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