Determination of neutron fluence using the plot of isotope variation of cadmium and gadolinium

The method described to determine the neutron fluence is based on the plot of the isotopic variation of Cd and Gd subjected to neutron irradiation in a research reactor. The isotopic ratios are measured by thermal ionization mass spectrometry. The results indicate that the fluence values obtained, using the variation in the ratios114Cd/113Cd,156Gd/155Gd and158Gd/157Gd show standard deviations varying from 0.3 to 6.6%. These values agree with the extrapolated values calculated using the short time Au activation method. The method appears to be useful for determining paleo neutron flux in natural samples and irradiated fuels.