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The Long and the Short of it: Some Fundamentals about Nuclear Isomers

The Long and the Short of it: Some Fundamentals about Nuclear Isomers. Paddy Regan Dept. of Physics, University of Surrey, Guildford, GU2 7XH, UK e-mail: p.regan@surrey.ac.uk. Isomers in Nature, nuclear astrophysics aspects 26 Al in r-p processed path, inversion of states

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The Long and the Short of it: Some Fundamentals about Nuclear Isomers

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  1. The Long and the Short of it:Some Fundamentals about Nuclear Isomers Paddy Regan Dept. of Physics, University of Surrey, Guildford, GU2 7XH, UK e-mail: p.regan@surrey.ac.uk

  2. Isomers in Nature, nuclear astrophysics aspects • 26Al in r-p processed path, inversion of states • 180Ta, nature’s only ‘stable’ isomer (power!) • 176Lu, cosmic chronometer and thermometer • All r-process path and structure of odd-odds !!! • Production and identification of isomers ? • Fusion-evap, projectile frag. Deep-inelastics, spallation, neutron capture… • electronic timing, proj. frag. • Mass separation for long-lived isomers • Cheating with isomer half-lives….undressing! • 74Kr (GANIL) bare, 201,200Pt (GSI) H-like

  3. 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). • Future ? Projectile fragmentation, undressing…..

  4. 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.

  5. Walker and Dracoulis, Physics World Feb. 1994

  6. From Walker and Dracoulis, Nature 399, p35 (1999) Ex>1MeV, T1/2>1ms (red), T1/2>1hour (black)

  7. From Walker and Dracoulis, Nature 399, p35 (1999)

  8. 74Kr, shape isomer a-decay to states in 208Pb. 212Po, high-spin a-decaying yrast trap. (also proton decaying isomers, e.g, 53Co PLB33 (1970) 281ff. E0 (ec) decay High-spin, yrast-trap (E3) in 212Fr K-isomer in 178Hf

  9. Seniority (spherical shell residual interaction) Isomers

  10. (a) Spin traps, eg. 26Al, (N=Z=13) 0+ state. (decays direct to 26Mg GS via superallowed Fermi b+…forking in rp-process 228.3 keV, T1/2=6.3 secs 0+, T=1 5+, T=0 (decays to 2+ states in 26Mg via forbidden, Dl=3 decays). 0 keV, T1/2=7.4x105 yrs Types of isomers (b) K-isomers, eg, 178Hf, K=8- state K=8-, I=8- K=0, I=8+ K=8, I=8- T1/2=4 secs Single particle spin can align on the axis of symmetry giving large K values. Decay selection rule requires Dl > DK, ie large K-changes require high (slow) multipoles. Instead of E1 decay, need M8! K=0, I=8+

  11. 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

  12. 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

  13. Some ‘special’, exotic cases! • 178Hf K-isomer with many branches….e.g., E5 decays. • 176Lu, chosmothermoter for s-process. • 26Al decay seen from space as example of nucleosynthesis, rp-process ‘by-pass’. • Nuclear batteries/gamma-ray lasers, can we de-excite the isomers ? (180Ta paper by PMW, GDD, JJC; 178Hf 31 yrs state?).

  14. Smith, Walker et al., submitted to Phys. Rev. C

  15. (a) Spin traps, eg. 26Al, (N=Z=13) 0+ state. (decays direct to 26Mg GS via superallowed Fermi b+…forking in rp-process 228.3 keV, T1/2=6.3 secs 0+, T=1 5+, T=0 (decays to 2+ states in 26Mg via forbidden, Dl=3 decays). 0 keV, T1/2=7.4x105 yrs Full-sky Comptel map of 1.8 MeV gammas in 26Mg following 26Al GS beta-decay.

  16. N=Z, isospin isomers, potentially important consequences for rp-process path. See e.g., Coc, Porquet and Nowacki, Phys. Rev. C61 (1999) 015801

  17. Astrophysical Consequences of Isomers 180Ta is ‘stable’ in its isomeric state, but its ground state decays in hours! Longstanding problem as to how the isomeric state is created in nature (via eg. S-process). Possible mechanism via heavier nuclei spallation or K-mixing of higher states in 180Ta.

  18. Figure from Wiescher, Regan and Aprahamian, Physics World, Feb 2002. For explanation see Walker, Dracoulis and Carrol Phys. Rev. C64 061302(R) (2001) K=9- isomer might be de-excited to 1+ ground state through intermediate path with states of Kp=5+.

