JP3938059B2 - Neutron detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a neutron detector used for a dosimeter for measuring a radiation dose in a nuclear power plant or the like.
[0002]
[Prior art]
Neutrons are radiation that has no electric charge and is difficult to detect directly because of its weak interaction with matter. Therefore, a neutron detector employs a method of detecting neutrons via protons and α rays generated by interaction with a substance having a large cross-sectional area (nuclear reaction cross-section) of interaction with neutrons. As one of the neutron detectors, a combination of a spherical ³ He neutron counter using ³ He, which has a large nuclear reaction cross section with thermal neutrons, and a spherical moderator made of thick polyethylene, etc. There is something.
[0003]
FIG. 4 is a conceptual diagram showing the configuration of an example of such a neutron detector.
The neutron detector 1 includes a ³ He neutron counter 11, a moderator 12, a signal processing circuit 14, and a base 13 that holds the former two and accommodates the signal processing circuit 14.
The nuclear reaction cross section between ³ He and thermal neutrons is very large, and protons are generated as a result of the nuclear reaction. ^{The 3} He neutron counter 11 detects neutrons, mainly thermal neutrons, by detecting the ionization current due to the protons. Moderator 12 made of thick polyethylene or the like, spherical with the same center as the spherical portion of the ³ He neutron counter tube 11, to slow the taller medium speed neutrons energy, to easily slow neutrons detected by the ³ He neutron counter tube 11 To do. The size of the moderator 12 is, for example, Φ350 mm.
[0004]
For reference, the nuclear reaction cross section of ³ He and neutrons is shown below with reference to the case where the neutron energy is 1 MeV.
Thermal neutrons: 8,700
10 keV: 13
0.1MeV: 3.1
1 MeV: 1
5 MeV: 0.56
[0005]
[Problems to be solved by the invention]
However, in the neutron detector 1 described above, the moderator 12 has an insufficient moderating effect on very high energy neutrons (ultrafast neutrons) of 50 MeV or more contained in cosmic rays and the like, and neutrons having energy of 100 MeV are insufficient. The detection sensitivity is reduced to 1/3 or less. In addition, when the measured dose value increases, it is necessary to distinguish between an increase in the background value due to cosmic rays and an increase in artificial neutrons (such as leaked neutrons due to shield deterioration) that must be a problem. Is difficult.
The problem of the present invention is that the detection sensitivity of neutrons (ultrafast neutrons) with energy of 50 MeV or more is improved, the detection accuracy of neutrons is high, and the main detected neutrons are super cosmic rays, etc. The purpose is to provide a neutron detector that can distinguish between fast neutrons and neutrons of several MeV or less.
[0006]
[Means for Solving the Problems]
The invention of claim 1 is a neutron detector comprising a medium-slow neutron detection means, a moderator surrounding the detection means and decelerating neutrons, and a signal processing circuit for processing a signal from the detection means, Two sets of detection means and a moderator are provided, and a sensitizing member that emits a plurality of neutrons by interacting with ultrafast neutrons included in cosmic rays or the like is provided outside one moderator.
Since there is a sensitizing member that emits multiple neutrons by interacting with ultrafast neutrons contained in cosmic rays etc. on the outside of one moderator, if ultrafast neutrons are included in the neutrons, Compared with the measured value from the moderator without the sensitizing member, the measured value from the moderator with the sensitizing member is larger, and the detection sensitivity of ultrafast neutrons is improved. The dose value of ultrafast neutrons in the measured dose value of neutrons can be calculated.
[0007]
The invention of claim 2 is a neutron detector comprising a medium-slow neutron detection means, a moderator surrounding the detection means and decelerating neutrons, and a signal processing circuit for processing a signal from the detection means, The moderator has an insertion hole, and is provided with a sensitizing member that is detachably inserted into the insertion hole and that emits a plurality of neutrons by interacting with ultrafast neutrons contained in cosmic rays or the like.
Depending on whether the sensitizing member is inserted into or removed from the insertion hole of the moderator, two measurement values based on the presence or absence of the sensitizing member can be obtained, so that the detection sensitivity of ultrafast neutrons is improved as in the first aspect of the invention. From the difference between the two measured values, it is possible to calculate the ultrafast neutron dose value that occupies the measured neutron dose value.
[0008]
The invention of claim 3 is a neutron detector comprising a medium-slow neutron detection means, a moderator surrounding the detection means and decelerating neutrons, and a signal processing circuit for processing a signal from the detection means, A sensitizing member that is detachably attached to the surface of the moderator and that emits a plurality of neutrons by interacting with ultrafast neutrons contained in cosmic rays or the like is provided.
By attaching / detaching the sensitizing member to / from the surface of the moderator, two measurement values based on the presence / absence of the sensitizing member can be obtained in the same manner as in the invention of claim 2. From this difference, it is possible to calculate the ultrafast neutron dose value that occupies the measured neutron dose value.
[0009]
According to a fourth aspect of the present invention, in any one of the first to third aspects, the sensitizing member is made of lead.
