8. Neutron Radiography and Radiation Application

概要

CO8-1 INTRODUCTION:
A compact 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 nuclear material detection method by using a neutron source of Califor- nium-252. In this method, a neutron source is rotated at a speed of thousands of rpm nearby a measurement object. Meanwhile, it is possible to detect nuclear materials by confirming the deformation of the time-distribution spec- trum obtained by a neutron detector near the object. The machine to rotate the neutron source is quite compact that its width, depth, and height is approximately 60 cm each. In the previous studies, we have accomplished fol- lowing two objectives. One is that this new method was verified with a neutron detector bank composed of He-3 proportional counters. Another is that a low-cost assay system setup was completed with the rotation machine and water Cherenkov detector. The water Cherenkov detector is much lower in cost than He-3 counters. The purpose of this year was to detect nuclear materials by using the low-cost assay system in KUCA.

CO8-2 INTRODUCTION:
A two-dimensional neutron detector with a gas electron multiplier (GEM) has been developed [1]. The GEM-based detector is called “nGEM” [2]. The nGEM is a gas-flow radiation detector having a 7:3 gas mixture of Ar and CO2. It has a compact body (52 × 25 × 5 cm) that includes a gas chamber and onboard electron- ics. In the onboard electronics, an application-specific integrated circuit for pulse shaping and a field-programmable gate array for online processing are also installed. The measurement data are transferred di- rectly to a computer through a network. The data transfer capability is limited by the Gigabit Ethernet. The hit in- formation, such as time of flight (TOF), hit position, and pulse width, is included in the 8-byte data format. The active area is 100 × 100 mm. For neutron detection, nGEM can be used to measure the number of charged particles originating from the n(10B, )7Li nuclear reac- tion. The thermal neutron efficiency of nGEM depends on the thickness of the enriched-boron layer on cathode aluminum plate. The thermal neutron efficiency simulat- ed by Geant4 [3] is shown in Figure 1. The maximum neutron efficiency is expected to be approximately 5% with a 2–3-m-thick 10B layer. Above the thicker layer region, the charged particles cannot come out from the 10 B layer because the particles from the n(10B, )7Li nu- clear reaction have a shorter range. Therefore, the thermal neutron efficiency decreases. In the thinner layer region, because it is difficult to control the layer thickness, the film formation accuracy becomes worse. To perform ab- solute measurements in some neutron experiments, neu- tron detectors must first evaluate the neutron efficiencies. In this article, the experimental results of neutron effi- ciency for nGEM are described. The neutron irradiation test was carried out using CN-3 [4] at the Kyoto Univer- sity Reactor (KUR).

CO8-3 INTRODUCTION:
Boron Neutron Capture Reaction (BNCR) is based on the nuclear reaction of 10B atom with thermal/epithermal neutron already applied to cancer treatment (BNCT) [1, 2]. As a new utilization method of BNCR, the purpose of this study is to establish a novel mutation breeding using BNCR.
The method attempts to mutagenesis by immersing plant seeds in a 10B-enriched boron compound, re-drying, and then irradiating the seeds with thermal neutrons to induce BNCR. Its mutagenic effect depends on chemical and physical factors such as 10B concentration, thermal neutron intensity, and irradiation time. In previous ex- periments, rice seeds were treated with 10B-enriched p-boronophenylalanine (BPA) [3] or BPA-fructose com- plexes (BPA-Fc) at concentrations of 10-1000 ppm for 16, 24, or 48 hours, irradiated with thermal neutrons, and then examined for germination rate to evaluate mutagen- icity. As a result, no decrease in germination rate was observed under these conditions; considering the solubil- ity of BPA, 1000 ppm is considered to be the upper limit, and as for the immersion time, no relationship with BNCR was observed for the 48h treatment, and simply the immersion time decreased the germination rate, indi- cating that 48h or longer is not appropriate. In other words, it was difficult to achieve a stronger treatment with BPA. In order to investigate a stronger treatment method that would confirm a decrease in germination rate, the experiment was conducted this time by changing the boron compound used to 10B-enriched boric acid (H310BO3).

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