Differential cross section measurement of first neutrons for the evaluation of Hiroshima and Nagasaki atomic-bomb dosimetry

辐射风险主要是基于广岛和长崎原子弹爆炸幸存者的辐射剂量。1980年前后，美国和日本开始对广岛和长崎原子弹剂量进行重新评估。此后，~（13）Eu、~（13）Co、~（13）Cl和~（13）Ni的剩余活度数据<152><60>< 36><63>逐渐积累，并显示出系统性差异。一个可能的解释是在空气中的中子输运计算，特别是氮和氧的截面的有效性。以前的作品的实验价值是缺乏前瞻性的方向。本工作的目的是详细地确定氮和氧的微分截面。(1)根据780 keV中子束准直实验，准直器聚乙烯块的厚度应大于20 cm。这一结果与Monte Carlo模拟结果一致。(2)为了确定氮和氧的微分截面，应利用氮氧化物和硅氧化物，并应扣除背景以获得足够的计数统计。(3)通过塑料闪烁体靶的中子散射实验，观察到了热中子散射和碳的散射中子。这一结果表明，减少热中子散射是必要的，以确定在前进方向上的截面。

The risk of radiation is mainly based on the radiation doses for Atomic-bomb survivors of Hiroshima and Nagasaki. Reevaluation of A-bomb doses of Hiroshima and Nagasaki started around 1980 both in US and Japan. Afterward, residual activity data of ^<152>Eu, ^<60>Co,^< 36>Cl and ^<63>Ni were gradually accumulated, and a systematic discrepancy has been revealed. A possible explanation of the discrepancy is neutron transport calculation in air, especially the validity of the cross section of nitrogen and oxygen. The experimental values of previous works are lacking at forward directions. The purpose of this work is to determine the differential cross sections of nitrogen and oxygen in detail.Summary of present work is as follows.(1) According to the neutron beam collimation experiment with 780 keV neutrons, collimator thickness of poliethylen block is necessary more than 20 cm. This result is in agreement with the Monte Carlo simulation results.(2) To determine the differential cross sections of nitrogen and oxygen, Nitrogen oxide and Si oxide should be utilized and background should be subtracted to get enough counting statistics.(3) According to the neutron scattering experiment with thin plastic scintillator as a target, thermal neutron scattering and scattered neutrons from carbon were observed. This result indicates the reduction of thermal neutron scattering is necessary to determine the cross section in the forward direction.