Euの化学分離と高純度化による原爆被曝試料の極微弱Eu-152残留放射能の測定

Abstract

Measurements of the specific radioactivity of residual neutron-induced radionuclides such as Eu-152 and Co-60 have been carried out to assess the validity of a series of computer calculations employed for atomic-bomb neutron dosimetry in Hiroshima and Nagasaki. However, the use of these nuclides for atomic-bomb neutron dosimetry has been limited by the following difficulties: 1) today, these radionuclides are found only at extremely low concentrations in materials exposed to the atomic bombs and 2) the neutrons that induced these radionuclides were thermal and epithermal, while the neutron dose received in Hiroshima and Nagasaki is attributable to fast neutrons. In order to overcome the first difficulty, a chemical procedure was established to extract Eu and Co from materials exposed to the atomic bomb. This chemical procedure has been successful for materials exposed to the atomic bomb within a 1,400 m slant distance from the explosion point over Hiroshima. At Nagasaki, materials exposed at distances greater than 1,200 m have never been subjected to the measurement of residual neutron-induced radionuclides. In this work, undertaken was the determination of the specific radioactivity of Eu-152 (half-life: 13.542 y) in a sample exposed to the Nagasaki atomic bomb at a place distant from the explosion point. However, because of radioactive decay during the approximately 60 years since 1945 and the great distance from the explosion point, the current specific radioactivity of Eu-152 in the sample is extremely low (estimated to be approximately 2 x 10(exp -4) Bq-Eu-152/mg-Eu), and a serious problem in the measurement of ultra low-level Eu-152 radioactivity arises due to interference from daughters of Ac-227 (half-life: 21.8 y). Hence, the chemical procedure for preparing a Eu-enriched counting source must be improved, and much attention must be given to the removal of Ac from the sample. An approximately 19 kg concrete sample was obtained from the outer surface of a building exposed to the Nagasaki atomic bomb at a distance of 1,596 m from the explosion point. From the concrete sample, approximately 9.6 kg of andesite (used as osteodentin) and approximately 7.7 kg of mortar were separated. Because preliminary neutron activation analysis demonstrated that the Eu concentration in the andesite fraction (0.96 ppm) was higher than the concentration in the mortar fraction (0.41 ppm), approximately 7.8 kg aliquots of andesite were subjected to a chemical procedure to separate rare-earth elements (REEs) including Eu. After total decomposition of the pulverized andesite by fusion with sodium hydroxide, removal of major elements (such as Si, Al, Fe, Ca, Na, K, and Mg) was carried out by precipitation, solvent extraction, and ion-exchange column methods. A crude specimen obtained in this manner was subjected to preliminary measurement of Eu and radionuclides. The crude specimen, which was enriched in REEs, was then subjected to total decomposition and chemical procedures to obtain a purified specimen, which was virtually free of major elements and Ac. To remove Ac from the REEs, the major elements were first removed, then solvent extraction with 1 M HDEHP-benzene was applied to the REE fraction in dilute nitric acid medium. Preliminary measurements of Eu and radionuclides in the crude specimen revealed that the content of Ac-227 daughters in the specimen was too high to determine the low-level Eu-152 radioactivity. A series of tracer experiments using Eu-152 and Ac-228 demonstrated that solvent extraction in a 1 M HDEHP-benzene/dilute HNO3 system is an efficient method to remove Ac from Eu. In the case of the actual crude specimen enriched in REEs, Eu and Ac were first extracted into 300 mL of 1 M HDEHP-benzene from 300 mL of 0.1 M HNO3 solution. The organic phase was then washed with five 300 mL portions of 0.2 M HNO3. Some results of the enrichment and purification of Eu from the sample exposed to the atomic bomb will be given. Although the overall recovery of Eu in the purified specimen is only approximately 26 percent, the removal of Ac from Eu has finally enabled the detection of Eu-152 radioactivity. The measurement of Eu-152 radioactivity in the purified specimen should be continued for at least 3 months.