Gamma Strength Functions and the
Brink-Axel Hypothesis
Peter von Neumann-Cosel
1 Gamma Strength Function
The GSF describes the average γ decay behavior of a nucleus. It depends on the
level densities at the initial and final energies. In general all multipoles allowed
for electromagnetic processes contribute but in practice E1 dominates. Thus, the
isovector giant dipole resonance (IVGDR) dominates the GSF at higher excitation
energies as indicated on the r.h.s. of Fig. 1. At lower energies M1 contributes to the
total GSF (although a few % only under most conditions).
As indicated in the scheme of decay and absorption in Fig. 1, for the special case
of γ decay to the g.s. the GSF can be related to the photoabsorption cross section
by the principle of detailed balance
f
E1 (E γ , J ) =
2J 0 + 1
2J + 1
1
(π ¯
hc) 2 E 3
γ
σ abs
(1)
where J, J 0 are the spins of excited and ground state, respectively, and for simplicity
the relation is written for the E1 component only. The brackets indicate averaging
over an energy interval.
P. von Neumann-Cosel ()
Institut für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany
e-mail: vnc@ikp.tu-darmstadt.de
© 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_18
155