Experimental investigation of the role of shell structure in quasifission mass distributions

Abstract

Background: To understand superheavy element synthesis reactions, quantifying the role of quantum shells in quasifission dynamics is important. In reactions with actinide nuclides, a wide peak in the binary quasifission mass yield is seen, centered close to the $^{208}\mathrm{Pb}$ mass. It is generally attributed to the $^{208}\mathrm{Pb}$ spherical closed shells causing a valley in the potential-energy surface, attracting flux to these mass splits. However, an early experiment studying $^{48}\mathrm{Ca}, ^{50}\mathrm{Ti}+^{238}\mathrm{U}$ reactions showed strong evidence that sequential fission plays an important role in generating the observed peak. These conflicting interpretations have not been resolved up to now.Purpose: This work aims to measure quasifission mass spectra for reactions with nuclei lighter than $^{208}\mathrm{Pb}$, having negligible sequential fission, to search for systematic features correlated with the proton shells known to affect low-energy fission mass distributions of the same actinide elements.Methods: Systematic measurements have been made at energies near and below the capture barriers (where quasifission is most prominent) of mass-angle distributions for fission following collisions of $^{48}\mathrm{Ti}$ projectiles with even-even nuclides from $^{154}\mathrm{Sm}$ to $^{200}\mathrm{Hg}$. Mean excitation energies above the ground-states ranged from 51 to 33 MeV, respectively.Results: With increasing compound nucleus atomic number ${\mathrm{Z}}_{CN}$, a rapid transition occurs from fission having characteristics of fusion-fission to fast quasifission. The heaviest reactions form $^{240}\mathrm{Cf}, ^{244}\mathrm{Fm}$, and $^{248}\mathrm{No}$. Low-energy fission of neighboring isotopes is mass asymmetric, correlated with proton number $Z=56$. However, peak quasifission yields are at mass-symmetry for all reactions. There appears to be a very small ($\ensuremath{\approx}3$%) systematic excess of yield correlated with $Z=56$, however this is at the limit of sensitivity of the experiment.Conclusions: No significant ($>3%$) systematic features are seen in the quasifission mass spectra that can be unambiguously identified as resulting from shells. This small influence may result from attenuation of shell effects due to the excitation energy introduced, even in these near-barrier reactions giving low excitation energies typical of superheavy element synthesis reactions.