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Aarhus 6 - CERN 5 - Göteborg 7 - Huelva 1 - Leuven 8 - Louvain la Neuve 3

Experiment IS444: Exploring Halo Effects in the Scattering of 11 Be on a Heavy Target at REX-ISOLDE.

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Aarhus 6 - CERN 5 - Göteborg 7 - Huelva 1 - Leuven 8 - Louvain la Neuve 3

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  1. Experiment IS444:Exploring Halo Effects in the Scattering of 11Be on a Heavy Target at REX-ISOLDE L. Acosta1, M.A.G.Álvarez2, M.V.Andrés2, C. Angulo3, M.J.G. Borge4, J.M. Espino2, L.M.Fraile5, H. Fynbo6, D. Galaviz4, J. Gómez-Camacho2, H.B. Jeppesen5, B. Jonson7, I. Martel1, A. Moro2, I. Mukha2, T. Nilsson7, G. Nyman7, F. Pérez-Bernal1, R. Raabe8, K. Riisager5, D. Rodríguez1, K. Rusek9, O. Tengblad4, M. Turrión4 and the REX-ISOLDE collaboration Aarhus6 - CERN5 - Göteborg7 - Huelva1- Leuven8 - Louvain la Neuve3 Madrid4- Sevilla2 - Warsaw9-Collaboration

  2. Why 11Be? Hansen, Jensen & Jonson Ann. Rev. Nucl. Part . Sci, 45 (1995) • Best halo nucleus is 11Li (B2n = 295 keV). But short T1/2 (8.5 ms). Low production at REX. • Measured 2n-Halo 6He • Next is 11Be (J = 1/2+). Bn = 504(6) keV Long T1/2 (13.8 s). • Extremely weakly bound 1st state (Bn=184 keV , J = 1/2-). Strongly coupled to the g.s. via dipole force t = 168(17) fs B(E1) = 0.36(3) W.u. Millener et al., PRC 28(83) 497 1s rrms = 6.0 fm 1p rrms = 5.7 fm

  3. Motivation • Halo nuclei, such as 11Be, are special. • The reaction mechanism involving halo nuclei are rather different from “normal” nuclei. • The dominant reaction channels for halo nuclei are elastic scattering and break-up. • Accurate measurements of elastic scattering and break-up are essential to understand the reaction mechanism of halo nuclei.

  4. Experimental Setup Nov-06 11Be 6 DSSDs (42-44 μm, 16x16 strips) 1 16 5 6 16 1 1 16 3 6 PADs ( 1500 μm) 4 16 1 1 16 2 1 16 1

  5. Results of preliminary IS444 run Experimental Setup • 11Be produced with a Ta-foil target • Purified 20Ne for the REX-Trap • Beam:11Be at 2.91 MeV/u. • Intensity: 3 104 pps • Beam time:21.9 h (120Sn) + 6.9 h (124Sn) + 17.1 h (197Au). • Targets:3.5 mg/cm2120Sn; 0.35 mg/cm2 124Sn; 0.5 mg/cm2197Au • Detector setup: 6 DSSD telescopes, (40 + 1500) µm.

  6. Particle identification

  7. Quasi-elastic scattering (gs+ 300 keV ½- )

  8. Break-up probability Ratio of 10Be break-up to 11Be quasi-elastic

  9. Energy distribution of 10Be fragments

  10. What have we learnt from IS444 run? • We can measure the scattering of 11Be on 120Sn, and separate 10Be events. • We cannot separate 11Be excitation. We cannot measure backward angles. • The quasi-elastic cross sections seem to deviate from Rutherford, as predicted by coupling to the continuum. More statistics is needed, for larger angles. • Break probability is very large, even larger than expected from CDCC calculations. • Measurements at larger angles are needed, to see the trend. • The target thickness of 3.5 mg/cm2 blurs the separation between elastic and break-up events. • A thinner target is desirable. • The energy distribution of the break-up fragment could be measured. • A better energy resolution is desirable to compare with theoretical calculation and disentangle the reaction mechanism.

  11. Proposed Experiment • 11Be produced with a Ta-foil target • Purified 20Ne for the REX-Trap • Beam energy: 2.91 MeV/u • Thinner target: 120Sn 1.2 mg/cm2 improve energy resolution • Reference target 197Au 1 mg/cm2 reference Rutherford cross sections • Angular coverage between 15 and 70 degrees • Thinner ΔE detectors 20 μm thick

  12. 40 mm 40 mm 15-450 20 mm 20 mm 45-700 Proposed Experiment

  13. Goals of Experiment • Observe the reduction in the elastic scattering cross sections in 11Be. Dipole Coulomb polarizability around = 30º, Coulomb + Nuclear break-up beyond = 30º. • Investigate the angular distribution of break-up cross sections, which lead to the production of 10Be. Elastic vs. Inelastic Break-up. • Investigate the energy distribution of the 10Be fragments produced in the collision. Direct break-up vs. Transfer to the continuum vs. Core excitation. Understand the reaction mechanism for 11Be

  14. Beam Time Request Events expected in our setup assuming I=3 104 pps • Stable beam, 12C: 3 shifts. • Stable beam, 9Be at 2.91MeV/u: 3 shifts. • Beam 11Be at 2.91 MeV/u: 19 shifts.

  15. Elastic Scattering: 6He + 208Pb @ 22 MeV • One channel calculations ( - - - ) unable to describe the scattering data • Coupling to the continuum needed ( ) : • Dipole polarizability • Nuclear Contributions

  16. Inelastic excitation and break-up • High probability for: • Inelastic excitation (- - -) • Break-up ( ) These probabilities depend strongly on the properties of the halo neutron Data on 6He @ LLN obtained with similar set-up allowed to obtain accurate data on breakup probability.

  17. 11Be on 3.5 mg/cm2120Sn

  18. 11Be @ 2.91 MeV/u on 120Sn at 55º

  19. Effect of target thickness 120Sn 1.2 mg/cm² 120Sn 3.5 mg/cm² Better separation of the two process

  20. Particle identification 120Sn 3.5 mg/cm²

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