Transcript Experiment IS444: Exploring Halo Effects in the Scattering of 11Be

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
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Why
Best halo nucleus is 11Li
(B2n = 295 keV).
But short T1/2 (8.5 ms).
Low production at REX.
11Be?
Hansen, Jensen & Jonson
Ann. Rev. Nucl. Part . Sci, 45 (1995)
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
t = 168(17) fs
B(E1) = 0.36(3) W.u.
Millener et al., PRC 28(83) 497
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-06
11Be
1
5
16
6 DSSDs
6
16
(42-44 μm,
16x16 strips)
1
1
16
3
4
16
1
1
16
1
16
1
2
6 PADs
( 1500 μm)
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: 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.
Particle identification
Quasi-elastic scattering
(gs+ 300 keV ½- )
Break-up probability
Ratio 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
Proposed Experiment
40 mm
40 mm
15-450
20 mm
20 mm
45-700
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
Events expected in our setup assuming I=3 104 pps
N.Events/h
16-26 deg
Elastic
Break-up
691
10.5
27-37 deg
130
9.6
•Stable beam, 12C:
3 shifts.
•Stable beam, 9Be at 2.91MeV/u:
3 shifts.
•Beam 11Be at 2.91 MeV/u:
19 shifts.
50-70 deg
15
4.3
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-up
High 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
120Sn
1.2 mg/cm²
Better separation of
the two process
120Sn
3.5 mg/cm²
Particle identification
120Sn
3.5 mg/cm²
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