JP2003066181A - Structure of moderator in moderation type instrument for measuring neutron spectrum - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a moderation type instrument for measuring neutron spectrum where a position-sensitive type thermal neutron detector is located in a neutron moderator and which measures the distribution of the energy of neutrons in a radiation field where neutrons that have various levels of energy exist. SOLUTION: The structure of a moderator in the measuring instrument is characterized in that the instrument can measure the position where neutrons are converted into thermal neutrons with high sensitivity by replacing a portion of the moderator on the surface side where the neutrons are incident, with a material whose hydrogen density is low and which has low performance of neutron moderation to enlarge the difference in the positions where the neutrons in a low-energy range are converted into the thermal neutrons and further, by placing a thermal neutron absorber in the moderator to restrain the thermal neutrons from diffusing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a moderator type neutron spectrum measuring instrument used for measuring a neutron energy distribution in a radiation field in which neutrons having various energies are present. By devising the structure of the moderator to be attenuated, the energy of neutrons in the low energy region can be measured.

[0002]

2. Description of the Related Art What is a moderating neutron spectrum measuring device?
As shown in, it is composed of a moderator and a position-sensitive thermal neutron detector that can measure the position where the thermal neutron is detected. Neutrons incident on the measuring instrument pass through the moderator and gradually lose energy. And finally, it becomes a very low energy neutron called thermal neutron. As shown in the figure, the position of this thermal neutron changes depending on the neutron energy at the time of incidence, so the energy of the incident neutron can be found by measuring the position distribution in which the thermal neutron exists. Therefore, a position-sensitive thermal neutron detector is installed at the center of the moderator so that the thermal neutron distribution in the moderator can be measured.

A conventional slow-down type neutron spectrum measuring instrument is
As shown in Fig. 1, one type of moderator is used, and no ingenuity has been made such as combining substances with different neutron moderating performance, or putting a substance that absorbs thermal neutrons into the moderator. . Therefore, it has been difficult to measure the energy of neutrons in the low energy region with the conventional measuring instrument.

[0004]

When a substance having a high capability of slowing down neutrons such as polyethylene or paraffin is used as a moderator, the difference in the position where the thermal neutrons are slowed down is small with respect to the neutrons having relatively low energy. Become. Since the energy distribution is obtained using this difference, it is difficult to obtain the energy distribution with neutrons in the low energy region.

Further, the neutrons incident on the measuring instrument are decelerated to thermal neutrons and then diffuse in the moderator. And the diffusion distance is relatively long. The diffusion of this thermal neutron causes the position where the incident neutron is decelerated to the thermal neutron and the position detected by the thermal neutron detector to scatter (diffusion effect). In particular, this diffusion effect appears remarkably for neutrons of relatively low energy in which the difference in position decelerated by thermal neutrons is small. This makes it difficult to measure the energy distribution for neutrons in the low energy region. The above two problems are solved by the present invention.

[0006]

The present invention is to improve the energy measuring performance of neutrons in the low energy region of a moderator neutron spectrum measuring instrument by devising a moderator structure. In improving the performance, neutrons in the low-energy region that are incident on the measuring instrument are decelerated to greatly expand the difference in the position where they become thermal neutrons, and the diffusion effect of the decelerated neutrons in the moderator to become thermal neutrons. We devised to realize the two points of suppression.

Specifically, in order to widen the difference in the position where the neutrons in the low energy region are decelerated to thermal neutrons,
A moderator having a lower hydrogen density (lower neutron moderating performance) than the polyethylene or paraffin moderator normally used on the neutron incident surface side of the measuring instrument is used. At the same time, in order to secure moderation performance for neutrons in the high energy region, a material having a high hydrogen density (high neutron moderation performance) such as polyethylene or paraffin is used for the moderator on the opposite side. The diffusion effect of thermal neutrons is suppressed by installing a thermal neutron absorbing material (cadmium, boron, etc.) in the moderator.

In determining the shape of the thermal neutron absorber, the distribution of the positions where the neutrons that have entered the moderator are decelerated by the thermal neutrons is calculated for each energy of the incident neutrons. Then, the shape of the thermal neutron absorber is determined from the distribution shape of the thermal neutrons in the moderator in the incident axis direction and the radial direction obtained by calculation.

That is, the present invention relates to a moderator type neutron spectrum measuring instrument in which a position-sensitive type thermal neutron detector is arranged in a neutron moderator and which measures a neutron energy distribution in a radiation field in which neutrons having various energies exist. , A low hydrogen density part of the neutron incident surface side of the moderator, by replacing the neutron moderating performance of a low substance, by expanding the difference in the position where neutrons in the low energy region are converted to thermal neutrons, Furthermore, the moderator structure of the measuring instrument is characterized in that the conversion position to thermal neutrons can be measured with high sensitivity by disposing a thermal neutron absorber in the moderator to suppress the diffusion of thermal neutrons.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A moderator-type neutron spectrum measuring instrument is one in which a position-sensitive type thermal neutron detector is arranged at the center of a cylindrical neutron moderator as shown in FIG. 1 to measure the thermal neutron distribution in the moderator. Is made possible. Then, the neutron energy is measured by utilizing the fact that the shape of the thermal neutron distribution in the moderator changes due to the energy of the neutrons incident on the measuring instrument.

In the present invention, the moderator structure is devised for the moderator neutron spectrum measuring instrument described above,
Improve energy measurement performance with neutrons in the low energy region. There are the following two points in devising the moderator structure.

