the beginning. According to the present invention, the surface of the semiconductor board far from the surface barrier or the pn junction is the incident surface of the radiation, and a plastic plate containing removable hydrogen atoms, a plastic plate containing detachable hydrogen atoms, and a boron atom are used to cover the incident surface. This is achieved by providing a plate or metal foil containing a 1000 .ANG. Embodiments of the present invention will be described below with reference to the drawings. In FIG. 2, the reference numeral 11 indicates, for example, a p-type ultra-high resistivity silicon plate of 1000 Ωcm or more,
An aluminum electrode layer 2 is deposited on one side, and a gold electrode layer 3 is deposited on the other side. As in the case of FIG. 1, the electrode layer 2 forms a surface barrier for the silicon plate 1.
The electrode layer 3 forms an ohmic contact. A power source 14 that applies a reverse bias to the surface barrier between the silicon plate 1 and the electrode layer 2 is connected to both electrode layers.
This power supply 14 has a variable output voltage. A depletion layer 5 is created in the silicon plate 11 due to the reverse bias. In the detector according to the invention, the side of the electrode layer 3 in ohmic contact is the radiation incident surface. This incident surface is covered with a plastic plate containing hydrogen atoms such as a polyethylene plate or a plate containing boron atoms such as a Boral plate 8, which covers it as a filter.
Also, a metal foil 9 such as aluminum foil can be removably attached. A very common method can be used for this removable attachment. For example, a male thread may be cut into a frame body (not shown) surrounding the plate 8 and foil 9, and the male thread may be screwed into a female thread (not shown) provided in a window of a container housing the silicon plate 11, or an airtight ring may be provided. The plate 8 and the foil 9 are placed in the opening of the window while
It is also possible to lower the bolt and apply tightening force from the top. When charged particles 6 such as α rays and β rays enter the depletion layer 5 from the electrode layer 3 side of such a detector, they lose their energy while forming pairs of electrons and holes. A large number of electron-hole pairs generated at that time are
It is swept toward both electrodes by the voltage applied by 4, is taken out as a current pulse to an external circuit, passes through an amplifier 7, and is counted by a detector (not shown). When electromagnetic radiation such as gamma rays or As a result, electron-hole pairs are created, which are detected as current pulses. In the case of neutron beams, since they have no electric charge, they do not have any effect on the orbital electrons or the Coulomb field of the atomic nucleus other than nuclear reactions. However, if the incident surface is covered with a plastic plate 8 containing hydrogen atoms, such as a polyethylene plate, when the fast neutron beam passes through the polyethylene plate, it excites the hydrogen atoms in the polyethylene, generating pronto. When this pronto enters the depletion layer 5, it is detected in the same process as the above-mentioned α ray. Further, when a Boral plate containing boron atoms is placed in place of the polyethylene plate 8, when a thermal neutron beam passes through the Boral plate, α rays are generated by the reaction ¹⁰ B+n→ ⁷ Li+α and detected. Next, a method for identifying and detecting the type of incident radiation using the detector according to the present invention will be described. (1) Detection of α rays For example, 5 MeV α rays are approximately 30μ in silicon.
It has a penetrating power of m, and when the polyethylene plate or Boral plate 8 and aluminum foil 9 are removed, even taking into account the absorption at the electrode 3, it is absorbed within a depth range of about 25 μm in silicon. Therefore, detection becomes possible only when the depletion layer 5 extends to this range. Now the thickness is 490μ
If the detector is manufactured using a p-type silicon plate 11 with m and resistivity of 20KΩcm, the width of the depletion layer 5 is approximated by 〓0.33√, where ρ: resistivity of silicon, v: reverse bias voltage. Therefore, when applying a reverse bias voltage of 100V, it becomes 467μm. Therefore, the end of the depletion layer 5 is located at a distance of 490-467=23 μm from the electrode layer 3 side, and the α rays reach the depletion layer 5 and are therefore detected. However, if an aluminum foil 9 or a polyethylene plate 8 is attached, the alpha rays are completely absorbed by these and are not detected, so that they can be determined to be alpha rays. (2) Detection of β rays For example, 1 MeV β rays have approximately 1200
Since it has a penetrating power of μm, if p-type silicon having the same resistivity and thickness as in the case of α rays is used, it can be detected regardless of the level of reverse bias. Beta rays are not absorbed when the aluminum foil 9 is attached, but are completely absorbed and undetected when the approximately 3 mm polyethylene plate or Boral plate 8 is attached, so they can be easily distinguished from the above-mentioned alpha rays or the below-mentioned γ rays. Can be identified. Note that in order to increase detection efficiency and detect low-energy β-rays, it is desirable to apply a reverse bias voltage of about 100 V in this example to make the depletion layer as wide as possible. (3) Detection of γ-rays and X-rays γ-rays or X-rays have strong penetrating power, and output pulses are detected regardless of the presence or absence of the polyethylene plate, Boral plate 8 or aluminum foil 9, and the level of reverse bias voltage. so it can be easily identified. (4) Detection of neutron beams As mentioned above, neutron beams do not have an electric charge, so they are detected only when a polyethylene plate or Boral plate 8 is attached.For fast neutron beams, protons generated in the polyethylene plate are detected. For thermal neutron beams, alpha rays generated in the Boral plate are detected. The reverse bias voltage is detected only when the reverse bias voltage is set high enough to reach the depletion layer close to the ohmic contact electrode side, for the same reason as in the case of detecting the alpha rays described above. For example, when a fast neutron beam of 2.5 MeV is irradiated, the range of protons generated in a polyethylene plate in silicon is about 70 μm. Therefore, when only the polyethylene plate 8 is attached, it can be detected, but when an aluminum foil 9 of several micrometers is attached between the polyethylene plate 8 and the silicon plate, it is no longer detected, so that a fast neutron beam can be determined. Similarly, in the case of a thermal neutron beam, it is detected when the Boral plate 8 is attached, and is no longer detected when the aluminum foil 9 is further attached, so that it can be confirmed that it is a thermal neutron beam. These are summarized in Table 1. In the table, the ○ mark indicates the case of detection, and the x mark indicates the case of no detection. Also, nf means fast neutron beam, and nth means thermal neutron beam.

