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

Abstract

Abstract. The distribution of electromagnetic dipole strength in 92, 98, 100 Mo has been investigated by photon scattering using bremsstrahlung from the new ELBE facility. The experimental data for well separated nuclear resonances indicate a transition from a regular to a chaotic behaviour above 4 MeV of excitation energy. As the strength distributions follow a Porter-Thomas distribution much of the dipole strength is found in weak and in unresolved resonances appearing as fluctuating cross section. An analysis of this quasi-continuum -here applied to nuclear resonance fluorescence in a novel way -delivers dipole strength functions, which are combining smoothly to those obtained from (γ, n)-data. Enhancements at 6.5 MeV and at ∼ 9 MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei.PACS. 21.10. .+k Dipole strength in heavy nucleiThe response of nuclei to dipole radiation is of special importance for the synthesis of the chemical elements in the cosmos: Particle thresholds may be crossed in hot or explosive scenarios leading to the production of new nuclides from previously formed heavier ones by dissociation in the thermal photon bath. This is likely to be the main path for the generation of the approximately 30 -40 neutrondeficient nuclides which cannot be produced in neutron capture reactions [1]. For the understanding and modelling of this so-called p-process the dipole strength function up to and near the particle thresholds has to be known accurately [2]. As shown previously [3], details of the dipole strength (now in n-rich nuclei) may as well have large consequences for the r-process path and also s-process branchings are influenced by nuclear excitations [4] induced by thermal photons. The experimental knowledge [5] on dipole strength is reasonably well established for many heavy and medium mass nuclei in the region of the giant dipole resonance (GDR) by (γ, xn) studies, which often also cover the region directly above the neutron threshold S n . At lower energies three features have been discussed to be of importance for processes in high-temperature cosmic environments: a) the fall-off [6-9] of the E1-strength on the low-energy slope of the GDR; b) the E1-strength between the ground-state (gs) and low energy excitations and its proper extension [10-12] into the regime (a); c) the occurrence of additional pygmy-resonances, [3,[13][14][15], which are assumed to be not as broad as the GDR, but wider as compared to the average level distance D -thus forming an intermediate structure enclosing many levels. Their low energy may well enhance their contribution to photo-dissociation processes in spite of their relatively low strength as compared to the GDR. In principle, also M1-transitions contribute to the dipole strength, but the average M1 strength is typically 1-2 orders of magnitude smaller as compared to E1, and they are frequently [1-3] not taken into account. A further approximation has to be introduced to estimate dipole strengths for transitions not connected to the g...

Exotic modes of excitation in atomic nuclei far from stability

“…In addition, photon scattering with high intensity beams at energies below and above the neutron separation threshold are planned at the superconducting electron accelerator ELBE [162,163]. The first studies at ELBE include photon-scattering on 92,98,100 Mo [164]. It is interesting to note that the data show an enhancement of the dipole transition strength around 9 MeV: in 92 Mo the pygmy strength is located below the neutron separation energy, whereas in 100 Mo it shifts above the neutron threshold.…”