Nilsson Model 50 years

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

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Break-up to multi-particle final states

• Characteristics
• Kinematics not fixed by conservation laws
• The mechanism of X-gtY can be studied
  sequential, simultaneous, democratic...
• Need complete kinematics measurement to fully
  characterise final state
• Connection to level structure closer for direct
  break-up.

Sequential
Direct
?
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A 9 Isobar
(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
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Experimental technique for multiparticle detection

• 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

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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

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Beta Filters
The beta particle taken as an alpha ? wrong
reconstruction ? beta filters needed
n
n

• Anticoincidences with
• the back detectors
• Efront-Ebacklt100 keV
• A cut in the total linear momentum
• ??Plt30 MeV/c
• E(deposited) gt 200 keV

n
n
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Double coincidences in 9Li decay
E(9Be) Esum 1.57 MeV (n?? breakup)
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Study of the 2.43 MeV state in 9Be
E Esum 1.57 MeV Esum lt 0.9 MeV
9Li beam 20 keV 40 ?g/cm2 C-foil
R-Matrix formalism Tail through 5He(gs)
E? (MeV)
0
0.6
Hyper-spherical harmonics Bochkarev , Sov J.
Nucl. Phys. 52 (1990) 964
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Spin Determination for states in 9Be
9Li
?
9Be
3/2-
Possible spins 5/2 ? A2-0.714 3/2 ? A20 1/2
? A21
?
n
5He
?(-)
3/2-
?
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Study of low lying levels 9Be
5He
8Be(2)
8Be(g.s.)
2.78 MeV level
0.9 ? Esum ? 1.3 MeV
J 1/2
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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

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Is any other level of 9Be contributing?
3 ? Esum ? 4 MeV
1.8 E?1 0.7 ? Esum ? 1.8 E?1 1.1
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Candidates in the literature?
Shell Model
(p,p) _at_ 180 MeV
Elevel 5.6(1) MeV, ? 1.33(36) MeV, J
3/2- Dixit et al., Phys. Rev. C 43(91)1758
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Fit alpha spectrum from 9Li decay
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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 J1/2 character
  of 2.78 MeV State
• Firm assignment of J7/2 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.)

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Collaborators
Århus University 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
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Calculation of B(GT) for the 11.81 MeV level in
9Be

• Normalisation. n?(4.5-5.5) (303) 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)

?-singles from 9Li decay
BGT 5.3 0.9
BGT 5.6 1.2 ? Nyman et al., NPA 510 (1990)
189
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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- 5/2-
Same spin and same width
a F. Ajzenberg-Selove NPA 490 (1988) 1
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Corrections

• Required corrections
• Energy losses in the C-foil
• Energy losses in detector deadlayer

Corrections as big as 400 keV!!
Corrections depending on the entrance angle
?geometry calibration needed Bergmann et al.,
NIM in press
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9B excitation energy
IAS

• 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

Esum (MeV)
Ep,?,?(keV)
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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 ? 9110 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
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Asymmetry in the A9 isobars

• Sequential decay
• ? open channels
• MC-Simulations
• ? Geometrical eff. angular correlations