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:

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Isomer Studies as Probes of Nuclear Structure in Heavy, Neutron-Rich Nuclei Dr. Paddy Regan Dept. of Physics University of Surrey Guildford, GU2 7XH

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 177 Lu ? On to the mid-shell ( 170 Dy).

What is an isomer ? Metastable (long-lived) nuclear excited state. ‘Long-lived’ could mean ~ seconds, shape isomers in alpha-clusters or ~10 15 years 180 Ta 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 208 Pb. 212 Po, high-spin  - decaying yrast trap E0 (ec) decay 74 Kr, shape isomer High-spin, yrast-trap (E3) in 212 Fr K-isomer in 178 Hf

Search for long (>100ms) K-isomers in neutron-rich(ish) A~180 nuclei. low-K high-K mid-K j K Walker and Dracoulis Nature 399 (1999) p35 (Stable beam) fusion limit makes high-K in neutron rich hard to synthesise

Expect 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~ rare-earth reg. High-  single particle orbitals from eg. i 13/2 neutrons couple together to give energetically favoured states with high-K (=  i ).

7 - ground state, 4x10 10 yrs 1- excited state, 4hrs  123 keV 176 Lu

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 ( 136 Xe) beams ? Should be ok, but will need some form of channel selection, eg. ASEP at GSI.

Modified 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) p868 Juutinen et al. Phys. Lett. 386B (1996) p80 Wheldon et al. Phys. Lett. 425B (1998) p239 Cocks et al. J. Phys. G26 (2000) p23 Krolas et al. Acta. Phys. Pol. B27 (1996) p493 Asztalos et al. Phys. Rev. C60 (1999)

A=184 A=185 A=186 A=183 A= MeV/u on to 186 W target in thermal ion source (TIS), tape speed 160 s. Mass selection achieved using dipole magnet in GSI Online mass separator (ASEP). 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 technique R=A o (1-exp(t/  Select cycle length for specific , add together multiple tape cycles.

Extract decay lifetime from grow-in curve Assumes only single component in the grow-in….what about when there are 2 components? 177 Tm-> 177 Yb T 1/2 =85 s 177 Yb-> 177 Lu 177 Hf isomer 1600 s 160 s A=177

K  =37/2 - isomer in 177 Hf (Chu et al. Phys. Rev. C6 (1972) p2259)      x            NB. 23/2 -  -decay isomer, known in 177 Lu.

              

177 Hf, K=37/ Lu, 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 178 Hf, how is this seen ?

Search for internal 177 Lu branch and decays on top of 177 Hf K  =37/2 - isomer. 160 s 1600 s s

‘Evidence’ for 177 Lu (    /2 - )   -Decaying Isomer K  =37/2 - T 1/2 =51 min isomer decay  rays observed in 177 Hf. Single component grow-in lifetime for Hf isomer not consisent with well established value. Hafnium is refractory, release is suppressed in thermal ion source. 178 Hf K  =8 - isomer observed with lifetime of (feeding) 178 Lu  -. Direct population of 182 Hf K  =8 - isomer not observed. Favoured K  =39/2 - state predicted by blocked Nilsson calcs. Calculations predict a simple      allowed GT decay between 177 Lu and 177 Hf isomers. Candidate for internal transition (or on isomer) found with lifetime consistent with 2 component fit (few minutes).

Future areas of beta-decaying isomers? Doubly-mid-shell nucleus, 170 Dy N=104, Z=66 (N p.N n =352=Maximum!). Appears to be a correlation between f values and N p N n for K=6 + isomers in A~180 region. (see P.Walker, J.Phys. G16 (1990) L233) Extrapolation suggests isomer in 170 Dy lives for hours….could be beta-decay candidate. 172 Hf, 174 Yb, 174 Hf, 176 Hf, 178 Hf, 178 W K=6 + isomers 170 Dy ? N=104 isotones, K=6 + energy Xu, Regan, Walker et al

Future Work ? Spectroscopy around ‘doubly mid-shell’ using ISAC 170 Dy Use (fragmentation) isomers as channel selection for new neutron rich spectroscopy.