Since substances having a high hydrogen density such as polyethylene and paraffin have a small difference in moderation performance with respect to neutrons in a low energy region, a difference in a position where incident neutrons are decelerated to thermal neutrons becomes small, and as a result, measurement is performed. The change in shape of the thermal neutron distribution is small, making energy measurement difficult.

As a solution to this problem, it is conceivable to use a substance having a lower hydrogen density than polyethylene or the like as a moderator to widen the difference in moderating performance for neutrons in the low energy region. However, if the moderator as a whole is changed to a substance such as polycarbonate having a low hydrogen density, it becomes impossible to obtain sufficient moderator performance for neutrons in the high energy region. Therefore, in the measuring instrument of the present invention, the polycarbonate moderator (neutron incident side)
A moderator structure that widens the difference in moderator performance of neutrons in the low energy region while ensuring moderator performance for neutrons in the high energy region has been realized by combining with and a polyethylene moderator.

The neutrons that have been decelerated into thermal neutrons move by diffusion over a relatively long distance in the moderator, and diffuse the positions where they become thermal neutrons and the positions measured by the thermal neutron detectors. The effect occurs. Therefore, the diffusion effect is suppressed by installing a thermal neutron absorber in the moderator and selectively removing thermal neutrons due to diffusion. This improves the energy measurement performance for neutrons in the low energy region where the diffusion effect becomes remarkable. Regarding the shape of the thermal neutron absorber, the position where the neutrons incident on the moderator are decelerated to become thermal neutrons is calculated in advance, and the optimum shape is obtained from the result.

[0015]

[Example] When neutrons of several kinds of energy are measured when only a polyethylene moderator that is commonly used as a neutron moderator is used, that is, when the moderator structure according to the present invention is not devised FIG. 2 shows the result of the simulation calculation of the output from the measuring instrument. It is shown that the energy measurement performance is higher as the shape of the measuring instrument output changes depending on the energy, but there is almost no shape difference in the low energy region of 100 keV or less, indicating that the energy measurement performance is poor. .

Therefore, as shown in FIG. 3, of the 100 cm moderator, the moderator on the neutron incident side 40 cm was replaced with a polycarbonate having a lower hydrogen density and lower neutron moderating performance than polyethylene, and the remaining 60 cm moderator was used. Polyethylene was used. Then, a simulation calculation was performed for a case where the moderator structure according to the present invention was devised, such as installing a thermal neutron absorber (using cadmium in the examples of the present invention) in the moderator.

The shape of the cadmium plate for absorbing thermal neutrons was determined according to the following procedure. In advance, it is calculated for various incident neutron energies where the neutrons are decelerated in the moderator and where they become thermal neutrons. Then, the correlation between the peak position of the thermal neutron distribution in the incident axis direction and the thermal neutron spread in the radial direction is obtained, and a graph as shown in FIG. 4 is created. The shape of the cadmium plate for thermal neutron absorption installed in the moderator is determined from the graph. Looking at the calculation results of the output of the measuring device devised according to the present invention shown in FIG. 5, the heat for incident neutron energy of 100 keV or less is compared with the result of FIG. 2 without devising the moderator structure according to the present invention. Differences are observed in the neutron distribution shape. From this, it is understood that the present invention improves the energy measurement performance for neutrons in the low energy region .

[0018]

EFFECTS OF THE INVENTION By changing a part of the moderator on the neutron incident side from polyethylene to a substance having a low hydrogen density and a low neutron moderating ability, the difference in the thermal neutron flux distribution shape in the moderator due to the energy of incident neutrons is widened. be able to.

Further, by disposing the thermal neutron absorbing material in the moderator in a shape corresponding to the thermal neutron flux distribution shape when neutrons are incident on the moderator, the accuracy of position information is lowered and the energy measurement performance is improved. It is possible to suppress the diffusion of thermal neutrons, which causes deterioration.

With these structures, it is possible to increase the difference in the thermal neutron distribution shape due to the neutron energy without significantly lowering the sensitivity in the moderator type neutron spectrum measuring instrument, and to improve the energy measurement performance even for lower energy neutrons. Becomes

[Brief description of drawings]

FIG. 1 is a view showing a structure of a conventional slowdown type neutron spectrum measuring instrument.

FIG. 2 is a diagram showing calculation results for neutrons of respective energies of a measuring instrument output when a simple polyethylene moderator is used as a conventional neutron moderator.

FIG. 3 is a view showing a structure of a moderator type neutron spectrum measuring device in which the moderator structure is devised according to the present invention.

FIG. 4 is a diagram showing an example of determining the shape of a thermal neutron absorbing material installed in the moderator of the present invention.

FIG. 5 is a diagram showing calculation results for neutrons of respective energies of a moderator type neutron spectrum measuring instrument output when the moderator structure of the present invention is used.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Michio Yoshizawa             4 of 2 Shirane, Shikata, Tokai-mura, Naka-gun, Ibaraki Prefecture               Japan Atomic Energy Research Institute Tokai Research Center (72) Inventor Makoto Yoshida             4 of 2 Shirane, Shikata, Tokai-mura, Naka-gun, Ibaraki Prefecture               Japan Atomic Energy Research Institute Tokai Research Center F term (reference) 2G075 CA08 CA43 FA06 FA19 FA20                       FC03 FC16

Claims (2)

[Claims] 1. A moderator structure in which a plurality of types of moderators having different moderating performances are combined in a moderator type neutron spectrum measuring instrument. 2. The moderator neutron spectrum measuring instrument, wherein the moderator has a structure in which a thermal neutron absorber is installed in an appropriate shape in the moderator.