PHYSICAL REVIEW C 86, 034606 (2012) Evidence of quasifission in asymmetric reactions forming the 250 Cf compound system C. Yadav, * R. G. Thomas, † and R. K. Choudhury Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India P. Sugathan, A. Jhingan, S. Appannababu, K. S. Golda, D. Singh, Ish Mukul, and J. Gehlot Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India E. Prasad Department of Physics, School of Mathematical and Physical Sciences, Central University of Kerala, Nileshwhar, 671314 Kasaragod, India H. J. Wollersheim GSI, D-64291 Darmstadt, Germany (Received 24 May 2012; published 17 September 2012) Mass-angle correlations and mass ratio distributions of fission fragments have been measured in 12 C + 238 U and 18 O + 232 Th reactions, both leading to the same compound nucleus 250 Cf (fissility = 0.86), with matching excitation energies at near and sub-barrier energies. In both systems, there is no mass-angle correlation in the entire energy region studied. However, it is found that the variance of the mass ratio distribution in the 18 O + 232 Th system is consistently higher than in the 12 C + 238 U system at all excitation energies. This points to the fact that mass equilibration has not been fully achieved in the 18 O + 232 Th reaction in comparison to the 12 C + 238 U reaction, indicating the presence of quasifission in the 18 O + 232 Th system. DOI: 10.1103/PhysRevC.86.034606 PACS number(s): 25.70.Jj, 25.70.Gh, 25.85.Ge I. INTRODUCTION Over the years, a large amount of work has been devoted to understanding the fission mechanism involving actinide nuclei with medium heavy-ion projectiles. These studies are important from the point of synthesis of superheavy nuclei in heavy-ion reactions mostly by complete fusion involving the amalgamation of the two colliding nuclei. The formation of heavy and superheavy elements is considered to proceed through three stages: (i) penetration of the Coulomb barrier between the colliding nuclei, (ii) formation of a compound nucleus (CN) after the system is captured inside the barrier, and finally (iii) the survival of the excited compound nucleus to produce evaporation residue (ER) against fission [1–4]. Though the first stage in the reaction is relatively easy to accomplish by providing sufficient kinetic energy to overcome the Coulomb repulsion, the formation of a compound system after capture is severely hindered by a fission-like process where the composite system reseparates before a compact compound nucleus can be formed. This process, known as quasifission (QF), is associated with incomplete relaxation in mass asymmetry within the time available for the reaction; i.e., the system reseparates prior to mass equilibration. The role of such a mechanism needs to be better understood to predict the formation probability of superheavy elements. The experimental features of quasifission differ from those of the true fusion-fission process in terms of the strong mass-angle correlations in the mass-angle distribution (MAD) * chandrabhan.yadav.29@gmail.com † rgthomas@barc.gov.in of fragments, the large mass width of the mass distribution, and large angular anisotropies [5,6]. Quasifission competes with CN formation as the combined system evolves toward a compact configuration. Many factors, such as deformation and orientation of the colliding nuclei, their mass asymmetry, the nuclear charge product (i.e., Coulomb factor Z P Z T , where Z P and Z T are the charge number of projectile and target, respectively), and the closed- shell nature, can potentially affect the quasifission process. The dynamical model proposed in the mid-1980s predicted the onset of quasifission for heavier systems, when the product Z P Z T > 1600 [7–9]. This was experimentally verified in the mid-1980s by using U and Pb projectiles on a number of targets ranging from O to Cr where strong mass-angle correlations have been observed for systems satisfying the above-mentioned criterion [5,6,10]. However, there are recent measurements where strong mass-angle correlations were observed even for systems with Z P Z T much less than 1600, indicating nonequilibration of the mass degree of freedom [11]. Berriman et al. [12] reported measurements of the reactions 12 C + 204 Pb, 19 F + 197 Au, and 30 Si + 186 W with Z P Z T around 800, each forming the same compound nucleus 216 Ra at the same excitation energies. It was observed that in the case of 19 F + 197 Au and 30 Si + 186 W, the fragment mass distributions were broader than that for the 12 C + 204 Pb reaction. A substantial reduction in ER cross sections for 19 F- and 30 Si- induced reactions was also observed. This reduction in ER cross section and broadened mass distributions were attributed to the presence of the quasifission process in 19 F + 197 Au and 30 Si + 186 W systems. Similar observations of broadened mass distributions have been observed in other systems [11,13,14], indicating the presence of quasifission even in less-fissile compound systems. 034606-1 0556-2813/2012/86(3)/034606(7) ©2012 American Physical Society