Background: A new type of asymmetric fission was observed in β-delayed fission of 180Tl [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 ∼180 at low and moderate excitation energies [Moller, Randrup, and Sierk, Phys. Rev. C 85, 024306 (2012); Andreev, Adamian, and Antonenko, ibid. 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 35Cl+144,154Sm reactions at Elab = 152.5, 156.1, and 163.7MeV 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 θlab = ±65.5◦ 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 35Cl+144Sm reaction. A clear flat top mass distribution has been observed for the 35Cl+144Sm reaction at the lowest beam energy. The mass distribution is very similar to that observed in the 40Ca+142Nd 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 35Cl+144Sm 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 40Ca+142Nd 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.