Gamma absorption and decay properties of atomic nuclei are of crucial importance in fundamental and applied nuclear physics research. They give information on the nuclear structure and are indispensable for cross-section calculations for a broad range of applications, such as next-generation nuclear reactors and for the description of the nucleosynthesis in explosive stellar environments.

One of the basic models used to calculate the electromagnetic strength distributions for levels at high excitation energy is based on the Brink-Axel hypothesis. This assumes that distribution for highly excited state is similar to that observed for the ground state. The magnetic dipole (M1) response on the ground state is dominated by a spin-flip resonance centered near 7-9 MeV. However, this contradicts with recent experiments that observe a low-energy enhancement in the M1 strength in ⁵⁶Fe and other nuclei.

With NuShellX we are able to calculate the complete gamma decay scheme for levels up to about 8 MeV in excitation in the pf shell model space. From these calculations we find a low-energy enhancement in the M1 strength function that qualitatively agrees with experiment. [1]. The figure shows the predicted gamma rays detected in the experiment based upon the theoretical gamma decay scheme for about 500 levels. The M1 distribution function shown in the lower-right hand corner shows the expected spin-flip resonance around 7 MeV. But also shows an increasing strength for low-energy M1 transitions.

A similar result was obtained in calculations for Gamow-Teller strength at high excitation energy in the sd shell [2]. The purpose for these calculations was to provide more realistic models for the electron capture in hot stellar environments where these highly excited states are thermally populated. Fig. 2 in [2] shows the Gamow-Teller strength function calculated as a function of the excitation energy of the initial state. At low excitation it is dominated by spin-flip states centered at 6-8 MeV. At higher excitation energy a lower component centered around zero energy also appears.

For M1 and GT these low-energy components are due to the diagonal single-particle matrix elements (e.g. f_7/2-f_7/2) that become involved only at high excitation. Both ground and excitation states also contain an off-diagonal spin-flip component (e.g. f_7/2-f_5/2). We propose a modified Brink-Axel model in which the M1 and GT distributions are at relatively high excitation energy are rather independent of spin and excitation and have two components, in contrast to the distribution on the ground and low-lying states that are dominated by only the spin-flip component.

References

[1] Large Low-Energy M1 Strength for ^56,57Fe Within the Nuclear Shell Model, B. A. Brown and R. C. Larsen, Phys. Rev. Lett. 113, 252502 (2014). [link to paper].

[2] Modification of the Brink-Axel Hypothesis for High Temperature Nuclear Weak Interactions, G. W. Misch, G. M. Fuller, and B. A. Brown, Phys. Rev. C 90, 065808 (2014). [link to paper].