Isomer Studies as Probes of Nuclear Structure in Heavy, Neutron-Rich Nuclei

Dr. Paddy Regan

Dept. of Physics

University of Surrey

Guildford, GU2 7XH

e-mail: p.regan@surrey.ac.uk

Outline of Talk

• What are isomers and what can you tell from them.

• Where do you find isomers ?

• How might you measure them ?

• Beta-decaying high-spin isomer(s) in 177Lu ?

• On to the mid-shell (170Dy).

What is an isomer ?

Metastable (long-lived) nuclear excited state.

‘Long-lived’ could mean

~10-19 seconds, shape isomers in alpha-clusters or

~1015 years 180Ta 9+->1+ decay.

Why/when do you get isomers?

If there is (i) large change in spin (‘spin-trap’)

(ii) small energy change

(iii) dramatic change in structure (shape, K-value)

What do isomers tell you ?

Isomers occur due to single particle structure. K-isomers probe both single particle and collective structure.

decay to states in 208Pb.

212Po, high-spin -decaying yrast trap

E0 (ec) decay

74Kr, shape isomer

High-spin, yrast-trap (E3) in 212Fr K-isomer in 178Hf

Search for long (>100ms) K-isomers in neutron-rich(ish) A~180 nuclei.

low-K high-K mid-K j

K

:rule sel. -K

Walker and Dracoulis Nature 399 (1999) p35

(Stable beam) fusion limit makes high-K in neutronrich hard to synthesise

82

126

50

82Expect to find K-isomers in regions where high-K orbitals are at the Fermi surface.

Also need large, axially symmetric deformation

(

Conditions fulfilled at A~170-190 rare-earth reg.

High- single particle orbitals from eg. i13/2 neutrons couple together togive energetically favoured states with high-K (=i).

7- ground state, 4x1010yrs

1- excited state, 4hrs

123 keV

176Lu

Aim to study decays of long-lived, high-spin isomers around A~180.

• How do you make high-spins ?

•Fusion-evaporation reactions (not neutron-rich)

•Projectile fragmentation (see MH talk), limit on lifetimes to <10ms

• Deep-inelastic/binary collisions with heavy (136Xe) beams ?

• Should be ok, but will need some form of channel selection, eg. ASEP at GSI.

2)12( LModified from Introductory Nuclear Physics, Hodgson, Gadioli and Gadioli Erba, Oxford Press (2000) p509

Aim? To perform high-spin physics in stable and neutron rich nuclei. Problem: Fusion makes proton-rich nuclei.Solutions? (a)fragmentation (b) binary collisions/multi-nucleon transfer

See eg. Broda et al. Phys. Rev Lett. 74 (1995) p868Juutinen et al. Phys. Lett. 386B (1996) p80Wheldon et al. Phys. Lett. 425B (1998) p239 Cocks et al. J. Phys. G26 (2000) p23Krolas et al. Acta. Phys. Pol. B27 (1996) p493Asztalos et al. Phys. Rev. C60 (1999) 044307

CCMMAX

MAX

TB

TLF

VER

L

LAA

L

2

2

31

2

1

1

7

2

:limit Rolling

A=184

A=185

A=186

A=183

A=182

136Xe @11.4 MeV/u on to 186W target in thermal ion source (TIS), tape speed 160 s.

Mass selection achieved using dipole magnet in GSI Onlinemass separator (ASEP).

keVEQ

Qe

EAu

Qe

vAuB

60 ,1

2.

Z selection by tape speed (ie. removing activity before it decays) and ion source choice.

See Bruske et al. NIM 186 (1981) p61

S. Al Garni et al. Surrey/GSI/Liv./Goettingen/Milano

Gate on electron ( or ec) at implantation point of tape drive, gives ‘clean’ trigger. Use add-back

Use grow-in curve techniqueR=Ao(1-exp(t/

Select cycle length for specific , add together multiple tape cycles.

)1(0

t

eIR

Extract decay lifetime from grow-in curve

Assumes only single component in the grow-in….what about when there are 2 components?

177Tm->177Yb

T1/2=85 s

177Yb->177Lu177Hf isomer 1600 s

160 sA=177

K=37/2- isomer in 177Hf (Chu et al. Phys. Rev. C6 (1972) p2259)

x

NB. 23/2- -decay isomer, known in 177Lu.

177Hf, K=37/2- 177Lu, K=39/2-

Configuration constrained PES calcs (by F.R.Xu)

Are there any shape effects ?

The PES suggest that the deformations of the two states are equal.

Hf (Z=72) has a higher ionisation potential and vapour temp. than Lu (Z=71). One would expect Hf to be hindered in its release from a TIS

178Hf, how is this seen ?

Search for internal 177Lu branch and decays on top of 177Hf K=37/2- isomer.

160 s

1600 s

16000 s

‘Evidence’ for 177Lu (/2-) -Decaying Isomer

• K=37/2- T1/2=51 min isomer decay rays observed in 177Hf.

• Single component grow-in lifetime for Hf isomer not consisent with well established value.

• Hafnium is refractory, release is suppressed in thermal ion source.• 178Hf K=8- isomer observed with lifetime of (feeding) 178Lu -.• Direct population of 182Hf K=8- isomer not observed.• Favoured K=39/2- state predicted by blocked Nilsson calcs.• Calculations predict a simple allowed GT

decay between 177Lu and 177Hf isomers.• Candidate for internal transition (or on isomer) found with lifetime

consistent with 2 component fit (few minutes).

,

1

21

exp

21

WT

Tf

Future areas of beta-decaying isomers?Doubly-mid-shell nucleus, 170DyN=104, Z=66 (Np.Nn=352=Maximum!).Appears to be a correlation betweenf values and NpNn for K=6+ isomers in A~180 region.(see P.Walker, J.Phys. G16 (1990) L233)

Extrapolation suggestsisomer in 170Dy lives forhours….could be beta-decay candidate.

172Hf, 174Yb, 174Hf, 176Hf, 178Hf, 178W K=6+ isomers

170Dy ?

N=104 isotones, K=6+ energy

Xu, Regan, Walker et al

Future Work ?

• Spectroscopy around ‘doubly mid-shell’ using ISAC 170Dy

• Use (fragmentation) isomers as channel selection for new neutron rich spectroscopy.