PHYSICAL REVIEW C VOLUME 33, NUMBER 6 Search for structure in the fusion of "Si + ~s 3osi and 'oSi + 'oSi E. F. Aguilera, ' J. J. Kolata, P. A. DeYoung, and J. J. Vega Physics Department, Uniuersity of ¹treDame, Votre Dame, Indiana 46556 (Received 27 January 1986) Excitation functions for the yields of all the residual nuclei from the ~ Si+~ Si and Si+ Si re- actions have been measured via the gamma-ray technique for center of mass energies in the region within one and two times the Coulomb barrier, using steps of 500 keV in the laboratory system. Thirteen nuclides were identified for the first reaction and ten for the other two. While no structure is shown by the data for the 'Si+2 Si reaction, we have found evidence for intermediate width structure in the 2a and apn channels in 'sSi+' Si, and for broad structure in the total fusion cross section for ~Si+ Si. Calculations using a barrier penetration model with one free parameter repro- duce the average behavior of the fusion cross sections quite well. I. INTRODUCTION The fusion of heavy ions, a phenomenon in which the nucleons of the reacting nuclei drastically rearrange to form a compound nucleus, has captured the interest of many nuclear physicists over a number of years. There are several features of fusion which are not yet well described by the models available. These include the ob- servations of subbarrier fusion, ' ~ the systematic behavior ' of the excitation functions for some light sys- tems such as ' 8+ '60 and '2C+ ' N, and the oscilla- tions observed in the excitation functions for several fusion reactions. Considerable experimental' ' and theoretical' 2' ef- fort has been dedicated to studying this last phenomenon, but no coherent explanation of the observations for the different systems has been given, and more work is clearly needed in order to fully understand the underlying phys- ics. In particular, very little experimental work has been done along these lines for systems involving nuclei with A ) 50. A search for possible structure in the fusion of three systems in this not-well-studied mass region was the motivation for this work. We present here data for the fusion of 2 Si + s 3cSi and 3cSi+ 3cSi, measured with relatively small energy steps (500 keV in the laboratory system), at center of mass ener- gies in the region between one and two times the Coulomb barrier. We used the gamma-ray technique, which allows high accuracy in the measurement of relative cross sec- tions and is therefore particularly appropriate for our pur- poses. In addition, the technique allows one to clearly identify individual evaporation channels so that a struc- ture which appears preferentially in only one or a few of these channels is more easily detected with this method. Special interest is attached to the Si+ Si system on the basis of existing evidence. The observations on many systems seem to indicate a general trend to favor fusion oscillations in reactions where target and projectile are identical alpha-particle nuclei. In addition, for energies above twice the Coulomb barrier the elastic and inelastic scattering of Si+ Si have revealed striking resonance behavior. The suggestion of surface-transparent op- tical potentials as a possible explanation for the observed gross structure increases the expectation that oscillations may also appear in the fusion cross sections. Finally, a research for structure in the fusion of this system has been carried out but no conclusive results were achieved. We used smaller energy steps and covered a wider energy range than those of the related experiment of Ref. 27. Additional interest in studying the fusion of Si+ Si systems comes from the fact that the ground-state shapes are rather different for Si and Si, being strongly oblate for the first and moderately prolate deformed for the last one. There would be some interest in investigating possi- ble effects of this deformation. Other studies of fusion of Si + Si and Si + Si have been recently report- ed, but in none of them has the excitation function been measured with sufficient detail as to produce very conclusive statements about the presence or absence of os- cillations. In the next section we describe the experimental method used in this work. The corresponding results and related discussion for each system are presented in Sec. III, while Sec. IV is dedicated to a comparison with model calcula- tions. Finally, a summary and the conclusions of this work are presented in Sec. V. II. EXPERIMENTAL TECHNIQUE AND DATA ANALYSIS The targets, made by vacuum evaporation of enriched 2sSi (99.9%, 35 p, g jcm) and 3 Si (95.2%, 24 )tsg/cm), were deposited onto a thick Au backing and then covered with a thin Au layer to retard oxidation. Beams of Si and 3cSi with typical intensities on target of 2 to 3 particle-nanoamps were obtained with the three-stage Van de Graaff facility at the University of Notre Dame. A Ge(Li) detector at 125' to the beam was used in the three experiments to determine the total yields of gamma rays, and another one at 90' was used in the case of Si+ uSi for comparison purposes. In order to have a better understanding of our spectra, an investigation of Doppler shift effects was done for Si+ sSi by placing the detector at the symmetric angle 55 and recording 33 1961 1986 The Amc;rican Physical Society