Study of the reactions induced by 6He K. C. C. Pires Universidade Tecnológica Federal do Paraná Brasil

Introduction and Purpose The Experiment 6He+9Be Total Reaction Cross Section Alpha Particles Production in the 6He+9Be Collision Inelastic 6He+9Be* Collision

Introduction Atomic nucleus: many-body system Light stable nuclei: N  Z Mass increase: N > Z (Coulomb repulsion) Far from stability line: Exotic Nuclei Excess of protons or neutrons number Very different properties in comparison with stable nuclei  300 stable nuclei  5000 nuclei far from stability line A lot of nuclei not observed but foreseen Large investigation area

Purpose To study the behaviour of the total reaction cross section in exotic light systems. Why? 6He+51V,58Ni,64Zn,120Sn  large total reaction cross section compared to stable systems 1) Physics Letters B 601 (2004) 20–26 2) Physics Letters B 647 (2007) 30–35 3) The Scientific program with RIBRAS (Radioactive Ion Beams in Brasil) http://scitation.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1139&Issue=1 What happens for very light systems? e.g. 6He+9Be

The RIBRAS System – 6He+9Be two superconducting solenoids able to select light secondary beams of nuclei far from stability line.

Two measurements: Elab(6He)=16.2 MeV Elab(6He)=21.3 MeV Production of 6He beam (I104pps) Production reaction: 9Be(7Li,6He)10B; 7Li  22.2 MeV and 26.1MeV; The 1st experiment: Primary beam intensities: 300 nAe; Secondary beam intensities: 2.4x104 pps.

Detection system: Four E-E telescopes formed by silicon detectors.

1st step to do an experiment using the RIBRAS facility is to calculate the solenoid current. First moment: Monte Carlo simulation Second moment: Experimental verification The solenoid current was varied to focus the 6He beam on the telescope. This procedure mitigates the effect of the beam divergence, on the angular resolution, of the particles scattered by the secondary target.

We measured the direct 6He beam placing a telescope at zero degree and reducing the primary beam to < 1nAe. relative intensities 70% 18% 10% Resolution: 1MeV

Production of alpha particles in the 6He +9Be collision A large alpha particles production was observed Could be produced in several processes: Breakup, transfer, fusion, etc. lab=15 Elig(6He) = 0,973 MeV  (n+n+4He) Elig(9Be) = 1,665 MeV  (n+4He+4He) We expect a large production of alpha particles!

Angular distributions involving these alpha particles were calculated in order to obtain information about the breakup of 6He projectile and 9Be target. lab=18 The cross section shape is well reproduced The calculation underestimates the experimental cross section and normalization factors were necessary Possible explanation: 9Be is unbound  breakup contribution of the inelastic channel could be important but it is not taken into account in CDCC calculations. Fusion-evaporation reactions could also contribute to the alpha yields.

Inelastic 6He+9Be* collisions Events related to the 6He nucleus with lower energy than the elastic scattering peak. 6He nucleus has no bound excited states  hypothesis  events correspond of 6He+9Be collision where 9Be states were excitated. But 9Be states are unbound  interpret these events as due to the breakup of the target. Coupled Channels Calculations using the Collective Model Formalism (CC)

Total Reaction Cross Section The total reaction cross section for 6He+9Be was obtained from elastic scattering analysis with: OM, CC, CDCC. The cross sections for other systems were obtained from the literature K.C.C. Pires et al – PRC83,064603 (2011). Method: remove geometric and coulomb barrier effects [PLB601(2004)20, PRC71(2005)017601] To compare different systems at different energies we make the transformation:

1st. Strip: nuclei strongly bound 2nd. Strip: systems involving projectiles and targets weakly bound 3rd. Strip: systems involving 6He = projectile weakly bound and halo nucleus. The total reaction cross section involving the 6He nucleus is probably higher.

Enhancements in the Total Reaction Cross Section reac(6He) = total reaction cross section induced by 6He, obtained from elastic scattering experiments for several systems  reac(6Li) = obtained from optical model calculations using the standard SPP Selected only experimental data taken at energies (Ered > 1.1 MeV) Coulomb barrier for reac(6Li) drops down rapidly and the enhancement effect becomes much more pronounced. Considerable enhancements in the total reaction cross section are observed when compared to the stable 6Li isobar.

Conclusions Most alpha particles produced in the 6He+9Be collision are probably due to contaminant beams. Calculation require a high normalization factors. Relative intensities of the contaminant beams: 71.8% (7Li) ,17.9% (6He) and 10.3% (4He). This indicate that the reactions induced by 7Li have a more important weight in the alpha particle production cross section. Total Reaction Cross Section: shows that the systems are grouped into three categories according to the reduced cross section magnitude: Smaller value: strongly bound stable systems weakly bound stable systems Higher value: 6He nucleus The total reaction cross sections shows a significant relative enhancement for reactions induced by 6He when compared to the stable 6Li isobar. This enhancement was observed for heavy targets and still persists for light targets where the Coulomb breakup is negligible.

Collaboration R. Lichtenthäler, A. Lépine-Szily, V. Guimarães, M.C. Morais, R. Pampa Condori, E. Leistenschneider Departamento de Física Nuclear, Instituto de Física, Universidade de São Paulo, Brazil. A.M. Moro, M. Rodriguez-Gallardo Departamento de FAMN, Facultad de Física, Universidad de Sevilla, Spain. A. Barioni Departamento de Física da Terra e do Meio Ambiente, Instituto de Física, Universidade Federal da Bahia, Brazil. D.R. Mendes Jr, V. Morcelle, P.N. Faria Instituto de Física, Universidade Federal Fluminense, Niterói, R.J. Brazil. M. Assunção Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, Campus Diadema, Brazil. J.C. Zamora TU Darmstadt, Germany. J.M.B. Shorto Instituto de Pesquisas Energéticas e Nucleares (IPEN-CNEN), Comissão Nacional de Energia Nuclear, São Paulo, Brazil.