Reevaluation of atomic-bomb neutron fluence through purification of europium separated from samples exposed to atom-bombing

目前用于计算广岛和长崎原子弹爆炸幸存者所受中子剂量的剂量学系统必须通过在广岛或长崎原子弹爆炸暴露的样品中再现中子诱发放射性核素的特定放射性来证明其有效性。但是，对于在距长崎原子弹爆炸点1170米以外的位置受原子弹中子照射的样品，没有获得原子弹中子诱发的放射性核素的特定放射性的数据。本研究开始于2003年，目的是在距长崎原子弹爆炸地点1595 m处，对暴露于原子弹爆炸中的样品，获得铕(Eu)-152（半衰期13.542 y）的高精度比放射性值。对暴露样品进行了约19公斤的化学分离，得到的富集Eu的粗样用伽马射线谱仪进行了测量。结果表明，残留Eu-152的超低放射性测量受到actinium (Ac)-227子核素的严重干扰。因此，本研究开发了从Eu中去除Ac的化学方法，并对富集Eu的粗样品进行了纯化。利用低能光子谱仪（LEPS）测量了铕-152的电子俘获衰变后发射的钐K-X射线，计算了铕-152的精馏分数。LEPS测量结果表明，eu152的实测比放射性比DS法计算值高出约10倍。由于测量/计算比值高，怀疑荧光x射线是增量的，即计数试样中的Sm被试样的α -射线和β -射线激发产生Sm- k - x射线，并对荧光x射线的产率进行了系统的研究。然后对荧光x射线的LEPS测量值进行校正，但发现校正不能有效降低测量/计算比。然后在金泽大学的Ogoya地下实验室（OUL）用超低能级伽马能谱仪测量了精炼后的eu组分。LEPS结果与OUL结果吻合较好。因此，实测值与计算值不一致的原因仍然是一个悬而未决的问题。少

The current dosimetry system (DS) to calculate neutron dose received by the survivor of atom-bombing in Hiroshima and Nagasaki must be proved its validity by reproducing specific radioactivity of neutron-induced radionuclides in samples exposed to atomic bomb in Hiroshima or Nagasaki. But, for the samples exposed to atomic-bomb neutrons at positions beyond 1170 m from the explosion point of Nagasaki atomic bomb, no data have been obtained for specific radioactivity of radionuclides induced with atomic-bomb neutrons.In the present research started in 2003, highly accurate value of specific radioactivity of europium (Eu)-152 (half-life 13.542 y) was aimed to obtain for a sample exposed to atomic-bomb explosion at position 1595 m from the explosion point of Nagasaki atomic bomb.About 19 kg of the exposed sample was subjected for the chemical separation of Eu, and crude specimen enriched in Eu thus obtained was measured with a gamma-ray spectrometer. As a result of the measurement, it beca … More me clear that the measurement of ultra-low-level radioactivity of residual Eu-152 was seriously interfered with daughter nuclides of actinium (Ac)-227. Hence, chemical procedure to remove Ac from Eu was developed in the present research, and purification of the crude specimen enriched in Eu was carried out. The refined Eu-fraction was counted by low-energy photon spectrometer (LEPS) by which samarium (Sm) K-X ray emitted after electron-capture decay of Eu-152 was measured. The result of LEPS measurement showed that measured specific radioactivity of Eu-152 was about ten times higher than the calculated one by DS methodology.For the reason of high measured/calculated ratio, incremental fluorescent X-ray was suspected, i.e. Sm in the counting specimen are excited with alpha-and beta-rays from the specimen to generate Sm-K-X ray, and a systematic study on the yield of fluorescent X-ray was carried out. The measured value by LEPS was then corrected for fluorescent X-ray, but it was found that the correction was not effective to lower the measured/calculated ratio. The refined Eu-fraction was then measured with ultra-low-level gamma spectrometer at Ogoya Underground Laboratory (OUL) of Kanazawa University. It was found that LEPS result and OUL result agreed well with each other. Hence, the reason of the disagreement between measured value and calculated value is still an open question. Less