REVISTA MEXICANA DE F ´ ISICA 50 SUPLEMENTO 2, 1–4 DICIEMBRE 2004 Fusion of the 8 Li+ 208 Pb system at near-barrier energies studied via xn evaporation E.F. Aguilera, E. Martinez-Quiroz, H. Garc´ ıa-Mart´ ınez, and A. G ´ omez-Camacho Departamento del Acelerador, ININ, Apartado postal 18-1027, C. P. 11801, M´ exico, D.F., M´ exico J.J. Kolata and G. Rogachev Physics Department, University of Notre Dame, Notre Dame, Indiana, 46556-5670 P.A. DeYoung, G.F. Peaslee, and P. Mears Physics Department, Hope College, Holland, Michigan, 49422-9000 F.D. Becchetti, J.H. Lupton, and Yu Chen Physics Department, University of Michigan, Ann Arbor, Michigan, 48109-1120 J.D. Hinnefeld Physics Department, Indiana University South Bend, South Bend, Indiana, 46694-7111 Recibido el 20 de enero de 2004; aceptado el 23 de mayo de 2004 The fusion excitation function for the radioactive projectile 8 Li on a 208 Pb target has been measured at energies near the Coulomb barrier. The results show that in the considered energy interval, the evaporation of four neutrons is the most relevant mechanism. However, at the highest energies used in the experiment, the 5n channel begins to appear. The preliminary experimental cross sections show a good agreement with the predictions of a simple evaporation calculation using the code PACE. Keywords: Radioactive nucleus; fusion; incomplete fusion. Se mide la func´ on de excitaci´ on de fusi´ on para el proyectil radioactivo 8 Li sobre un blanco de 2 08Pb a energ´ ıas cercanas a la barrera Coulombiana. Los resultados muestran que a estas energ´ ıas la evaporaci´ on de cuatro neutrones es el mecanismo mas relevante. Sin embargo, par las energ´ ıas mas altas usadas en el experimento, el canal 5n empieza a aparecer. Los valores preliminares de las secciones eficaces experimentales muestran un buen acuerdo con las predicciones de un c´ alculo simle de evaporaci´ on usando el c ´ odigo PACE. Descriptores: N´ ucleo radiactivo; fusi´ on; fusi ´ on incompleta. PACS: 25.60.-t; 25.60.Pj; 25.70.-z 1. Introduction Much work lately has been done with weakly-bound light nuclei, both, stable and short-lived, i.e., radioactive. The 6 He+ 209 Bi system, for instance, recently has been found to present extremely interesting features. Its sub-barrier fusion excitation function shows a considerable enhancement with respect to simple barrier-penetration model predictions [1] and yet the direct transfer/breakup processes dominate in this energy region [2, 3]. This behaviour for the weakly-bound neutron-halo nucleus 6 He strikingly contrasts that for nor- mal nuclei, for which the fusion process usually exhausts the total cross section at energies sufficiently below the barrier. Indeed, for normal nuclei the direct reaction channels tend to close down when the energy decreases toward the bar- rier energy. This is consistent with the so called threshold anomaly that has been observed for many systems, consisting of a rapid variation, around the barrier, of the optical poten- tial obtained from elastic data. The imaginary strength be- comes smaller while the real one increases when the energy decreases around the barrier. For weakly-bound nuclei, however, transfer and breakup channels may remain open even below the barrier, as ob- served in refs. [2, 3] for the 6 He+ 209 Bi system. Consistently, a systematic optical model analysis of the corresponding elas- tic data shows no threshold anomaly for this system [4]. Such an absence of threshold anomaly has been also observed for other weakly-bound projectiles such as 6 Li [5]. Through the coupling to direct reaction channels, this peculiar behaviour is expected to affect also the fusion process [4, 6] thus posing a challenge on both theorists and experimentalists to perform complete studies involving all relevant channels in specific systems with light weakly-bound projectiles. Along these lines, considerable progress recently has been made with the 8 Li+ 208 Pb system for which both, elas- tic angular distributions and reaction channels leading to 7 Li and 4 He have been measured for energies around the bar- rier [7, 8]. The interaction barrier for this system was found to be reduced by approximately 4 MeV relative to that of the 7 Li+ 208 Pb reaction previously measured [5], indicating a strong effect of the weakly bound valence neutron on the reaction cross section near the Coulomb barrier. In fact, a