Fast neutrons measured in copper from the Hiroshima atomic bomb dome.

The first measurements of (63)Ni produced by A-bomb fast neutrons (above approximately 1 MeV) in copper samples from Hiroshima encompassed distances from approximately 380 to 5062 m from the hypocenter (the point on the ground directly under the bomb). They included the region of interest to survivor studies (approximately 900 to 1500 m) and provided the first direct validation of fast neutrons in that range. However, a significant measurement gap remained between the hypocenter and 380 m. Measurements close to the hypocenter are important as a high-value anchor for the slope of the curve for neutron activation as a function of distance. Here we report measurements of (63)Ni in copper samples from the historic Hiroshima Atomic Bomb Dome, which is located approximately 150 m from the hypocenter. These measurements extend the range of our previously published data for (63)Ni providing a more comprehensive and consistent A-bomb activation curve. The results are also in good agreement with calculations based on the current dosimetry system (DS02) and give further experimental support to the accuracy of this system that forms the basis for radiation risk estimates worldwide.

Neutron-induced 63Ni in copper samples from Hiroshima and Nagasaki: a comprehensive presentation of results obtained at the Munich Maier-Leibnitz Laboratory.

Those inhabitants of Hiroshima and Nagasaki who were affected by the A-bomb explosions, were exposed to a mixed neutron and gamma radiation field. Few years later about 120,000 survivors of both cities were selected, and since then radiation-induced late effects such as leukemia and solid tumors are being investigated in this cohort. When the present study was initiated, the fast neutron fluences that caused the neutron doses of these survivors had never been determined experimentally. In principle, this would have been possible if radioisotopes produced by fast neutrons from the A-bomb explosions had been detected in samples from Hiroshima and Nagasaki at distances where the inhabitants survived. However, no suitable radioisotope had so far been identified. As a contribution to a large international effort to re-evaluate the A-bomb dosimetry, the concentration of the radionuclide (63)Ni (half-life 100.1 years) has been measured in copper samples from Hiroshima and Nagasaki. These measurements were mainly performed at the Maier-Leibnitz-Laboratory in Munich, Germany, by means of accelerator mass spectrometry. Because the (63)Ni had been produced in these samples by fast A-bomb neutrons via the reaction (63)Cu(n,p)(63)Ni, these measurements allow direct experimental validation of calculated neutron doses to the members of the LSS cohort, for the first time. The results of these efforts have already been published in a compact form. A more detailed discussion of the methodical aspects of these measurements and their results are given in the present paper. Eight copper samples that had been significantly exposed to fast neutrons from the Hiroshima A-bomb explosion were investigated. In general, measured (63)Ni concentrations decreased in these samples with increasing distance to the hypocenter, from 4 x 10(6 ) (63)Ni nuclei per gram copper at 391 m, to about 1 x 10(5 ) (63)Ni nuclei per gram copper at about 1,400 m. Additional measurements performed on three large-distant copper samples from Hiroshima (distance to the hypocenter 1,880-7,500 m) and on three large-distant copper samples from Nagasaki (distance to the hypocenter 3,931-4,428 m) that were not exposed significantly to A-bomb neutrons, suggest a typical background concentration of about 8 x 10(4 ) (63)Ni nuclei per gram copper. If the observed background is accounted for, the results are consistent with state-of-the-art neutron transport calculations for Hiroshima, in particular for those distances where the victims survived and were included in the life span study cohort.

Measuring fast neutrons in Hiroshima at distances relevant to atomic-bomb survivors.

Data from the survivors of the atomic bombs serve as the major basis for risk calculations of radiation-induced cancer in humans. A controversy has existed for almost two decades, however, concerning the possibility that neutron doses in Hiroshima may have been much larger than estimated. This controversy was based on measurements of radioisotopes activated by thermal neutrons that suggested much higher fluences at larger distances than expected. For fast neutrons, which contributed almost all the neutron dose, clear measurement validation has so far proved impossible at the large distances (900 to 1,500 m) most relevant to survivor locations. Here, the first results are reported for the detection of 63Ni produced predominantly by fast neutrons (above about 1 MeV) in copper samples from Hiroshima. This breakthrough was made possible by the development of chemical extraction methods and major improvements in the sensitivity of accelerator mass spectrometry for detection of 63Ni atoms (refs 8-11). When results are compared with 63Ni activation predicted by neutron doses for Hiroshima survivors, good agreement is observed at the distances most relevant to survivor data. These findings provide, for the first time, clear measurement validation of the neutron doses to survivors in Hiroshima.

Accelerator mass spectrometry of 63Ni at the Munich Tandem Laboratory for estimating fast neutron fluences from the Hiroshima atomic bomb.

After the release of the present dosimetry system DS86 in 1987, measurements have shown that DS86 may substantially underestimate thermal neutron fluences at large distances (>1,000 m) from the hypocenter in Hiroshima. This discrepancy casts doubts on the DS86 neutron source term and, consequently, the survivors' estimated neutron doses. However, the doses were caused mainly by fast neutrons. To determine retrospectively fast neutron fluences in Hiroshima, the reaction 63Cu(n, p)63Ni can be used, if adequate copper samples can be found. Measuring 63Ni (half life 100 y) in Hiroshima samples requires a very sensitive technique, such as accelerator mass spectrometry (AMS), because of the relatively small amounts of 63Ni expected (approximately 10(5)-10(6) atoms per gram of copper). Experiments performed at Lawrence Livermore National Laboratory have demonstrated in 1996 that AMS can be used to measure 63Ni in Hiroshima copper samples. Subsequently, a collaboration was established with the Technical University of Munich in view of its potential to perform more sensitive measurements of 63Ni than the Livermore facility and in the interest of interlaboratory validation. This paper presents the progress made at the Munich facility in the measurement of 63Ni by AMS. The Munich accelerator mass spectrometry facility is a combination of a high energy tandem accelerator and a detection system featuring a gas-filled magnet. It is designed for high sensitivity measurements of long-lived radioisotopes. Optimization of the ion source setup has further improved the sensitivity for 63Ni by reducing the background level of the 63Cu isobar interference by about two orders of magnitude. Current background levels correspond to a ratio of 63Ni/Ni<2x10(-14) and suggest that, with adequate copper samples, the assessment of fast neutron fluences in Hiroshima and Nagasaki is possible for ground distances of up to 1500 m, and--under favorable conditions--even beyond. To demonstrate this capability, we have measured successfully 6Ni/Ni ratios as low as (3.5 +/- 0.6) x 10(-13). The latter are, based on DS86, representative of a ratio expected from a typical Hiroshima copper sample at about 1,300-m ground range.