J.D. Bierman, S. Kailas,^* S.S. Kapoor,^* J.P. Lestone, J.F. Liang, D.M. Nadkarni,^* D.J. Prindle, A.A. Sonzogni and R. Vandenbosch

It has been observed¹ that the anisotropies for actinide targets are well accounted for by the transition state model for the lighter projectiles ¹²C, ¹⁰B and ⁹Be but are larger than expected for the heavier projectiles ¹⁶O and ¹⁹F. These observations have been interpreted¹ as an entrance channel effect arising from contributions of fission-like events from pre-equilibrium fission expected to arise only in the case of the heavier projectiles, on the basis of the variation of the liquid drop model driving force at the saddle in the mass asymmetry degree of freedom. The target-projectile combinations having an entrance channel mass asymmetry = (A_T - A_P)/(Ai_T + A_P) less than about 0.88 (¹⁶O and ¹⁹F) exhibit anomalous anisotropies. The Businaro-Gallone critical asymmetry (_BG) value where the driving force changes direction has been estimated to be about 0.9 in this mass and charge region. For values of greater than _BG the driving force favours amalgamation of the nascent partners (fusion and compound nucleus formation), whereas for smaller values the smaller partner gains in mass at the expense of the heavier, and the dinuclear system may reseparate as a fission-like event without K-equilibration and formation of a compound nucleus.

The above study,¹ however, involved formation of different compound nuclei and the data for the anomalous systems were at energies not very far above the fusion barrier. An alternative correlation with the energy relative to the barrier rather than with the entrance channel mass asymmetry has been offered.² We have performed a more definitive experiment by studying two entrance channels which lead to the same compound nucleus. The ¹²C+²³⁶U(=0.903) entrance channel has greater than the Businaro-Gallone critical asymmetry and the ¹⁶O+²³²Th(=0.871) has smaller than the critical asymmetry. At an excitation energy of 62 MeV (well above the fusion barrier for both entrance channels) one can also match the average angular momentum. The experiment was performed using beams from the tandem-booster accelerator. Inclusive single-fragment anisotropies were obtained from Si surface barrier detectors. We also used three large-area segmented gas detectors. These were primarily used for measurements of fragment-fragment coincidences in order to determine the folding angle distributions. We also determined inclusive (singles) fission fragment angular distributions from the gas detectors, and the anisotropies from these measurements agree well with the Si detector results. We report here the average value of the independent singles determinations with the two kinds of detectors. We focus here on the single-fragment anisotropies, as these were the kind of results that led to the motivation of the present measurement. Interpolating between the measured anisotropies gives 2.05±0.1 for the O + Th system and 1.96± 0.1 for the C + U system at this excitation energy. As these two values are the same within the experimental error there is no evidence that the entrance channel mass asymmetry relative to the Businaro-Gallone critical asymmetry plays any role in determining the fission anisotropy at energies well above the fusion barrier.