Systematics of γ-Ray Strength Functions
Within the Shell Model
J. E. Midtbø, A. C. Larsen, T. Renstrøm, F. L. Bello Garrote, and E. Lima
The γ -ray strength function f (E γ ) plays an important role in many areas of
nuclear physics. Notably, it finds widespread use in reaction rate calculations for
nucleosynthesis networks [1]. In recent years, a topic of much debate has been
the behavior of the γ -ray strength function at the very lowest E γ energies, below
∼2 MeV. It has been observed experimentally for a large number of nuclei that
the strength function increases as E γ approaches zero (a complete list is given in
Ref. [2]). The presence of such a low-energy enhancement (LEE) can have a large
impact on neutron-capture rates [3].
In this contribution, we present our findings from a large-scale survey on the LEE
within the framework of the full-configuration shell model [2]. We have calculated
M1 strength functions for nuclei in two mass regions, comprising all nuclei in the
sd shell and isotopes of Ni, Cu, Ga, Ge, As, and Se atop a 56 Ni core. We use the
massively parallel shell-model code KSHELL [4] and calculate hundreds of energy
levels of many spins and (when available) both parities for each nucleus, as well as
all allowed M1 transitions between the levels. We then compile a level density and
extract the M1 γ -ray strength function.
A plot of the calculated relative steepness for each nuclide is shown in Fig. 1.
The calculations reveal systematic trends:
1. The LEE is steeper near shell closures.
2. It is generally steeper in the fpg shell than the sd shell, and seems to steepen
with mass number. This can also be interpreted as an increase as a function of
the availability of high- orbitals, as suggested in Ref. [5].
3. For both calculated regions, the low-energy enhancement is steeper on the
neutron-rich side than on the proton-rich side. We interpret this as a preference
J. E. Midtbø () · A. C. Larsen · T. Renstrøm · F. L. Bello Garrote · E. Lima
Department of Physics, University of Oslo, Oslo, Norway
e-mail: j.e.midtbo@fys.uio.no
© This is a U.S. government work and not under copyright protection
in the U.S.; foreign copyright protection may apply 2021
J. Escher et al. (eds.), Compound-Nuclear Reactions, Springer Proceedings in
Physics 254, https://doi.org/10.1007/978-3-030-58082-7_24
203
Within the Shell Model
J. E. Midtbø, A. C. Larsen, T. Renstrøm, F. L. Bello Garrote, and E. Lima
The γ -ray strength function f (E γ ) plays an important role in many areas of
nuclear physics. Notably, it finds widespread use in reaction rate calculations for
nucleosynthesis networks [1]. In recent years, a topic of much debate has been
the behavior of the γ -ray strength function at the very lowest E γ energies, below
∼2 MeV. It has been observed experimentally for a large number of nuclei that
the strength function increases as E γ approaches zero (a complete list is given in
Ref. [2]). The presence of such a low-energy enhancement (LEE) can have a large
impact on neutron-capture rates [3].
In this contribution, we present our findings from a large-scale survey on the LEE
within the framework of the full-configuration shell model [2]. We have calculated
M1 strength functions for nuclei in two mass regions, comprising all nuclei in the
sd shell and isotopes of Ni, Cu, Ga, Ge, As, and Se atop a 56 Ni core. We use the
massively parallel shell-model code KSHELL [4] and calculate hundreds of energy
levels of many spins and (when available) both parities for each nucleus, as well as
all allowed M1 transitions between the levels. We then compile a level density and
extract the M1 γ -ray strength function.
A plot of the calculated relative steepness for each nuclide is shown in Fig. 1.
The calculations reveal systematic trends:
1. The LEE is steeper near shell closures.
2. It is generally steeper in the fpg shell than the sd shell, and seems to steepen
with mass number. This can also be interpreted as an increase as a function of
the availability of high- orbitals, as suggested in Ref. [5].
3. For both calculated regions, the low-energy enhancement is steeper on the
neutron-rich side than on the proton-rich side. We interpret this as a preference
J. E. Midtbø () · A. C. Larsen · T. Renstrøm · F. L. Bello Garrote · E. Lima
Department of Physics, University of Oslo, Oslo, Norway
e-mail: j.e.midtbo@fys.uio.no
© This is a U.S. government work and not under copyright protection
in the U.S.; foreign copyright protection may apply 2021
J. Escher et al. (eds.), Compound-Nuclear Reactions, Springer Proceedings in
Physics 254, https://doi.org/10.1007/978-3-030-58082-7_24
203