The Long and the Short of it: Some Fundamentals about Nuclear Isomers Paddy Regan Dept. of Physics, University of Surrey, Guildford, GU2 7XH, UK

Isomers in Nature, nuclear astrophysics aspects – 26 Al in r-p processed path, inversion of states – 180 Ta, nature’s only ‘stable’ isomer (power!) – 176 Lu, 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! – 74 Kr (GANIL) bare, 201,200 Pt (GSI) H-like

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). Future ? Projectile fragmentation, undressing…..

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.

Walker and Dracoulis, Physics World Feb. 1994

E x >1MeV, T 1/2 >1ms (red), T 1/2 >1hour (black) From Walker and Dracoulis, Nature 399, p35 (1999)

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

Seniority (spherical shell residual interaction) Isomers

Types of isomers (b) K-isomers, eg, 178 Hf, K=8 - state K=0, I=8 + K=8, I=8 - T 1/2 =4 secs Single particle spin can align on the axis of symmetry giving large K values. Decay selection rule requires  ie large K- changes require high (slow) multipoles. K=8 -, I=8 - K=0, I=8 + Instead of E1 decay, need M8! (a) Spin traps, eg. 26 Al, (N=Z=13) 0 + state. 5 +, T=0 0 keV, T 1/2 =7.4x10 5 yrs 0 +, T= keV, T 1/2 =6.3 secs (decays direct to 26 Mg GS via superallowed Fermi  + …forking in rp-process (decays to 2 + states in 26 Mg via forbidden,  l=3 decays).

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 ).

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

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

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

Full-sky Comptel map of 1.8 MeV gammas in 26 Mg following 26 Al GS beta-decay. (a) Spin traps, eg. 26 Al, (N=Z=13) 0 + state. 5 +, T=0 0 keV, T 1/2 =7.4x10 5 yrs 0 +, T= keV, T 1/2 =6.3 secs (decays direct to 26 Mg GS via superallowed Fermi  + …forking in rp-process (decays to 2 + states in 26 Mg via forbidden,  l=3 decays).

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

Astrophysical Consequences of Isomers  Ta 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 180 Ta.

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

7 - ground state, 4x10 10 yrs 1- excited state, 4hrs  123 keV 176 Lu 176 Lu survival depends on not exciting  -decaying isomer

Bohr and Mottelson, Phys. Rev. 90, 717 (1953) Wrong spin for isomer (I  >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.

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.)

In-beam, electronic technique (  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

100 Mo MeV GAMMASPHERE + CHICO TLFs BLFs elastics

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

primary beam 1GeV/u Production target Central focus, S2 Final focus, S4  E(Z 2 ) catcher degrader dipole, B  scint MW=x,y 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) >LISE M. Pfutzner et al. Phys. Lett. B444 (1998) p32 -> FRS

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

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

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

74 Kr isomer from 92 Mo 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.

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

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

136 Sb 135 Te Use FRS to select projectile fission products (forward boosted ones). Note transmission a few %. T 1/2 =565(50) ns state in 136 Sb (Z=51, N=85) M. Mineva et al. Eur. Phys. J. A11 (2001) p9-13

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

33 ns isomer in 195 Os (last stable 192 Os), useful test of structure in prolate/oblate shape coexistence region. 194 Os Wheldon et al. Phys. Rev. C63 (2001) (R) First id of ‘doubly mid-shell’ nucleus, 170 Dy (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.

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

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

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

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

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

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)

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

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.

Basic Technical Requirements for Studies with Isomers Beam pulsing, good t=0 reference for short (ns) lifetimes. In-flight separator (eg. FMA, LISE, FRS...) for ~microsecond-ms decays. Tape drive/helium jet system for 10ms->hours lifetimes Traps, cyclotrons etc. for longer lived species