JPH0736447B2 - Semiconductor neutron detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a boron isotope ¹⁰ when a thermal neutron beam is incident.
The present invention relates to a semiconductor neutron detection element that utilizes α rays generated by the reaction with B.

[Conventional technology]

The principle of the semiconductor radiation detecting element is to form a diode structure by any method such as a pn junction, a semiconductor-metal Schottky junction, or a heterojunction between a single crystal semiconductor and an amorphous semiconductor, and apply a reverse bias voltage to the diode. The voltage is applied to expand the depletion layer in the semiconductor, and the electron-hole pairs generated by the radiation flying into the depletion layer are counted and detected as current pulses.

Also with respect to radiation, x-rays, α-rays, β-rays and γ-rays directly generate electron-hole pairs in the semiconductor depletion layer, and therefore radiation can be detected as it is. On the other hand, since the neutron beam has no electric charge, it has no effect on the Coulomb field of the orbital electrons and electron nuclei except for the nuclear reaction, so that electron-hole pairs do not occur in the semiconductor depletion layer and the neutron beam Is not possible with the above method. Therefore, as a neutron ray detection method, neutron rays are transmitted through a substance having a large neutron absorption cross section, and α rays are generated by a neutron transmutation reaction,
There is a method by detecting the electron-hole pair generated by the α ray in the semiconductor depletion layer.

As a specific example, using a boron isotope ¹⁰ B having a large scattering cross section for thermal neutron rays, according to the reaction shown by the following formula, α rays generated from boron when thermal neutron rays are incident There are methods to detect ( ⁴ He) and ⁷ Li nuclei. ¹⁰ B + n → ⁷ Li + α ( ⁴ He) (1) FIG. 2 shows the cross-sectional structure and detection principle of a thermal neutron beam detecting element using this method, which is known, for example, in Japanese Patent Laid-Open No. 61-17477. Then, as disclosed in, for example, Japanese Patent Laid-Open Nos. 59-218732 and 59-219462, plasma CVD is applied to the window portion of the surface protective film 24 covering the upper surface of the n-type silicon substrate 21.
Containing coating 22 is in contact more which is formed by law, p ⁺ layer 23 thereunder
Are formed, and an electrode 25 is provided on the upper surface of the boron coating film 22 and an electrode 26 is provided on the lower surface of the substrate 21. When a reverse bias of −V _B is applied to this device and the thermal neutron beam 1 is irradiated in the state where the depletion layer 27 is generated, the neutron beam of the formula (1) is exchanged with ¹⁰ B contained in the boron film 22. When a conversion reaction occurs and α-rays 3 shown by solid lines or ⁷ Li nuclei 4 shown by broken lines which make 180 ° with each other reach the depletion layer 27, electron-hole pairs are generated, and these are not shown in the amplification circuit. Is detected through the counting circuit.

To increase the neutron sensitivity of this device, the amount of ¹⁰ B should be increased as can be seen from the equation (1). The boron concentration in the boron film formed by the plasma CVD method known in the above publication is
At 1.0 × 10 ²³ atoms / cm ² , it has reached a value close to the atomic density of boron concentration. Therefore, the enrichment ¹⁰ B described in the specification of Japanese Patent Application No. 62-323240 relating to another applicant's patent application
If the boron film is formed thicker than diborane gas containing, a highly sensitive neutron beam detection element can be obtained.

Examples of the method using concentrated ¹⁰ B include, for example, concentrated
HM Mann and FE Janar is a method of applying a solution containing ¹⁰ B to a silicon wafer with a brush and then heat-treating it to form a pn junction and detecting thermal neutrons using the reaction of equation (1).
IGG ver d is a method of forming pn junction by implanting ¹⁰ B into the silicon substrate surface by the ion implantation method and then heat-treating it by ek by US magazine IRE Trans. "NS-9, No3 (1962) page 200.
tsiteli and others by Soviet magazine Prib.Tekh.Eksp. "No.3,
(1979) page 81.

[Problems to be Solved by the Invention]

A film containing a large amount of ¹⁰ B is formed by the above method,
In order to generate a large amount of α rays according to the formula (1) and further increase the sensitivity, it is necessary to increase the number of electron-hole pairs that reach the α ray or ⁷ Li nucleus depletion layer.

The purpose of the present invention is not only to generate a large amount of α-rays and ⁷ Li nuclei, but also to generate α-rays or ⁷ Li nuclei by electron-
An object of the present invention is to provide a semiconductor neutron beam detecting element that is effectively used for generating hole pairs.

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides a neutron beam detecting element having a boron film containing concentrated isotope ¹⁰ B deposited on an n-type semiconductor substrate and having a p-type doping layer under the film. In, the thickness of the boron coating is thinner than the range of α rays in the boron coating.

[Action]

The range of α-rays in the boron coating is about 4.4 μm, and the range of ⁷ Li nuclei is about 1.4 μm, which is extremely short. Therefore, if the boron coating is too thick, the α-rays generated near the surface of the boron coating ⁷ Li nuclei are simply absorbed in the boron coating and do not reach the depletion layer. Therefore, if the thickness of the boron coating is made thinner than the range of the boron coating for α rays longer than that for ⁷ Li nuclei, at least almost all of the α rays that fly to the depletion layer side contribute to the generation of electron-hole pairs. The neutron sensitivity is improved.

〔Example〕

FIG. 1 is a partially enlarged view of the boron film 22, the p ⁺ layer 23 and the depletion layer 27 shown in FIG. 2, in which the boron film 22 is d ₁ and the p ⁺ layer 23 is d _2.
Shall have a thickness of. On the other hand, α in the boron film
The range of line 2 is l ₁ , and the range of ⁷ Li nucleus 4 is l ₂ .

