Fission fragment mass distributions in ${}^{35}\mathrm{Cl}+{}^{144,154}\mathrm{Sm}$ reactions

Background: A new type of asymmetric fission was observed in $\beta$-delayed fission of ${}^{180}\mathrm{Tl}$ [Phys. Rev. Lett. 105, 252502 (2010)] as symmetric mass distribution would be expected based on conventional shell effects leading to the formation of $N=50$ fragments. Following this observation, theoretical calculations were carried out which predict asymmetric mass distribution for several mercury isotopes around mass region of $\sim 180$ at low and moderate excitation energies [Moller, Randrup, and Sierk, Phys. Rev. C 85, 024306 (2012); Andreev, Adamian, and Antonenko, Phys. Rev. C 86, 044315 (2012)]. Studies on fission fragment mass distribution are required in this mass region to investigate this newly observed phenomenon.

Purpose: The fission fragment mass distributions have been measured in ${}^{35}\mathrm{Cl}+{}^{144,154}\mathrm{Sm}$ reactions at $E_{\mathrm{lab}}=152.5,156.1,\mathrm{and}163.7\mathrm{MeV}$ populating compound nuclei in the mass region of ∼180 with variable excitation energy and neutron number to investigate the nature of mass distribution.

Method: The fission fragment mass distribution has been obtained by measuring the “time of flight (TOF)” of fragments with respect to the beam pulse using two multiwire proportional counters placed at ${\theta }_{\mathrm{lab}}=\pm 65.5^{\circ }$ with respect to the beam direction. From the TOF of fragments, their velocities were determined, which were used to obtain mass distribution taking the compound nucleus as the fissioning system.

Results: For both systems, mass distributions, although, appear to be symmetric, could not be fitted well by a single Gaussian. The deviation from a single Gaussian fit is more pronounced for the ${}^{35}\mathrm{Cl}+{}^{144}\mathrm{Sm}$ reaction. A clear flat top mass distribution has been observed for the ${}^{35}\mathrm{Cl}+{}^{144}\mathrm{Sm}$ reaction at the lowest beam energy. The mass distribution is very similar to that observed in the ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ reaction, which populated a similar compound nucleus, but for the pronounced dip in the symmetric region [Phys. Rev. C 91, 064605 (2015)].

Conclusions: The present study shows that the mass distribution deviates from that expected on the basis of a pure liquid drop model in the mass region of ∼180, indicating a contribution from asymmetric fission. The contribution from asymmetric fission is more pronounced for the ${}^{35}\mathrm{Cl}+{}^{144}\mathrm{Sm}$ reaction as evident from the large deviation of the fission fragment mass distribution from the single Gaussian fit. This is consistent with the observation of an asymmetric component in the ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ reaction in a recent study [Phys. Rev. C 91, 064605 (2015)]. The contribution from asymmetric component is also consistent with the theoretical predictions by Moller et al. [Phys. Rev. C 85, 024306 (2012)], although the magnitude of the effect appears to be smaller.

Figure 1

(a) Plot of timing spectrum for the ${}^{35}\mathrm{Cl}+{}^{144}\mathrm{Sm}$ reaction at $E_{\mathrm{lab}}=156.1\mathrm{MeV}$ (gated with ${\theta }_{\mathrm{c}.\mathrm{m}.}={82}^{\circ }-{99}^{\circ }$). Fission fragments and elastically scattered particles are marked in the figure. (b) Plot of ${\theta }_{\mathrm{c}.\mathrm{m}.}$ vs mass ratio ($M_R=\frac{M_1}{M_1+M_2}$, where $M_1$ and $M_2$ are the mass numbers of coincident fission fragments). (c) Plot of “${\varphi }_2-{\varphi }_1$” vs “${\theta }_{\mathrm{c}.\mathrm{m}.,1}+{\theta }_{\mathrm{c}.\mathrm{m}.,2}$,” subscripts 1 and 2 refer to the fragments detected in detectors 1 and 2, respectively.

Figure 2

Fission fragment mass distribution in the ${}^{35}\mathrm{Cl}+{}^{144}\mathrm{Sm}$ reaction at (a) $E_{\mathrm{lab}}=152.5\mathrm{MeV}$, (b) 156.1 MeV, and (c) 163.7 MeV. The filled symbols are experimental data, and the solid lines are fit to the data. The deviation of the experimental data from the fit is shown in the panel below the respective mass distribution (see text for details).

Figure 3

Same as Fig. 2 but for the ${}^{35}\mathrm{Cl}+{}^{154}\mathrm{Sm}$ reaction.

Figure 4

(a) Comparison of fission fragment mass distribution in the ${}^{35}\mathrm{Cl}+{}^{144}\mathrm{Sm}$ reaction with that in the ${}^{40}\mathrm{Ca}+{}^{142}\mathrm{Nd}$ reaction from Prasad et al. [12]. Data from Ref. [12] have been normalized to the present data using the area in the $M_R$ range of $0.33-0.65$. (b) is the same as (a) but for the logarithmic scale.