Article,
Investigation of the reaction Ge-74(p,gamma)As-75 using the in-beam method to improve reaction network predictions for p nuclei
Phys. Rev. C, (September 2012)
DOI: 10.1103/PhysRevC.86.035802
Abstract
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 Ge-74(p,gamma)As-75 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
Se-74. 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 <= E-p <= 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 As-75.
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 Ge-74, 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 As-74(n,gamma)As-75 rate.
Conclusions: Between data and statistical model prediction a factor of
about two was found. Nevertheless, the improved astrophysical reaction
rate of Ge-74(p,gamma) (and its reverse reaction) is only 28\% larger
than the previous standard rate. The prediction of the
As-74(n,gamma)As-75 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.
