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Sub-Barrier Fusion and Elastic Scattering in S + Ni Systems

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Sub-Barrier Fusion and Elastic Scattering in S + Ni Systems CRN/PN 90-23 • STRASBOURG! "^^^^^_H ^^_H I Sub-Barrier fusion and elastic scattering in S + Ni systems RJ. TIGHE, JJ. VEGA, Genbao UU, A. MORSAD*. and JJ. KOLATA Physics Department, University of Notre Dame, Notre Dame, In. 46556 S.H. FRICKE, H. ESBENSEN, and S. LTVNDOWNE Physics Division, Argonne National Laboratory. Argonne, IL 60439 P/jys. Rev. C (to be published) Centre de Recherches Nucléaires, IN2P3-CNRS/Université Louis Pasteur, B.P. 20. F67037 STHASBOURG CEDEX, France CENTRE DE RECHERCHES NUCLEAIRES STRASBOURG IN2P3 UNIVERSITE CNRS LOUIS PASTEUR SUB-BARRIER FUSION AND ELASTIC SCATTERING in S+Ni SYSTEMS RJ. Tighe, JJ. Vega, Genbao Liu, A.Morsad*. and J J.Kolata Physics DepL, Univ. of Notre Dame, Notre Dame, IN 46556 S.H. Fricke, R Esbensen, and S. Landowne Physics Division, Argonne National Laboratoiy, Argonne, IL 60439 ABSTRACT 32 The elastic scattering of S on 58.64^1 a^j fusiOn excitation functions for 32s+58,64i»ji and 34S^64Ni have been measured at energies near the Coulomb barrier. Our results differ in several important respects from previous measurements on these systems. Coupled- channels calculations which explicitly allow for collective inelastic excitations and single- nucléon transfer reactions simultaneously reproduce the main features of the new elastic scattering and fusion data. I. INTRODUCTION In recent years there has been considerable interest in heavy-ion reactions at energies near to and well below the Coulomb barrier. Observations of enhanced low energy heavy ion fusion rates, as compared with the predictions of conventional barrier penetration models, have accelerated the exploration of how nuclear structure influences the fusion process. At the opposite end of the reaction spectrum, optical model analyses of heavy ion elastic scattering data are found to require strongly energy-dependent complex potentials in the region of the barrier. In particular, the magnitude of the imaginary part of the optical- model potential has been found to decrease, and the magnitude of the attractive real potential to increase, as the bombarding energy is lowered. It is natural to expect that increased low energy fusion rates should be correlated with increased attraction in the optical model potential1, since both phenomena reflect the dynamic polarizability of the colliding nuclei. The interpretation of these recent developments relies upon having a consistent body of data for both fusion and elastic scattering cross sections. The measurements of the S + Ni systems by the Legnaro group2»3 have played a prominent role in discussions of the physical basis behind the observed phenomena. An analysis of the 32St58^64Ni elastic- scattering data presented in Réf. 2 tried to directly correlate the energy dependence of the optical model potentials with the corresponding low-energy fusion data3 using an energy- dependent barrier penetration model as suggested in Réf. 1. However, the theoretical basis of this procedure has been questioned in Réf. 4. A later analysis of the 32St5^64Ni elastic scattering and fusion data was carried out by Udagawa, et al.5 in the context of an absorption model, and a recent discussion of this particular analysis is given in Réf. 6. Quite recently7, the Legnaro group has published tables of their 32St58^64Ni elastic scattering data. There are, however, some unusual features of the 32St58^64Ni data presented in Refs. 2 and 3 which must be clarified before one attempts to draw conclusions from them. In the former case, elastic scattering angular distributions for 32S + 58-64Ni, near to and below fhe Coulomb barrier, are characterized by strong deviations from Rutherford scattering at angles that are well forward of the expected "grazing" angle. For example, the 32 58 experimental S + Ni elastic angular distribution at E0n = 56.7 MeV (4 MeV below the barrier) begins to deviate from Rutherford scattering at 9crn = 80°, whereas "normal" nuclear potentials predict little or no deviation even at 8cm = 160°. Coulomb excitation of low-lying excited states of the target and/or projectile, which might conceivably provide a mechanism for explaining part of the observed behavior8, is in fact far too weak to make a significant impact on the predictions. As emphasized in Réf. 5, this seems to imply that an anomalously large, unknown direct reaction process is occurring in these systems. With regard to the fusion channel, we note that the barrier parameters extracted from the analysis presented in Réf. 3 do not follow the systematics as closely as one might expect, nor do they show the same sensitivity to nuclear structure effects that we have found in detailed 9 10 studies of the ^Cl + 58.60.62.64Ni ^112?Ai + 70.72,73.74,76Ge systems - in the same general mass and charge