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"Investigation of the reaction 74Ge(p,gamma)75As
using the in-beam method to improve reaction network predictions
for p nuclei"
A. Sauerwein et al.
Physical Review C 86 (2012) 035802, direct download
Background: Astrophysical models studying the origin of
the neutron-deficient p nuclides require knowledge of
proton capture cross sections at low energy. The
production site of the p nuclei is still under discussion
but a firm basis of nuclear reaction rates is required
to address the astrophysical uncertainties. Data at
astrophysically relevant interaction energies are scarce.
Problems with the prediction of charged particle
capture cross sections at low energy were found in
the comparisons between previous data and calculations
in the Hauser-Feshbach statistical model of compound reactions.
Purpose: A measurement of 74Ge(p,gamma)75As at low proton
energies, inside the astrophysically relevant energy region,
is important in several respects. The reaction is directly
important because it is a bottleneck in the reaction flow
which produces the lightest p nucleus 74Se. It is also an
important addition to the data set required to test
reaction-rate predictions and to allow an improvement
in the global p+nucleus optical potential required in such calculations.
Method: An in-beam experiment was performed, making it
possible to measure in the range 2.1 to 3.7 MeV,
which is for the most part inside the astrophysically
relevant energy window. Angular distributions of
the gamma-ray transitions were measured with high-purity
germanium detectors at eight angles relative to the beam axis.
In addition to the total cross sections, partial cross
sections for the direct population of 12 levels were determined.
Results: The resulting cross sections were compared to
Hauser-Feshbach calculations using the code smaragd.
Only a constant renormalization factor of the calculated
proton widths allowed a good reproduction of both total
and partial cross sections. The accuracy of the calculation
made it possible to check the spin assignment of some states
in 75As. In the case of the 1075-keV state, a double state
with spins and parities of 3/2- and 5/2- is
needed to explain the experimental partial cross sections.
A change in parity from 5/2+ to 5/2- is required for
the state at 401 keV. Furthermore, in the case of 74Ge,
studying the combination of total and partial cross sections
made it possible to test the gamma width, which is essential
in the calculation of the astrophysical 74As(n,gamma)75As rate.
Conclusions: Between data and statistical model prediction
a factor of about two was found. Nevertheless, the improved
astrophysical reaction rate of 74Ge(p,gamma)
(and its reverse reaction) is only 28% larger
than the previous standard rate. The prediction
of the 74As(n,gamma)75As rate (and its reverse)
was confirmed, the newly calculated rate differs
only by a few percent from the previous prediction.
The in-beam method with high-efficiency detectors
proved to be a powerful tool for studies in nuclear
astrophysics and nuclear structure.