Comparison and improvement of sediment chronology models using 210Pb and other natural and artificial radioisotopes

Abstract

The present work deals with the problems presented in the gamma spectroscopy when it is used as method to quantify the activity concentration of natural and man-made radionuclides in sediment samples and its use to determine the age of depth profiles. In the frame of this thesis, four sediment cores corresponding to three projects have been measured, modeled and interpreted using sediment models based on unsupported lead.

Measurements of radionuclides with low energy emission (<100keV) as 210Pb or 241Am via gamma spectrometry used for sediment dating present the problem of self-absorption within the bulk material. It supposes a challenge because eficiency calibrations need to be customized from sample to sample to avoid this problem. LabSOCS/ISOCS is a commercial software based on Monte Carlo method used to compute eficiency curves, however it use requires precise information about sample shape and also chemical composition of the sediment which in many cases is unknown.

The first part of this thesis deals with gamma spectrometry and the possible variation of the activity of 210Pb in case of unknown the chemical composition of the sediment and its consequence in the age model outputs. A new method described in chapter seven to calculate pseudo-chemical composition based on one transmission experiment using the multi-gamma emitter 155Eu has been development and published as original method.

The second part of this work deals with the sediment chronology applied to four sediment cores from lakes with high anthropogenic impact. Classical sediment models have been applied and compared its outputs. Additionally migration modeling of 137Cs has been done only in case of the lake Tegel in order to understand its behavior and contribute with extra information to the chronology. Chronologies have been complemented with historical information using time markers.