PHYSICAL REVIEW C 103, 064605 (2021) 6,7 Li + 27 Al reactions close to and below the Coulomb barrier J. Kühtreiber, P. Hille, O. Forstner , * H. Friedmann , † A. Pavlik , and A. Priller University of Vienna, Faculty of Physics,1090 Wien, Austria (Received 30 March 2020; revised 11 February 2021; accepted 6 May 2021; published 9 June 2021) The reactions 6,7 Li + 27 Al were compared with the reactions 17,18 O + 16 O which form the same compound nuclei 33,34 S. Cross-section data for the reactions 6,7 Li + 27 Al were derived from experimentally determined γ -ray production cross sections for transitions in several residual nuclei. For the reactions 17,18 O + 16 O experi- mental results from the literature were used. It could be shown that the weakly bound projectiles 6,7 Li undergo not only fusion processes, but also breakup reactions quite in contrast to the tightly bound projectiles 18 O and 17 O, respectively. Especially below the Coulomb barrier such direct reactions play an important role in competition to complete fusion. Calculations based on the statistical model agree well with the available data for 17,18 O + 16 O, but failed to represent the behavior of the experimentally determined production cross sections for the evaporation residues in the reactions 6,7 Li + 27 Al. But coupled-channel codes and calculations based on a nucleus-nucleus proximity potential are able to reproduce the energy dependence of the complete fusion cross sections. DOI: 10.1103/PhysRevC.103.064605 I. INTRODUCTION Radioactive beams have opened the way to regions in the nuclide chart which could not be reached by other methods (see, e.g., Geissel et al. [1]). Since radioactive projectiles are often weakly bound, breakup before complete fusion is a matter of interest. Additionally, nuclear reactions close to and below the Coulomb barrier (CB) are of eminent impor- tance during the formation of the elements by astrophysical processes. In many cases the cross sections are too small to be experimentally determined. Therefore, it is necessary to understand the mechanism of these nuclear reactions to estimate the excitation functions by model calculations. It is not yet clear if for weakly bound projectiles a breakup or a partial fusion contribute substantially to nuclear reaction close and below the CB. Tripathi et al. [2] as well as the measurements of Dasgupta et al. [3]( 7 Li + 209 Bi), Figueira et al. [4]( 6,7 Li + 144 Sm), and Rath et al. [5]( 6 Li + 144 Sm) support these ideas. A relative large number of experiments deal with the question of complete fusion (CF) suppression close to and below the CB concerning reactions of light parti- cles with different types of target material. Here only a few of these publications are mentioned. Hinde et al. [6] also observed a fusion suppression but depending on the atomic number of the target. Interestingly some authors, such as Parkar et al. [7]( 7 Li + 12 C), Mukherjee and co-workers [8] ( 6,7 Li + 16 O), [9]( 6 Li + 12,13 C), [10]( 12 C + 7 Li), and Ray et al. [11]( 6,7 Li + 24 Mg) did not see any fusion suppression. These different behaviors may be explained by a decreasing * Present address: University Jena, Institut für Optik und Quan- tenelektronik, 07743 Jena, Germany. † Corresponding author: harry.friedmann@univie.ac.at breakup effect with a decreasing atomic number of the target as observed by Hinde et al. [6] and Pakou et al. [12]. De Barbará et al. [13] did not observe fusion suppression in the 9 Be + 27 Al system and for 6 Li + 27 Al. For 7 Li + 27 Al de Barbará et al. state a possible fusion suppression but do not draw final conclusions due to problems with the analysis of this data set. Gasques et al. [14] found a systematic suppres- sion of weakly bound nuclei 6,7 Li, 9 Be, and 10,11 B on targets of 208 Pb and 209 Bi as a function of breakup threshold energy. However, most often significant fusion enhancement close or below the CB could be observed by neutron-rich projec- tiles [15–17]. Microscopic calculations are unable to explain this behavior. More insight into the reaction process may be deduced from an isotopic chain of weakly bound nuclei with increasing neutron number (e.g., Refs. [18,19] and similar work). The reactions 7 Li + 27 Al and 6 Li + 27 Al are of special interest to improve the database for such projectiles: These data should give better insight whether the fusion process is preceded or competed by a breakup process (see Canto et al. [20]). To study these reactions experimentally, 6,7 Li ions were accelerated to laboratory energies between 6 and 13 MeV and the formed reaction products in an 27 Al foil were measured by γ -ray spectrometry. The excitation functions for the formation of the most important nuclear reaction products were determined and compared with the results of investiga- tions on the reaction of the tightly bound nuclei 17,18 O + 16 O (Thomas et al. [21]) leading to the same compound nuclei 33,34 S. Since in this case both reaction partners are magic nuclei it can be expected that breakup is of minor importance and CF is the dominant process. For a meaningful comparison it is necessary to choose beam energies in order to get over- lapping excitation energies for the compound nuclei produced in both reactions. Obviously, the spin and parity distributions in the highly excited compound nuclei will not be identical. 2469-9985/2021/103(6)/064605(8) 064605-1 ©2021 American Physical Society