Workshop Report on Atomic Bomb Dosimetry—Review of Dose Related Factors for the Evaluation of Exposures to Residual Radiation at Hiroshima and Nagasaki

Abstract

Groups of Japanese and American scientists, supported by international collaborators, have worked for many years to ensure the accuracy of the radiation dosimetry used in studies of health effects in the Japanese atomic bomb survivors. Reliable dosimetric models and systems are especially critical to epidemiologic studies of this population because of their importance in the development of worldwide radiation protection standards. While dosimetry systems, such as Dosimetry System 1986 (DS86) and Dosimetry System 2002 (DS02), have improved, the research groups that developed them were unable to propose or confirm an additional contribution by residual radiation to the survivor's total body dose. In recognition of the need for an up-to-date review of residual radiation exposures in Hiroshima and Nagasaki, a half-day technical session was held for reports on newer studies at the 59 th Annual HPS Meeting in 2014 in Baltimore, MD. A day-and-a-half workshop was also held to provide time for detailed discussion of the newer studies and to evaluate their potential use in clarifying the residual radiation exposure to atomic bomb survivors at Hiroshima and Nagasaki. The process also involved a re-examination of very early surveys of radioisotope emissions from ground surfaces at Hiroshima and Nagasaki and early reports of health effects. New insights were reported on the potential contribution to residual radiation from neutron-activated radionuclides in the airburst's dust stem and pedestal and in unlofted soil, as well as from fission products and weapon debris from the nuclear cloud. However, disparate views remain concerning the actual residual radiation doses received by the atomic bomb survivors at different distances from the hypocenter. The workshop discussion indicated that measurements made using thermal luminescence and optically stimulated luminescence, like earlier measurements, especially in very thin layers of the samples, could be expanded to detect possible radiation exposures to beta particles and to determine their significance plus the extent of the various residual radiation areas at Hiroshima and Nagasaki. Other suggestions for future residual radiation studies are included in this workshop report.

Retrospective and emergency dosimetry in response to radiological incidents and nuclear mass-casualty events: A review

“…The tile had been located in the roof eaves of the Old Hiroshima University building H-4 at a ground range of 1324 m (Young and Kerr, 2005). After deconvolution of the depth-dose profile, the value of external beta dose in the subsurface (∼1Gy) was reported to be consistent with an estimate of several hundred mGy above neutron-activated soil at the Hiroshima hypocenter (Kerr et al, 2015). The average dose (depth range 2-22 mm) is 7% larger than the value adopted at this location in the DS02 model (measurements by Ichikawa et al, 1987), but given the combination of the use of two different measurement techniques (TL multiple grain and OSL single grain) and the differences in instrumentation during the intervening 30 years, this represents a very good level of agreement for dosimetry based on measurements with quartz.…”

“…The average dose (depth range 2-22 mm) is 7% larger than the value adopted at this location in the DS02 model (measurements by Ichikawa et al, 1987), but given the combination of the use of two different measurement techniques (TL multiple grain and OSL single grain) and the differences in instrumentation during the intervening 30 years, this represents a very good level of agreement for dosimetry based on measurements with quartz. As noted by Kerr et al (2015), the significant difference between the end-point energies of beta emitters associated with the main neutron-activated products in soil (all less than 3 MeV; 28 Al, 56 Mn, 24 Na and 46 Sc) and in the brackish waters of the river system ( 38 Cl; 4.9 MeV) may provide the opportunity to establish the contribution of the latter in areas of the site affected by flooding by deconvolution of the beta particle depthdose profiles obtained using a slice and single grain measurement technique. Also, if building structures with sufficiently thick ceramic layers are available (i.e., >10 cm), and preferably samples from locations with comparable source geometries, there is the opportunity to test the current interpretation of the physical dosimetry that the average energy of gamma photons increased linearly with ground range, from 2.4 MeV at 500 m to 4 MeV at 2200 m at both Hiroshima and Nagasaki (Egbert et al, 2007).…”

“…Further examination of the differences between measured and calculated values of gamma dose was undertaken at two recent workshops (Kerr et al, 2013 andKerr et al, 2015). Notwithstanding the overall agreement obtained between the luminescence and calculated values of free-in-air kerma with ground distance over a range of three orders of magnitude of dose (Fig.…”

“…Cullings et al (2006) provide an informative overview of the key stages of the evolution of the dosimetry system (DS), and details of these are given in a series the published reports, including the initial tentative dosimetry system T65D (Milton and Shohoji, 1968), DS86 (Roesch, 1987) and the current system, DS02 (Young and Kerr, 2005). A series of interleaved workshops have served to review and develop agendas for further improvement of the dosimetry system (Kerr et al, 2015 andKerr et al, 2013).…”

“…On the one hand it can be argued (RERF, 2012) that within this ground range the health effects do not indicate a substantial underestimate of the dose and, close to the hypocenter, where ground activation was high, survivors could not gain access because of the physical destruction and conflagration. A differing view advanced, based on a multi-step cancer model (Kerr et al, 2015) is that a dose of ∼1-2 Gy arising from exposure to sources of residual radiation would be required to match the prediction of the model and this would be comparable to the initial radiation dose (DS02) at ∼500 m from the hypocenter (∼1350-1450 m at Hiroshima; ∼1450-1550 m at Nagasaki), making it a significant contribution at distances where many survivors were located, and thus representing an important departure in the currently adopted physical model for the dosimetry.…”