Workshop on Photon Strength Functions and Related Topics

Gamma-ray emission following neutron capture in medium-weight and heavy nuclei has been a subject of continuing interest for more than five decades. Although copious experimental data have been accumulated and many attempts to describe this phenomenon in terms of related theoretical models have been undertaken, a number of important issues still remain unsolved or even untouched. Within the statistical approach the quantities of central importance for understanding the γ emission are photon strength functions (PSFs). There are two reasons why these entities deserve deeper studies:

First, in line with Brink’s hypothesis, the PSFs carry invaluable information about various vibrational modes coupled to the excited nuclear levels, including the levels in the quasicontinuum. Thus the shape and size of the tail of the giant electric dipole resonances built on excited levels can be subject to examination at energies below the neutron threshold. An example of the usefulness of studying the PSFs is the assessment that scissors-mode vibrations of deformed nuclei are not a property of only their ground states, as seen from nuclear resonance fluorescence (NRF) experiments, but all their excited levels. It implies that the interaction responsible for the scissors-mode vibrations is a fundamental attribute of deformed neutron and proton fluids. In general, the physics behind PSFs is closely or complementarily related to the widely-studied phenomenon of NRF.

Second, the knowledge of PSFs is a crucial prerequisite for rigorous processing of raw data yielded by the state-of-the-art, high-granularity γ-calorimetric systems that have been installed at the large-scale neutron time-of-flight facilities such as DANCE at Los Alamos and n_TOF at CERN. Detailed understanding of these complex detector systems is essential, because they are of primary importance for precise measurements of neutron capture cross sections for next-generation nuclear technologies, as well as for the present needs of the increasing nuclear astrophysics community.

Understanding these systems and understanding PSFs are interconnected. Indeed, the multi-parametric data accumulated from the γ calorimeters carry unique information on behavior of γ cascades accompanying the neutron capture. These systems make possible to learn more about PSFs, in particular in the region of low γ-ray energies below 4 MeV where the available data are very scarce. These new capabilities present significant future opportunities. Two promising techniques bring additional physical information in this general idea: the technique of two-step γ cascades following thermal neutron capture and the analysis of first-generation γ-ray spectra from 3He-induced γ emission.

The main mission of the Workshop is to bring together specialists from various fields of research related directly or indirectly to PSFs, to assess, exchange and share our most recent developments and formulate joint tasks aimed towards better understanding the physics of γ decay of nuclear levels below the neutron threshold.

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