Entrance channel effects on the quasifission reaction channel in Cr + W systems

Background: Formation of a fully equilibrated compound nucleus is a critical step in the heavy-ion fusion reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. In particular, the impact of changing the neutron richness on the quasifission process is not well known. A previous study of Cr + W reactions at a constant $13\%$ above the Coulomb barrier concluded that an increase in neutron richness leads to a decrease in the prominence of the quasifission reaction channel.

Purpose: The dynamics of quasifission for reactions with varying neutron richness was explored at a constant excitation energy, closer to the interaction barrier than the previous work, to see if the correlation between neutron richness and quasifission is valid at lower energies.

Methods: Mass distributions were measured at the Australian National University for eight different combinations of Cr + W reactions, using the kinematic coincidence method. To eliminate the effect of differing excitation energies, measurements were made at beam energies chosen to give 52 MeV of excitation energy in all the compound nuclei.

Results: A curvature parameter, describing the shape of the mass distributions, was determined for the fission-like fragment mass distributions for each reaction, and compared to various reaction parameters known to influence quasifission.

Conclusions: The present work demonstrates that, at energies near the interaction barrier, the beam energy with respect to the barrier is as important as neutron-richness effects in determining the quasifission characteristics in these Cr + W reactions involving statically deformed target nuclei, and both are important considerations for future heavy and superheavy element production reactions.

Figure 1

Schematic scale diagram of the CUBE detector setup from above. The definitions of $\theta$ and $r$ are indicated. The coordinates ($r,\theta ,\mathrm{\Phi }$) at the center of the CUBE and at the center of the two MWPCs are indicated, in millimeter and degrees.

Figure 2

The mass-angle distributions are shown for all eight Cr + W systems from the present work at $E^{*}=52.0\mathrm{MeV}$. The color scale (top right) indicates the number of events per pixel, which is proportional to $d^2\sigma /d\theta d{M}_R$. In the (a–d) first and (i–l) third rows, the MADs corresponding to the mass and angle of the fragment detected in the back MWPC are shown. In the (e–h) second and (m–p) fourth rows, the MADs corresponding to the front MWPC are shown. For each system, the projectile, target, and number of neutrons relative to ${}^{230}\mathrm{Cf}$ are given.

Figure 3

The projected mass distributions are shown for all eight Cr + W systems from the present work at $E^{*}=52.0\mathrm{MeV}$. The black points represent the experimental data. Experimental uncertainties are smaller than the size of the points. The solid (blue) line shows a quadratic fit to the fission-like region. For each system, the projectile, target, compound nucleus, and number of neutrons relative to ${}^{230}\mathrm{Cf}$ are given.

Figure 4

The curvature parameters determined from the fit of the fission-like region of the mass distributions are shown as a function of ${(N/Z)}_{\mathrm{CN}}$. The points corresponding to the reactions forming ${}^{236}\mathrm{Cf}$ are highlighted by (green) circles.

Figure 5

The curvature parameter determined for each system as a function of (a) compound nucleus fissility (${\chi }_{\mathrm{CN}}$) and (b) mass asymmetry ($\alpha$). The points corresponding to the reactions forming ${}^{236}\mathrm{Cf}$ are highlighted by (green) circles.

Figure 6

The curvature parameter determined for each Cr + W system as a function of the maximum rotational energy $E_{\mathrm{rot}}$ (MeV). The points corresponding to the reactions forming ${}^{236}\mathrm{Cf}$ are highlighted by (green) circles.

Figure 7

Illustration of (a) an aligned and (b) an anti-aligned collision between a spherical projectile and a prolate target.

Figure 8

The curvature parameter determined for each system as a function of $E_{\mathrm{c}.\mathrm{m}.}/V_{\mathrm{B}}$ for all Cr + W systems where $V_{\mathrm{B}}$ is taken as (a) the aligned barrier, (b) the average barrier, and (c) the anti-aligned barrier. The dashed line denotes the barrier in each panel. The points corresponding to the reactions forming ${}^{236}\mathrm{Cf}$ are highlighted by (green) circles.