Dosimetry System 2002 (DS02)

RERF has been conducting studies of long-term health effects on A-bomb survivors. To determine whether health effects are caused by radiation exposure, first we need to know the radiation dose to the various organs of each survivor. The dosimetry system provides us information on the dose. The radiation protection community throughout the world depends on the estimates of risk from exposure to radiation defined by the epidemiological studies conducted at RERF, and those estimates of risk depend on the accuracy with which the radiation doses are determined.

The first system to estimate individual radiation doses was a tentative dosimetry system called T57D, the establishment of which was announced in 1957. T57D was then modified and replaced with T65D. Both of these tentative methods for dose estimation were empirical systems based on nuclear measurements. As computers became available with sufficient power to calculate the neutron and gamma doses to the organs of survivors through the shielding provided by structures and the human body, DS86 was developed and implemented in 1986. That dosimetry system was used at RERF until recently. After DS86 began to be used, it was pointed out that some of the calculations did not match the neutron activation measured in samples of materials in Hiroshima, especially beyond 1.5 km from the hypocenter. In an effort to resolve this discrepancy, various studies were initiated both in the United States and Japan.

In December 2000, the U.S. and Japanese governments convened a meeting of experts to discuss a unified approach to resolve the neutron discrepancy. At that meeting it was decided to establish a Joint U.S.-Japan Working Group to reassess the A-bomb dosimetry. The working group was extremely active, holding eight meetings until January 2003, during which the Joint Working Group reached specific conclusions. Then, in January 2003, the U.S. and Japanese governments established a Joint Senior Review Group, composed of four members from each country (Japanese chair: Dr. Wataru Mori, U.S. chair: Dr. Warren K. Sinclair), which approved the findings and recommendations of the Joint Working Group. On 15 March 2003, DS02 was established as a new dosimetry system to replace DS86.

Comparison between DS86 and DS02 does reveal many small improvements. Generally speaking, however, DS86 estimates are not very different from DS02 estimates and have proved to be mostly correct. Due to recent advances in computer technology, DS02 allows more complicated and detailed calculations than DS86 did, as well as detailed simulations of the atomic bomb explosions and of the radiation’s release and dispersal. Many improvements have been made, including dose estimates, with consideration paid to the more detailed shielding conditions of individual survivors, which has resulted in greatly improved accuracy of the radiation dose estimates.

The discrepancies between neutron calculations and actual measurements beyond 1.5 km from the hypocenter in Hiroshima, the circumstance that triggered the review of DS86, have been resolved. The discrepancies between neutron calculations and activation measurements at less than 0.5 km from the hypocenter in Hiroshima were resolved when the burst height was raised by 20 m. Thanks to significant improvements in accuracy of the measurements, neutron measurements up to 1.5 km from the hypocenter closely correspond with DS02 calculations. Beyond 1.5 km from the hypocenter, measurements become too uncertain to be used to verify the radiation calculations. Because the calculations are carried out in exactly the same manner at all distances and those calculations now match the measurements at all distances where the measurements are considered reliable, we are confident that the DS02 calculations accurately compute the dose to survivors at all distances from the detonations. The differences between DS86 and DS02 are shown in the table below.