regime. In order to gain a better insight into the behavior of the S + Ni systems, we have made new measurements of the elastic scattering and fusion channels near the barrier. Our measurements disagree with the previous results in several important respects, and in fact do not show the unusual features noted above. The new elastic scattering and fusion data have been simultaneously analyzed in the context of a coupled-channels barrier penetration model, and a good overall description of the data is achieved. The results of the experiments, presented in the next section, are followed by a discussion of the theoretical calculation. II. EXPERIMENTALMETHODANDRESULTS A. Elastic Scattering As mentioned above, the low-energy elastic scattering results of Réf. 2 could possibly be interpreted5 by invoking a significant flux-loss to unmeasured reaction channels. We therefore used a kinematic coincidence technique, which should be a sensitive way to search for strong inelastic and/or transfer channels that might be responsible for the observed anomalies. Two sets of two silicon surface-barrier position sensitive detectors (PSDs) each were placed on either side of the beam at a distance of 131mm from the target. Both detectors of one set were collimated using an array of six slits, each 2.4mm wide by 12.7mm high and separated from the adjoining slit by a center-to-center distance of 9mm. Thus, twelve angles could in principle be measured simultaneously with a single setting of the detector array, but the angle of incidence on the outermost slit of each detector was such that it did not give reproducible results and only ten angles were simultaneously measured. The two-detector array was oriented such that the plane of the collimator was perpendicular to the line from the target to the center-point of the array, and the length of this line was 131mm. The solid angle subtended by each slit therefore varied according to its distance from the target, and also because of the changing orientation of the normal to each slit relative to the target direction. The two PSDs on the opposite side of the beam were uncollimated. Each was 52mm long by 15mm high, and this array was also perpendicular at its centerpoint to the line to, and at a distance of 131mm from, the target. The coincidence efficiency in this geometry can be computed from kinematics. However, die elastic-scattering angular distribution was in fact determined from the singles rather than the coincidence counting rate so that efficiency corrections were not needed in this case. The center-of-mass (cm) angular range from 40° to 160° was covered with four settings of the detector arrays. There was considerable overlap between detector angles measured at these settings, so that many normalization checks were possible. We were also able to obtain elastic scattering information by de;ecting the recoiling Ni ions in many cases. The data set typically included 45-50 individual measurements at each beam energy. The targets consisted of 187 jig/cm2 58Ni and 160 jig/cm2 64Ni self-supporting foils. The isotopic purity of the 58Ni foil was >99.8%, while that of 64Ni was 97.3%, with 1 % 58.60NJ and 0.7% 62Ni contaminants. Further discussion of the determination of the thickness and isotopic purity of the targets can be found in Réf. 9. The beam current was collected in a magnetically-suppressed Faraday cup. The absolute elastic-scattering cross section was obtained by normalizing to Rutherford at the most forward angles. In all cases, the ratio to Rutherford was found to be very close to unity over the cm range from 40° to 80°, so that at least 15 data points could be used to determine the normalization at each energy. The elastic-scattering angular distributions obtained in this experiment are compared with the results of the previous work, as tabulated in Réf. 7, in Fig. 1. Consider, for 32 58 example, elastic scattering in the St Ni system at Ecn,= 56.0 MeV (corrected for energy loss to the center of the target). In marked contrast to the Legnaro results at nearly the same energy, no deviation from Rutherford scattering beyond the experimental uncertainty is observed over the entire cm angular range from 40° to 160° covered in the present experiment. 32 Strong population of the first excited states of S or 58,64Nj v{a inelastic scattering was not evident at any angle or beam energy and for either target, except for the two most backward angles at the highest energy measured for 32S + 58js|i. On the other hand, from the latter two data points it was determined that the experimental energy resolution (1.2 MeV FWHM) was marginal for separating thes? HVO states from the clastic yield. Therefore, we compare our data (Fig. 1) with theoretical predictions for true elastic scattering (dashed curves), as well as with the sum of elastic scattering plus inelastic scattering to the first excited states of 32S and 58.64Ni (solid curves).
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