Observation of mass-asymmetric fission of mercury nuclei in heavy ion fusion

Background: Mass-asymmetric fission has been observed in low energy fission of ${}^{180}\mathrm{Hg}$. Calculations predicted the persistence of asymmetric fission in this region even at excitation energies of 30–40 MeV.

Purpose: To investigate fission mass distributions by populating different isotopes of Hg using heavy ion fusion reactions.

Methods: Fission fragment mass-angle distributions have been measured for two reactions, ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ and ${}^{13}\mathrm{C}+{}^{182}\mathrm{W}$, populating ${}^{182}\mathrm{Hg}$ and ${}^{195}\mathrm{Hg}$, respectively, using the Heavy Ion Accelerator Facility and CUBE spectrometer at the Australian National University. Measurements were made at beam energies around the capture barrier for the two reactions and mass ratio distributions were obtained using the kinematic reconstruction method.

Results: Asymmetric fission has been observed following the population of ${}^{182}\mathrm{Hg}$ at an excitation energy of 22.8 MeV above the saddle point. A symmetric peaked mass ratio distribution was observed for ${}^{195}\mathrm{Hg}$ nuclei at a similar excitation energy above the saddle point.

Conclusions: Mass-asymmetric fission has been observed in neutron deficient Hg nuclei populated via heavy ion fusion for the first time. The results are consistent with observations from beta-delayed fission measurements and provide a proof-of-principle for expanding experimental studies of the influence of shell effects on the fission processes.

Figure 1

Experimental MAD scatter plots for ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ reaction at different beam energies. The $\frac{E_{\mathrm{c}.\mathrm{m}.}}{V_B}$ values and CN excitation energies $E^{*}$ are given at the top of each plot. The angular window used for producing the $M_R$ distribution is indicated in (e).

Figure 2

Experimental MAD scatter plots for ${}^{13}\mathrm{C}+{}^{182}\mathrm{W}$ reaction at different beam energies. The angular window used for producing the $M_R$ distribution is indicated in (c).

Figure 3

The $M_R$ distributions of fission from (a) ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ and (b) ${}^{13}\mathrm{C}+{}^{182}\mathrm{W}$ reactions at different CN excitation energies. The yields are scaled to show different energies in a single plot. The excitation energy and scaling factors used are shown in parentheses.

Figure 4

(a) The fragment $M_R$ distributions of ${}^{180}\mathrm{Hg}$ reported in Ref. [8] at $E^{*}=10.44$ MeV. The mass distribution in Ref. [8] was reported with a bin size of 2 amu. The same distribution (scaled) is also shown with a bin size of 4 amu for comparison. (b) $M_R$ distributions of the fission observed following the reaction ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ at $E^{*}=33.6$ MeV with a bin size of 3.64 amu. (c) $M_R$ distribution obtained by subtracting $25\%$ contribution from the fission of ${}^{180}\mathrm{Hg}$. A Gaussian fit to this distribution is shown using the dot-dashed line (blue). Dashed line (red) represents the sum of Gaussian fit values and $25\%$ contribution from (a).

Figure 5

Experimental fission fragment $M_R$ distribution for ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ at $E^{*}=33.6$ MeV is compared with Möller's predcitions for the same compound nucleus ${}^{182}\mathrm{Hg}$ at $E^{*}=20$ MeV (red dashed line) and 40 MeV (blue dot-dashed line). The mass distributions obtained using the Brownian shape motion treatment in Ref. [22] were converted to $M_R$ distributions and scaled for this plot.