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Exploring the shores of the “Island of Inversion”: the structure of neutron-rich Al isotopes

Exploring the shores of the “Island of Inversion”: the structure of neutron-rich Al isotopes. INTC-2003-039/P-185. Th. Kröll 1 , T. Behrens 1 R. Krücken 1 , T. Faestermann 1 , R. Gernhäuser 1 , M. Mahgoub 1 ,

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Exploring the shores of the “Island of Inversion”: the structure of neutron-rich Al isotopes

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  1. Exploring the shores of the “Island of Inversion”: the structure of neutron-rich Al isotopes INTC-2003-039/P-185 Th. Kröll1, T. Behrens1 R. Krücken1, T. Faestermann1, R. Gernhäuser1, M. Mahgoub1, M. Münch1, P. Maierbeck1, C. A. Ur2, E. Farnea2, R. Chapman3, M. Labiche3, X. Liang3, K.-M. Spohr3, N. Amzal3, H. Scheit4, O. Niedermaier4, M. Lauer4, D. Schwalm4, M. Pantea5, O. Kester6, P. Mayet7, D. Weisshaar8, T. Davinson9, J. Cederkall10, and the REX-ISOLDE and MINIBALL collaborations 1TU München – 2INFN Padova – 3University of Paisley – 4MPI Heidelberg 5TU Darmstadt –6LMU München – 7KU Leuven – 8Universität zu Köln 9University of Edinburgh – 10CERN Genève

  2. Proposal • Study of the neutron-rich nuclei 31Al and 32Al • Physics goal: • Determination of the single particle configurations • Þ Breaking of the magic number N = 20 • One neutron transfer reaction: 2H (30,31Al, 31,32Al) 1H • 30,31Al beams obtained from REX-ISOLDE • Experimental set-up: • - MINIBALL (g detection) • - DSSSD (particle detection)

  3. Motivation N=20 isotones 34Si - spherical 33Al - ??? 32Mg - deformed Magic number N = 20 Residual interaction(s) Þ breaking of magic number N = 20 Island of Inversion Ground state of nuclei consists of configurations with two neutrons excited to intruder fp-states Þ Deformation 34Si l l 33Al 31,32Al ... shores of the island are NOT known yet!!!

  4. Shell model Gap between normal (0p0h)- and intruder (2p2h)-configuration Island of Inversion Û nuclei withdeformed ground states p p Al p u 34Mg p u p p u 32Mg Al outside of island Inversion can be described … but, e.g.: 32Mg: E(4+)/E(2+) = 2.6 34Mg:E(4+)/E(2+) = 3.2 … deformed configuration is favoured!!! 7

  5. Monte Carlo shell model (MCSM) Probabilities of different configurations in the ground state of N=20 isotones Inversion can be described too … and Substantial mixing of normal sd-configurations and intruder fp-configurations in 33Al Þ 33Al is exactly at the shore of the island 33Al normal sd-shell intruder from fp-shell

  6. 31Al isotope Ground state of 31Al is outside of the “Island of Inversion”

  7. 32Al isotope Deduced from decay of 956 keV isomere Experiment Theory Inversion of 2+ and 4+ No consistent sd shell model description for 32Al can be achieved …

  8. Proposed experiment One neutron transfer reaction in inverse kinematics 2H (30,31Al, 31,32Al) 1H Spectroscopic factor S measures essentially overlap of actual and shell model wavefunction of transferred nucleon • Experimental observables: • excitation energies • Þ single particle energies • cross sections s • Þ spectroscopic factors S • particle angular distributions • Þangular momenta l • g angular distributions • particle-g-correlations • Þspins I Physics goal: determination of the configurations Þ deeper insight into the residual interactions

  9. Beam energy Integrated cross section depends on configuration to which neutron is transferred 3.1 MeV/u is the optimal beam energy

  10. Angular distribution Ebeam = 3.1 MeV/u fLab(31,32Al) < 4° Q-values: 2H (30Al, 31Al) 1H : 4.93 MeV 2H (31Al, 32Al) 1H : 1.95 MeV Þ Protons will be emitted in all directions Larger angular range in CM system Larger integrated cross section Detecting the protons in forwardorbackward direction? 2H (36S, 37S) 1H (3/2-, 646 keV, Q = 2.08 MeV)

  11. Experimental set-up MINIBALL DSSSD DE-E-telescope Beam Dump Detector PPAC MINIBALL Gamma- spectroscopy • Particles • angles • energies • ID Beam monitor Beam impurities Elastic scattering for normalisation Þ absolute cross sections 2H (30Al, 31Al)1H 2H (30Al, 30Al)2H 12C (30Al,30Al)12C 2H (30Al, 29Al)3H }

  12. Example 1H »21 h Preliminary Courtesy H. Scheit (IS410, Run in 2002)

  13. Rate estimate ISOLDE yields [atoms/mC] (standard UCx/graphite target, RILIS) Improvement of RILIS yield: 2 (3-10 possible) eREX = 3% (as obtained for 30Mg) eph,MINIBALL = 10% eDSSSD = 93% dTarget = 10 mm (» 1 mg/cm2) deuterated PE foil (CD2) 30Al-beam, 5 mb cross section Þ Rate = 19 h-1 p-g-coincidences

  14. Beam time request 3 days 30Al-beam on target: 1400 p-g-coincidences (5 mb) Þ observational limit » 0.5 mb (» 100 p-g-coincidences required) 7 days 31Al-beam on target: 320 p-g-coincidences (5 mb) Þ observational limit » 1.5 mb (» 100 p-g-coincidences required) We request 30 shifts (10 days) of beam time

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