Geospatial
Analysis of 137Cs in Hiroshima Soil Cores Collected in 1976 and 1978
Harry
M. Cullings
Statistics Department, Radiation Effects Research
Foundation
Abstract
In 1976 and 1978 soil
samples were collected at a large number of locations in the area
of Hiroshima at distances up to 30 km from the hypocenter, to measure
radioisotopes such as 137Cs for the purpose of detecting local radioactive
fallout from the Hiroshima atomic bomb. The lack of any obvious
pattern in these measurements that might correspond to fallout from
the Hiroshima bomb has been a source of scientific curiosity over
the years. Much has been learned in the past 35 years about the
behavior of such radioactive materials in the environment and the
interferences caused by worldwide ("global") deposition
of radioactive fallout from testing of nuclear weapons in the 1950s
and 1960s. However, the analysis of the data from these soil samples,
to detect any pattern from the Hiroshima bomb, remains a statistically
and analytically challenging problem. We know that the 137Cs in
soil from global fallout is large, and it is more recent than any
that could remain from the Hiroshima bomb in 1945. Because there
are so many variables that affect the retention and migration of
the deposited radioactivity in soil, which we presently cannot include
in a statistical model, there is a large variation in the measurements
from place to place, and it has a spatial structure. To be valid
a statistical analysis must consider the resulting "spatial
autocorrelation" in the data.
Although an analysis of the data including a careful estimation
of the variable effect of long-term rainfall on local deposition
of global fallout does not suggest a pattern consistent with local
radioactive fallout from the Hiroshima bomb, an important question
that remains is how large such a deposition from the Hiroshima bomb
would have to have been to be evident in these data. Before an analysis
could begin to address the question of how large a deposition of
local fallout in 1945 would be detectable in these samples with
the interference of global fallout, there is a need to better understand
the variables related to soil, terrain and weather that affected
the deposition and migration in soil of any radioactivity deposited
on the soil surface, and the extent, if any, to which their effect
over time on the fallout from the Hiroshima atomic bomb may have
been different from their effect on global fallout.
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