Photon Strength Functions of Heavy Nuclei: Achievements and Open Problems

Abstract

Experimental studies of γ decay of highly excited levels in nuclei started as early as 60 years ago. Soon, it turned out that this decay is strongly dominated by E1 transitions and that it is closely related to the giant electric dipole resonances built not only on the ground state, but also on each excited level, including the levels in the quasicontinuum. These findings brought the first evidence for the electric dipole vibrations coupled to the excited levels and demonstrated viability of Brink’s concept. Nevertheless, with growing information on γ rays following neutron capture it emerged that extrapolation of the photonuclear cross sections to the energy region below the neutron separation energy leads in some cases to overpredictions of the E1 photon strength. Several theories were developed, but this disproportion has not yet been satisfactorily accounted for. New information deduced from the data on two‐step γ cascades following the thermal neutron capture and from the data yielded by γ‐calorimetric (n. γ) measurements at isolated neutron resonances seems promising for further studies of the E1 photon strength functions. The paper is focused on two issues: on the observed deficit of photon strength in transitional nuclei at γ‐ray energies above 5 MeV and on the recently observed strong enhancement of the photon strength in nuclei near mass number A≈100 at low energies. The main problems to be solved are formulated.