Z. Phys. A 351, 195-198 (1995) ZEITSCHRIFT FOR PHYSIK A 9 Springer-Verlag 1995 Near-barrier fission of the system 12C+ 23aTh A. Karnik, S. Kailas, A. Chatterjee, P. Singh, A. Navin, D.C. Biswas, D.M. Nadkarni, A. Shrivastava, S.S. Kapoor Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay400 085, India Received: 24 May 1994/Revisedversion: 3 August 1994 Abstract. The fission fragment angular distributions for the system 12C + 232Th have been measured in the energy range 0.97 < Ec.m./VB < 1.22. The measured anisotropies have been compared with the predictions of the standard saddle point statistical model, using the second moment of the compound nucleus spin distribution (I 2) deduced from a Wong model fit to the fission excitation function. While the measured anisotropies agree with the standard saddle point statistical model for E .... /VB > 1.05, they are abnormally large at lower energies. This is also true for the existing measurements of 160 and 19F -+- 232Th sys- tems. Since for this system pre-equilibrium fission contri- butions are not expected, this anomaly indicates strong channel coupling effects leading to an increase in (/2) at sub-barrier energies. PACS: 25.85 Ge 1. Introduction The surprising observation of abnormally large values of fission fragment anisotropies as compared to the standard saddle point statistical model (SSPSM) predictions in heavy-ion induced fission has been a subject of several investigations [1-7] in recent years. These investigations seem to indicate that two different mechanisms may be operating, one below the fusion barrier and the other above the fusion barrier, leading to anomalous values of anisotropies. At bombarding energies mostly above the fusion barrier, the measurement of fission-fragment angu- lar distributions for 9Be, l~ 12C, 160 and 19F+232Th, 237Np systems [4, 8, 9] revealed that while the measured anisotropies are consistent with the SSPSM calculations for the projectiles 9Be, I~ and 12C, they are anomalously large for the heavier projectiles 160 and 19F. This obser- vation of an entrance channel dependence of measured anisotropies was interpreted earlier as evidence for a small admixture of pre-equilibrium fission events in addition to compound nucleus fission events in the case of 160 and 19 F projectiles. This interpretation was supported by the fact that for these two cases one expects formation of composite systems across the Businaro-Gallone point, which subsequently evolve towards compound nucleus formation via a mass-symmetric dumb-bell shape, in which case only pre-equilibrium fission with nearly mass symmetric distributions is possible. On the other hand, anomalously large anisotropies have also been reported at sub-barrier and near-barrier energies for the systems 160 + 2~ 232Th and t2C + 236U by Vandenbosch et al. [2] and as pointed out by them, the anomalous values of anisotropies can arise due to a different mecha- nism, namely, from an enhancement of compound nucleus (l 2) values at these energies caused by coupling to other degrees of freedom. In an earlier investigation [10] we have shown that the measured anisotropies for the 19F + 232Th system at near-barrier energies are anoma- lous. Recently Zhang et al. [11] have shown that the anisotropies measured for 12C + Z37Np and 11B + 23sU systems at near- and sub-barrier energies are also anoma- lous. From these measurements it appears that the obser- vation of abnormal values of fission anisotropies at sub- barrier and near-barrier energies is independent of the entrance channel, implying that the enhancement of (12 ) is the dominant mechanism. The motivation for the pre- sent measurement of fission anisotropies at near-barrier energies for the 12C + 232Th system is to further investi- gate the barrier effect on measured anisotropies, in a sys- tem for which pre-equilibrium fission has been shown to be absent from our earlier work at above barrier energies 1-4] in order to isolate specifically fusion-barrier depen- dent effects in influencing the observed fission fragment anisotropies. A preliminary account of this work has al- ready been reported [12]. 2. Experimental details and results The present experiment was performed using the 12C beam from the BARC-TIFR 14 UD Pelletron accelerator at Bombay. The fission fragment angular distributions were measured at laboratory bombarding energies of 62.8,