8. Neutron Radiography and Radiation Application

概要

INTRODUCTION: Devices for detecting nuclear mate- rials have become increasingly important with the in- crease of today’s global terrorism. In particular, a com- pact and low-cost non-destructive assay system to detect hidden nuclear material is required in the fields of nuclear security. We have therefore developed an innovative nu- clear material detection method by using a neutron source of Californium-252, which is capable to assemble such system. In this method, a neutron source is rotated at a speed of thousands of rpm nearby the target. Meanwhile, it is possible to detect nuclear materials by confirming the deformation of the time-distribution spectrum obtained by a neutron detector near the target. The machine to ro- tate the neutron source is quite compact that its width, depth, and height is 60 cm each. We have installed the machine at an experimental laboratory in the KUCA. In the previous study, we presented experimental verifica- tions for this new method [1]. However, we used expen- sive detectors based on He-3, so that we have studied to develop a low-cost detector for this new method. A water Cherenkov detector has a potential ability as a low-cost neutron detector. Unfortunately, our experiments in this year are severely limited due to the influence of the COVID-19.

EXPERIMENTS: The left side of the Fig.1 shows a water Cherenkov detector. Four PMTs (Photomultiplier tube) are mounted on the aquarium. The diameter of the PMT is 2 inches. The aquarium (25x25x30cm) is a com- mercial item and reasonable price. It is covered with a black sheet and boron sheet in order to prevent light and thermal neutrons. The aquarium is filled with the gado- linium aqueous solution with approximately 0.5wt%. The gadolinium was added in order to increase the amount of the Cherenkov light. This detector can be constructed at a much lower cost than He-3 detectors. Since high energy gamma ray, for example from hydrogen in water, also causes the Cherenkov light, we discriminate neutron sig- nal by using pulse height difference. Measurements of the neutron time distribution were performed by a mul- ti-channel scaler (MCS) that is synchronized with the disc rotation. The right side of the Fig.1 shows a rotation machine to rotate a neutron source. The neutron source was installed at the outer periphery of a disc of 32 cm diameter. The maximum rotation speed is 4000 rpm.

RESULTS: Figure 2 shows an example of experimental results of a blank sample (only polyethylene blocks) un- der the condition that the rotation speed is 3000 rpm. The neutron source was closest to the polyethylene blocks approximately at 8500 micro-seconds in Fig. 2. Since the neutron source, the polyethylene blocks and the neutron detector system have a complicated geometry, the dips in the times spectrum appear before and after the peak. We have confirmed that this Cherenkov detector is able to measure as same as He-3 detectors. It means that we can construct a new detection system for nuclear materials that is low cost and transportable. Further improvement of the Cherenkov detector is possible and desirable. Therefore, more detailed experimental study with nuclear materials would be required.

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