Device and method for measuring neutrons

Abstract

To provide a neutron measurement device which expands neutron measurement range by offering variable sensitivity.SOLUTION: A neutron measurement device 30 is provided, comprising a cylindrical electrode 12 with a hollow section 11, a wire electrode 15 located on a center axis of the cylindrical electrode 12, sealing members 16 (16a, 16b) for supporting both ends of the wire electrode 15 and sealing the hollow section 11, and a pressure variable unit 20 configured to vary the pressure of a reaction gas 17 encapsulated in the hollow section 11.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE Embodiments of the present invention relate to neutron measurement techniques with tunable sensitivity. [Background technology]

[0002] Neutron detectors include gas-amplified proportional counters, such as the He-3 detector. In order to include multiple objects with significantly different neutron emission rates in the measurement range, the sensitivity of the neutron detector (also called detection efficiency) must be changed successively.

[0003] The reason is that when the neutron sensitivity is low, the statistical accuracy is not met, and the neutron counting rate, which is represented by the detection signal of neutrons per unit time, drops and the error increases. On the other hand, when the neutron sensitivity is high, the measurement range is exceeded, the neutron counting rate rises, and the signals overlap, resulting in a large error. In such cases, the sensitivity of the neutron detector should be changed.

[0004] Examples of situations that require such measures include measuring unpredictable neutron emission rates in unknown spaces, such as waste generated during the dismantling of aging nuclear facilities or the internal investigation of an accident plant. Furthermore, it is effective to set the neutron sensitivity higher when measuring in environments with significantly high gamma rays, such as near radioactively contaminated areas. In addition, as decommissioning work at nuclear power plants is expected to increase, it is thought that there will be an increasing demand in the future to expand the neutron measurement range and inspect the contamination of objects removed from the plant. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-80157 [Patent Document 2] Japanese Patent Publication No. 62-55855 Summary of the Invention [Problem to be solved by the invention]

[0006] A simple measure to expand the neutron measurement range is to prepare multiple detectors with different sensitivities in advance and replace them as necessary, but this measure is difficult to implement because it requires manpower to replace the detectors and it is expected that it will be difficult to get close to the detectors when the object to be measured is highly radioactive.

[0007] The gas amplification factor characteristic, which indicates the sensitivity of a proportional counter type neutron detector, is mainly determined by three factors: the applied voltage, the tube diameter, and the internal gas pressure. Other factors include the type of internal gas and the neutron sensitive material.

[0008] If it is desired to change the sensitivity of a neutron detector after it has been manufactured, this can be achieved by changing any of these three elements. However, when changing the applied voltage, not only does the sensitivity to neutrons change, but the sensitivity to other radiation (for example, gamma rays) also changes, and the noise level also changes, so it is not an easy change to make. In addition, changing the tube diameter is not easy either, as it is almost synonymous with replacing the neutron detector itself, since it is manufactured from metallic materials such as stainless steel.

[0009] The internal gas pressure is the pressure of the rare gas filled inside the neutron detector for signal amplification, and corresponds to the number of molecules of the rare gas filled inside. This internal gas pressure is filled at a specified pressure during the manufacturing process, and the gas inlet hole is sealed by caulking, welding, etc. so that it does not easily open. In this way, conventional neutron detectors have been designed and manufactured by fixing the above three elements in an optimal state so that noise does not affect the neutron detection signal while satisfying the required measurement range.

[0010] The embodiments of the present invention have been made in consideration of the above circumstances, and have an object to provide a neutron measurement technique that makes the sensitivity variable and expands the neutron measurement range. [Means for solving the problem]

[0011] In the neutron measurement device of the embodiment, the device comprises a cylindrical electrode having a hollow portion, a wire electrode located on the central axis of the cylindrical electrode, a sealing member that supports both ends of the wire electrode and seals the hollow portion, and a pressure variable portion that varies the pressure of a reaction gas sealed in the hollow portion. Effect of the Invention

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention provide a neutron measurement technique that provides tunable sensitivity and extended neutron measurement range. [Brief description of the drawings]

[0013] [Figure 1] FIG. 2 is a longitudinal sectional view of a detector of the neutron measurement device according to the embodiment of the present invention. [Diagram 2] 5A and 5B are diagrams illustrating the operation of a pressure variable part of a neutron measurement device according to an embodiment. [Diagram 3] 1 is an overall configuration diagram of a neutron measurement device according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a vertical cross-sectional view of a detector 10 of a neutron measurement device 30 (Fig. 3) according to an embodiment of the present invention. As described above, the detector 10 of the neutron measurement device 30 includes a cylindrical electrode 12 having a hollow portion 11, a wire electrode 15 located on the central axis of the cylindrical electrode 12, and sealing members 16 (16a, 16b) that support both ends of the wire electrode 15 and seal the hollow portion 11.

