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 wellseparated 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.Pc Single-particle levels and strength functions -24.10.Lx Monte Carlo simulations (including hadron and parton cascades and string breaking models) -25.20.Dc Photon absorption and scattering -26.50.+x Nuclear physics aspects of novae, supernovae, and other explosive environments 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 [6,7,8,9] of the E1-strength on the lowenergy slope of the GDR; a e-mail: e.grosse@fz-rossendorf.de b) the E1-strength between the ground-state (gs) and low energy excitations and its proper extension [10,11,12] 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...

Relativistic quasiparticle random phase approximation in deformed nuclei

“…The experimental data points show two enhancements at around 9 MeV and 10.5 MeV, in good agreement with our results, which predict a K π = 1 − mode peak at 8.9 MeV and a double K π = 0 − mode peak at 10.5 MeV. In addition to these, Figure 7.10 shows another theoretical K π = 0 − peak at 7.4 MeV, which can be linked to the 7 MeV enhancement found in the results from reference [134]. In that same study, another peak was found at 6.5 MeV with strong dipole strength.…”

“…The red and blue curves correspond to the K π = 0 − and K π = 1 − excitation modes, respectively. The experimental data was taken from references [14,134]. Interestingly enough, at a first glance to Figure 7.9 the NL3 parameter set seems to perform better, in agreement with the Lorentzian fits parameters.…”

“…The dipole strength close to separation energies in several Mo isotopes was measured in a recent experiment [134]. We shall concentrate in 100 Mo, as it was the only deformed nucleus which was studied.…”

“…The black data points with error bars correspond to the experimental results of references [14,134]. The experimental centroid energy is situated at 18 MeV.…”

“…Figure 7.10 is a detailed plot of the E1 strength as calculated with DD-ME2 parameter set in the energy interval 6-12 MeV. The plotted experimental data up to 8 MeV is from reference [134], while for higher energies, above the neutron separation threshold, it was taken from reference [14]. The experimental data points show two enhancements at around 9 MeV and 10.5 MeV, in good agreement with our results, which predict a K π = 1 − mode peak at 8.9 MeV and a double K π = 0 − mode peak at 10.5 MeV.…”