Residual Radioactivity Measurement in Hiroshima and Nagasaki for the Evaluation of DS86 Neutron Fluence

Residual Eu activity produced by neutrons from the Nagasaki atomic bomb has been measured in seven mineral samples located up to 1142m from the epicenter. Europium was chemically separated from the sample and gamma-ray measurement was carried out with a well-type Ge detector. Deduced specific activities were compared with previous measurements and with activation calculation based on the DS86 neutron fluence. Present results are slightly higher than the calculation at far distances. However, systematic discrepancy as has been observed in Hiroshima is not clear. Further measurements for samples beyond 1000m from the hypocenter are necessary to ensure the discrepancy problem. INTRODUCTION A new dosimetry system DS86 has been assessed in 1987 for survivors of the Hiroshima and Nagasaki atomic bombings. In the low-energy neutron evaluation of activation data, a systematic discrepancy has been observed between the residual Co data measured by Hashizume et al. (1) and activation calculation (2) for both cities. However, this problem was not clarified at that time. The Eu data in Hiroshima and Nagasaki were not accurate enough to examine the calculation. Thereafter radioactivity data were accumulated for Eu (3,4), Co (5,6,7,8), and Cl (9) up to 1500 m from the hypocenter. The results revealed a systematic discrepancy between the measured data and the calculation in Hiroshima. The discrepancy shows that calculation is over estimated near the hypocenter and vice versa beyond about 1000 m. To determine the nature of the problem, interest has been concentrated on Nagasaki data to establish whether such discrepancy exists there as well as in Hiroshima. If calculation agrees with measurements in Nagasaki, the discrepancy observed in Hiroshima is not due to uncertainties in neutron transport calculation in air but to calculation of the neutron output spectrum from the Hiroshima bomb. Residual activity measurement of Eu in mineral samples in Nagasaki were performed by Nakanishi et al. (10) and Okajima and Miyajima (11), but a large deviation in Nagasaki data was not enough to ensure the Co discrepancy indicated by Hashizume’s data (1). Straume et al. (12) measured Cl at three locations up to 1250 m slant range in Nagasaki, they observed good agreement between the measurements and calculation in contrast to Hashizume’s Co data. Recently Nakanishi et al.(13) reported Eu data at about 1100 m in Nagasaki. We collected mineral samples to determine specific activity of Eu:Eu up to 1140 m. The sample preparation and measurements are in progress. Preliminary results are shown in this report and compared with the calculations based on DS86 neutron. MATERIALS AND METHODS Seven mineral samples up to 1039 m from the hypocenter were collected for Eu measurement. These were surface samples faced to the epicenter without any shielding. Sampling location, material, altitude from the ground level as well as sea level, distances (ground range:distance between the sample and the hypocenter, slant range:distance between the sample and the epicenter ) are given in Table 1. Mineral samples were ground to powder under 100 mesh. Because of the difficulty to detect gamma rays from Eu for powdered samples except sample 1 and 2, all powdered samples were chemically treated to enrich the europium concentration as described previously (4). Enriched samples were pressed into a polypropylene test tube (13 mm diameter×7.5 cm height) for measurement with a well-type germanium (Ge) detector. Stable europium content in the final form was determined by neutron activation analysis utilizing the Kinki University reactor and also Cf neutron source at Research Institute for Radiation Bilology and Medicine, Hiroshima University. The europium content in enriched samples are given in Table 1.