Oxygen stable isotope measurements on a gravestone exposed to the Hiroshima A-bomb explosion and the “Dosimetry System 1986”

The “Dosimetry System 1986”, DS86, established theoretical neutron spectra and fluences of the atomic-bomb explosions in Hiroshima and Nagasaki correlated with the radiation disease rate of survivors. The actual neutron fluence and spectrum from the Hiroshima explosion can be derived by use of different radioisotopes produced by neutron capture reactions due to different neutron capture cross-sections.

In a gravestone 107 m from the hypocentre of the explosion in Hiroshima, the radioisotopes³⁶Cl,⁴¹Ca,⁶⁰Co,¹⁵²Eu and¹⁵⁴Eu were already measured. From this a neutron spectrum can be derived which is much harder than the neutron spectrum used in DS86 provided the gravestone represents a closed system. To check this, petrographic and oxygen stable isotope investigations were performed on the gravestone as well as on a not-irradiated sample coming from the same quarry as the gravestone. The results of the petrographic and geochemical studies both negate the possibility of major isotopic exchange with rainwater.