PHYSICAL REVIEW C 86, 064321 (2012) The 8 Li( p,α) 5 He reaction at low energies, and 9 Be spectroscopy around the proton threshold D. R. Mendes, Jr., 1 A. L´ epine-Szily, 1 P. Descouvemont, 2 R. Lichtenth¨ aler, 1 V. Guimar˜ aes, 1 P. N. de Faria, 1 A. Barioni, 1 K. C. C. Pires, 1 V. Morcelle, 1 R. Pampa Condori, 1 M. C. Morais, 1 E. Leistenschneider, 1 C. E. F. Lima, 1 J. C. Zamora, 1 J. A. Alcantara, 1 V. Zagatto, 1 M. Assunc ¸˜ ao, 3 and J. M. B. Shorto 4 1 Departamento de F´ ısica Nuclear, Instituto de F´ ısica da Universidade de S ˜ ao Paulo, Caixa Postal 66318, 05315-970, S˜ ao Paulo, SP, Brazil 2 Physique Nucl´ eaire Th´ eorique et Physique Math´ ematique, C.P. 229, Universit´ e Libre de Bruxelles (ULB), B-1050 Brussels, Belgium 3 Departamento de Ciˆ encias Exatas e da Terra, Universidade Federal de S˜ ao Paulo, Campus Diadema, S˜ ao Paulo, SP, Brazil 4 Instituto de Pesquisas Energ´ eticas e Nucleares (IPEN-CNEN), Comiss˜ ao Nacional de Energia Nuclear, S˜ ao Paulo, SP, Brazil (Received 14 February 2012; revised manuscript received 14 September 2012; published 26 December 2012) We present a direct measurement of the low-energy 8 Li(p,α) 5 He cross section, using a radioactive 8 Li beam impinging on a thick target. With four beam energies, we cover the energy range between E c.m. = 0.2 and 2.1 MeV. An R-matrix analysis of the data is performed and suggests the existence of two broad overlapping resonances (5/2 + at E c.m. = 1.69 MeV and 7/2 + at E c.m. = 1.76 MeV). At low energies our data are sensitive to the properties of a subthreshold state (E x = 16.67 MeV) and of two resonances above threshold. These resonances were observed in previous experiments. The R-matrix fit confirms spin assignments, and provides partial widths. We propose a new 8 Li(p,α) 5 He reaction rate and briefly discuss its influence in nuclear astrophysics. DOI: 10.1103/PhysRevC.86.064321 PACS number(s): 21.10.Hw, 21.10.Tg, 25.40.Hs, 27.20.+n I. INTRODUCTION The availability of radioactive beams provides new op- portunities in nuclear physics [1]. On the one hand, recent experiments involving radioactive beams have been very successful in nuclear astrophysics [2], where many stellar scenarios involve short-lived nuclei [3]. On the other hand, radioactive beams provide a probe of the nuclear structure in unusual conditions of excitation energy and isospin. Many experiments have been performed with various beams such as 6 He or 11 Li (see references in Ref. [1]). At energies near the Coulomb barrier or above, these experiments provide valuable information on the structure of exotic nuclei. In this work, we present a measurement of the 1 H( 8 Li,α) 5 He cross section at low energies. This experiment has been performed at Radioactive Ion Beams in Brazil (RI- BRAS) [4,5] with a 8 Li beam (τ 1/2 ≈ 0.8 s). One of our goals is to investigate the 9 Be structure near the proton threshold (16.89 MeV) through the 8 Li(p,α) 5 He reaction. The 9 Be level scheme is well known at low excitation energies [6,7], but the high-energy region is still uncertain. The 1 H( 8 Li,α) 5 He reaction allows the precise determination of several resonance parameters: energies, spins, and proton and α widths. A transfer reaction offers several advantages. In particular, the isospin of the exit channel limits the population to T = 1/2 states in 9 Be, and interferences with the Coulomb interaction, which are dominant in elastic-scattering experiments, are absent in a transfer reaction. Our experiment also addresses some issues for stellar models. Reactions associated with 8 Li play a role in nuclear astrophysics [8]. In particular the 8 Li(α, n) 11 B reaction is expected to affect nonstandard Big Bang nucleosynthesis (see Ref. [9] and references therein) and has been inves- tigated by various groups (see, for example, Ref. [10] and references therein). More recently, it was suggested that this reaction could also affect r -process nucleosynthesis [11]. Consequently, the role of other reactions involving 8 Li is an important issue which is addressed by the present experiment. The 8 Li(p,α) 5 He cross section was measured at a single energy (1.5 MeV) about twenty years ago [12]. Here we provide the experimental cross section over a wide energy range (from 0.2 to 2.1 MeV), which allows us to determine a more reliable reaction rate. The paper is organized as follows. In Sec. II, we present the experimental setup and the conditions of the experiment. Section III is devoted to an R-matrix analysis of the data and to a discussion of 9 Be spectroscopy near the proton threshold. We briefly discuss the associated reaction rate. Concluding remarks and outlook are presented in Sec. IV. II. EXPERIMENTAL METHOD AND RESULTS A. Experimental setup The 1 H( 8 Li,α) 5 He reaction was studied with the RIBRAS facility, installed at the 8-UD Pelletron Tandem of the Univer- sity of S˜ ao Paulo. The use of a 8 Li beam hitting the hydrogen atoms of a (CH 2 ) n plastic target, in inverse kinematics, made it possible to reach low energies in the center-of-mass reference frame. We give here a short description of the experimental equipment; more detail can be found in Refs. [4,5]. This facility consists of two superconducting solenoids with a 6.5-T maximum central magnetic field and a 30-cm clear warm bore. The 7 Li 3+ primary beam was accelerated by the Pelletron Accelerator at energies between 16 and 22 MeV and the beam current was typically 300 nA. The 8 Li 3 + beam was produced by the 9 Be( 7 Li, 8 Li) 8 Be transfer reaction (Q = 0.367 MeV) and focused by the first solenoid onto the secondary target. In this work we have used a 9 Be foil of 16 μm thickness as the production target. A tungsten stopper located behind the Be foil measured and integrated the primary beam current. The stopper and a collimator at the entrance of the solenoid bore defined the angular acceptance of the system; the angles of the secondary 8 Li beam with respect to the magnetic field in the solenoid (our z axis) varied between, respectively, 2 ◦ –6 ◦ at the entrance and 1.5 ◦ –4.5 ◦ at the exit of the solenoid. 064321-1 0556-2813/2012/86(6)/064321(10) ©2012 American Physical Society