Effect of N/Z and dissipation in the fission of ^212,214,216Ra nuclei via neutron multiplicity measurements

Full text: Pre-scission neutron multiplicity (𝑉_𝑝𝑟𝑒) is one of the best probes to understand the evolution of the compound nucleus formed in heavy ion fusion. Measured 𝑉_𝑝𝑟𝑒 is observed to be larger than the standard statistical model (SSM) predictions in many cases and were attributed to the dynamical delay or dissipation involved in fission. A few attempts have been made to understand the effect of neutron shell closure, N/Z and dissipation in fission dynamics. The deduced dissipation strength is shown to have a strong temperature dependence in some of these works. Contradicting results are also reported. A correlation between the shell closure and dissipation strength is also worked out in a few cases. We measured the pre-scission neutron multiplicity for the ³⁰Si+^182,184,186W reactions populating ^212,214,216Ra compound nuclei. Among the CN populated, ²¹⁴Ra has neutron shell closure (N=126) and others are two neutrons away on either sides. It is observed that the measured 𝑉_𝑝𝑟𝑒 values increase with increasing N/Z of the compound nuclei at all excitation energies. However the measured 𝑉_𝑝𝑟𝑒 does not show any noticeable effect of shell closure at N=126. Statistical model analysis of the 𝑉_𝑝𝑟𝑒 excitation function has been performed including the collective enhancement of level density (CELD), shell correction at fission barrier and level density, K- orientation effect in fission width and dissipation. The strength of pre-saddle dissipation was fixed by reproducing the evaporation residue cross section for the ³⁰Si+¹⁸⁶W reaction and varied the strength of post-saddle dissipation according to the measured 𝑉_𝑝𝑟𝑒 values. The measured pre values are observed to be larger than the Bohr-Wheeler predictions indicating the effect of dissipation. Strength of the deduced dissipation coefficient does not show any effect of neutron shell closure in the measured excitation energies and does not vary with the N/Z of the fissioning nuclei. Most importantly, the dissipation strength does not show any temperature dependence unlike reported earlier. Emission of pre-saddle neutrons is observed to be energy independent. A substantial contribution to 𝑉_𝑝𝑟𝑒 comes from the post-saddle phase of shape evolution. CELD and K-orientation effects are also observed to be significant in these nuclei. (author)