Cascadeγdecay study of108Agfollowing thermal and resonance neutron capture in<

Abstract

With the aim to obtain information on the E1 and M 1 photon strength functions at ␥-ray energies below the neutron separation energy, we studied two-step ␥ cascades following the capture of thermal neutrons in 107 Ag. For this purpose, we undertook an experiment with the dedicated facility for two-step ␥ cascades at the Ř ež research reactor. The obtained data were discussed in conjunction with previous results from resonance neutron capture measurements with the same isotope, obtained at the GELINA facility of the Institute for Reference Materials and Measurements. The cascade ␥ decay of the 108 Ag compound nucleus has been simulated with the aid of the Monte Carlo algorithm DICEBOX assuming several models for photon strength functions. To interpret the results of the experiments, the outcome from these simulations was confronted with the observed cascade-related quantities. The results indicate that the E1 photon strength function below the neutron binding energy is suppressed with respect to the conventional Brink-Axel model and that the M 1 and/or possibly E2 photon strengths may play an important role in the decay of compound nucleus at excitations below Ϸ3 MeV.

Photon Strength Functions at the Low-Energy Tail of GEDR

“…In TSC experiment, the γ-ray spectra belonging to all TSCs connecting the initial level arising from thermal neutron capture with preselected low-lying levels in the product nuclei are accumulated using two Ge detectors, see e.g. [14,15]. The method does not allow direct determination of f (XL) but it requires comparison of experimental spectra with results of simulations based on different assumptions on f (XL) and level density.…”

Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

“…Information about energy-dependent dipole strength functions can be obtained from data on multi-step gammadecays following n-capture [17] or direct reactions [12], from inelastic electron scattering [18] or elastic photon scattering [14,15,19]. For the region above the n-threshold the electromagnetic strength in a very large number of stable nuclei has been experimentally determined by observing the neutrons emitted after the excitation by quasimonochromatic photons [5,20].…”

Nuclear dipole strength in the tail of the giant dipole resonance (GDR)

“…The analysis of the data is performed by carefully accounting for non-nuclear contributions [26] to the scattering and by extended investigations on the influence on inelastic scattering [22,23]. As will be shown in the following, this hitherto unexplored method yields a strength function at clear variance to previous work [12,13].…”

“…Previous attempts to experimentally derive the strength down to energies far below the GDR have suffered from serious deficiencies: (a) in (n, ) studies [12][13][14] only a narrow region is reached in the 1 st step of the de-excitation cascade; (b) photon scattering experiments [15] neglecting the continuum part of the spectra can deliver correct strength information only up to limited excitation energy [16]; (c) experiments performed with He-ions [17] populate a diffuse distribution in spin. To properly assess observations [15,18,19] on additional 'pygmy' structures in photo-absorption data a proper knowledge of the underlying smooth strength function is urgently needed.…”