JPH053551B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to a neutron detection device using a boron thin film.

[Prior art and its problems]

The principle of a semiconductor radiation detector is to form a diode structure using any method such as a PN junction, a semiconductor-metal Schottky junction, or a crystalline semiconductor-amorphous semiconductor heterojunction, and then apply a reverse bias voltage to the diode. Accordingly, a depletion layer is expanded in the semiconductor, and electron-hole pairs generated by the irradiated radiation flying into the depletion layer are counted and detected as current pulses.

Semiconductor materials include germanium (Ge) and silicon (Si). Currently, silicon (Si) is used industrially in radiation dosimeters because the material is easily available. Overwhelmingly many. Recently, high-purity, high-resistivity silicon has come to be used due to the demand for low-voltage operation.

Regarding radiation, X-rays, α-rays, β-rays, and γ-rays directly generate electron-hole pairs within the semiconductor depletion layer, and can be detected using conventional methods. Since it has no electric charge, it does not have any effect on the orbital electrons or the Coulomb field of the atomic nucleus other than the nuclear reaction.Therefore, no electron-hole pairs are generated in the semiconductor depletion layer, and neutron beams can be detected using conventional methods. It is impossible. Therefore, as a neutron beam detection method, neutron beams are transmitted through a material with a large neutron absorption cross section, and α rays are generated by a neutron transmutation reaction, and the α rays generate electron-hole pairs within the semiconductor depletion layer. However, there is a method to detect this and detect the neutron beam.

As a specific ^example , ¹⁰ B (n,
There is a method using the α) reaction to detect α rays generated from boron when a neutron beam is incident, according to the reaction shown in the following equation.

¹⁰ B(n,α) reaction: ₁₀₅ B+ ₁₀ n→ ₇₃ Li+ ₄₂ He However, boron has an extremely high melting point of 2000°C or higher, making it difficult to easily form a single layer of boron. Even if a boron layer could be formed,
There was a problem in that the substrate itself, which should detect the alpha rays, would be destroyed or deteriorated due to the high temperature.

Particularly in semiconductor radiation detectors, there is a problem that thermal distortion occurs at high temperatures, resulting in deterioration of device characteristics. In particular, since semiconductors used in radiation detectors have high purity and high specific resistance, a low-temperature process is required in the manufacturing process. It is said that thermal strain begins to occur in high-purity, high-resistivity silicon at temperatures above 400°C. Therefore, high-temperature processes of 800° C. or higher, which are commonly used in semiconductor processes, are not suitable for semiconductor radiation detectors.

For example, a neutron detector that combines a boron phosphide layer and a semiconductor PN junction is known, but this has the following problems.

Since a thermal decomposition method is used to form the boron phosphide layer, the semiconductor substrate is exposed to a high temperature of 900° C., which inevitably causes deterioration of semiconductor characteristics for the reasons mentioned above.

The boron isotope ^10B is found in nature ( ^10B :
^11B ≒ 1:4), but even if ^10B is added to a higher concentration, phosphorus is included in the boron phosphide layer using phosphin (PH ₃ ), so the concentration of ^10B decreases. do. Therefore, neutron detection sensitivity decreases.

Since the thickness of the fabricated boron phosphide layer is as much as 20 μm, the generated alpha rays are reduced by self-absorption, which reduces the neutron detection sensitivity.

As described above, a neutron detection device that combines a boron phosphide layer and a semiconductor has problems.

In addition, neutron detectors that do not use semiconductors have complex structures, poor stability, low detection sensitivity, and poor counting characteristics. Furthermore, non-semiconductor neutron detectors also have the disadvantage of being extremely large and heavy.

Neutron detection elements have the above-mentioned drawbacks, so if you try to use a moderator to detect fast neutron beams, etc., unless you use a semiconductor detector, the detection part is large and cannot be mounted around it. The moderator to be installed is also extremely large and heavy, making it difficult to put it into practical use.

In order to measure fast neutrons of each energy using a moderator, it is necessary to place a moderator with a thickness corresponding to each energy.
Unless the detection element is miniaturized like a semiconductor, the amount of moderator will be enormous.

