PHYSICAL REVIEW C 87, 014619 (2013) Fusion cross sections of 8 B + 28 Si at near-barrier energies A. Pakou, 1 E. Stiliaris, 2 D. Pierroutsakou, 3 N. Alamanos, 4 A. Boiano, 3 C. Boiano, 5 D. Filipescu, 6 T. Glodariu, 6 J. Grebosz, 7 A. Guglielmetti, 5 M. La Commara, 8 M. Mazzocco, 9 C. Parascandolo, 9 K. Rusek, 10 A. M. S´ anchez-Ben´ ıtez, 11 C. Signorini, 9 O. Sgouros, 1 F. Soramel, 9 V. Soukeras, 1 E. Strano, 9 L. Stroe, 6 N. Toniolo, 12 D. Torresi, 9 and K. Zerva 1 1 Department of Physics and HINP, The University of Ioannina, 45110 Ioannina, Greece 2 Institute of Accelerating Systems and Applications and Department of Physics, University of Athens, Athens, Greece 3 INFN – Sezione di Napoli, via Cinthia, I-80126 Napoli, Italy 4 CEA-Saclay, DAPNIA-SPhN, Gif-sur-Yvette, France 5 Universita’ degli Studi di Milano and INFN – Sezione di Milano, via Celoria 16, I-20133 Milano, Italy 6 “Horia Hulubei” National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 7 IFJ-PAN, Krakow, Poland 8 Dipartimento di Scienze Fisiche and INFN – Sezione di Napoli, via Cinthia, I-80126 Napoli, Italy 9 Dipartimento di Fisica and INFN – Sezione di Padova, via F. Marzolo 8, I-35131 Padova, Italy 10 Heavy Ion Laboratory, University of Warsaw, Pasteura 5a, 02-093 Warsaw, Poland 11 Departamento de F´ ısica Aplicada, Universidad de Huelva, E-21071 Huelva, Spain 12 INFN – Sezione di Padova, Padova, Italy (Received 12 November 2012; revised manuscript received 9 January 2013; published 28 January 2013) Fusion cross sections were measured for 8 B + 28 Si at near-barrier energies by detecting the alpha particles produced in the evaporation process. The results present a small suppression with respect to one-barrier penetration model predictions, which could be attributed to incomplete fusion processes and do not differ appreciably from fusion cross sections obtained with weakly bound but stable projectiles on the same target. Comprehensive comparisons of fusion cross sections at sub- and near-barrier energies with various light weakly bound projectiles support a simple tunneling probability with slight modifications due to coupled-channel effects. DOI: 10.1103/PhysRevC.87.014619 PACS number(s): 25.60.Pj, 25.70.−z I. INTRODUCTION Fusion of two nuclei can in principle be understood as a quantum tunneling effect of two structureless objects in a potential depending only on the distance between their centers [1]. Under this scenario it can be described by one- barrier penetration models (BPM) [2]. However, at near-barrier energies and for complex nuclei, the influence of static and dynamic effects, relevant to the detailed structure of both projectile and target nuclei [3] and the reaction mechanisms involved [4], strongly alter this image, and fusion cross sections below and near barrier energies can be interpreted via coupled-channel formalisms [5,6]. It is expected that for exotic nuclei the influence of a neutron (proton) halo or skin will enrich our knowledge relevant to the structure and reaction mechanisms [7–11] but also will lead to the appropriate evidence for producing drip-line nuclei and superheavy ele- ments. While our knowledge on fusion with neutron-rich light projectiles starts to build up [12–18], studies with proton-rich nuclei are scarce. Only one fusion measurement at sub-barrier energies on 8 B + 58 Ni was recently reported [19]. 8 B is a proton drip-line beta-decaying nucleus, attracting strong interest due to its role in the production of high-energy neutrinos in the sun [20–23] and its unusual structure with a possible proton halo [24,25]. The last issue is still in an exploratory stage since the Coulomb force may prevent the growth of the halo at distances out of the Coulomb radius. Therefore it is interesting to test the behavior of this nucleus in a fusion process and compare it with that presented by other weakly bound nuclei on various targets. It should be noted that the first fusion measurement reported for both sub- and near-barrier energies [19] very large fusion cross sections, compatible with a BPM prediction only after an elongation of the interaction radius by ∼26%. This experimental finding should be confronted also for other targets. In general, studies with 8 B are scarce, as this beam is produced in flight only in a few laboratories and with low intensities. Therefore special tools had to be invoked to perform a fusion measurement. Adopting the idea of total reaction cross-section measurements, where a silicon detector is used as an active target and the detector itself acts as a calorimeter, we have proceeded with a fusion measurement of 8 B + 28 Si at near-barrier energies between 20 and 35 MeV. This type of technique was developed by Warner and collab- orators [26–28] for total reaction cross-section measurements but it was applied mainly for intermediate energies for stable and exotic nuclei [29–33]. Very recently it was adopted for near-barrier energies with lithium projectiles [34,35]. The results were promising and gave the boost for applications with radioactive beams. The present fusion measurement is based on the assumption that, during the collision of 8 B with silicon, neither the breakup nor a transfer process can produce alpha particles, but an evaporation process can. The transfer of three protons and a neutron to silicon is considered highly improbable, and in any case the high Q value of the reaction (Q = 12.3 MeV) guarantees the discrimination between alphas from transfer and alphas from fusion. A transfer of a deuteron with Q = 3.26 MeV leads to 6 Be which in turn could give alphas with energies overlapping the fusion ones. However, according to preliminary calculations this probability is very small, and the 014619-1 0556-2813/2013/87(1)/014619(8) ©2013 American Physical Society