  Although $^{79}$Se represents an important branching in the weak s process,
the stellar neutron capture cross sections to this isotope have not yet been
measured experimentally. In this case, experimental data is essential for
evaluating the important branching in the s-process reaction path at $^{79}$Se.
The total cross section of $^{78}$Se at a stellar energy of kT = 25 keV has
been investigated with a combination of the activation technique and
accelerator mass spectrometry (AMS), since offline decay counting is
prohibitive due to the long terrestrial half life of $^{79}$Se (2.80$\pm$0.36
$\times10^5$ y) as well as the absence of suitable $\gamma$-ray transitions.
The preliminary result for the total Maxwellian averaged cross section is
$<\sigma>_{30 keV}$= 60.1$\pm$9.6 mbarn, significantly lower than the previous
recommended value. In a second measurement, also the partial cross section to
the 3.92 min-isomer was determined via $\gamma$-spectroscopy and yielded
$<\sigma>_{30 keV}$(part.)= 42.0$\pm$2.0 mbarn.
