  Extremely metal-poor stars are keys to understand the early evolution of our
Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of
metal-poor turn-off stars, whereas most of the previously known extremely
metal-poor stars are giant stars. Sixty five turn-off stars (preselected from
SDSS spectra) have been observed with the X-Shooter spectrograph at the ESO VLT
Unit Telescope 2, to derive accurate and detailed abundances of magnesium,
silicon, calcium, iron, strontium and barium. We analysed medium-resolution
spectra (R ~ 10 000) obtained with the ESO X-Shooter spectrograph and computed
the abundances of several alpha and neutron-capture elements using standard
one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres. Our
results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor
turn-off stars as observed in metal-poor giant stars. We found a significant
spread of the [alpha/Fe] ratios with several stars showing sub-solar [Ca/Fe]
ratios. We could measure the abundance of strontium in 12 stars of the sample,
leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium
in two stars of the sample. One of the stars (SDSS J114424-004658) shows both
very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex).
