Decay of excited nuclei produced inKr78,82+

Abstract

Decay modes of excited nuclei are investigated in 78,82 Kr+ 40 Ca reactions at 5.5 MeV/nucleon. Charged products were measured by means of the 4π INDRA array. Kinetic-energy spectra and angular distributions of fragments with atomic number 3 Z 28 indicate a high degree of relaxation and are compatible with a fissionlike phenomenon. Persistence of structure effects is evidenced from elemental cross sections (σ Z ) as well as a strong odd-even staggering (o-e-s) of the light-fragment yields. The magnitude of the staggering does not significantly depend on the neutron content of the emitting system. Fragment-particle coincidences suggest that the light partners in very asymmetric fission are emitted either cold or at excitation energies below the particle emission thresholds. The evaporation residue cross section of the 78 Kr+ 40 Ca reaction is slightly higher than the one measured in the 82 Kr+ 40 Ca reaction. The fissionlike component is larger by ∼25% for the reaction having the lowest neutron-to-proton ratio. These experimental features are confronted to the predictions of theoretical models. The Hauser-Feshbach approach including the emission of fragments up to Z = 14 in their ground states as well as excited states does not account for the main features of σ Z . For both reactions, the transition-state formalism reasonably reproduces the Z distribution of the fragments with charge 12 Z 28. However, this model strongly overestimates the light-fragment cross sections and does not explain the o-e-s of the yields for 6 Z 10. The shape of the whole Z distribution and the o-e-s of the light-fragment yields are satisfactorily reproduced within the dinuclear system framework which treats the competition among evaporation, fusion-fission, and quasifission processes. The model suggests that heavy fragments come mainly from quasifission while light fragments are predominantly populated by fusion. An underestimation of the cross sections for 16 Z 22 could signal a mechanism in addition to the capture process.

Methods for a quantitative evaluation of odd-even staggering effects

“…Recently, staggering effects have been observed also in heavy ion collisions from few MeV/nucleon up to Fermi energies (15 ≤E/A≤ 50 MeV/nucleon) [22,23,24,25,26,27,28,29,30]. These experiments have stirred renewed interest in staggering phenomena.…”

“…The staggering has been usually ascribed to nuclear structure effects that manifest in the decay of the excited reaction products or already in the reaction mechanism, if part of the reaction proceeds through low excitation energies [26]. However, in collisions at intermediate energies the preferred interpretation is that the odd-even staggering effect depends on the structure of the nuclei produced near the end of the evaporation chain [19,25].…”

Isospin effects and symmetry energy studies with INDRA

“…In this line an experiment was performed with INDRA to study the 78,82 Kr+ 40 Ca reactions at 5.5 A MeV [63]. At this low incident energy, around the Coulomb barrier, medium-mass compound nuclei are formed in a controlled way in terms of excitation energy and angular momentum.…”

Persistence of odd-even staggering in charged-fragment yields from112Sn+58Ni collisions at 35 MeV/nucleon

“…They may manifest themselves just in the reaction mechanism, if part of the reaction proceeds through low excitation energies [8]. However, in collisions at intermediate energies the preferred interpretation is that structure effects are restored in the final products of hot decaying nuclei and that the odd-even staggering depends-in a complex and presently not very well understood way-on the structure of the nuclei produced near the end of the evaporation chain [3,6].…”