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 \times 10 ^ { 5 } y ( ( 1 ) ) ) 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 .