Nilsson Model 50 years Low-Lying resonant states in the 9Be continuum María José García Borge Århus-Göteborg-ISOLDE-Madrid-York Collaborations Outline: Motivation Experimental Tools & Analysis Methods Excited states in 9Be E < 9 MeV Summary and Outlook

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 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

? Break-up to multi-particle final states Sequential X Direct Y Characteristics Kinematics not fixed by conservation laws The mechanism of X->Y can be studied sequential, simultaneous, democratic... Need complete kinematics measurement to fully characterise final state Connection to level structure closer for direct break-up. Y E,G X E1  G1 Sequential Direct ?

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

Experimental technique for multiparticle detection 9Li n C-foil ISOL method -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

Analysis Method Precise determination of the source position The radioactive beam is completely stopped in the thin carbon foil  it decays at rest  linear momentum conservation The uncertainties coming from the finite size of the pixel Triple coincidences Identification of particles Removal of beta contamination Double coincidences Reconstruction of the third particle P1+ P2+ P3=0 less precise

Beta Filters The beta particle taken as an alpha  wrong reconstruction  beta filters needed n n Anticoincidences with the back detectors |Efront-Eback|<100 keV A cut in the total linear momentum ΔP<30 MeV/c E(deposited) > 200 keV n n

Double coincidences in 9Li decay 9Li beam @ 20 keV Ultra-thin entrance window Detectors Tengblad et al., NIMA525 (2004)458 E*(9Be) = Esum + 1.57 MeV (n breakup)

Study of the 2.43 MeV state in 9Be E*= Esum + 1.57 MeV Esum < 0.9 MeV 9Li beam 20 keV 40 g/cm2 C-foil R-Matrix formalism Tail through 5He(gs) E (MeV) 0.6 Hyper-spherical harmonics Bochkarev , Sov J. Nucl. Phys. 52 (1990) 964 E (MeV) 0.6

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- 

Study of low lying levels 9Be 5He 8Be(2+) 6  Esum  7 MeV J = 5/2 (p,p’) data PRC43(91)1758 7.94 MeV Level 8Be(g.s.) 2.78 MeV level 0.9  Esum  1.3 MeV J = 1/2

Contributions of the known -fed levels of 9Be 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 E, MeV Missing Intensity

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

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

Fit alpha spectrum from 9Li decay New Level Singles

Summary & Outlook Beta-delayed multi-particle emission is a powerful tool If study in full kinematics Decay mechanism E, , spin... 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=7/2 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.) FUTURE: Break up of the 2.34 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

Collaborators B.R. Fulton Århus University Chalmers Univ of Technology C.Aa. Diget H.O.U. Fynbo H. Jeppesen K. Riisager 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

Calculation of B(GT) for the 11.81 MeV level in 9Be -singles from 9Li decay Normalisation. n(4.5-5.5) = (30±3) x10-4 Corrections: 0.335 of  in (4.5-5.5) MeV Energy dependence of “f” (1.1) Part of 11.81 Mev peak out of the range (0.76) BGT= 5.3 ± 0.9 BGT =5.6 ± 1.2  Nyman et al., NPA 510 (1990) 189

Comparison of 9Li & 9C decays There is no asymmetry in the beta-decay of 9C and 9Li to the ground states of 9Be and 9B. With respect to the mirror transitions to the high energy region 9Be 9B Energy 11.81(0.15) MeV a 12.19 (0.04) MeV Width 400(30) keV a 450 (20) keV Spin 5/2- Same spin and same width a F. Ajzenberg-Selove NPA 490 (1988) 1

9B excitation energy Esum (MeV) Ep,,(keV) Sequential Decay of 12.2 MeV State  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) Bergmann et al., NPA692 (2001) 427 IAS Esum (MeV) Ep,,(keV)

Beta feeding to the 11-12 MeV region in 9Be -emission  5He(gs)-channel Fit of the high energy peak gating on the 5He(3/2-) channel 11.81 MeV state  91±10% 11.28 MeV state  9 % (e,p)-scattering on 9Be assumed J = 7/2 Only the participation of the 11.81 MeV state in 9Be for the beta feeding is considered

Asymmetry in the A=9 isobars Sequential decay  open channels MC-Simulations  Geometrical eff. & angular correlations 9C  9B (12.19 MeV, 5/2) B(GT) = 1.20 (15) / 1.58(16) [PRC61(2000) 064310]    8Be(0+) + p   8Be(2+) + p    8Be(4+ ) + p 5 He(3/2-)+    5 He(1/2-) +  Ref 0.090 (10) 0.085 (14)   0.25 (7) 0.18 (3)     -   0.60 (7) 0.74 (8)   0.06 (4) This work, NPA 692(2001)427 - PRC61(2000)      8Be(0+) + n   8Be(2+) + n    8Be(4+ ) + n   5 He(3/2-)+    5 He(1/2-) +    0.02(0.01)   0.11(0.06)    0.12(0.08)   0.28(0.06)   0.47(0.07) This work Y. Prezado, Phys. Lett B576 (2003) 55 9Li  9Be (11.8 MeV, 5/2) B(GT) = 5.3 (9)