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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

Why study -decay of Light Nuclei ?

- “Exact” A-body calculations possible for A12

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

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 (A40)

- 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

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

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

- 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)

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-

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

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)

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

Contributions of the known -fed levels of 9Be

Missing Intensity

E, MeV

- Sequential Decay
- 11.81 MeV State8Be(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

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 E1 + 0.7 Esum 1.8 E1 + 1.1

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

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

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.)

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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