Shell model description of dipole strength at low energy and its impact on reaction rates

Radiative strength functions (RSF) and level densities (LD) are fundamental properties of the atomic nucleus, which govern the formation and decay of excited nuclei. They are basic inputs for evaluation of neutron capture cross sections, which are critically important to a breadth of scientific fields, for example, in astrophysical models of nucleosynthesis and stellar evolution [1,2]. The knowledge of neutron-capture cross-sections is also necessary to optimize the design of nuclear power reactors and determine the ideal parameters for the burn-up of the long-lived nuclear waste [3]. Direct measurements of neutron capture cross sections are however limited to stable nuclei and those with long half-lives. Thus most of the applications resort to large sets of theoretical reaction rates which are evaluated based on simple statistical assumptions, e.g. radiative strength functions are described by global parameterizations which can be adjusted to photodissociation data. However, such RSF miss important structure effects at low energy. In particular, a low-energy enhancement of the RSF was discovered recently in experiment [4]. With the enhancement established in stable nuclei [5], its nature and extent still remain unknown.

In this talk I will address the recent attempts of describing the low-energy enhancement of the radiative strength within the large-scale shell-model framework [6,7]. The results will be presented for both E1 and M1 components in medium mass nuclei and compared to experiment and other theoretical models. The difference between the radiative and photoabsorption strength obtained within the shell model will be also discussed. Finally, I will show the consequences of incorporating the low-energy behavior of the RSF established in the shell model to the corresponding QRPA description  and its impact on the radiative neutron capture.

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2. M. Arnauld, S. Goriely and K. Takahashi, Physics Reports 450, 97 (2007).
3. N. Colonna et al., Energy Envir. Sci. 3, 1910 (2010).
4. A. Voinov et al., Phys. Rev. Lett. 93, 142504 (2004).
5. http://www.mn.uio.no/fysikk/english/research/about/infrastructure/OCL/nuclear-physicsresearch/compilation/
6. K. Sieja, Phys. Rev. Lett.119, 052502 (2017).
7. K. Sieja, EPJ Web of Conferences 146, 05004 (2017).