Lead has a high effect as a sensitizing member, is easily available, and is not a very expensive material.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the neutron detector according to the present invention will be described using examples. In addition, the same code | symbol is attached | subjected to the part of the same function as a prior art.
[First embodiment]
FIG. 1 is a conceptual diagram showing a configuration of a first embodiment of a neutron detector according to the present invention.
This neutron detector 1a includes two sets of ³ He neutron counters 11 and a moderator 12, a lead sensitizing material 15 as a sensitizing member covering the outside of one moderator 12 (left side in FIG. 1), and two ^three A signal processing circuit 14a for processing a signal from the He neutron counter 11, a display circuit 16, a neutron detector comprising a ³ He neutron counter 11, a moderator 12 and a lead sensitizer 15 are held, and a signal processing circuit 14a And a gantry 13a for accommodating the display circuit 16.
[0011]
^{Since the 3} He neutron counter 11 and the moderator 12 are the same as those in the prior art, the description thereof is omitted.
The lead sensitizing material 15 as a sensitizing member is a member for increasing the detection sensitivity of ultrafast neutrons such as cosmic rays, and is made of lead and has a spherical shell shape with a thickness of 15 mm. Since lead has a large nuclear reaction cross-sectional area that emits a plurality of neutrons by reacting with ultrafast neutrons, it functions as a member that increases the detection sensitivity of ultrafast neutrons. Therefore, by providing the lead sensitizing material 15, the count value when ultrafast neutrons are present is increased, and the sensitivity reduction in the ultrahigh energy region in the conventional neutron detector is improved.
[0012]
The output signals of the two ³ He neutron counters 11 are input to the signal processing circuit 14a, and the respective count values are counted, and the count values of the left ³ He neutron counter 11 and the right ³ He neutron counter 11 are totaled. The numerical value is compared and the difference is obtained. Since the left ³ He neutron counter 11 is completely surrounded by the lead sensitizer 15, it efficiently detects neutrons emitted from the lead sensitizer 15, but the right ³ He neutron counter 11 The neutron emitted from the lead sensitizing material 15 is not detected so much because it is away from the lead sensitizing material 15. Therefore, the number of neutrons emitted from the lead-sensitized material 15 by ultrafast neutrons is estimated from the difference between the two count values and the size and arrangement of each member, and the dose of ultrafast neutrons is estimated. When the difference between the two count values is small, it indicates that there are few ultrafast neutrons, and when the difference is large, there are many ultrafast neutrons.
[0013]
The ultrafast neutron dose value calculated in this way is added to the other neutron dose values, and is displayed on the display circuit 16 as a neutron dose value. If necessary, the dose value of ultrafast neutrons is also displayed on the display circuit 16.
If the size and thickness of the lead sensitizing material 15 are selected as optimum values, it is possible to calculate the neutron dose value only from the output signal of the left ³ He neutron counter 11. In this case, the output signal of the right ³ He neutron counter 11 is only required to calculate the dose ratio of ultrafast neutrons. Therefore, when it is not necessary to calculate the dose ratio of ultrafast neutrons, the right ³ He neutron counter 11 and moderator 12 can be omitted.
[0014]
The first embodiment includes two sets of ³ He neutron counters and a moderator. However, the second and third embodiments described below are designed to be lightweight and portable. ^{The 3} He neutron counter and moderator are combined into one set, and the sensitizing member is detachable, and the neutron count value corresponding to the presence or absence of the sensitizing member is counted by attaching / detaching the sensitizing member.
[Second Embodiment]
FIG. 2 is a conceptual diagram showing the configuration of the second embodiment.
In this neutron detector 1b, a moderator 12a having an insertion hole 121 for inserting a lead sensitizing plate 15a, which is a sensitizing member, is used as a moderator, and the case 17 for housing the signal processing circuit 14b is suitable for carrying. In order to obtain a curved shape, it is attached to the side surface of the moderator 12a, and a handle (not shown) is also attached to the case 17.
[0015]
In order to make it portable, the diameter of the moderator 12a was reduced to 200 mm, and the lead intensifying plate 15a was also lightened. The dimensions of the lead intensifying plate 15a are, for example, 15 mm thick, 200 mm long, and 150 mm wide.
An example of the effect of the lead intensifying plate 15a is shown below.
In the background counting, 1.2 cpm was counted when the lead sensitizing plate 15a was removed, whereas 1.6 cpm was counted when the lead sensitizing plate 15a was inserted. The increased 0.4 cpm corresponds to the neutrons emitted from the lead intensifying plate 15a by the ultrafast neutrons included in the cosmic rays.
[0016]
In addition, in the 65 MeV neutron field counting by the accelerator, 240 cps was counted with the lead intensifying plate 15a removed, whereas 770 cps was counted with the lead intensifying plate 15a inserted, and the lead intensifying plate Insertion of 15a increased the count value approximately 3-fold.
[Third embodiment]
FIG. 3 is a conceptual diagram showing the configuration of the third embodiment.
