1. 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

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
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. ? 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 ?

4. 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

5. 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

6. 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

7. 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

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

9. Study of the 2.43 MeV state in 9Be
E*= Esum 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

10. 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- 

11. 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

12. 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

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

14. 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

15. Fit alpha spectrum from 9Li decay
New
Level
Singles

16. 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

17. 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

18. Calculation of B(GT) for the 11.81 MeV level in 9Be
-singles from 9Li decay
Normalisation. n( ) = (30±3) x10-4
Corrections:
0.335 of  in ( ) MeV
Energy dependence of “f” (1.1)
Part of 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

19. 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

20. 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)

21. 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

22. 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) ]
   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)