Both 236U and 239Pu are produced naturally in uranium ores via neutron capture on 235U and 238U, respectively. Neutrons in the Earth's crust are produced almost entirely from (α,n)-reactions and spontaneous fission of 238U. Neutron fluxes however, depend not only on the number of neutrons produced but also on the elemental composition of the rock. Therefore, concentrations of 236U and 239Pu in uranium ores is characteristic for different grades and compositions.
Only a limited set of data is currently available for these nucleogenic isotopes. In the case of 239Pu, these data have been obtained either with α-spectroscopy or in more modern works with thermal ionization mass spectroscopy. The benefits of AMS measurements are obvious; smaller samples, shorter measurement time and better discrimination against molecular interferences. We are in the process of performing AMS measurements on naturally produced 236U and 239Pu in a wide variety of uranium ores.
An essential adjunct to these measurements has been the development of the sample preparation protocol. This ensures both efficient extration of the 239Pu and 236U from the ore and chemical equilibrium between the 239Pu in the ore and the typically 4pg of 242Pu that is added for normalisation.
First results for both 236U and 239Pu will be presented for selected ore samples. In parallel to this measurement program, a model is being developed to calculate 236U and 239Pu concentrations for given ore compositions. The essential features of this model will be described. In addition, the possibility of using the combination of 236U and 239Pu as a neutron flux monitor to validate the nucleogenic 36Cl production calculations, which can be a source of significant error in young exposure ages, will be discussed.
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