Hiroshima neutron fluence on a glass button from near ground zero

A decorative glass button that was uncovered at a location that is 190 +/- 15 m from directly beneath the atomic explosion at Hiroshima on 6 August 19 45 has been scanned for,induced fission tracks produced mostly by the therm al neutrons from the bomb due to interactions with the trace uranium that i s normally present in silicate glasses. In surveying 4.14 cm(2) at 500 X ma gnification, 28 tracks were seen. From a calibration irradiation in a nucle ar reactor we infer that the neutron fluence in 1945 was 5.7(+/-1.1) X 10(1 1) cm(-2); and, allowing for shielding by the structure in which the button was probably located, the free-air (i.e., outside) value is estimated as 1 .5(+/-0.5) x 10(12) cm(-2). A limit has been placed on possible fading of t he radiation-damage tracks that could increase the fluence by at most a fac tor of 1.27. The values bracket the calculated value of 9 X 10(11) given in DS86 but are higher than the 3.6 X 10(11) inferred from induced radionucli des for the distance given. The difference is, however, within the observed variability of the two types of results.