  The synthesis of heavy, proton rich isotopes in the astrophysical
gamma-process proceeds through photodisintegration reactions. For the improved
understanding of the process, the rates of the involved nuclear reactions must
be known. The reaction 128Ba(g,a)124Xe was found to affect the abundance of the
p nucleus 124Xe. Since the stellar rate for this reaction cannot be determined
by a measurement directly, the aim of the present work was to measure the cross
section of the inverse 124Xe(a,g)128Ba reaction and to compare the results with
statistical model predictions. Of great importance is the fact that data below
the (a,n) threshold was obtained. Studying simultaneously the 124Xe(a,n)127Ba
reaction channel at higher energy allowed to further identify the source of a
discrepancy between data and prediction. The 124Xe + alpha cross sections were
measured with the activation method using a thin window 124Xe gas cell. The
studied energy range was between E = 11 and 15 MeV close above the
astrophysically relevant energy range. The obtained cross sections are compared
with statistical model calculations. The experimental cross sections are
smaller than standard predictions previously used in astrophysical
calculations. As dominating source of the difference, the theoretical alpha
width was identified. The experimental data suggest an alpha width lower by at
least a factor of 0.125 in the astrophysical energy range. An upper limit for
the 128Ba(g,a)124Xe stellar rate was inferred from our measurement. The impact
of this rate was studied in two different models for core-collapse supernova
explosions of 25 solar mass stars. A significant contribution to the 124Xe
abundance via this reaction path would only be possible when the rate was
increased above the previous standard value. Since the experimental data rule
this out, they also demonstrate the closure of this production path.