Now, near the surface of the boron film 22, thermal neutrons 10 and 11 are ¹⁰ B5.
When α-rays 3, ⁷ Li nuclei 4 are generated by reacting with α, d ₁ > l ₁ , d ₁
When> l ₂ , all α rays 3 and ⁷ Li nuclei 4 are absorbed in the coating film 22 and do not reach the depletion layer 27, so that they do not contribute to neutron sensitivity. When the thermal neutron beam 12 reacts with ¹⁰ B5 at a position deeper than the surface, only α-rays such as 32 out of α-rays 31, 32, ⁷ Li nuclei 41, 42 that reach the depletion layer 27 reach the depletion layer and the neutron Contributes to sensitivity. In addition, like thermal neutrons 13 and 14, boron film 2
When reacting with ¹⁰ B5 near the interface between 2 and the semiconductor substrate 21,
The α rays 3 and ⁷ Li nuclei 4 contribute to the neutron sensitivity with almost a half probability, except for those that go to the side of the boron coating such as 43 and 44.

As can be seen from the above description, the thickness d ₁ of the boron coating 22 is different for each range l ₁ and l _{2 in the} α-ray or ⁷ Li nucleus boron coating.
If it is thicker, even if a dose of thermal neutrons such as 15 to 19 reacts with ¹⁰ B5 in the boron coating 22, it will enter into the coating like α rays 35,36,37 generated by 15 to 17. Some of them are absorbed and contribute to the neutron sensitivity are thermal neutron rays 17 to 19 that react with ¹⁰ B5 in the vicinity of the interface between the coating 22 and the semiconductor substrate 21.
The neutron sensitivity is reduced because it is limited to α-rays 3 and ⁷ Li nuclei 4 generated at.

These facts will be explained below using equations.

(1) In the case of d ₁ ≦ l ₂ Let the intensity of the neutron beam irradiating the boron coating of thickness d _{1 be} I ₀ (pieces / cm ² · s), α rays generated in the coating and ⁷ Li
The number of nuclei reaching the depletion layer is It is represented by. Where N is the number of ¹⁰ B in the boron coating,
σ is the scattering cross section of ¹⁰ B.

(2) When l ₂ ≤ d ₁ ≤ l ₁ The number of α rays that are generated in the boron film and reach the depletion layer is However, the number of ⁷ Li nuclei is become.

(3) When d ₁ ≧ l ₁ The numbers of α-rays and ⁷ Li nuclei generated in the boron film and reaching the depletion layer by the reaction with thermal neutrons are I ₀ respectively. And I ₀ Becomes

The curve 61 in FIG. 3 shows the result calculated from the numerical values of (1), (2), and (3) above, assuming that l ₁ = 4.4 μm and l ₂ = 1.4 μm. The film thickness of the elementary coating is shown, and the vertical axis shows the relative ratio of the thermal neutron sensitivity. The thermal neutron relative sensitivity increases with the increase of d ₁ , but its slope changes depending on the above conditions (1), (2), and (3). The curve 62 is a neutron standard radiation source with a 40 mm-thick polyethylene moderator on the neutron beam detection element formed with various thickness boron films.
The relative ratio of the measurement results obtained when irradiating with ²⁵² C _f is shown. The reason why the curve 62 does not match the curve 62 is that the generated α rays and ⁷ Li nuclei reach the depletion layer with a certain probability and that the p ⁺ layer 23
In addition to the dead layer, α rays and ⁷ L generated inside the boron film 22
The boron film becomes an insensitive layer in the i-nucleus, and reaches the depletion layer after passing through these insensitive layers, so that the maximum output pulse wave height becomes small, and in addition, in order to remove the mixed γ-ray component, For example, by counting only the output pulse heights higher than this.

From Fig. 3, the relationship between the thickness d _{1 of the} boron coating containing ¹⁰ B and the thermal neutron sensitivity becomes clear, and the maximum thermal neutron sensitivity is that the thickness d _{1 of the} boron coating is α in the boron coating. It was proved that it was obtained when the line range was less than l ₁ .

[Effects of the Invention] According to the present invention, the thermal neutron sensitivity decreases when the thickness of the boron coating containing ¹⁰ B exceeds the range of α-rays in the boron coating. By making the thickness thinner than the range of α rays, it is possible to obtain a detection element having high neutron sensitivity. Therefore, unlike the conventional case, an unnecessary thick film is not formed, so that the manufacturing process is shortened and the cost can be reduced. Further, the optimum thickness of the boron coating according to the present invention can be similarly applied to the case of coating the inner wall of the proportional counter for neutron detection using gas.
The thermal neutron beam detection element obtained in the present invention is lightweight and small in size when combined with a moderator for neutron beams such as paraffin of a predetermined shape, and it has been impossible in the past to detect highly sensitive neutron beam for individuals. Dosimeters have become easier to obtain.

[Brief description of drawings]

FIG. 1 is a sectional view conceptually showing the vicinity of a reaction generation site between a thermal neutron beam and ¹⁰ B in one embodiment of the present invention, and FIG. 2 is a sectional view of a detection element in one embodiment of the present invention, The figure is a diagram showing the relationship between the boron coating thickness and the thermal neutron sensitivity. 10 to 19: thermal neutron beam, 21: n-type silicon substrate, 22: boron film, 23: p ⁺ layer, 25, 26: electrode, 27: depletion layer, 3: α ray, 4: ⁷ Li
Nuclear, 5: ¹⁰ B.

Claims (1)

[Claims] 1. An isotope ¹⁰ B enriched on an n-type semiconductor substrate.
A boron coating containing P is deposited, and p is placed under the boron coating.
A semiconductor neutron detecting element having a boron-doped layer, wherein the thickness of the boron coating is smaller than the range of α rays in the boron coating.