In this neutron detector 1c, a moderator 12 having the same shape as that of the conventional example and having no insertion hole is used as a moderator, and a part of a spherical shell that can be placed on the surface of the moderator 12 as a sensitizing member. A lead intensifying plate 15b having a shape corresponding to the above is used. The signal processing circuit 14b and the case 17 are the same as those in the second embodiment.
[0017]
As in the second embodiment, it is possible to discriminate counting by ultrafast neutrons contained in cosmic rays, depending on whether the lead sensitizing plate 15b is placed on the moderator 12 or removed as shown in FIG. it can.
Although not shown in FIG. 3, a small protrusion or the like is provided at the apex of the moderator 12, a recess or a through hole is provided in the center of the lead sensitizing plate 15b, and the protrusion or the like is fitted to the recess or the through hole. A structure may be used, and the lead sensitizing plate 15b may be screwed as necessary. With this structure, the lead intensifying plate 15b can be easily positioned and fixed, and is convenient for carrying.
[0018]
In the above embodiment, only the case where the ³ He neutron counter is used as the medium slow neutron detection means has been described. However, the ³ He neutron counter is formed by another detection means such as a ¹⁰ B film on the surface. It is also possible to replace it with a semiconductor detector.
[0019]
【The invention's effect】
According to the first aspect of the present invention, two sets of detection means and a moderator are provided, and a sensitizing member that emits a plurality of neutrons by interacting with ultrafast neutrons included in cosmic rays or the like is provided outside one of the moderators. Therefore, when the detection sensitivity of ultrafast neutrons is improved and the ultrafast neutrons are included in the neutrons, the sensitizing member is compared with the measured value from the moderator without the sensitizing member. The measured value from the moderator side equipped with is larger, and the dose value of ultrafast neutrons in the measured neutron dose value can be calculated from the difference between the two.
[0020]
Therefore, according to the present invention, there is provided a neutron detector with high detection accuracy and capable of distinguishing whether the main detected neutrons are ultrafast neutrons such as cosmic rays or neutrons of several MeV or less. can do.
In the invention of claim 2, the moderator has an insertion hole, is detachably inserted into the insertion hole, and emits a plurality of neutrons by interacting with ultrafast neutrons contained in cosmic rays or the like. Since the sensitizing member is provided, two measured values based on the presence or absence of the sensitizing member can be obtained depending on whether the sensitizing member is inserted into or removed from the insertion hole of the moderator. The detection sensitivity of neutrons is improved, and the dose value of ultrafast neutrons occupying the measured dose value of neutrons can be calculated from the difference between the two measured values.
[0021]
In the invention of claim 3, since the sensitizing member is detachably attached to the surface of the moderator and emits a plurality of neutrons by interacting with ultrafast neutrons contained in cosmic rays or the like, it is sensitized. By attaching / detaching the member to / from the surface of the moderator, two measurements based on the presence or absence of the sensitizing member are obtained, the detection sensitivity of ultrafast neutrons is improved, and the measured neutron dose value is calculated from the difference between the two measurements. The dose value of ultrafast neutrons occupying the inside can be calculated.
In the invention of claim 4, the sensitizing member is made of lead. Lead has a high sensitization effect of ultrafast neutrons, is easily available, is not a very expensive material, and is an optimal material as a sensitizing member.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing the configuration of a first embodiment of a neutron detector according to the present invention. FIG. 2 is a conceptual diagram showing the configuration of a second embodiment. FIG. 3 shows the configuration of a third embodiment. Schematic diagram [Fig. 4] Schematic diagram showing the configuration of an example of a conventional neutron detector [Explanation of symbols]
1, 1a, 1b, 1c neutron detector
11 ³ He Neutron Counter
12, 12a Moderator
13, 13a
14, 14a, 14b Signal processing circuit
15 Lead sensitizer
15a, 15b Lead intensifier
16 Display circuit
17 cases

Claims (4)

A neutron detector comprising a medium / low-speed neutron detection means, a moderator surrounding the detection means and decelerating neutrons, and a signal processing circuit for processing a signal from the detection means,
2 sets of detection means and moderator,
Provided on the outside of one moderator is a sensitizing member that emits multiple neutrons by interacting with ultrafast neutrons contained in cosmic rays, etc.
A neutron detector characterized by that. A neutron detector comprising a medium / low-speed neutron detection means, a moderator surrounding the detection means and decelerating neutrons, and a signal processing circuit for processing a signal from the detection means,
There is an insertion hole in a part of the moderator,
It is detachably inserted into this insertion hole, and includes a sensitizing member that interacts with ultrafast neutrons contained in cosmic rays and the like to emit a plurality of neutrons,
A neutron detector characterized by that. A neutron detector comprising a medium / low-speed neutron detection means, a moderator surrounding the detection means and decelerating neutrons, and a signal processing circuit for processing a signal from the detection means,
It is detachably mounted on the surface of the moderator and includes a sensitizing member that interacts with ultrafast neutrons contained in cosmic rays and the like to emit a plurality of neutrons.
A neutron detector characterized by that. The sensitizing member is made of lead,
The neutron detector according to any one of claims 1 to 3, wherein

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