Physics Letters B 764 (2017) 109–113
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Radiative neutron capture: Hauser Feshbach vs. statistical resonances
D. Rochman
a,∗
, S. Goriely
b
, A.J. Koning
c,d
, H. Ferroukhi
a
a
Reactor Physics and Systems Behavior Laboratory, Paul Scherrer Institute, Villigen, Switzerland
b
Institut d’Astronomie et d’Astrophysique, CP-226, Université Libre de Bruxelles, 1050 Brussels, Belgium
c
Nuclear Data Section, IAEA, Vienna, Austria
d
Uppsala University, Uppsala, Sweden
a r t i c l e i n f o a b s t r a c t
Article history:
Received
6 September 2016
Received
in revised form 2 November 2016
Accepted
10 November 2016
Available
online 16 November 2016
Editor:
W. Haxton
Keywords:
Statistical
Hauser–Feshbach model
Radiative
neutron captures
Nuclear
astrophysics
The radiative neutron capture rates for isotopes of astrophysical interest are commonly calculated on
the basis of the statistical Hauser Feshbach (HF) reaction model, leading to smooth and monotonically
varying temperature-dependent Maxwellian-averaged cross sections (MACS). The HF approximation is
known to be valid if the number of resonances in the compound system is relatively high. However, such
a condition is hardly fulfilled for keV neutrons captured on light or exotic neutron-rich nuclei. For this
reason, a different procedure is proposed here, based on the generation of statistical resonances. This
novel technique, called the “High Fidelity Resonance” (HFR) method is shown to provide similar results
as the HF approach for nuclei with a high level density but to deviate and be more realistic than HF
predictions for light and neutron-rich nuclei or at relatively low sub-keV energies. The MACS derived
with the HFR method are systematically compared with the traditional HF calculations for some 3300
neutron-rich nuclei and shown to give rise to significantly larger predictions with respect to the HF
approach at energies of astrophysical relevance. For this reason, the HF approach should not be applied
to light or neutron-rich nuclei. The Doppler broadening of the generated resonances is also studied and
found to have a negligible impact on the calculated MACS.
© 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Nuclear reactions of astrophysical interest often concern unsta-
ble
or even exotic species for which no experimental data exist.
Although significant efforts have been devoted in the past decades,
experimental information only covers a minute fraction of the en-
tire
data set required for nuclear astrophysics. Moreover, the en-
ergy
range for which experimental data is available is restricted
to the small range that can be studied by present experimen-
tal
setups. For all unknown cases, only theoretical predictions can
fill the gaps. One of these specific examples concerns the rapid
neutron-capture process (r-process) called for to explain the origin
of about half of the elements heavier than iron observed in na-
ture
(for a review, see [1]). The r-process is believed to take place
in environments characterized by high neutron densities, such that
successive neutron captures can proceed into neutron-rich regions
well off the β-stability valley. It involves a large number (typically
five thousands) of unstable nuclei for which many different proper-
*
Corresponding author.
E-mail
address: dimitri-alexandre.rochman@psi.ch (D. Rochman).
ties have to be determined and cannot be obtained experimentally.
One of such fundamental properties concern the radiative neutron
capture reaction.
The
radiative neutron capture is traditionally estimated within
the statistical Hauser–Feshbach model [2,3]. The model makes the
fundamental assumption that the capture process takes place with
the intermediary formation of a compound nucleus (CN) in ther-
modynamic
equilibrium. The energy of the incident particle is
then shared more or less uniformly by all the nucleons before re-
leasing
the energy by particle emission or γ -de-excitation. In the
Hauser–Feshbach approach, the formation of a CN is usually justi-
fied
by assuming that the nuclear level density (NLD) in the CN at
the projectile incident energy is large enough to ensure an aver-
age
statistical continuum superposition of available resonances [4].
For medium- and heavy-mass nuclei lying within the valley of
β-stability, the CN capture at energies of astrophysical interest is
known to be the dominant reaction mechanism [1,3,4].
Three
main ingredients of the CN determine the characteristics
of the capture rates in the HF model: the NLD, the particle optical
models and the γ -strength function. For low energy neutrons, in
the keV region, the most important HF ingredients remain the NLD
and the γ -strength function, since the radiative cross section is not
http://dx.doi.org/10.1016/j.physletb.2016.11.018
0370-2693/
© 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.