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Production of Exotic Nuclear Isomers in Fragmentation and Deep-Inelastic Reactions

Production of Exotic Nuclear Isomers in Fragmentation and Deep-Inelastic Reactions. Paddy Regan Dept. of Physics, University of Surrey, Guildford, GU2 7XH, UK e-mail: p.regan@surrey.ac.uk. Outline of Talk Where do you find (long-lived) isomers ? Restrictions ? Isomer predictions.

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Production of Exotic Nuclear Isomers in Fragmentation and Deep-Inelastic Reactions

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  1. Production of Exotic Nuclear Isomers in Fragmentation and Deep-Inelastic Reactions Paddy Regan Dept. of Physics, University of Surrey, Guildford, GU2 7XH, UK e-mail: p.regan@surrey.ac.uk

  2. Outline of Talk • Where do you find (long-lived) isomers ? • Restrictions ? • Isomer predictions. • Towards the neutron rich…. • Deep-inelastic reactions and results • Projectile Fragmentation, effects of atomic stripping

  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.

  4. Walker and Dracoulis, Physics World Feb. 1994 Winnie the Pooh, (trapped by a potential barrier !) A.A. Milne (1927)

  5. 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 T1/2=0.3ms High-spin, yrast-trap (E3) in 212Fr K-isomer in 178Hf

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

  7. Bohr and Mottelson, Phys. Rev. 90, 717 (1953) NB. wrong spin for isomer !!! Ip>11 shown later to be Kp=8-, Korner et al. Phys. Rev. Letts. 27 , 1593 (1971) K-value and detailed spectroscopy very imporant in understanding isomers.

  8. low-K mid-K high-K j K Search for long (>100ms) K-isomers in neutron-rich(ish) A~180 nuclei. Walker and Dracoulis Hyp. Int. 135 83 (2001) (Stable beam) fusion limit makes high-K in neutron rich hard to synthesise Alaga, Alder, Bohr and Mottelson, Mat. Fys. Medd. 29 no 9 (1955)

  9. Can use single particle (‘Weisskopf’) estimates for transitions rates for a given multipolarity. (Eg (keV) , T1/2(s), Firestone and Shirley, Table of Isotopes (1996). Hindrance (F) (removing dependence on multipolarity and Eg) is defined by Reduced Hindrance ( fn ) gives yardstick for ‘goodness’ of K- quantum number and validity of K-selection rule (possibly a measure of axial symmetry?) fn ~ 100 typical value for ‘good’ K isomer (see Lobner Phys. Lett. B26 (1968) p279) ‘Forbiddenness’ in K isomers

  10. ‘Classic’ 31 yr 16+ isomer in 178Hf , spin-trap + K-forbidden (NB. Idaho invention!) Smith,Walker et al., in press Phys. Rev. C

  11. Smith, Walker et al., in press Phys. Rev. C

  12. 170Dy, double mid-shell, best case yet for ‘pure’ K-isomer ? (see PHR, Oi et al. Phys. Rev. C65 (2002) 037302)

  13. Ways to make 178Hf 31 yr isomer ? • Neutron capture • 176Yb(a,2n)178Hf • 176Yb(9Be,a3n)178Hf (see Dracoulis talk) • Coulomb excitation (Hayes et al., PRL (2002)) • Deep-inelastic heavy-ion binary reactions • Projectile Fragmentation

  14. 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.

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

  16. C=A+B Z (protons) B Locus of b-stable nuclei A N (neutrons) Towards the Neutron-Rich ? Fusion-evap. great for high-spins, BUT….stable beams/targets create predominantly neutron-deficient nuclei.

  17. projectile Final fragment Excited pre-fragment target hotspot Projectile Fragmentation Reactions Energy (velocity) of beam > Fermi velocity inside nucleus ~30 MeV/u Can ‘shear off’ different combinations of protons and neutrons. Large variety of exotic nuclear species created, all at forward angles with ~beam velocity. Some of these final fragments can get trapped in isomeric states. Problem 1: Isotopic identification. Problem 2: Isomeric identification.

  18. In-Flight Technique Using Projectile Fragmentation 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@GSI or LISE3@GANIL to ID 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 Zs. Podolyak et al. Phys. Lett. B491 (2000) p225 -> FRS M. Pfutzner et al. Phys Rev. C65 (2002) 064604 -> FRS

  19. 92Mo fragmentation on natNi target 76Rb 69Se 67Ge C. Chandler et al. Phys. Rev. C61 (2000) 044309

  20. 74Kr isomer from 92Mo fragmentation at GANIL. 456 keV 2+->0+ transitions decays (a) too fast (500 ns flight time) & (b) too slow for measured value of 2+ state (~25 ps) ?