【table】

[Table] In the above example, a silicon plate provided with a surface barrier was used, but a silicon plate provided with a pn junction can be used in exactly the same way. The surface barrier or pn junction may be provided not only on one side of the semiconductor board but also on both sides. This is because a voltage that causes a reverse bias to the surface barrier or pn junction on one side causes a forward bias to the surface barrier or pn junction on the other side, so that the detector operates in exactly the same way. Therefore, as long as the relationship between the direction of voltage application and the direction of radiation incidence is correct, it is also possible to use an element that is symmetrical on both sides. In addition to silicon, compound semiconductors such as CdTe and GaAs are also effective as semiconductors. As a filter, other general metal foils can be used instead of aluminum foil, and other general plastic plates can be used instead of polyethylene plates, and all three types of filters can be used when the amount of radiation to be detected is limited. It is clear from Table 1 that this is not necessary. As described above, according to the present invention, the type of incident radiation can be identified and detected simply by using a detection element with the same structure as the conventional one, inverting the incident surface, making the applied reverse bias adjustable, and preparing a filter. A semiconductor radiation detector can be obtained. Therefore, all radiation can be detected with one detector, making it extremely effective in terms of handling and economy.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing how a conventional semiconductor radiation detector is used, and FIG. 2 is a connection diagram showing how a semiconductor radiation detector according to an embodiment of the present invention is used. DESCRIPTION OF SYMBOLS 11...Silicon plate, 2...Surface barrier contact electrode layer, 3...Ohmic contact electrode layer, 5...Depletion layer, 6
... Incident radiation, 8 ... Polyethylene plate or Boral plate, 9 ... Aluminum foil, 14 ... Voltage variable power supply.

Claims (1)

[Claims] 1 A surface barrier or a pn junction is formed on a semiconductor plate, and a reverse bias is applied to the formed depletion layer. Electron/hole pairs generated by the incidence of radiation are transferred to a current through electrodes provided on both sides of the semiconductor plate. In the semiconductor board, the surface of the semiconductor board far from the surface barrier or p-n junction is the incident surface of the radiation, and a plastic plate containing detachable hydrogen atoms and a plate containing boron atoms cover the incident plane. Alternatively, a semiconductor radiation detector characterized in that a metal foil is provided, and both electrodes are connected to a power source that applies a reverse bias of an adjustable magnitude.