Z. Phys. A - Hadrons and Nuclei 338, 51-60 (1991) z''~ Hadrons for Physik A and Nuclei 9 Springer-Vertag 1991 Decay of the compound nucleus j 79Au formed in the cold fusion reaction 9~ 89y. U. Gollerthan 1, T. Brohm 1, H.-G. Clerc 1, E. Hanelt 1, M. Horz ~, W. Morawek ~, W. Schwab 1, K.-H. Schmidt 2, F.P. HeBberger 2, G. Miinzenberg 2, V. Ninov, R.S. Simon 2, j.p. Dufour 3, and M. Montoya 4 1 Institut ffir Kernphysik, Technische Hochschule Darmstadt, W-6100 Darmstadt, Federal Republic of Germany 2 GSI Darmstadt, Postfach 1105 52, W-6100 Darmstadt 1 l, Federal Republic of Germany 3 Centre d'Etudes Nucl6aires de Bordeaux-Gradignan, Le Haut Vigneau, F-33175 Gradignan Cedex, France 4 Universidad Nacional de Igenieria, Lima, Peru Received July 18, 1990 For the compound nucleus 179Au formed at an excita- tion energy of 26 MeV in the fusion reaction 9~ 89y, the energy spectra of promptly emitted protons, e parti- cles and 7 rays were measured in concidence with the evaporation residues. On the basis of the measured total decay energy, the 1 p and 1 e decay channels were sepa- rated from all other evaporation-residue channels. The energy spectra and absolute cross sections, together with previously measured excitation functions for various de- cay channels, are successfully described by statistical- model calculations with the Monte Carlo code CODEX. PACS: 21.10.MA; 25.70.Gh; 27.70. +q Introduction A compound nucleus formed by the fusion of two stable, heavy nuclei usually is "exotic" in several respects at the same time. It may carry a considerable amount of excitation energy and angular momentum, and, being very neutron deficient, it is far removed from the valley of fl stability. In the present work, we tried to separate the effects arising from a large neutron deficiency from those of high excitation energy and angular momentum by studying the decay of the extremely neutron-deficient compound nucleus 179Au formed in a cold fusion reac- tion with the low initial excitation energy of 26 MeV. The angular momentum of the compound nucleus was limited to relatively small values (0 to 30h), too, since the emitted charged particles and 7 rays were detected in coincidence with the evaporation residues, thus ex- cluding from observation high angular-momentum states which preferentially decay by fission. A very sensitive tool for the investigation of the com- pound nuclear system is the emission of protons and particles since their energy distribution may reflect the nuclear matter distribution at the time of emission. For the process reverse to evaporation, namely the fusion * This work is part of the Ph.D. thesis of U. Gollerthan of protons and e particles, respectively, with stable nuclei in their ground states, Vaz and Alexander [1] in a com- prehensive treatment of existing experimental data have deduced nuclear interaction potentials. Taking these po- tentials as a reference, in many papers [2-10] a signifi- cant reduction of the barrier against the emission of charged particles has been deduced by comparing the measured energy distributions of charged particles with calculations based on the statistical model. In t94Hg compound nuclei at an excitation energy of 98 MeV and at angular momenta up to 45 h the interpretation of the measured energy spectra of evaporated ~ particles leads to a barrier reduction of about 10% [2]. For the com- pound nuclei 156Er, 194Hg, and Z37Bk at excitation ener- gies between 65 and 195 MeV even larger barrier reduc- tions (2(~30%) and unexpected differences between the proton and e emission have been deduced [3]. At ex- treme excitation energies ranging from 100 to 400 MeV, temperature-dependent barrier reductions for e-particle emission have been found in the system l~N+154Sm [5]. In several papers [4, 6-8] in which proton emission as well as e-particle emission of hot nuclei was measured simultaneously and interpreted with the computer code GANES [-11], nuclear shapes with axis ratios of 1.7 [4] to 2.5 [8] had to be assumed in order to explain the e-particle data. Even much higher deformations (axis ra- tio up to 3.0 and more) were necessary to describe the proton spectra at the same time. These results lead to new ideas [-6, 8] about the charged-particle emission pro- cess. According to these suggestions low-energy protons may be emitted due to extreme tails of a very diffuse nuclear matter distribution. The effect of a possible nu- clear stratosphere on the barrier transmission of protons and c~ particles has been treated theoretically, too [12]. Doubts about the correct interpretation of the experi- mental data came up when the measured energy spec- trum of e particles [6] emitted from excited 67Ga nuclei (E* =90 MeV) was compared with calculations [13] of the statistical-model code CASCADE [,14]. In contrast to calculations [-6] performed with GANES [,11], where