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

辐射风险主要基于广岛和长崎原子弹幸存者的辐射剂量。美国和日本在1980年左右开始重新评估广岛和长崎的原子弹剂量。此后，^&lt；152&gt；Eu，^&lt；60&gt；Co，^&lt；36&gt；Cl和^&lt；63&gt；Ni的剩余活度数据逐渐积累，并显示出系统的差异。这种差异的一个可能解释是中子在空气中的输运计算，特别是氮和氧的横截面的有效性。前人的工作在前瞻方向上缺乏实验值。这项工作的目的是详细测定氮气和氧气的微分截面。主要工作如下：(1)根据780keV中子的中子束准直实验，聚乙稀的准直器厚度必须大于20 cm。这一结果与蒙特卡罗模拟结果一致。(2)为了确定氮和氧的微分截面，应利用氮氧化物和氧化硅，并减去本底以获得足够的计数统计。(3)以薄塑料闪烁体为靶的中子散射实验，观察到了热中子散射和碳的散射中子。这一结果表明，减少热中子散射对于确定正向截面是必要的。

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.