M. Blann

Fusion ofNi58,60,62with 113-170 MeVCa40ions

Results of measurements of the fusion excitation functions of /sup 58,60,62/Ni targets with 113 --170 MeV (lab) /sup 40/Ca ions are reported. These results are used to extract fusion barrier heights and radii by several different methods. Subbarrier fusion cross sections are analyzed in terms of static deformations. Possible importance of dynamic deformation is discussed. Conclusions as to the role of static deformation are similar to those from the /sup 35/Cl+/sup 58,60,62/Ni fusion system; a large part of the subbarrier behavior can be attributed to this cause.

Alpha decay amplification in superdeformed nuclei: An important new mechanism of nuclear de-excitation at high angular momenta

Abstract Alpha decay rates of superdeformed nuclei are predicted to exhibit up to hundredfold emission rate amplifications when transmission coefficients are calculated taking cognizance of deformation rather than for spherical nuclei. For the reaction 40 Ar + 109 Ag it is shown that several heretofore unexplained experimental observations can be understood by this potentially new de-excitation mechanism.

Mechanisms of fragment production in heavy-ion reactions at intermediate energies

Comparative analysis of three typical models of nuclear disintegration, the statistical multifragmentation model (SMM), the quantum statistical model (QSM) and a generalized evaporation model (GEM), is carried out. The thermodynamical properties of a decaying system as well as observable characteristics in heavy ion collisions predicted by the different models are discussed. It is shown that these models yield quite similar results for low charge yields at higher excitation energies (E/A>6 MeV per nucleon) and it is suggested that the coincidence measurements of the intermediate mass fragment multiplicity and the neutron and proton multiplicity (or alternatively, the total bound charge) may be very useful for deducing the decay mechanism. The GEM is shown to differ from the other models in predicting a high Z residue peak.

Superdeformed Nuclei: A New Frontier

The importance of considering the deformation of nuclei at high angular momenta in computing transmission coefficients for their decay is the major topic of this work. Deformations based on the rotating liquid drop model were used to generate transmission coefficients versus compound nucleus angular momentum. The results were then used in a Hauser-Feshbach code which included fission competition to assess the ultimate importance of deformation modified transmission coefficients. It was found that for a broad range of prolate nuclei (superdeformed) the course of deexcitation predicted changes totally from predominant fission to predominant α decay because of a new mechanism called cluster-decay amplification. The phase space relationships responsible for this new mechanism are presented. It is shown that this predicted new decay mode of superdeformed nuclei is consistent with a large body of existing experimental results, though more explicit experiments must be completed to confirm the new mechanism.

Double differential cross sections for (p, xn) reactions of64Zn,65Cu and89Y with 26MeV protons

Double differential cross sections for the inclusive production of neutrons from64Zn,65Cu and89Y bombarded with 26.0 MeV protons have been measured with time-of-flight techniques. The data reduction is discussed with respect to the influence of background corrections in the continuous part of the spectra, and a short comparison with preequilibrium model calculations is presented, showing good agreement with predictions of both, the hybrid and the geometry-dependent hybrid model including multiple particle emission. The question of how to treat pairing remains open.

THE ISOTOPIC EFFECT AS A CLOCK FOR NUCLEAR FRAGMENTATION

Experimental ratios between the emission rates of light fragments (Li, Be, B and C) from 112,(116)Sn and 124Sn targets are compared to predictions from a model which combines the Boltzmann master equation to describe the dynamics and preequilibrium particle emission and the statistical multifragmentation model to describe the final breakup. Since secondary decay and the choice of freeze-out volume affect these ratios very little this comparison can be used to estimate the excitation energy at the instant of fragmentation in a self-consistent way. The preequilibrium nucleon emission governs the time dependence of the excitation energy available for fragment production which is nearly the same for all Sn isotopes. Therefore the yield ratios can be used to estimate the fragmentation time. Data from 1 GeV proton- and 200 MeV 4He induced reactions indicate short fragmentation times - 20-40 fm/c while 32 A MeV 14N induced reactions seems to exhibit much longer times - 300-400 fm/c.

Equilibration and Multifragmentation in Heavy Ion Reactions

Modeling of multifragmentation measurements from heavy ion reactions generally requires separate treatment of the initial fast part of the reaction, during which energetic nucleons are emitted, and of a quasi-equilibrated system where sufficient degrees of freedom have been excited, so that statistical approaches may be applied. Some of the more sophisticated fast cascade models, e.g., Quantum molecular dynamics (QMD), might also produce fragment yields, however, transport models have not yet been able to satisfactorily reproduce fragmentation properties of nuclear reactions.

Photon and Pion Production in Heavy Ion Collisions

Why has so much effort been expended in the measurement and interpretation of the phenomena of photon and γ-ray yields in heavy ion collisions? Several motivations may be given: 1. These data may give information on the early reaction time scale during the most violent part of the reaction. 2. They may provide a probe of the nuclear equation of state (EOS). 3. Their interpretation via reaction models may suggest interesting new mechanisms for these reactions, and in any case provide a better understanding of mass and energy transport. Of these (3) may be the strongest motivation, in that much more straightforward measurements of emitted nucleons probably will give better answers to (1) and (2) than the more convoluted interpretation of the production of secondary reaction products such as photons and mesons.

Fusion of /sup 58,60,62/Ni with 113 --170 MeV /sup 40/Ca ions

Results of measurements of the fusion excitation functions of /sup 58,60,62/Ni targets with 113 --170 MeV (lab) /sup 40/Ca ions are reported. These results are used to extract fusion barrier heights and radii by several different methods. Subbarrier fusion cross sections are analyzed in terms of static deformations. Possible importance of dynamic deformation is discussed. Conclusions as to the role of static deformation are similar to those from the /sup 35/Cl+/sup 58,60,62/Ni fusion system; a large part of the subbarrier behavior can be attributed to this cause.

Fusion of Ni-58, Ni-60, Ni-62 with 113-170 MeV Ca-40 ions

Results of measurements of the fusion excitation functions of /sup 58,60,62/Ni targets with 113 --170 MeV (lab) /sup 40/Ca ions are reported. These results are used to extract fusion barrier heights and radii by several different methods. Subbarrier fusion cross sections are analyzed in terms of static deformations. Possible importance of dynamic deformation is discussed. Conclusions as to the role of static deformation are similar to those from the /sup 35/Cl+/sup 58,60,62/Ni fusion system; a large part of the subbarrier behavior can be attributed to this cause.