JPH053550B2 - - 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 semiconductor radiation detectors is PN junction, semiconductor-metal Schottky junction, or crystal semiconductor-
A diode structure is formed by any method such as an amorphous semiconductor heterojunction, and a reverse bias voltage is applied to the diode, thereby expanding a depletion layer in the semiconductor and causing radiation generated by flying into the depletion layer. The electron-hole pairs that occur 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 charge, it does not have any effect on the Coulomb field of orbital electrons or atomic nuclei other than nuclear reactions, and therefore within the semiconductor depletion layer,
No electron-hole pairs are produced and detection of the neutron beam is not possible with conventional methods. 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 using boron isotope ¹⁰ B, which has a large scattering cross section for neutron beams.
Using the (n, α) reaction, according to the reaction shown by the following formula,
There is a method of detecting alpha rays generated from boron when a neutron beam is incident.

¹⁰ B(n,α) reaction: ¹⁰ ₅ B+ ¹ ₀ n→ ⁷ ₃ Li+ ⁴ ₂ He However, boron has an extremely high melting point of 2000° C. or higher, and it is 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 element 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,
800℃ commonly used in semiconductor process
The above high temperature 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, and deterioration of semiconductor characteristics is inevitable for the reasons mentioned above.

The boron isotope ¹⁰ B is found in nature as ¹⁰ B: ¹¹ B
It exists at a ratio of ≒1:4, but in addition to this, ¹⁰ B
Even at higher concentrations, the concentration of ¹⁰ B decreases because phosphorus is included in the boron phosphide layer using phosphine (PH ₃ ). Therefore, neutron detection sensitivity decreases.

Since the fabricated boron phosphide layer is as thick 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.

[Purpose of the invention]

It is an object of the present invention to provide a neutron detection device having a simple structure, high stability, and high detection sensitivity.

[Key points of the invention]

In order to achieve this objective, the present invention performs glow discharge using diborane gas ( ¹⁰ B ₂ H ₆ ), which is prepared by selectively concentrating the boron isotope ¹⁰ B, to increase the concentration of ¹⁰ B when forming a boron thin film. It is characterized by forming a thin film containing boron.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. Hereinafter, in all of the examples disclosed herein, the boron thin film shown is made of the above-mentioned boron isotope.
It is a boron thin film containing a high concentration of ¹⁰ B. 1st
2 and 2 show the case of a detector having a surface barrier type structure. For example, in the case of P-type silicon, an aluminum electrode 6 as a barrier metal and a gold electrode 7 as an ohmic contact are vacuum-deposited, respectively. A boron thin film 8 is formed on the surface of at least one of the electrodes 6 and 7. A thermal neutron beam of 4
When the thermal neutron beam passes through the boron thin film 8, α rays 5 are generated due to the neutron transmutation reaction ¹⁰ B+ ¹ ₀ n→ ⁷ Li+α, and the α rays deplete the depletion formed in the silicon semiconductor 1. The thermal neutron beam can be detected because it penetrates into the layer and generates electron-hole pairs, resulting in a current pulse. When a boron thin film 8 is formed on each of the surfaces of both electrodes 6 and 7 as shown in FIG. 2, α rays generated on the back surface are also detected, so that the thermal neutron detection efficiency is the same as that of the embodiment shown in FIG. 1. It can be further enhanced.

3, 4, and 5 show the case of a pn junction type radiation detection element, in which a boron thin film 8 is selectively formed on an n-type silicon substrate 9. A boron interstitial layer, that is, a p ⁺ layer 10, is formed on the n-type silicon substrate 9 directly under the boron thin film 8. This p ⁺ layer is 10 ²¹ ~
It has an extremely high boron concentration of 10 ²² atmos/ ^cm3 ,
Moreover, since it is boron containing a high concentration of ¹⁰ B, thermal neutron beams can be detected efficiently. This p ⁺ layer containing a high concentration of ¹⁰ B was formed using the method disclosed in Japanese Patent Application No. 1983-93218 filed by ^the present applicant. It is formed by using gas contained in

In addition, in order to further increase the detection efficiency of thermal neutron beams, electrodes 6 and 7 are arranged as shown in FIGS. 4 and 5.
A boron thin film 8 is also formed on the surface. the result,
A neutron detection element with high detection efficiency can be obtained.

FIG. 6 is a structural sectional view of a surface barrier type detection element. A p ⁺ layer 10, that is, an ohmic contact layer, is formed directly under the boron thin film 8, and a boron thin film 8 is also formed on the electrode.

FIG. 7 is a structural diagram of a heterojunction element in which amorphous silicon is formed on the surface of a single-crystal silicon semiconductor.

