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

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 ?

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

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  1. 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

  2. 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).

  3. 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.

  4. 74Kr, shape isomer a-decay to states in 208Pb. E0 (ec) decay 212Po, high-spin a-decaying yrast trap High-spin, yrast-trap (E3) in 212Fr K-isomer in 178Hf

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

  6. Expect to find K-isomers in regions where high-K orbitals are at the Fermi surface. Also need large, axially symmetric deformation (b2>0.2, g~0o) Conditions fulfilled at A~170-190 rare-earth reg. 82 50 126 High-W single particle orbitals from eg. i13/2 neutrons couple together to give energetically favoured states with high-K (=SWi). 82

  7. 1- excited state, 4hrs DE=123 keV 7- ground state, 4x1010yrs 176Lu

  8. 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.

  9. 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) 044307 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 Modified from Introductory Nuclear Physics, Hodgson, Gadioli and Gadioli Erba, Oxford Press (2000) p509

  10. A=186 A=185 A=184 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 Online mass separator (ASEP). See Bruske et al. NIM 186 (1981) p61 Z selection by tape speed (ie. removing activity before it decays) and ion source choice. S. Al Garni et al. Surrey/GSI/Liv./Goettingen/Milano

  11. Use grow-in curve technique R=Ao(1-exp(t/t)) Gate on electron (b or ec) at implantation point of tape drive, gives ‘clean’ trigger. Use add-back Select cycle length for specific t, add together multiple tape cycles.

  12. Extract decay lifetime from grow-in curve 177Tm->177Yb T1/2=85 s 177Yb->177Lu 1600 s 177Hf isomer A=177 Assumes only single component in the grow-in….what about when there are 2 components? 160 s

  13. p(7/2+[514] . 9/2-[514] )x n(7/2-[514] . 9/2+[624] . 5/2-[512]) = Kp=37/2- NB. 23/2-b-decay isomer, known in 177Lu. Kp=37/2- isomer in 177Hf (Chu et al. Phys. Rev. C6 (1972) p2259)

  14. p (7/2+[404] . 9/2-[514] . 7/2-[523]) n (7/2-[514] . 9/2+[624]) = Kp=39/2-

  15. Are there any shape effects ? The PES suggest that the deformations of the two states are equal. 177Lu, K=39/2- 177Hf, K=37/2- b Configuration constrained PES calcs (by F.R.Xu)

  16. 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 ?

  17. Search for internal 177Lu branch and decays on top of 177Hf Kp=37/2- isomer. 16000 s 1600 s 160 s

  18. ‘Evidence’ for 177Lu (Kp=39/2-) b--Decaying Isomer • Kp=37/2- T1/2=51 min isomer decay g 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 Kp=8- isomer observed with lifetime of (feeding) 178Lu b-. • Direct population of 182Hf Kp=8- isomer not observed. • Favoured Kp=39/2- state predicted by blocked Nilsson calcs. • Calculations predict a simple p 7/2-[523] -> n 5/2-[512] 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).

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

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

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