[0015] In the embodiment, the neutron measurement device 30 is a proportional counter. The reactive gas 17 sealed in the neutron measurement device 30 may be an inert gas such as helium 3 (He-3) that undergoes a nuclear reaction with the incident neutrons n, but is not particularly limited thereto.

[0016] A neutron moderator 18 made of a material such as polyethylene is provided on the outer periphery of the cylindrical electrode 12, and the hollow portion 11 is filled with a reaction gas 17. A high voltage of 1000 to 2000 V is applied between the cylindrical electrode 12 and the wire electrode 15 located on its central axis.

[0017] The wire electrode 15 has a function of collecting electric charges generated due to the reaction between the incident neutrons n and the reactive gas 17. The electric charges collected in this manner are output from the detector 10 to the outside as a detection signal s.

[0018] Electrically insulating sealing members 16 (16a, 16b) are provided on both ends of the cylindrical electrode 12. These sealing members 16 (16a, 16b) hermetically seal the hollow portion 11 of the cylindrical electrode 12, support both ends of the wire electrode 15, and electrically insulate them from each other.

[0019] One of the sealing members 16a is provided with a connector 13 that connects to a wire that applies a high voltage between the cylindrical electrode 12 and the wire electrode 15 and a wire that outputs a detection signal s from the wire electrode 15. Furthermore, one of the sealing members 16a is provided with a through hole 14 to which a base end of a gas pipe 23, the tip of which communicates with the pressure variable unit 20, is connected.

[0020] When neutrons n enter the detector 10, they are slowed down by the neutron moderator 18, and the high-speed neutrons are converted into thermal neutrons with low energy. These thermal neutrons are then absorbed by and interact with He atoms, which are neutron-sensitive substances in the reaction gas 17, generating an electric charge in the hollow portion 11 of the cylindrical electrode 12.

[0021] The generated charge is collected by the wire electrode 15 by the applied voltage and becomes a detection signal s, which is output from the detector 10. This detection signal s is a small pulse-like signal caused by the incident neutron n.

[0022] 2(A) and (B) are diagrams illustrating the operation of the pressure variable unit 20 of the neutron measurement device 30 according to the embodiment. As described above, the neutron measurement device 30 includes the pressure variable unit 20 that varies the pressure of the reaction gas 17 sealed in the hollow portion 11.

[0023] 2A, the volume of the internal space 21 of the pressure variable part 20 is changed to be larger, thereby decreasing the pressure of the reaction gas 17 in the hollow part 11 of the detector 10. On the other hand, in FIG. 2B, the volume of the internal space 21 of the pressure variable part 20 is changed to be smaller, thereby increasing the pressure of the reaction gas 17 in the hollow part 11 of the detector 10.

[0024] Here, increasing / decreasing the pressure of the reactive gas 17 sealed in the hollow portion 11 of the detector 10 is synonymous with increasing / decreasing the number of atoms of the reactive gas 17 present in the hollow portion 11. In other words, increasing / decreasing the pressure of the reactive gas 17 increases / decreases the number of charges generated by reaction with the neutrons n, improving / decreasing the sensitivity of the detector 10.

[0025] This pressure varying unit 20 includes a syringe 22 that holds reaction gas 17 in internal space 21, a gas pipe 23 that connects hollow portion 11 of cylindrical electrode 12 and internal space 21 of syringe 22, a plunger 25 that is inserted into syringe 22 to determine the volume of internal space 21, and a drive unit 26 that displaces plunger 25 to vary the volume of internal space 21.

[0026] The drive unit 26 is, for example, a rotary motor, which rotates the feed screw shaft and converts the rotation into translational motion while preventing the threaded plunger 25 from co-rotating. The volume of the internal space 21 defined by the tip of the plunger 25 and the inner surface of the syringe 22 is variable according to the amount of displacement of the plunger 25.