This is fabricated by depositing amorphous silicon on the surface of single crystal silicon by plasma CVD, and then depositing 20 boron thin film layers 20 by plasma CVD using diborane ¹⁰ B ₂ H ₆ gas containing a high concentration of ¹⁰ B. It was formed.

This detection element uses plasma CVD, and all element manufacturing processes are performed at a low temperature of 200°C or less, so there is almost no thermal deterioration of the high-purity silicon 9, and single-crystal silicon with a resistivity of 10 kΩ-cm or more is used. Even so, there is no thermal deterioration.

For example, when calculating the sensitivity of a silicon wafer with a surface area of 10 cm ² , the neutron beam flux density
At 100nV, the counting rate A=Nδφ where N is the atomic density, δ is the scattering cross section, and φ is the flux density.

= 10 cps, and a highly sensitive neutron detector can be easily formed. Furthermore, if multiple layers of this boron thin film are formed, the sensitivity will of course be further improved.

Here, the conditions for manufacturing this device are shown below.

Silicon single crystal: Specific resistance 10 kΩ-cm or more (40φ) Amorphous silicon: DC plasma using monosilane [SiH ₄ (10% H ₂ base)]
Produced using CVD method.

●Plasma CVD method Substrate temperature: 200℃ Pressure: 10.0Torr Applied voltage: 700V Boron thin film: DC plasma using diborane [B ₂ H ₆ (1000ppmH ₂ base)]
Produced using CVD method.

●Plasma CVD method Substrate temperature: 200°C Pressure: 2.0 Torr Applied voltage: 560V Since the structure of the radiation detector according to the present invention is similar to that of conventional detectors, it is naturally possible to detect radiation other than thermal neutrons. For example, in the case of gamma rays in this semiconductor detector, the spectrum shown in 11 in FIG. 8 is shown in order to detect secondary electron beams due to the photoelectric effect and the Compton effect.

As mentioned above, when neutrons enter this detector and pass through the boron thin film, they react with the boron isotope ¹⁰ B (n, α) to generate α rays of 2.30 MeV. Therefore, the spectrum shown at 12 in FIG. 7 is shown. As shown in FIG. 8, normally detectable gamma rays and alpha rays caused by thermal neutron beams can be clearly distinguished.

Furthermore, if a moderator such as polyethylene is placed on the neutron beam entrance window of this detector, fast neutron beams can also be detected. In other words, when a fast neutron beam enters the moderator, the protons ejected by elastic collisions enter the depletion layer formed within this detector and generate electron-hole pairs, so it can be detected in the same way as other radiation. sell.

In the embodiments disclosed here, silicon is used for the semiconductor substrate, but of course the present detector can sufficiently detect the neutron beam even if germanium or compound semiconductors such as GaAs or CDTE are used. The reason for this is that the aforementioned boron thin film is formed by the plasma CVD method using glow discharge, but the temperature applied to the semiconductor substrate is 300°C or less, so the boron thin film can be formed without causing thermal distortion on the semiconductor substrate. This is because a good radiation detector can be produced.

〔Effect of the invention〕

According to the present invention, thermal neutron beams can be detected without attaching a neutron transmutation material to the front surface of a radiation detection element as in the prior art. Moreover, since this boron thin film is in close contact with the radiation detection element, it causes ^{a 10} B (n, α) reaction with the boron isotope ¹⁰ B, and since the boron thin film itself is thin at around 1500A, self-absorption of α rays is also possible. Since the small amount of α rays generated efficiently penetrates the semiconductor substrate, the detection efficiency of thermal neutron beams is increased, and at the same time, the thickness of the boron thin film is 500 to 1,600 Å, making it possible to make the dead layer width extremely thin. As a result, there is no particular problem in detecting alpha, beta, and gamma rays.

[Brief explanation of the drawing]

Figures 1 and 2 are cross-sectional views of a surface barrier type radiation detector, Figures 3, 4, and 5 are cross-sectional views of a pn junction type radiation detector, and Figure 6 is a cross-sectional view of a surface barrier type radiation detector. 7 is a sectional view of a heterojunction type radiation detector, and FIG. 8 is a spectrum of count rate versus pulse height when a neutron beam is measured using the semiconductor radiation detector according to the present invention. 1...p-type silicon substrate, 4...neutron beam, 5
... alpha rays, 6 and 7 ... electrodes, 8 ... boron thin film, 9 ... n-type silicon substrate, 10 ... p ⁺ region,
11... γ-ray spectrum, 12... alpha-ray spectrum, 20... amorphous silicon.

Claims (1)

[Claims] 1. A neutron detection device characterized by combining a semiconductor PN junction, a semiconductor-metal Schottky junction, or a crystalline semiconductor-amorphous semiconductor heterojunction, and a boron thin film containing a high concentration of boron isotope ¹⁰ B.