Interference due to Sm-X-rays to the determination of ultra-low-level 152Eu separated from a sample exposed to Nagasaki atomic-bomb

We have undertaken the determination of the specific radioactivity of 152Eu (half-life: 13.542 y) in a concrete sample exposed to Nagasaki atomic-bomb at a place 1595 m from the explosion point. Chemical separation to prepare europium-enriched sample was performed for a 7.8 kg sample, and low-energy photon spectrometry was carried out. In the spectrometry, 39-40 keV Sm X-rays emitted after electron-capture decay of 152Eu was measured because of the highest sensitivity. During careful counting, we found an excess 152Eu activity compared to that estimated by DS02. It was suspected that the europium-enriched sample contains Sm, and Sm generates fluorescence X-rays. Hence, mock-up samples were prepared to assess the interference due to the fluorescence X-rays of Sm to the accurate determination of 152Eu. It was confirmed that the intensity of fluorescence X-rays of Sm increases in direct proportion to the alpha/beta-activity and beta maximum energy in the mock-up samples. The measured value was corrected for the fluorescence X-rays, but it is still highest than estimated by DS02. (author)