Mar. 27, 2002 Scientists are still studying the after-effects of the nuclear disaster caused by an atomic bomb dropped on Hiroshima on August 6, 1945. The amount of neutron fluence, for example, has been calculated from the site to establish what might be a safe level of exposure for humans. (Neutron fluence is the number of neutrons from the blast found per square centimeter.) The advent of nuclear accidents such as Three Mile Island and potential nuclear accidents in the future makes accurate information imperative. But scientists have recently realized that there are discrepancies in earlier estimates and have been looking for ways to resolve them. According to the latest research, the standards for a safe level of exposure to humans might be too conservative.
Analyzing the neutron-induced fission tracks on certain objects found in Hiroshima enables scientists to calibrate the nuclear fluence. Geologist Robert L. Fleischer (Union College in New York), S. Fujita (Radiation Effects Research Institute in Japan), and M. Hoshi (Research Institute for Nuclear Medicine and Biology in Japan) have published their findings on a glass button found near ground zero in Hiroshima in the Health Physics Society Journal.
More recently, Union College geology student Jonathan MacDonald joined the team to determine the track density of fission tracks in porcelain fragments that were also found near ground zero. This study complements the previous work.
“At face value, these results are fluence values from a specific bomb event that will hopefully never be used again. These fission events occurred when thermal neutrons from the bomb caused fission in the normal trace amounts of uranium in the material,” MacDonald explained. “We determined the track density on the exterior glaze (glass), on the outside of one porcelain fragment and in a cross section of the glaze in another.”
While the value of the one sample nearly matches the previous estimates, the other one is 2 to 2.5 times higher. MacDonald will report the new values at the Geological Society of America’s North Eastern Section Meeting on March 25.
“If correct, the latter value may be important for regulation of human exposures in the U.S., because many of the current regulations are based on theoretical calculated values,” MacDonald said. “If this latter dose is correct—being higher than what is currently assumed—then the current standards should be less restrictive.”
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