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

TLDR

Let's extract content. Mechanism: "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 $^{64}\mathrm{Ni}$." Purpose/Findings: The sentence labeled [Purpose, Findings] says: "A new method of interpretation of experimental pre-scission neutron multiplicities ν_pre and mean kinetic energies ε_ν 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±15)×10^−21 s for symmetric fission." Findings: multiple sentences. Summarize: All reactions show ν_pre falls rapidly with increasing mass asymmetry; for fusion-fission partly due to reduction of dynamical fission time scale with mass asymmetry.

Abstract

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.