How strong is the absorption in the 12C + 20Ne system

2014 A statistical model calculation of the compound nuclear contribution to the 12C + 20Ne elastic scattering reaction has been performed. The results indicate that compound elastic scattering may account for most of the cross-section observed at angles larger than 90° c.m., supporting the proposal that the optical potential for the 12C + 20Ne system is considerably more absorptive than for the 16O + 16O system. LE JOURNAL DE PHYSIQUE LETTRES TOME 37, JUILLET-AOUT 1976, Classification J~/n s/c~ Abstracts 4.375 We reported recently [1] on the elastic scattering of 12C by 2°Ne. This study was motivated by a desire to elucidate the relative importance of direct and compound channels in determining the surface transparency responsible for the pronounced structure and large cross-sections for elastic scattering of 160 by 160. We had suggested previously [2] that these features of 160 + 160 scattering are primarily a consequence of the difficulty for the direct reaction channels (rather than compound nuclear channels) to carry away the angular momentum brought in by the entrance channel. The inhibition of the direct channels in this case is due to unfavorable Q values associated with the closed shell structure of both the target and projectile. This circumstance is no longer present in the 12C + 2°Ne system, where the Q values for inelastic scattering and transfer reactions are more favorable. Thus if the direct channels are important one would expect to see a difference in the absorption for the 12C + 2°Ne system compared to the 160 + 160 system, whereas if the compound nuclear channels are important the absorption should be similar since the same compound nucleus is formed for both systems. An excitation function measured [1] at 70° c.m. for 12C + 2°Ne indeed showed considerably smaller cross-sections and less structure compared to 160 + 160 scattering, consistent with expectations if direct channels are dominant in determining the absorption. In a recent communication [3] to this journal results (*) Work supported in part by U. S. Energy Res. & Dev. Admin. of another study of 12C + 2°Ne elastic scattering were reported. In this work the excitation functions at more backward angles, 90° and 1300, were measured as well as at 700 c.m. At the more backward angles and higher energies the observed cross-sections are considerably larger than the quite small crosssections predicted by the optical potential obtained [1] in the fit to the earlier data. The back-angle crosssections were intermediate in magnitude between those predicted by the 12C + 2°Ne potential and j 60 + 160 potential [4]. It was thus suggested that the 12C + 2°Ne potential was somewhat too absorptive and that the absorption in the 12C + 2°Ne system is intermediate between that in the 160 + 160 and the 180 + 180 systems. It seemed likely to us that the observed crosssections at back angles in the 12C + 2°Ne system were of the approximate magnitude to be expected from compound nuclear processes. (A compound nuclear contribution of this approximate magnitude had been previously established [5] in the 160 + 160 system.) The excitation functions also exhibited structure suggestive of Ericson fluctuations. We have therefore performed a Hauser-Feshbach calculation to see if indeed compound nuclear cross-sections of the magnitude observed are expected. The calculations have been performed with the code STATIS [6], using the level density [7] and optical model parameters given in table I. The exit channels considered were chosen so as to include all of the important heavy ion exit channels which compete most effectively with the compound Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:01976003707-8016100 L-162 JOURNAL DE PHYSIQUE LETTRES