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接受的中子剂量的剂量测量系统(DS)必须通过再现暴露在广岛或长崎原子弹下的样品中中子诱发的放射性核素的比放射性来证明其有效性。但是，在距长崎原子弹爆点1170m以外的位置暴露于原子弹中子的样品，还没有获得原子弹中子诱发放射性核素比活度的数据。本研究始于2003年，旨在获得离长崎原子弹爆点1595m的原子弹爆炸样品高精度的Eu152比放射性数值(半衰期为13.542年)。将约19公斤暴露的样品进行Eu的化学分离，并用伽马能谱仪测量由此得到的富含Eu的粗样品。作为测量的结果，它是…更清楚的是，残留Eu-152的超低水平放射性测量严重干扰了Ac(Ac)-227的子体核素。为此，本研究开发了从Eu中脱除Ac的化学方法，并对富含Eu的粗品进行了纯化。用低能光子谱仪(LEPS)测量了Eu-152电子俘获衰变后的Sm(Sm)K-X射线，计算了Eu-152的精制Eu组分。LEPS测量结果表明，Eu-152的测量比活度约为DS方法计算值的10倍，由于测量/计算比值较高，因此怀疑是增量荧光X射线，即计数样品中的Sm被样品中的α和β射线激发而产生Sm-K-X射线，并对荧光X射线的产额进行了系统的研究。然后对LEPS的测量值进行了荧光X射线校正，但发现校正不能有效地降低测量/计算比。然后在金泽大学Ogoya地下实验室(OUL)用超低能级伽马能谱仪测量了精制的Eu组分。结果表明，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是一个悬而未决的问题。较少

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