New techniques for probing nuclei shape around A=70

David Jenkins

Shape coexistence

• Shape coexistence long-predicted for A=70 region• Prolate and oblate shell gaps at N=Z=34 and 36• Oblate shapes are very rare• Weak claim for oblate shape in 68Se based on

moment of inertia• Weak claim for oblate shape in 72Kr based on

comparison of B(E2) with theoretical results (A. Gade, PRL 95, 022502 (2005).)

• Moral: Need new approaches to the problem!

Reorientation in Coulomb excitation

70SeCO Molecular beam extraction (ISOL technique)

1.4 GeV p-beam (900 ms) with Ib ~ 2.5 A

1.5 x 1013 protons per pulse

Zr(IV)02 ISOLDE GPS target (500 – 600 A)

yield: 3 x 105 molecules of 70SeCO per C

MK7 plasma ion source 70SeCO+ molecular ions

~ 6 x 105 ions transported to injection plate of REXTRAP each second (optimal running conditions)

70Se12C16O 98 a.m.u.

Isobaric contaminants A = 98

Break up 70SeCO inside EBISand charge breed up to a q = 19+ charge state (A/q ~ 3.68)

eliminates isobars!

REX-ISOLDE ~ 2.4%

Ib(70Se) ~ 1.4 x 104

delivered to MB target

104Pd(70Se,70Se) @ 2.94 MeV/u

“normal kinematics”

Coulomb excitation of 70Se

A.M. Hurst et al. Phys. Rev. Lett. (in press)

Conclusion:Oblate solution ruled out at 2-sigma level

A remeasurement of 2+ lifetime could restrict this even further

Preferred solution for 2+ state is weakly-deformed prolate

Implies evolution from weakly to strongly deformed prolate as a function of spin

Where are the oblate shapes?

Isobaric symmetry demands:If shape coexistence is present in Tz=1 nuclei e.g. 70Se then it must be present in the T=1 states in neighbouring odd-odd N=Z nuclei

Odd-odd N=Z

Fascinating laboratory for Fascinating laboratory for studying interplay of T=0 and studying interplay of T=0 and T=1 statesT=1 states

Very unusual low level Very unusual low level density for odd-odd nuclei density for odd-odd nuclei e.g. only 1 state below 1 MeV e.g. only 1 state below 1 MeV in in 7070BrBr

Evidence for np-pairing in Evidence for np-pairing in both low lying states and high both low lying states and high spin rotational bandsspin rotational bands

How to study odd-odd N=Z

• Nuclei are difficult to produce without reactions close to 40Ca+40Ca at near-barrier energies

• Production cross-sections are low (<1% of total cross-section)• Residues are too slow at focal plane of separator e.g. FMA to

use ion chamber to identify Z• Most measurements done with neutron detectors + charge

particle detectors to select e.g. pn or pn channel

Recoil-decay tagging

Recoil-beta tagging

RITU+GREAT

Test case: 74Rb

Proof-of-principle

• natCa (36Ar, pn) 74Rb• Ebeam = 103 MeV• τ½ (74Rb) = 65 ms• β+

endpoint ~ 10 MeV• σ ~ 10 μb

High energy positrons

Varying the beta gate size

1 10 MeV

3 10 MeV

6 10 MeV

Identification of 74Rb

A.N. Steer, et al., NIM A565, 630 (2006)

74Rb level scheme from RBT

Intensity?

Unknown case:78Y

• Nothing known about 78Y except 0+ superallowed decay and (5+) beta-decaying isomer

• RBT technique applied using 40Ca(40Ca,pn)78Y reaction

• Cross-section should be very similar to 74Rb

• 90% of flux proceeds to low-lying isomer

• Isomer is too long-lived for effective tagging

Coulomb Energy Differences

Extremely sensitive to nuclear structure effects:

•Rotational alignment mechanism •Correlations of pairs of particles •Changes in deformation•The evolution of nuclear radii D.D. Warner et al., Nature Physics 2, 311 (2006)

CEDs for A~70

Difference in np and NN pairs gives CED rise of ~12 keV/J

Uniform upward trend for deformed nuclei except:

