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Hirschegg’06: Astrophysics and Nuclear Structure

Hirschegg’06: Astrophysics and Nuclear Structure. Low-Lying resonant states in 9 Be. María José García Borge Århus-Göteborg-ISOLDE-Madrid-York Collaborations. Outline: Motivation Asymmetries in A= 9 isobar Excited states in 9 Be accessible in the -decay of 9 Li Summary and Outlook.

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Hirschegg’06: Astrophysics and Nuclear Structure

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  1. Hirschegg’06: Astrophysics and Nuclear Structure Low-Lying resonant states in 9Be María José García Borge Århus-Göteborg-ISOLDE-Madrid-York Collaborations Outline: Motivation Asymmetries in A= 9 isobar Excited states in 9Be accessible in the -decay of 9Li Summary andOutlook Eexc (9Be) Q

  2. Why study -decay of Light Nuclei ? • “Exact” A-body calculations possible for A12 reaching lowest energy states for I ≤ 9/2 • Green Funtion Monte-Carlo methods • Non-core Shell-model • Asymetries in mirror beta transitions • The (n,)9Be + 9Be(,n)12C Competes with triple- in n-rich scenarios • Importance of the + n  5He(, )9Be • Experimentally -decay provides • a clean way to feed unbound states • Break-up mechanism not fixed by kinematics

  3. Mirror asymmetry principle and Systematics b+ : p→n + e+ +  b- : n→p + e- +  E.C. : p + e-→n +  ft- ft+ n p n p Systematics of experimental values (A40) • Isospin symmetry breaking  • asymmetry in mirror b-decays  = 4.8 (4) % Thomas et al., AIP Conf. Proc 681, p. 235 • Charge independence hypothesis of nuclear interactions • symmetry of analog btransitions

  4. A = 9 Isobar Large asymmetries δ ≈ 3 δ=1.2±0.5 δ ≈ 0 Nyman et al., NPA 510 (1990) 189 Mikolas et al., PRC 37 (1988) 766 F. Ajzenberg-Selove, NPA 490 (1988) 1

  5. 9Li n C-foil Experimental technique for multiparticle detection • ISOL method • point-like pure sources • -decay to populate state of interest • clean and selective • Use DSSSDs for complete kinematics • Large solid angle (rare events) • High Segmentation (avoid summing) • Effective Readout

  6. 9B high excited states • Sequential Decay of 12.2 MeV State via 8Be(gs), 8Be(2+), 5Li(gs) and 5Li(1/2) • R-Matrix-formalism applied. • MC-simulations to account for efficiencies of each channel • Results E: 12.19(4) MeV • : 450(20) keV • J: 5/2 • BGT: 1.20(15) • U.C. Bergmann et al., Nucl. Phys. A692 (2001) 427 IAS Esum(MeV) Esum (MeV) Ep,,(keV)

  7. Spin Determination for states in 9Be Fit of the angular distribution breakup  the 5He(3/2-) channel Rev. Mod. Phys. 25 (1953) 729 9Li  9Be 3/2- Possible spins: 5/2  A2=-0.714 3/2  A2=0 1/2  A2=1  n 5He ?(-) 3/2- 

  8. A = 9 Isobar 13.257 =0.45 5/2-  = 3.4(7) δ ≈ 3 5/2- δ=1.2±0.5 54.1(15)%  = 0.032(3) δ ≈ 0 PLB576 (2003)55 NP A692(2001)427 3/2- (1/2,5/2)- (1/2)- 3/2- 3/2- Mikolas et al., PRC 37 (1988) 766 Nyman et al., NPA 510 (1990) 189 F. Ajzenberg-Selove, NPA 490 (1988) 1

  9. Study of the 2.43 MeV, 5/2- state in 9Be 5He 8Be(2+) Fit Fit Data Data E*= Esum + 1.57 MeV Esum < 0.9 MeV 8Be(gs) R-Matrix formalism Tail through 5He(gs) 0 0.3 0.6 E (MeV) Hyper-spherical harmonics Bochkarev , Sov J. Nucl. Phys. 52 (1990) 964 0 0.3 0.6 E (MeV)

  10. Study of low lying levels 9Be 5He 8Be(2+) 7.94 MeV Level 6  Esum 7 MeV J = 5/2 (e,e’p) Unpbl. Tilley,NPA745(04)155 8Be(g.s.) 2.78 MeV level 0.9  Esum  1.3 MeV J = 1/2

  11. Contributions of the known -fed levels of 9Be Missing Intensity E, MeV • Sequential Decay • 11.81 MeV State8Be(gs), 8Be(2+), 5He(gs), 5He(1/2-), 8Be(4+) • 7.94 MeV State 5He(gs), 8Be(gs) • 2.78 MeV State 8Be(2+), 5He(gs) 2.48 MeV state(Bocharev et al., Sov. J. Nucl. Phys. 52(90)964) • R-Matrix-formalism applied. • MC-simulations to account for efficiencies of each channel Incoherent sum of all channels

  12. Is any other level of 9Be contributing? J= 3/2 Elevel = 5.0(5) MeV, = 2.0(2) MeV 3  Esum 4 MeV 1.8 E1 + 0.7  Esum 1.8 E1 + 1.1

  13. Candidates in the literature? Elevel = 5 MeV, = 2 MeV, J = 3/2- Elevel = 5 MeV, = 2 MeV, J = 3/2- Shell Model (p,p’) @ 180 MeV Mikolas et al., PRC 37 (1988) 766 Elevel = 5.6(1) MeV, = 1.33(36) MeV, J = 3/2- Dixit et al., Phys. Rev. C 43(91)1758

  14. Fit alpha spectrum from 9Li decay New Level Singles Langevin et al.,Nucl. Phys. A366 (1981) 449 Nyman et al., Nucl. Phys. A510 (1990) 189

  15. Summary & Outlook • FUTURE: • Break up of the 2.43 MeV level in 9Be • 11Li: Disentangle the breakup of the 18.1 MeV state in 11Be • Comparison of BGT distribution between 11Li and its core 9Li • The beta-decay asymmetry in the A= 9 isobar system studied • for the gs and high excited ( 12 MeV) states in 9Be & 9B • 12MeV Sequential breakups for p and n • Confirmed large asymmetry  = 3.4 (1.0) • Beta asymmetry to the g.s. negligible   must be due to • differences in the structure of the two final state resonances • The low lying resonance states in 9Be have been investigated via -delayed particle emission from 9Li. • Angular correlations used for firm spin determination • First exp. determination of the J=1/2 character of 2.78 MeV State • Firm assignment of J=5/2 for the 7.94 MeV • Confirmation of broad 3/2- state at 5 MeV, = 2 MeV • Evidence of the contribution of decay via 5He(g.s.)

  16. Collaborators Århus University C.Aa. Diget H.O.U. Fynbo H. Jeppesen K. Riisager U. Bergmann Chalmers Univ of Technology B. Jonson M. Meister G. Nyman T. Nilsson K. Wilhelmsen Inst. Estructura de la Materia L.M. Fraile Y. Prezado O. Tengblad University of York B.R. Fulton

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