HIROSHIMA NEUTRON FLUENCE FROM GLASS THAT WAS NEAR GROUND ZERO

There is a continuing need for mapping of neutron doses at ground level in Hiroshima for August 6, 1945 – observed values being discrepant with the detailed calculations that are given in the report DS86. Neutron-fluence results are of basic importance for the setting of radiation standards for human exposure.

Fleischer noted in 1987 that ordinary silicate glass, which typically has about a part per million of uranium, could become a simple, retrospective dosimeter for neutrons by using the 235U(n, f) reaction. Recent attempts at locating materials to do such dosimetry have finally been successful, although most of the submitted glasses were either too distant from the explosion or were visibly altered in shape – the effect of heating that would have erased the bomb-produced tracks.

The first example was a decorative glass button that was uncovered at a location that was almost directly beneath the atomic explosion. In that case the free-air (i.e. outside) value is estimated as 1.5(+0.5) x 1012 neutrons-cm-2. The more recent advance has come by finding tracks in porcelain – the thin layer on the surface of which is also glass – a detector of nuclear fission tracks. We first used a reactor to irradiate several samples found near ground zero at Hiroshima for their uranium content, which turned out to be 4-7 wt ppm – an encouraging result that is significantly higher than is common for ordinary glass. So far, bomb induced tracks have been found in one of the samples leading us to a neutron fluence that will be presented.