Volume 171, number 1 PHYSICS LETTERS B 17 April 1986 PARTIAL WAVE CROSS SECTIONS FOR 34S + 13°Te FUSION ~ R.D. FISCHER, A. RUCKELSHAUSEN, G. KOCH, W. KI]HN, V. METAG, R. MI]HLHANS, R. NOVOTNY, H. STRC)HER IL Physikalisches lnstitut, Universiti~t Giessen, D-6300 Giessen, Fed. Rep. Germany H. GROGER, D. HABS, H.W. HEYNG, R. REPNOW, D. SCHWALM Max-Planck-lnstitut J'ftr Kernphysik, D-6900 Heidelberg, Fed. Rep. Germany and Physikalisches lnstitut, Universitgtt Heidelberg, D-6900 Heidelberg, Fed. Rep. Germany W. REISDORF and R.S. SIMON Gesellschaft fftr Schwerionenforschung, D-6100 Darmstadt, Fed. Rep. Germany Received 4 February 1986 Angular and multiplicity distributions for the 7-decay of 164Er compound nuclei have been measured with the Darmstadt-Heidelberg Crystal Ball. Combining this information with the fusion cross section of (500 + 50)mb, a compound nuclear spin distribution has been derived which almost exhausts the unitarity limit for lower partial waves. Near the limiting angular momentum of (52 + 2)h the distribution of partial wave cross sections exhibits a steeper fall-off than predicted by barrier fluctuation and coupled channel calculations. The knowledge of compound nuclear spin distribu- tions is of fundamental importance for a detailed un- derstanding of heavy-ion fusion reactions. Since most of the compound nucleus angular momentum is re- moved by ?-emission, ?-ray multiplicity distributions provide the most direct probe of the initial/-distribu- tion leading to fusion. While early experiments were limited to measuring the first and second moment of the ?-multiplicity distribution [1] the advent of mod- ular 4n detector systems has opened up the possibil- ity to directly measure the entire distribution [2]. Recently a direct relation has been pointed out [3,4] between enhanced subbarrier fusion cross sec- tions and compound nuclear spin distributions at energies around the interaction barrier. In contrast to standard fusion models rather broad distributions ex- tending to high partial waves are predicted, caused by coupling to channels like inelastic excitations [5] or Part of the Thesis of R.D. Fischer, Universitat Giessen (1986). particle transfer [6]. Moreover, coupled channel cal- culations [7] indicate that fusion may not exhaust the reaction cross section at low spins. These theoretical considerations have initiated measurements of average multiplicities [8] and multi- plicity distributions [9]. This letter presents for the first time partial wave cross sections in heavy-ion fu- sion derived from a combined measurement of evapo- ration residue cross sections and spin distributions. The latter have been deduced from the full informa- tion on angular and multiplicity distributions provided by a modular 4rr detector system. Thus, a direct com- parison between experimental spin distributions and various fusion model predictions can be made. The compound nucleus 154Er has been chosen since its decay leads to only a few residual nuclei which exhibit strongly collective behaviour, facilitating the conversion of multiplicity distributions into spin distributions. The experiment was performed at the Darmstadt-Heidelberg Crystal Ball [10] with 158 of the 162 NaI modules subtending 97% of the full solid 33