Exploring the shores of the Exploring the shores of the “Island of Inversion”: “Island of Inversion”:

the structure of neutron-rich Al isotopesthe structure of neutron-rich Al isotopes

Th. Kröll1, T. Behrens1 R. Krücken1, T. Faestermann1, R. Gernhäuser1, M. Mahgoub1, M. Münch1, P. Maierbeck1, C. A. Ur2, E. Farnea2, R. Chapman3, M. Labiche3, X. Liang3,

K.-M. Spohr3, N. Amzal3, H. Scheit4, O. Niedermaier4, M. Lauer4, D. Schwalm4, M. Pantea5, O. Kester6, P. Mayet7, D. Weisshaar8, T. Davinson9, J. Cederkall10,

and the REX-ISOLDE and MINIBALL collaborations

1TU München – 2INFN Padova – 3University of Paisley – 4MPI Heidelberg 5TU Darmstadt – 6LMU München – 7KU Leuven – 8Universität zu Köln

9University of Edinburgh – 10CERN Genève

INTC-2003-039/P-185

ProposalProposal

• Study of the neutron-rich nuclei 31Al and 32Al

• Physics goal: Determination of the single particle configurations Breaking of the magic number N = 20

• One neutron transfer reaction: 2H (30,31Al, 31,32Al) 1H

• 30,31Al beams obtained from REX-ISOLDE

• Experimental set-up: - MINIBALL ( detection) - DSSSD (particle detection)

MotivationMotivation

Residual interaction(s)

breaking of magic number N = 20

Island of InversionGround state of nuclei consists

of configurations with two neutronsexcited to intruder fp-states

Deformation

Magic number N = 20

31,32Al

N=20 isotones 34Si - spherical 33Al - ??? 32Mg - deformed

... shores of the island are NOT known yet!!!

34Si

33Al

Shell modelShell model

Inversion can be described … but, e.g.: 32Mg: E(4+) / E(2+) = 2.634Mg: E(4+) / E(2+) = 3.2 … deformed configuration is favoured!!!

Island of Inversion nuclei withdeformed ground states

Gap between normal (0p0h)- and intruder (2p2h)-configuration

Al outside of island

34Mg

32Mg

Al

MMonte Carlo shell model (MCSM)onte Carlo shell model (MCSM)

33Al

Inversion can be described too … and

Substantial mixing of

normal sd-configurationsand

intruder fp-configurations in 33Al

Probabilities of different configurations in the ground state of N=20 isotones

intruder from fp-shell normal sd-shell

33Al is exactly at theshore of the island

3131Al isotopeAl isotope

22/3

22/1

62/5

52/5%57 dsdd

Ground state of 31Al is outside

of the “Island of Inversion”

3232Al isotopeAl isotope

TheoryDeduced from

decay of956 keV isomere

Inversion of 2+ and 4+

Experiment

No consistent sd shell model description for 32Al can be achieved …

Proposed experimentProposed experiment

One neutron transfer reaction in inverse kinematics

2H (30,31Al, 31,32Al) 1H

Physics goal: determination of the configurations

deeper insight into the residual interactions

Experimental observables: • excitation energies single particle energies• cross sections spectroscopic factors S• particle angular distributionsangular momenta l• angular distributions particle--correlationsspins I

Spectroscopic factor S measures essentially overlap of

actual and shell model wavefunction of transferred nucleon

Beam energyBeam energy

3.1 MeV/u is the optimal beam energy

Integrated cross sectiondepends on configuration

to which neutron is transferred

Angular distributionAngular distribution

Ebeam = 3.1 MeV/u

Lab(31,32Al) < 4

Q-values:2H (30Al, 31Al) 1H : 4.93 MeV2H (31Al, 32Al) 1H : 1.95 MeV

Protons will be emitted inall directions

2H (36S, 37S) 1H (3/2-, 646 keV, Q = 2.08 MeV)

Detecting the protons in forward or backward

direction?

Larger angular rangein CM system

Larger integratedcross section

Experimental set-upExperimental set-up

MINIBALLDSSSD

E-E-telescopePPAC

MINIBALLBeam Dump

Detector

2H (30Al, 31Al) 1H2H (30Al, 30Al) 2H

12C (30Al, 30Al) 12C2H (30Al, 29Al) 3H

}Elastic scatteringfor normalisation

absolute

cross sections

Gamma-spectroscopy

Particles• angles• energies• ID

Beammonitor

Beamimpurities

Courtesy H. Scheit (IS410, Run in 2002)

ExampleExample

1H

Preliminary

21 h

Rate estimateRate estimate

30Al 31Al 32Al 33Al 34Al 35Al

2.5106 2.5105 5103 490 86 4

ISOLDE yields [atoms/C] (standard UCx/graphite target, RILIS)

30Al-beam, 5 mb cross section

Rate = 19 h-1 p--coincidences

Improvement of RILIS yield: 2 (3-10 possible)

REX = 3% (as obtained for 30Mg)

ph,MINIBALL = 10%

DSSSD = 93%

dTarget = 10 m ( 1 mg/cm2) deuterated PE foil (CD2)

Beam time requestBeam time request

3 days 30Al-beam on target: 1400 p--coincidences (5 mb) observational limit 0.5 mb (100 p--coincidences required)

7 days 31Al-beam on target: 320 p--coincidences (5 mb) observational limit 1.5 mb (100 p--coincidences required)

We request 30 shifts (10 days) of beam time