  19. 1- excited state, 4hrs DE=123 keV 7- ground state, 4x1010yrs 176Lu survival depends on not exciting b-decaying isomer 176Lu

  20. Bohr and Mottelson, Phys. Rev. 90, 717 (1953) Wrong spin for isomer (Ip>11 shown later to be 8- by Korner et al. Phys. Rev. Letts. 27, 1593 (1971)). K-value and real spectroscopy very imporant in understanding isomers.

  21. How do you measure isomers ? • ns : Use in-beam electronic techniques (eg. start-stop) • ns -> ms: In-flight technique, projectile fragmentation. • 100 ms -> hours: On-line mass-separator (eg. GSI set-up). • > hours: (Mass diffs. in eg, traps, coupled cyclotrons etc.)

  22. In-beam, electronic technique (g-g-t) eg, PHR et al. Nucl. Phys. A586 (1995) p351 Fusion-evaporation reaction with pulsed beam (~1ns), separated by fixed period (~500ns). Using coincidence gamma-rays to see across isomer

  23. 100Mo + 136Xe @ 750 MeV GAMMASPHERE + CHICO BLFs TLFs elastics

  24. Isomer gating very useful in DIC experiments. Test with known case…..

  25. Production target Central focus, S2 Final focus, S4 primary beam Pb @ 1GeV/u dipole, Br degrader degrader MW=x,y scint catcher scint DE(Z2) scint (veto) Use FRS (or LISE3) to ID exotic nuclei. Transport some in isomeric states (TOF~ x00ns). Stop and correlate isomeric decays with nuclei id. eg. R. Grzywacz et al. Phys. Rev. C55 (1997) p1126 -> LISE C.Chandler et al. Phys. Rev. C61 (2000) 044309 ->LISE M. Pfutzner et al. Phys. Lett. B444 (1998) p32 -> FRS

  26. 76Rb 69Se 67Ge Chandler et al. Phys. Rev. C61 (2000) 044309

  27. Heaviest odd-odd,N=Z gammas, isobaric analog states ?86Tc, C. Chandler et al. Phys. Rev. C61 (2000) 044309

  28. 8+ isomer in 78Zn, real evidence of 78Ni shell closure. J.M.Daugas et al. Phys. Lett. B476 (2000) p213

  29. 74Kr isomer from 92Mo fragmentation at GANIL. 456 keV appears to decay (a) too fast (500 ns flight time) and (b) too slow for measured value of 2+ state (~25 ps). Effect of undressing the nucleus of its e- and switching off electron-conversion decay mode…see later.

  30. from Bouchez et al., Phys. Rev. Lett. 90 082502 (2003)

  31. Gamma-gamma analysis on 200Pt isomer (21 ns!), Caamano et al. Nucl. Phys. A682 (2001) p223c; Acta Phys. Pol. B32 (2001) p763

  32. M. Mineva et al. Eur. Phys. J. A11 (2001) p9-13 Use FRS to select projectile fission products (forward boosted ones). Note transmission a few %. T1/2=565(50) ns state in 136Sb (Z=51, N=85) 135Te 136Sb

  33. 170Dy, double mid-shell, may be best case yet for ‘pure’ K-isomer see PHR et al. Phys. Rev. C65 (2002) 037302

  34. First id of ‘doubly mid-shell’ nucleus, 170Dy (N=104, Z=66). K=6+ isomers predicted for well deformed N=104 nuclei. TRS calcs (F.Xu) predict a very ‘stiff’, highly deformed prolate nucleus. Could be the best K-isomer? Data from M.Caamano et al. 33 ns isomer in 195Os (last stable 192Os), useful test of structure in prolate/oblate shape coexistence region. 194Os Wheldon et al. Phys. Rev. C63 (2001) 011304(R)

  35. C. Schlegel et al.Physica Scripta T88 (2000) p72 High spins (>35/2) populated

  36. Proton drip line isomer physics from 208Pb fragmentation. N=74 chain of Kp=8- isomers. Next in chain would be 140Dy, proton decay daughter of (deformed) 141Tb. Isomers orginally seen in fusion-evap (ANU data) A.M.Bruce et al. Phys. Rev. C50 (1994) p480 and C55 (1997) p620

  37. M. de Jong et al. Nucl. Phys. A613 (1997) p435 M. Pfutzner et al. Phys. Rev. C & Acta. Phys. Pol. (submitted)

  38. Isomeric Ratio Calculations M. Pfutzner et al. Phys Rev. C65, 064604 (2002)

  39. M. Pfutzner et al. Phys Rev. C65, 064604 (2002)

  40. 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

  41. Online-Mass Separation Technique Select by mass Select by decay times Surrey/GSI/Liverpool, 136Xe+Tanat Lifetimes from grow-in curve

  42. 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

  43. 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.

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