Annual dose assessment for luminescence and ESR dating: evaluation of nuclides mobility

The mobility of nuclides depends on their distribution within different sediment phases. Using a soil science approach, three main phases can be distinguished, which, according to increasing stability, are:

— The mobile phase, which includes elements in aqueous solution or elements weakly bonded at the surface of minerals.

— The phase that can be mobilised, in oxides, amorphous or weakly crystallised minerals. It can possibly move.

— The resistant phase, which cannot move.

The proportion of nuclides in the different phases, analysed together with other paedological data, can give information on the local stability of the radioactivity and possibly allow evaluation of error limits and corrections that should be applied to the present dose rate for dating purpose. In the worst cases this can lead to the conclusion that dating would not be reliable. Two case studies show the potential of the approach, which still remains more qualitative than quantitative. In an archaeological site (Ladoux, Puy de Dôme, France), streaming surface water brings radioactivity characterised by radium in excess. In volcanic tephra (La Vestide, Bas Vivarrais, France), nuclide mobility is found to be negligible; therefore observed disequilibrium in the U series pre-existed the eruption.