フレーク状ターゲットの14MeVでの後方散乱反応断面積のベンチマーク実験法の詳細検討

1.Introduction

The elastic scattering reaction cross section data commonly show smaller in backward angles compared to those of forward angles when the energy of the incident neutron is high. However, in the case of gap streaming phenomenon in the neutronics design of the fusion reactor, the reaction cross section value of back scattering may have a large influence on the calculation result. Until now, there was a difference reported between experimental and calculated values of neutron benchmark experiments using a DT neutron source. The cause is not yet clear, however it has been pointed out that the reaction cross section value of large angle scattering could be uncertain. For this problem, the author’s group developed a benchmark method for large angle scattering cross sections and has carried out experiments with an iron sample for the last few years. In the next step, we will consider benchmark experiments for other elements contained in fusion structural materials like W, Si and so on. In this case we expect that it may be required to cope with non-solid samples such as powders, e.g., Si flake. In this study, we propose a method to realize an accurate large angle scattering cross section benchmark that can eliminate the influence of the case of the sample and the wall of the irradiation room when conducting experiments with powder samples filled in the case.

2.Experimental method

In this experiment, a shadow bar is positioned in front of the DT neutron source, and a silicon target filled in a cylindrical case is placed behind it. A Nb foil that detects neutrons is placed on the edge of the shadow bar. The shadow bar has the role of shielding neutrons that are directly incident on the Nb foil from the radiation source. We use two shadow bars, the one of which is thin, and the other is thick. The thin shadow bar (S1) measures the contribution of large angle scattering neutrons, and the thick shadow bar (S2) measures the contribution of neutrons reflected by the wall. For each of these two shadow bars, two experiments are conducted with and without flake silicon in the case, and in total four experimental results are carried out. The four results are used to estimate the contribution of neutrons from the wall and the contribution of large angle scattering neutrons from the silicon target accurately.

3.Result and discussion

Based on the above experimental principle, we performed numerical simulations using MCNP-5 in order to extract only the contribution of large angle scattering neutron. To clarify the neutron path incident on Nb, we flagged five cells, i.e., shadow bar, wall, Si target, container, and its lid, then examined the contribution of 31 pathways (=5C1 + 5C2 + 5C3 + 5C4 + 5C5). As a result of physical consideration, it was found that 19 paths were needed to be considered in detail because we could remove several paths through which neutrons cannot physically pass. The large angle scattering neutron contribution by the route through lid and target, PATH, is the value we want to obtain as the final experimental result. It was found that the contribution of large angle scattering neutrons can be estimated from the measured Nb reaction rates of four experiments using the following equation.

PATH =(S1tc) -(S1c) -((S2tc) -(S2c)), where tc : Experiment with Si target in the case, c : Experiments with only the case (without Si target).

4.Future plan

The next step is to carry out experiments with the present method and then to find a way to feedback the obtained results to the evaluated nuclear data.