Dynamic studies of 11Li and its core 9Li on 208Pb near the Coulomb barrier

Dynamic studies of 11Li and its core 9Li on 208Pb near the Coulomb barrier M. Cubero1, M.J.G. Borge1, L. Acosta2, M. Alcorta1, M.A.G. Alvarez5,6, C. Diget3, D. Galaviz4, J.P. Fernández-García5, J. Gomez-Camacho5,6, J.A. Lay5, M. Madurga1, I. Martel2, A.M. Moro 5, I. Mukha5, A.M. Sánchez- Benítez2, A. Shotter7, O. Tengblad1, P. Walden7. 1 Inst. Estructura de la Materia, CSIC, Serrano 113bis, E28006 Madrid, Spain. 2 Departamento de Física Aplicada, Universidad de Huelva, E-21071, Huelva, España 3 Department of Physics, University of York, York, UK 4 CFNUL, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003, Lisboa, Portugal 5 Departamento de FAMN, Universidad de Sevilla, E-41080 Sevilla, España 6 Centro Nacional de Aceleradores, Universidad de Sevilla-Junta de Andalucía-CSIC, Av. Thomas A. Edison, 41092 Sevilla, España 7 TRIUMF, Vancouver, British Columbia, Canada, V6T 2A3 * email: [email protected] Abstract. We measured the scattering of the halo nucleus 11Li and its core 9Li on the lead target at TRIUMF at energies below and around to the Coulomb barrier. We report here on our preliminary analysis of the inclusive breakup reaction. Keywords: Halo nuclei, 11Li, 9Li, Elastic scattering, Breakup channel. PACS: 25.70.Bc; 25.60.Dz; 25.60.Gc; 25.60.Bx; 21.10.Gv; 27.20.+n INTRODUCTION In the past 20 years there has been an interest in the nuclear physics community to study the halo structure. 11Li is the archetype halo nucleus composed of a 9Li core and two neutrons with very low binding energy (S2n=369.15 (65) keV [1]). Recent theoretical calculations predicted a deviation from the standard elastic cross-section for stable nuclei, due to the distortion of the wave function of this loosely bound system when passing near a strong Coulomb field. To study this effect an experiment to measure the elastic and breakup cross-sections of 11Li on 208Pb at energies below and around the Coulomb barrier was performed in ISAC II (Isotope Separator and Accelerator) at TRIUMF laboratory in Vancouver, Canada. Both 11Li and 9Li were measured with the same center of mass energies using the same setup. This will help us disentangle the contribution of the 11Li halo to the process and to precisely determine the optical model parameters describing the scattering process. 211 CREDIT LINE (BELOW) TO BE INSERTED ON THE FIRST PAGE OF EACH PAPER CP1231, La Rábida 2009, International Scientific Meeting on Nuclear Physics, Basic Concepts in Nuclear Physics: Theory, Experiments, and Applications, edited by J. A. Caballero, C. E. Alonso, M. V. Andrés, J. E. García-Ramos, and F. Pérez-Bernal © 2010 American Institute of Physics 978-0-7354-0776-3/10/$30.00 EXPERIMENTAL SET-UP AND DATA ANALISIS We used a set of four telescopes with angular coverage from 10º to 140º. The forward telescopes consisted of 40 μm DSSSD (Double Side Silicon Strip Detector) backed by a 500 μm Silicon PAD. The backward telescopes have a very thin 20 μm SSSD (Single Side Silicon Strip Detector) backed by a 60 μm DSSSD to separate the contribution for the elastic 11Li and breakup, 9Li nuclei. See figure 1. We selected the doubly magic nucleus, 208Pb, as a target to avoid any extra effect contributing to the breakup process. The target thickness chosen was 1.45 mg·cm-2 as a compromise between sufficient statistics and extra broadening due to the straggling. The target was at 15º with respect to the beam in order to avoid shadowing from the target frame on the detector. (a) (b) FIGURE 1. Experimental set-up and pixel-by-pixel analysis. (a) Artistic view of the setup. The polar angle distribution is shown on the DSSSD. (b) The picture displays the bi-dimensional plot ∆E versus ∆E+E of the scattering events of the 11Li beam on 208Pb at 2.2 MeV·u-1 for the angular range 20º to 35º. We measured 9Li beam at 2.67 and 3.27 MeV·u−1 energies for a total of 19h with the 1.45 mg·cm−2 target of 208Pb. In order to select the optimal thickness of the target and to characterize the optical potential we studied 9Li at 3.27 and 3.67 MeV·u−1 with a target of 1.9 mg ·cm−2 during 41.5h. Afterwards we studied 11Li at 2.2 and 2.7 MeV·u−1 during about 100h with the target of 1.45 mg·cm−2. During the experiment the detectors were calibrated with the standard α-sources 148Gd, 239Pu, 241Am and 244Cm. In the data analysis several conditions were used. (i) Energy thresholds were determined from the α-sources. (ii) The difference between the energy in front and back strips is less than 70 keV so that multi-scattering from light particles like betas as well as charge sharing between strips was avoided. Figure 1 (b) shows the elastic and breakup events of 11Li at energies well below the Coulomb barrier. The considerable amount of breakup events observed at these close to forward angles indicates that the effect on the scattering process of the loosely bound structure of 11Li is higher than expected. REFERENCES 1. M. Smith et al., Phys. Rev. Lett. 101, 202501 (2008). 2. A. Moro et al., Phys. Rev. C 75, 064607 (2007). 212