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

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

= 168(17) fsB(E1) = 0.36(3) W.u.

Millener et al., PRC 28(83) 497

Hansen, Jensen & Jonson Ann. Rev. Nucl. Part . Sci, 45 (1995)

1s rrms = 6.0 fm 1p rrms = 5.7 fm

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.

Experimental Setup Nov-0611Be

6 DSSDs(42-44 μm,

16x16 strips)

6 PADs( 1500 μm)

1 2

34

5 6

1

16

16

16

16

16

16

1

1

1

1

1

Results of preliminary IS444 run

•11Be produced with a Ta-foil target

•Purified 20Ne for the REX-Trap •Beam: 11Be at 2.91 MeV/u.

• Intensity: 3 104 pps

• Beam time: 21.9 h (120Sn) + 6.9 h (124Sn) + 17.1 h (197Au).

• Targets: 3.5 mg/cm2 120Sn; 0.35 mg/cm2 124Sn; 0.5 mg/cm2 197Au

• Detector setup: 6 DSSD telescopes, (40 + 1500) µm.

Experimental Setup

Particle identification

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

Break-up probabilityRatio of 10Be break-up to 11Be quasi-elastic

Energy distribution of 10Be fragments

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.

Proposed Experiment•11Be produced with a Ta-foil target

• Purified 20Ne for the REX-Trap

• Beam energy: 2.91 MeV/u

• Thinner target: 120Sn 1.2 mg/cm2 improve energy resolution

• Reference target 197Au 1 mg/cm2 reference Rutherford cross sections

• Angular coverage between 15 and 70 degrees

• Thinner ΔE detectors 20 μm thick

20 m

40 m

20 m

40 m

15-450

45-700

Proposed Experiment

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

Beam Time Request

•Stable beam, 12C: 3 shifts.

•Stable beam, 9Be at 2.91MeV/u: 3 shifts.•Beam 11Be at 2.91 MeV/u: 19 shifts.

N.Events/h

16-26 deg

27-37 deg

50-70 deg

Elastic 691 130 15Break-

up10.5 9.6 4.3

Events expected in our setup assuming I=3 104 pps

Elastic Scattering: 6He + 208Pb @ 22 MeV

One channel calculations ( - - - ) unable to describe the scattering data

Coupling to the continuum needed ( ) :

Dipole polarizability Nuclear

Contributions

Inelastic excitation and break-upHigh probability for:

Inelastic excitation (- - -)

Break-up ( )

These probabilities depend strongly on the properties of the halo neutron

Data on 6He @ LLN obtained with similar set-up allowed to obtain accurate data on breakup probability.

11Be on 3.5 mg/cm2 120Sn

11Be @ 2.91 MeV/u on 120Sn at 55º

Effect of target thickness

Better separation of the two process

120Sn 1.2 mg/cm²

120Sn 3.5 mg/cm²

Particle identification

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

120Sn 3.5 mg/cm²