A=78 - flat

A=70 - strongly down

A=70 data from G. de Angelis, EPJ A12, 51 (2001) and

D.G. Jenkins et al., PRC 65, 064307 (2002)

CED(J)=Ex(J,T=1,Tz<)-Ex(J,T=1,Tz>)

Effect of shape change

2=-0.3 2=0.35

2=0.18 2=0.35

CED=-7 keV

CED=-75 keV

R. Sahu et al., J. Phys. G 13, 603 (1987)

TRS calculations: T. Mylaeus et al., J. Phys. G 15, L135 (1989)

Coulomb energies calculated after S. Larsson, Phys. Scri 8, 17 (1973).

Conclusions

• Reorientation is viable technique even at very low beam currents (104 /s)

• Coulex supports weak prolate shape for 70Se 2+

• Recoil-beta-tagging: Powerful technique for study of odd-odd N=Z

• Coulomb energy differences are sensitive to nuclear shape

• CED for A=70 supports stretching of prolate shape as function of spin

Plans for future measurements

• Coulomb excitation: Extend to N=Z nuclei: 68Se and 72Kr

• 72Kr is possible if we use B(E2) from MSU to constrain matrix elements

• Recoil-beta-tagging: Search for Tz=-1 nuclei e.g.70Kr

• Study mirror symmetry in A=71 i.e. 71Kr

RBT Collaboration

B.S. Nara Singh1, A.N. Steer1, D.G. Jenkins1, R. Wadsworth1, P. Davies1, R. Glover1, N.S. Pattabiraman1, T. Grahn2, P.T. Greenlees2, P. Jones2,

R. Julin2, M. Leino2, M. Nyman2, J. Pakarinen2, P. Rahkila2, C. Scholey2, J. Sorri2, J. Uusitalo2, P.A. Butler3, M. Dimmock3, R. D. Herzberg3,

D.T. Joss3, R.D. Page3, J. Thomson3, R. Lemmon4, J. Simpson4, B. Blank5, B. Cederwall6, B. Hadinia6, M. Sandzelius6

• Department of Physics, University of York, Heslington, York YO10 5DD, UK • Department of Physics, University of Jyväskylä, P.O. Box 35, FIN-40351, Jyväskylä,

Finland • Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, UK • CCLRC Daresbury Laboratory, Keswick Lane, Warrington WA4 4AD, UK • Centre d’Etudes Nuclèaires de Bordeaux-Gradignan, F-33175 Gradignan Cedex,

France• Royal Institute of Technology, Roslagstullsbacken 21, S-106 91 Stockholm, Sweden.

REX-ISOLDE and Miniball collaborations:A.M. Hurst1, P.A. Butler1, D.G. Jenkins2, G.D. Jones1, E.S. Paul1, M. Petri1, C. Barton, A. Steer2, R. Wadsworth2, J.F. Smith3, T. Davinson4, I. Stefanescu5, J. Van de Walle5, O. Ivanov5, F. Ames6, J. Cederkall6,7, P. Delahaye6, S. Franchoo6, G. Georgiev6, U. Koster6, T. Sieber6, F. Wenander6, A. Ekstrom7, T. Czosnyka8, J. Iwanicki8, M. Zielinska8, H. Scheit9, J. Eberth10, N. Warr10, D. Weisshaar10, M. Pantea11, M. Munch12, S. Siem13, C. Sunde13, N. Syed13, A. Goergen14, E. Clement14, A. Buerger14.

1Department of Physics, University of Liverpool, UK2Department of Physics, University of York, UK

3Department of Physics and Astronomy, University of Manchester, UK4Department of Physics and Astronomy, University of Edinburgh, UK

5IKS, Catholic University of Leuven, Belgium6PH Division, CERN, Geneva, Switzerland

7Department of Physics, University of Lund, Sweden8Heavy Ion Laboratory, University of Warsaw, Poland

9MPI, University of Heidelberg, Germany10IKP, University of Cologne, Germany

11IKP, Darmstadt Technical University, Germany12Department of Physics, Munich Technical University, Germany

13Department of Physics, University of Oslo, Norway14CEA, Saclay, France