[0027] In response to the displacement of the plunger 25, the reactive gas 17 moves through the gas piping 23. In response to the movement of the reactive gas 17 in the gas piping 23, the pressure of the reactive gas 17 changes in the hollow portion 11 that is in communication with the internal space 21 and has a constant volume. In this manner, in the neutron measurement device 30 according to the embodiment, the sensitivity to neutrons n can be varied by increasing or decreasing the pressure of the reactive gas 17 in the detector 10.

[0028] 3 is an overall configuration diagram of a neutron measurement device 30 according to an embodiment. Here, a pressure gauge 27 is installed in a gas pipe 23 and outputs a measured value p of the pressure of the reaction gas 17. The pressure gauge 27 may be installed in the hollow portion 11 of the cylindrical electrode 12 or the internal space 21 of the syringe 22.

[0029] When a neutron n emitted from the radioactive material 39 enters the detector 10, it reacts with the reactive gas 17 sealed inside, generating an electric charge, and a detection signal s is output.

[0030] Here, if the pressure of the reaction gas 17 is low, the pulse peak value of the detection signal s becomes small, and the sensitivity of the detection signal s decreases. As a result, the pulse peak value falls outside the lower limit of the measurement range, the statistical accuracy is not satisfied, the apparent neutron count rate c decreases, and the error increases.

[0031] Furthermore, if the pressure of the reaction gas 17 is high, the detection signal s will exceed the upper limit of the measurement range. As a result, the noise component will be erroneously detected as the pulse height value, and the apparent neutron count rate c will increase, resulting in a large error.

[0032] In this way, if the detection signal s output from the detector 10 deviates from the measurement range of neutrons n, the pressure variable unit 20 is controlled to vary the pressure of the reaction gas 17 in the detector 10 so that it falls within the measurement range.

[0033] The neutron measurement device 30 is equipped with a first accommodation section 31 that accommodates the pressure variable unit 20 and an amplifier 35 that amplifies the detection signal s transmitted from at least one of the cylindrical electrode 12 and the wire electrode 15 when a neutron n is incident, and a second accommodation section 32 that accommodates a signal processing circuit 36 ​​that processes the amplified detection signal s, an operation terminal 38 that inputs an operation target value a of the pressure variable unit 20 based on the detection signal s, and a control section 37 that controls the pressure variable unit 20 based on this operation target value a and adjusts the pressure of the reaction gas 17.

[0034] The amplifier 35 is connected to the wire electrode 15 (FIG. 1) of the detector 10 via a signal cable 41, and amplifies and outputs the detection signal s, which is an input pulsed minute signal. The signal processing circuit 36 ​​is connected to the amplifier 35 via a transmission cable 42, and performs counting processing on the input amplified detection signal S, and outputs the neutron count rate c. The neutron count rate c is the count of pulses per unit time, and is calculated discretely and displayed on the operation terminal 38. The signal processing circuit 36 ​​can also measure the distribution of the maximum pulse height values ​​of the pulses of the detection signal S. By such pulse height distribution analysis, the radioactive nuclide that emits neutrons n can be identified and displayed on the operation terminal 38.

[0035] Furthermore, the operation terminal 38 inputs the operation target value a of the pressure variable unit 20 to the control unit 37 based on the processing result (neutron count rate c) of the detection signal s (and also based on the current measurement value p of the pressure gauge 27 as appropriate). The control unit 37 is connected to the drive unit 26 of the pressure variable unit 20 via a control cable 45, and translates the plunger 25 based on the operation target value a to adjust the measurement value p of the pressure of the reaction gas 17.

[0036] When the radioactive material 39 irradiates a high radiation dose that prevents an operator from approaching, the detector 10 is positioned by a machine such as a robot. The installation position of the first housing unit 31 is away from the position of the radioactive material 39, indicates a radiation dose that allows an operator to stay for a certain period of time, and houses the amplifier 35 and the pressure variable unit 20.

[0037] The second housing section 32 is installed at a position farther away from the position of the first housing section 31, exhibits a low radiation dose so that an operator can stay for a longer period of time, and houses a signal processing circuit 36, a control section 37, and an operation terminal 38.

