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

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.