1. Experiment IS444: Exploring Halo Effects in the Scattering of 11Be on a Heavy Target at REX-ISOLDE
L. Acosta1, M.A.G.Álvarez2, M.V.Andrés2, C. Angulo3, M.J.G. Borge4, J.M. Espino2, L.M.Fraile5, H. Fynbo6, D. Galaviz4, J. Gómez-Camacho2, H.B. Jeppesen5, B. Jonson7, I. Martel1, A. Moro2, I. Mukha2,
T. Nilsson7, G. Nyman7, F. Pérez-Bernal1, R. Raabe8, K. Riisager5,
D. Rodríguez1, K. Rusek9, O. Tengblad4, M. Turrión4
and the REX-ISOLDE collaboration
Aarhus6 - CERN5 - Göteborg7 - Huelva1 - Leuven8 - Louvain la Neuve3
Madrid4 - Sevilla2 - Warsaw9 -Collaboration

2. Why 11Be?
Best halo nucleus is 11Li (B2n = 295 keV) But short T1/2 (8.5 ms) Low production at REX.
Measured 2n-Halo 6He
Next is 11Be (J = 1/2+).
Bn = 504(6) keV
Long T1/2 (13.8 s).
Extremely weakly bound 1st state (Bn=184 keV , J = 1/2-). Strongly coupled to the g.s. via dipole force
Hansen, Jensen & Jonson Ann. Rev. Nucl. Part . Sci, 45 (1995)
t = 168(17) fs
B(E1) = 0.36(3) W.u.
Millener et al., PRC 28(83) 497
1s rrms = 6.0 fm p rrms = 5.7 fm

3. Motivation Halo nuclei, such as 11Be, are special.
The reaction mechanism involving halo nuclei are rather different from “normal” nuclei.
The dominant reaction channels for halo nuclei are elastic scattering and break-up.
Accurate measurements of elastic scattering and break-up are essential to understand the reaction mechanism of halo nuclei.

4. Experimental Setup Nov-06
11Be
6 DSSDs
(42-44 μm,
16x16 strips) 5 1 16 6 16 1 1 16 3
6 PADs
( 1500 μm) 4 16 1 1 16 2 1 16 1

5. Results of preliminary IS444 run
Experimental Setup
11Be produced with a Ta-foil target
Purified 20Ne for the REX-Trap
Beam: 11Be at 2.91 MeV/u.
Intensity: pps
Beam time: h (120Sn) h (124Sn) h (197Au).
Targets: 3.5 mg/cm2 120Sn; 0.35 mg/cm2 124Sn; 0.5 mg/cm2 197Au
Detector setup: 6 DSSD telescopes, ( ) µm.

6. Particle identification

7. Quasi-elastic scattering
(gs+ 300 keV ½- )

8. Ratio of 10Be break-up to 11Be quasi-elastic
Break-up probability
Ratio of 10Be break-up to 11Be quasi-elastic

9. Energy distribution of 10Be fragments

10. What have we learnt from IS444 run?
We can measure the scattering of 11Be on 120Sn, and separate 10Be events.
We cannot separate 11Be excitation. We cannot measure backward angles.
The quasi-elastic cross sections seem to deviate from Rutherford, as predicted by coupling to the continuum.
More statistics is needed, for larger angles.
Break probability is very large, even larger than expected from CDCC calculations.
Measurements at larger angles are needed, to see the trend.
The target thickness of 3.5 mg/cm2 blurs the separation between elastic and break-up events.
A thinner target is desirable.
The energy distribution of the break-up fragment could be measured.
A better energy resolution is desirable to compare with theoretical calculation and disentangle the reaction mechanism.

11. Proposed Experiment 11Be produced with a Ta-foil target
Purified 20Ne for the REX-Trap
Beam energy: MeV/u
Thinner target: 120Sn 1.2 mg/cm improve energy resolution
Reference target 197Au 1 mg/cm reference Rutherford cross sections
Angular coverage between 15 and 70 degrees
Thinner ΔE detectors 20 μm thick

12. Proposed Experiment
20 mm
40 mm
15-450
45-700

13. Goals of Experiment
Observe the reduction in the elastic scattering cross sections in 11Be.
Dipole Coulomb polarizability around = 30º,
Coulomb + Nuclear break-up beyond = 30º.
Investigate the angular distribution of break-up cross sections, which lead to the production of 10Be.
Elastic vs. Inelastic Break-up.
Investigate the energy distribution of the 10Be fragments produced in the collision.
Direct break-up vs. Transfer to the continuum vs. Core excitation.
Understand the reaction mechanism for 11Be

14. Events expected in our setup assuming I=3 104 pps
Beam Time Request
Events expected in our setup assuming I=3 104 pps
N.Events/h
16-26 deg
27-37 deg
50-70 deg
Elastic
691
130 15
Break-up
10.5
9.6
4.3
Stable beam, 12C:
3 shifts.
Stable beam, 9Be at 2.91MeV/u: 3 shifts.
Beam 11Be at 2.91 MeV/u:
19 shifts.

15. Elastic Scattering: 6He + 208Pb @ 22 MeV
One channel calculations ( ) unable to describe the scattering data
Coupling to the continuum needed ( ) :
Dipole
polarizability
Nuclear
Contributions

16. Inelastic excitation and break-up
High probability for:
Inelastic excitation (- - -)
Break-up ( )
These probabilities depend strongly on the properties of the halo neutron
Data on LLN obtained with similar set-up allowed to obtain accurate data on breakup probability.

17. 11Be on 3.5 mg/cm2 120Sn

18. MeV/u on 120Sn at 55º

19. Effect of target thickness
120Sn 1.2 mg/cm²
120Sn 3.5 mg/cm²
Better separation of the two process

20. Particle identification
120Sn 3.5 mg/cm²