[0038] Assume that a neutron measurement is performed on a certain radioactive material 39. If the neutron count rate c indicates a very low value on the operation terminal 38, the sensitivity of the detection signal s is suspected to be insufficient. In this case, the operation target value a is set on the operation terminal 38 so that the pressure varying unit 20 operates to increase the measured value p of the pressure of the reaction gas 17 filled in the detector 10.

[0039] This causes the plunger 25 to be pushed into the syringe 22, moving the reaction gas 17 from the pressure variable part 20 to the detector 10, and increasing the measured pressure p of the reaction gas 17 in the hollow part 11 of the detector 10. This improves the sensitivity of the detection signal s.

[0040] On the other hand, when the neutron count rate c indicates an extremely high value on the operation terminal 38, the detection signal s is suspected to be oversensitive. In this case, the operation target value a is set on the operation terminal 38 so that the pressure varying unit 20 operates to reduce the measured value p of the pressure of the reaction gas 17 filled in the detector 10.

[0041] As a result, the plunger 25 is pulled up from the syringe 22, the reaction gas 17 moves from the detector 10 to the pressure variable part 20, and the measured value p of the pressure of the reaction gas 17 in the hollow part 11 of the detector 10 decreases. This eliminates the oversensitivity of the detection signal s.

[0042] The mechanism for increasing or decreasing the measured pressure value p of the reaction gas 17 may be incorporated into the control unit 37 as a function for automatically inputting the operating target value a that provides an appropriate neutron count rate c.

[0043] Although the pressure varying unit 20 including the syringe 22 and the plunger 25 has been exemplified as a method for varying the pressure of the reaction gas 17 sealed in the hollow portion 11 of the detector 10, the present invention is not limited to this. For example, as another method for varying the pressure, a system for filling the hollow portion 11 of the detector 10 with the reaction gas 17 and a system for discharging the same may be separately provided.

[0044] According to at least one of the embodiments of the neutron measurement device described above, the pressure of the enclosed reactive gas can be changed to vary the sensitivity, thereby expanding the neutron measurement range.

[0045] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. [Explanation of symbols]

[0046] 10...detector, 11...hollow portion, 12...cylindrical electrode, 13...connector, 14...through hole, 15...wire electrode, 16 (16a, 16b)...sealing member, 16a...sealing member, 17...reactive gas, 18...neutron moderator, 20...pressure variable portion, 21...internal space, 22...syringe, 23...gas piping, 25...plunger, 26...drive portion, 27...pressure gauge, 30...neutron measurement device, 31...first storage portion, 32...second storage portion, 35...amplifier, 36...signal processing circuit, 37...control portion, 38...operation terminal, 39...radioactive material, 41...signal cable, 42...transmission cable, 45...control cable, a...operation target value, c...neutron count rate, p...pressure measurement value, s...detection signal.

Claims

1. A cylindrical electrode having a hollow portion; a wire electrode located on a central axis of the cylindrical electrode; a sealing member that supports both ends of the wire electrode and seals the hollow portion; a pressure varying unit that varies the pressure of the reaction gas sealed in the hollow portion.

2. 2. The neutron measurement device according to claim 1, The pressure varying unit is a syringe that holds the reaction gas in an internal space; a gas pipe communicating the hollow portion of the cylindrical electrode with the internal space of the syringe; a plunger inserted into the syringe to define a volume of the internal space; and a drive unit that displaces the plunger to vary the volume of the internal space.

3. 3. The neutron measurement device according to claim 2, A neutron measurement device comprising a pressure gauge that measures the pressure of the reaction gas in at least one of the hollow portion of the cylindrical electrode, the internal space of the syringe, and the gas pipe connecting the two.

4. In the neutron measurement device according to claim 1 or 2, a first housing portion housing an amplifier that amplifies a detection signal transmitted from at least one of the cylindrical electrode and the wire electrode when a neutron is incident thereon, and the pressure variable portion; A neutron measurement device comprising: a signal processing circuit that processes the amplified detection signal; an operation terminal that inputs an operating target value of the pressure variable unit based on the detection signal; and a second accommodation unit that accommodates a control unit that controls the pressure variable unit based on the operating target value and adjusts the pressure of the reaction gas.

5. A cylindrical electrode having a hollow portion; a wire electrode located on a central axis of the cylindrical electrode; a sealing member that supports both ends of the wire electrode and seals the hollow portion, A neutron measurement method in which the pressure of the reaction gas sealed in the hollow portion is varied.

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