The Long and the Short of it: Some Fundamentals about Nuclear Isomers Paddy Regan Dept. of Physics,...

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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– 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

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…..

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.

Walker and Dracoulis, Physics World Feb. 1994

Ex>1MeV, T1/2>1ms (red), T1/2>1hour (black)

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

decay to states in 208Pb.

212Po, high-spin -decaying yrast trap. (also proton decaying isomers, e.g, 53Co PLB33 (1970) 281ff.

E0 (ec) decay

74Kr, shape isomer

High-spin, yrast-trap (E3) in 212Fr K-isomer in 178Hf

Seniority (spherical shell residual interaction) Isomers

Types of isomers

(b) K-isomers, eg, 178Hf, K=8- state

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 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. 26Al, (N=Z=13) 0+ state.

5+, T=0 0 keV, T1/2=7.4x105 yrs

0+, T=1 228.3 keV, T1/2=6.3 secs(decays direct to 26Mg GS via superallowed Fermi+…forking in rp-process

(decays to 2+ states in 26Mgvia forbidden, l=3 decays).

82Expect 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~170-190 rare-earth reg.

High- single particle orbitals from eg. i13/2 neutrons couple together togive 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

:rule sel. -K

Walker and Dracoulis Nature 399 (1999) p35

(Stable beam) fusion limit makes high-K in neutronrich hard to synthesise

• 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?).

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

Full-sky Comptel map of 1.8 MeV gammas in 26Mg following 26Al GS beta-decay.

(a) Spin traps, eg. 26Al, (N=Z=13) 0+ state.

5+, T=0 0 keV, T1/2=7.4x105 yrs

0+, T=1 228.3 keV, T1/2=6.3 secs(decays direct to 26Mg GS via superallowed Fermi+…forking in rp-process

(decays to 2+ states in 26Mgvia 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) 015801

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 180Ta.

Figure from Wiescher, Regan and Aprahamian, Physics World, Feb 2002.For explanation see Walker, Dracoulis and CarrolPhys. Rev. C64 061302(R) (2001) K=9- isomer might be de-excited to

1+ ground state through intermediatepath with states of K=5+.

7- ground state, 4x1010yrs

1- excited state, 4hrs

123 keV

176Lu 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

100Mo + 136Xe @ 750 MeVGAMMASPHERE + CHICO

elastics

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

primary beamPb @ 1GeV/u

Production target

Central focus, S2Final focus, S4

E(Z2)cu

catcher

degraderdegrader

dipole, B

scintscint

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) 044309 ->LISE M. Pfutzner et al. Phys. Lett. B444 (1998) p32 -> FRS

Chandler et al. Phys. Rev. C61 (2000) 044309

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

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

B476 (2000) p213

74Kr isomer from 92Mo fragmentationat GANIL. 456 keV appears to decay (a) too fast (500 nsflight 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. 90 082502 (2003)

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

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)

M. Mineva et al. Eur. Phys. J. A11 (2001) p9-13

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

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)

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.

C. Schlegel et al.Physica Scripta T88 (2000) p72

High spins (>35/2) populated

Proton drip line isomer physicsfrom 208Pb fragmentation.N=74 chain of K=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) p480and C55 (1997) p620

)1(exp

ratio,isomer predicts model off-cut sharp

210178.0

)1(exp

fJ jth

JJdJPR

M. de Jong et al. Nucl. Phys. A613 (1997) p435

M. Pfutzner et al. Phys. Rev. C & Acta. Phys. Pol. (submitted)

effimp

TOFTOF

expexp

Isomeric Ratio Calculations

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

2)12( LModified 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) p868Juutinen et al. Phys. Lett. 386B (1996) p80Wheldon et al. Phys. Lett. 425B (1998) p239 Cocks et al. J. Phys. G26 (2000) p23Krolas et al. Acta. Phys. Pol. B27 (1996) p493Asztalos et al. Phys. Rev. C60 (1999) 044307

CCMMAX

:limit Rolling

Online-Mass Separation Technique

Select by massSelect by decay times

Lifetimes from grow-in curve

Surrey/GSI/Liverpool,

136Xe+Tanat

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 Onlinemass 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 techniqueR=Ao(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

The Long and the Short of it: Some Fundamentals about Nuclear Isomers Paddy Regan Dept. of Physics,...

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