Gamma-ray thermoluminescence measurements: a record of fallout deposition in Hiroshima?

Abstract

In certain Hiroshima neighborhoods, radiation measurements using thermoluminescence dosimetry (TLD) exceed what can be explained by the initial gamma-ray doses and uncertainties from the Dosimetry System 2002 (DS02). This problem was not previously recognized as being isolated to certain parts of that city. The ratio between TLD measurements and DS02 dose calculations for gamma rays appear to grow larger than unity up to more than three with increasing ground range, but closer examination shows the excess TLD dose (0.1, 0.2, or possibly up to 0.8 Gray) is correlated with certain neighborhoods and could be due to radioactive fallout. At Nagasaki, the TLD measurements do not show this same excess, probably because there were no TLD measurements taken more than 800 m downwind (eastward) from the Nagasaki hypocenter, so that any small excess TLD dose was masked by larger initial gamma-ray doses of 25-80 Gray in the few downwind samples. The DS02 Report had noted many measurements lower than the DS02 calculation for several Nagasaki TLD samples, independent of ground range. This was explained as being the result of previously unaccounted urban shielding which was observed from Nagasaki pre-bomb aerial photos. However, the Hiroshima excess TLD dose issue was not resolved. If the excess TLD doses at Hiroshima are an indication of fallout, it may be possible to use additional TLD studies to make better estimates of the locations and radiation doses to survivors from the fallout after the bombings at both cities.

Workshop Report on Atomic Bomb Dosimetry--Residual Radiation Exposure: Recent Research and Suggestions for Future Studies

“…Egbert) Measurements of thermoluminescence (TL) of tiles in several areas in Hiroshima indicated measured doses larger by up to 0.8 Gy than could be explained by exposure to initial radiation from the detonation. This excess dose was particularly noticeable in four areas around the city (Egbert and Kerr 2012). It was suggested that this was due to some sources of radiation that are unique to the Hiroshima detonation.…”

“…Thus, the most likely source of a significant residual radiation exposure to survivors near the hypocenter areas at Hiroshima and Nagasaki appears to result from (1) the lofting of neutron-activated radionuclides from soil and other environmental materials into the atmosphere by the ground shock from the blast wave of the explosion (see Figs. 1 and 2) and (2) the horizontal scouring and sweeping of ground and river water by the blast wave, which moved some neutron-activated materials to larger ground distances (Egbert and Kerr 2012).…”

“…It is possible to make other general statements about the source and amount of fallout from an explosion by referring to a weapon's shock-scaled height of burst (SHOB), which is defined as SHOB = HOB Â Y j1/3 by G.D. Spriggs (Egbert and Kerr 2012). The SHOB for an explosion is used to assign it to one of several regimes that describe the potential for mixing of (1) the cap and the debris stem containing the fission products and actinides (e.g., uranium and plutonium) plus neutron-activated weapon materials and (2) the dust stem from the explosion containing neutron-activated radionuclides from soil and other materials on the ground.…”

“…The possibility of some minimal contribution to the total whole body doses by residual radiation exposure was considered briefly in the DS86 and DS02 documents. Due to both time and funding limitations, it was not possible to review in detail the residual radiation exposures from neutron activation products and fission product fallout in the environment as a part of the DS02 study, and it was expected that an up-to-date review would require ongoing efforts by numerous research groups (Egbert and Kerr 2012). Thus, a special half-day technical session and a fullday workshop were held on residual radiation exposures to the atomic bomb survivors at Hiroshima and Nagasaki during the 57th Annual Meeting of the Health Physics Society in Sacramento, California, 22Y26 July 2012.…”

“…Thus, a special half-day technical session and a fullday workshop were held on residual radiation exposures to the atomic bomb survivors at Hiroshima and Nagasaki during the 57th Annual Meeting of the Health Physics Society in Sacramento, California, 22Y26 July 2012. The primary purpose of the half-day technical session was to review results of recently reported studies on residual radiation in both the United States and Japan (Aoyama and Oochi 2011;Egbert and Kerr 2012), while the purpose of the one-day workshop was to provide time for detailed discussions of these newer studies and to evaluate their potential use in clarifying residual radiation exposures to the atomic-bomb survivors at Hiroshima and Nagasaki. Organizers of the technical session and workshop were Joseph F. Weiss and Isaf Al-Nabulsi of the U.S. Department of Energy; co-chairs for the technical session were Harry M. Cullings and Tetsuji Imanaka; and co-chairs for the workshop were George D. Kerr and Stephen D. Egbert.…”