Recently, with the increase in neutron sources such as nuclear reactors and accelerators, we are investigating what kind of neutron beams are being generated, that is, what kind of energy neutron beams are being generated, and are investigating the effects of neutron beams on the human body. There is an urgent need to take appropriate measures.

However, as mentioned above, it is currently difficult to accurately detect fast neutron beams in real time, so a device called a Bonner ball is actually used to detect fast neutron energy spectrum images.

This uses gold ( ¹⁹⁷ Au) in the neutron detection part,
Around it, several moderators such as polyethylene with different thicknesses are used to correspond to each energy.
Activation of gold after neutron beam irradiation ( ¹⁹⁷ Au→ ¹⁹⁸ Au)
is used to detect neutron beam spectrum images.

This will be miniaturized to some extent, but it will only become clear after being irradiated with neutron beams.
It is not measured in real time.

[Purpose of the invention]

The present invention is ^directed to the boron isotope ^10B (n,
It is a semiconductor neutron detector using α) reaction. This involves creating a detector using a boron thin film containing a high concentration of ¹⁰ B, placing a semiconductor element with high neutron detection efficiency in the center, and varying the thickness of the moderator.
The purpose of this invention is to provide an apparatus that can obtain energy spectrum images of fast neutron beams in real time.

[Key points of the invention]

The present invention involves forming a boron thin film containing a high concentration of the boron isotope ¹⁰ B on the surface of a semiconductor substrate at a low temperature of 200°C or less, which has been considered difficult in the past. The sensing element was fabricated entirely in a low-temperature process by forming a heterojunction diode structure that was deposited at a low temperature below 0.9°C. As a result, a highly sensitive neutron beam detection element was fabricated without thermally straining the semiconductor substrate or deteriorating its characteristics. This system uses several or more devices to obtain energy spectrum images of fast neutron beams in real time.

This is because the neutron beam detection part is highly sensitive, small,
Since it has become possible to make it lightweight, it has also become possible to make the moderator smaller and lighter.

[Embodiments of the invention]

FIG. 1 is a cross-sectional view of a neutron beam detection element portion. An amorphous silicon film 2 was deposited on the surface of a P-type single crystal silicon 1 substrate by plasma CVD under the following conditions.

●Plasma CVD conditions Gas used: Monosilane [SiH ₄ (10% H ₂ base)] Substrate temperature: 200°C Pressure: 10.0 Torr Applied voltage: 700 V (DC) Also, on the back side, a boron thin film containing a high concentration of ¹⁰ B 3 was formed by plasma CVD method. The conditions are shown below.

Gas used: Ciborane [ ¹⁰ B ₂ H ₆ (1000 ppm _{H 2} base)] Substrate temperature: 200°C Pressure: 2.0 Torr Applied voltage: 560 V (DC) After this, metal is deposited as electrodes 5 and 6. A reverse bias voltage is applied to electrodes 5 and 6 to form a depletion layer in the single crystal silicon substrate. Here, the neutron beam 6 enters and causes _{a 10} B (n1α) reaction in the boron thin film 3, generating α rays 7, which are detected as current pulses within the depletion layer.

FIG. 2 shows the detection element 10 of FIG.
This equipment is capable of measuring fast neutron beams. When the neutron beam 6 comes, it causes the moderator 11 to reach the detection element 10 along a trajectory as shown in FIG.

In this example, it was found that the measurement neutron beam energy range was 0.025 eV to 14 MeV.

〔Effect of the invention〕

According to this invention, a neutron beam energy spectrum image can be measured in real time. In addition, it can be used as a monitor to indicate the influence of neutron beams on the human body by changing the thickness of the moderator in several types in accordance with the ICRP recommendations that indicate the degree of influence of neutron beams on the human body.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a neutron beam detection element, and FIG. 2 is a sectional view of a detection device using the element of FIG. 1... P-type single crystal silicon, 2... Amorphous silicon, 3... Poron thin film, 4, 5... Electrode, 6...
...Neutron beam, 7...α ray, 10...Detection element, 1
1... Moderator.

Claims (1)

[Claims] 1. A neutron detection device comprising a detector combining a heterojunction in which an amorphous semiconductor is deposited on the surface of a single crystal semiconductor and a boron thin film, and a moderator placed around the detector.