Energy levels of light nuclei A=8,9,10

Energy levels of light nuclei A = 16-17

Langevin description of fusion, deep-inelastic collisions and heavy-ion-induced fission

Energy levels of light nuclei A = 18–19☆

The most interesting process of collective flow of nuclear matter is the fission process. The time-scale of this process at moderate to high excitation energies is reviewed. Various methods to measure the time associated with large scale collective flow of nuclei in the range of 10^(-21) to 10^(-17) s are compared. Pre- and post-scission multiplicities of light particles in particular neutrons are shown to be a sensitive probe of the dynamics of nuclear fission. A systematics of pre- and post-scission neutron multiplicities associated with nuclear fission is presented. The magnitude of nuclear dissipation deduced from pre- and post-scission neutron multiplicities is compared with theoretical estimates.

Dynamics of nuclear fission

Pre-scission and post-scission neutron yields have been measured as a function of projectile mass, compound nucleus fissility, and fission mass split and total kinetic energy (TKE) for 27 fusion-fission and quasifission reactions induced by beams of $^{16,18}\mathrm{O}$, $^{40}\mathrm{Ar}$, and $^{64}\mathrm{Ni}$. A new method of interpretation of experimental pre-scission neutron multiplicities ${\ensuremath{\nu}}_{\mathrm{pre}}$ and mean kinetic energies ${\mathrm{\ensuremath{\varepsilon}}}_{\ensuremath{\nu}}$ allows the extraction of fission time scales with much less uncertainty than previously, all fusion-fission results being consistent with a dynamical time scale of (35\ifmmode\pm\else\textpm\fi{}15)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ s for symmetric fission. All reactions show that ${\ensuremath{\nu}}_{\mathrm{pre}}$ falls quite rapidly with increasing mass asymmetry; evidence is presented that for fusion-fission reactions this is partly due to a reduction of the dynamical fission time scale with mass asymmetry. For quasifission, the data indicate that the pre-scission multiplicity and mean neutron kinetic energy are very sensitive to the final mass asymmetry, but that the time scale is virtually independent of mass asymmetry. It is concluded that for fusion-fission there is no dependence of ${\ensuremath{\nu}}_{\mathrm{pre}}$ on TKE, while for $^{64}\mathrm{induced}$ quasifission reactions, a strong increase of ${\ensuremath{\nu}}_{\mathrm{pre}}$ with decreasing TKE is observed. This is probably largely caused by neutron emission during the acceleration time of the fission fragments in these fast reactions. Interpretation of post-scission multiplicities in terms of fragment excitation energies leads to deduced time scales consistent with those determined from the pre-scission data.

Neutron emission as a probe of fusion-fission and quasifission dynamics.

The time scale of fission at initial nuclear temperatures of about 5 MeV is deduced from the number of neutrons evaporated prior to and after scission in the reactions /sup 144/Sm and /sup 154/Sm+(838 MeV) /sup 32/S. The prescission lifetime for fission with symmetric mass splits is longer than for asymmetric mass splits. For symmetric mass splits the excitation energy at scission is only about 60 MeV and independent of the initial excitation energy which is consistent with a prescission lifetime in the order of 10/sup -20/ s.

Observation of cold scission of highly excited fissioning nuclei.

Pre- and post-scission neutron multiplicities for the reaction {sup 169}Tm({sup 36}Ar,{ital f}) at {ital E}{sub lab}=205 MeV were measured in coincidence with fission fragments of different masses and total kinetic energies. The mass and total kinetic energy dependence of the total neutron multiplicity as well as the width of the out-of-plane fission fragment correlation angle are well described by evaporation calculations. An average time before scission of several 10{sup {minus}20} s is deduced from the average pre-scission neutron multiplicity. The mass dependence of the post-scission neutron multiplicity is consistent with an energy division at scission proportional to the mass of the fragments. For the first time clear evidence for an increase in pre-scission neutrons with increasing total kinetic energy values has been observed. Possible interpretations of this unexpected behavior are discussed.

Analysis of pre- and post-scission neutrons emitted in the reaction 169Tm(36Ar,f) at Elab

Neutron emission in coincidence with fusion-fission events and evaporation residues was measured in the heavy-ion reactions $^{141}+(316$ MeV) $^{40}$Ar and $^{175}$Lu+(192 MeV) ${\mathrm{}}^{12}$C. Both reactions are leading to similar composite systems and excitation energies as the previously investigated reaction $^{165}$Ho${+\mathrm{}}^{20}$Ne. In order to determine the lifetimes of the composite systems prior to scission and to study entrance channel and angular-momentum effects, the results for all three systems are compared.

Fusion-fission dynamics at high excitation energies studied by neutron emission.

Fission of {sup 197}Au,{sup 232}Th+838-MeV {sup 32}S projectiles was studied by measuring fragment coincident neutrons. Neutron energy spectra were decomposed into preequilibrium, prescission, and postscission contributions with a constrained moving source analysis. Excitation energies deduced from the transferred linear momentum are consistent with calculations applying the Boltzmann master equation, and with an energy balance based upon the experimental neutron multiplicities and charged particles from evaporation calculations. The time scale of fission derived from the prescission neutron multiplicities extends from 5{times}10{sup {minus}21} to 3{times}10{sup {minus}20} s; it does not depend on the initial excitation energy, is about a factor of 2 longer for symmetric than for asymmetric fragmentations, and increases with the fissility of the primary reaction system. For all mass splits, the excitation energy left at scission is 50--60 MeV.

M. Lehmann

Papers

Neutron emission as a probe of fusion-fission and quasifission dynamics.

Observation of cold scission of highly excited fissioning nuclei.

Analysis of pre- and post-scission neutrons emitted in the reaction 169Tm(36Ar,f) at Elab

Fusion-fission dynamics at high excitation energies studied by neutron emission.

Time scales for binary fragmentations of highly excited, fissile nuclei from 32S+197Au, 232Th.