  21. Ex=509 keV, T1/2~20 ns 0+ 2+ 456 keV gamma E0, 0+->0+ e- conversion decay 0+ undressing (to fiddle the decay probability) Fully stripping the nucleus of its atomic electrons (in-flight) ‘switches off’ the electron conversion decay branches. Result is that the bare nuclear isomeric lifetime is increased compared to ‘atomic’ value. (important in explosive stellar scenarios).

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

  23. 208Pb beam at 1 GeV/u allows production of (a) neutron-rich heavy (A>160) and (b) high-spin isomers, Schlegel et al.Physica Scripta T88 (2000) p72 High spins (>35/2) populated

  24. Gamma-gamma analysis on 200Pt isomer (21 ns!), M. Caamano et al. Nucl. Phys. A682 (2001) p223c; Acta Phys. Pol. B32 (2001) p763 stripping effect again (a la 74Kr).

  25. M. Pfutzner, PHR et al. Phys Rev. C65, 064604 (2002) Higher spins for greater DA.

  26. (iii) (ii) (i) Ebeam ~15-20% above Coulomb barrier beam target Z N Can not use fusion-evaporation reactions to study high-spin states in beta-stable and neutron-rich systems. Use deep-inelastic reactions.

  27. z x q1 q2 f1 f2 y

  28. Ge TLF beam qtlf,ftlf qblf,fblf BLF Simon et al., Nucl. Inst. Meth. A452, 205 (2000) Rochester Group TOF ~5-10 ns. ns-ms isomers can de-excite in be stopped byCHICO position detector. Delayed gs can still be viewed by GAMMASPHERE.

  29. 198Pt +136Xe @ 850 MeV, Dobon, Wheldon, PHR et al.,

  30. 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)

  31. Target-like fragment isomers from 136Xe+198Pt DIC, Valiente-Dobon et al., (Surrey/Rochester/Berkeley/Manchester/bPaisley/Daresbury collaboration)

  32. Target-like fragment isomers from 136Xe+198Pt DIC, Valiente-Dobon et al., (Surrey/Rochester/Berkeley/Manchester/bPaisley/Daresbury collaboration)

  33. Isomers in Nature, nuclear astrophysics aspects • 26Al in r-p processed path, inversion of states • 180Ta, nature’s only ‘stable’ isomer (nuclear battery ?) • 176Lu, cosmic chronometer and thermometer • r-process path and structure of odd-odd nuclei • 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

  34. Summary of some ‘special’, exotic cases! • 178Hf K-isomer with many branches….e.g., E5 decays. • 176Lu, cosmothermoter for two phases in 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?). • Lengthing the half-life…stripping of 74Kr, 201Pt etc.

  35. Thanks! • Bertram Blank (Bordeaux) et al., GANIL • Zsolt Podolyak (Surrey) et al. GSI • Carl Wheldon (Surrey/GSI) Berkeley expts. • Surrey PhD students, Katie Chandler, Jose Javier Valiente-Dobon, Monica Camaano, Arata Yamamoto, Sareh Al-Garni for hours and hours of analysis etc. • Physics comments/help from Phil Walker, Bill Gelletly (Surrey), Dave Warner (Daresbury) + many others! • Money from EPSRC (UK)

  36. 100Mo + 136Xe @ 750 MeV GAMMASPHERE + CHICO, PHR et al. Submitted to Phys. Rev. C. (Surrey, Rochester, Berkeley, Manchester) BLFs TLFs elastics

  37. Kinematics and angular mom. input calcs (assumes ‘rolling mode’) for 136Xe beam on 100Mo target. Estimate ~ 25hbar in TLF for ~25% above Coul. barrier. For Eb(136Xe)~750 MeV, in lab qblf~30o and qtlf~50o. 100Mo +136Xe (beam) DIC calcs.

  38. Identification of new isomeric state in 136Ba, N=80 isotone.

  39. N=80 isotonic chain, 10+ isomers (nh11/2)-2I=10+

  40. Structure of 8+ final state changes from 134Xe -> 136Ba ? Isomer decay also depends on structure of final state N=80, (nh11/2)-210+ isomers

  41. 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

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

  43. 136Xe @11.4 MeV/u on to 186W target in thermal ion source (TIS), tape speed 160 s. A=186 A=185 Mass selection achieved using dipole magnet in GSI Online mass separator (ASEP). A=184 A=183 See Bruske et al. NIM 186 (1981) p61 Z selection by tape speed to remove activity. A=182 S. Al Garni, PhD thesis, Surrey (2002) Surrey/GSI/Liv./Goettingen/Milano

  44. 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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