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

Paddy ReganDept. 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.

• Towards the neutron rich….

• Deep-inelastic reactions and results

• Projectile Fragmentation, effects of atomic stripping

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.

Winnie the Pooh, (trapped by a potential barrier !)

A.A. Milne (1927)

Walker and Dracoulis, Physics World Feb. 1994

E0 (ec) decay

74Kr, shape isomer

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

decay to states in 208Pb.

212Po, high-spin -decaying yrast trap. (also proton decaying isomers, e.g, 53Co PLB33 (1970) 281ff).T1/2=0.3s

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

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

Bohr and Mottelson, Phys. Rev. 90, 717 (1953)

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

Search for long (>100ms) K-isomers in neutron-rich(ish) A~180 nuclei.

low-K high-K mid-K j

K

:rule sel. -K

Walker and Dracoulis Hyp. Int. 135 83 (2001)

(Stable beam) fusion limit makes high-K in neutronrich hard to synthesiseAlaga, Alder,

Bohr and Mottelson,Mat. Fys. Medd. 29 no 9 (1955)

‘Forbiddenness’ in K isomersCan use single particle (‘Weisskopf’) estimates for transitions rates for a given multipolarity.(E (keV) , T1/2(s), Firestone and Shirley, Table of Isotopes (1996). sAETM

sAETE

sETM

sAETE

EAT

W

W

W

W

/

83/2572/1

103/4562/1

13-352/1

153/2362/1

21

101.31010.32

103.0 1052.92

101.8 1020.21

106.1 1076.61

keV 500 180, for Estimates Weisskopf

Hindrance (F) (removing dependence on multipolarity and E is defined by

rates trans. Weisskopfand expt. of ratio 2/1

2/1

WT

TF

Reduced Hindrance ( f) gives yardstick for ‘goodness’ of K- quantum number andvalidity of K-selection rule (possibly a measure of axial symmetry?)

K

TTFf W ,

/1

2/1

2/1/1f~ 100 typical value for ‘good’ K isomer (see Lobner Phys. Lett. B26 (1968) p279)

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

‘Classic’ 31 yr 16+ isomer in178Hf , spin-trap + K-forbidden (NB. Idaho invention!)

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

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

Ways to make 178Hf 31 yr isomer ?

• Neutron capture • 176Yb(,2n)178Hf • 176Yb(9Be,3n)178Hf (see Dracoulis talk)• Coulomb excitation (Hayes et al., PRL (2002))• Deep-inelastic heavy-ion binary reactions• Projectile Fragmentation

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.

(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 K=5+ (see Walker, Dracoulis and Carroll Phys. Rev. C64 061302(R) (2001))

Towards the Neutron-Rich ?Fusion-evap. great for high-spins, BUT….stable beams/targets create predominantly neutron-deficient nuclei.

Z (p

roto

ns)

N (neutrons)

A

B

C=A+B

Locus of -stablenuclei

Projectile Fragmentation Reactions

hotspot

Excited pre-fragment

Finalfragment

projectile

target

Energy (velocity) of beam > Fermi velocity inside nucleus ~30 MeV/uCan ‘shear off’ different combinations of protons and neutrons.Large variety of exotic nuclear species created, all at forward angleswith ~beam velocity. Some of these final fragments can get trapped in isomeric states.

Problem 1: Isotopic identification. Problem 2: Isomeric identification.

primary beamPb @ 1GeV/u

Production target

Central focus, S2Final focus, S4

E(Z2)

cueB

QA

FTO

catcher

degraderdegrader

dipole, B

scintscint

MW=x,y

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

In-Flight Technique Using Projectile Fragmentation

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

67Ge

69Se

76Rb

92Mo fragmentation on natNi target

74Kr isomer from 92Mo fragmentationat 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) ?

undressing (to fiddle the decay probability)

0+

2+

0+

456 keV gamma

eEEEtot ,2,20

E0, 0+->0+

e- conversiondecay

Ex=509 keV, T1/2~20 ns

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).

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

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

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).

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

Can not use fusion-evaporation reactions to study high-spin states in beta-stable and neutron-rich systems.

Use deep-inelastic reactions.

Z

N

Ebeam ~15-20% above Coulomb barrier

beam

target

(i) (ii) (iii)

-1

cos1

by calculated then is correctionDoppler The

coscoscoscossinsinsinsin)cos( where

)cos(r.r by given is angleray -fragment/ the

k )cos( , j )sin()sin( ,i )cos()sin(

k, and j i, rsunit vectoCartesian For

2

2,1'

2121212112

122121

1,2

EE

rr

rzryrx

z

x

y

Simon et al., Nucl. Inst. Meth. A452, 205 (2000)

BLF

TLF

beam tlftlf

blfblf

Ge

TOF ~5-10 ns.ns-s isomers can de-excite in bestopped by CHICO position detector. Delayeds can still be viewedby GAMMASPHERE.

Rochester Group

198Pt +136Xe @ 850 MeV, Dobon, Wheldon, PHR 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)

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.

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

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

• 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

• 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.

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)

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

TLFs

BLFs

elastics

0

10

20

30

40

50

%>Ecoul

Ltlf (roll)

v/c graz tlf

Linear(%>Ecoul)

0102030405060

620 648 677 705 733 761 790E_beam (MeV)

blf_graz

tlf_graz

lmax/10

Kinematics and angular mom. input calcs (assumes ‘rolling mode’) for 136Xe beam on 100Mo target.

Estimate ~ 25hbar in TLFfor ~25% above Coul. barrier. For Eb(136Xe)~750 MeV, in labblf~30o and tlf~50o.

100Mo +136Xe (beam) DIC calcs.

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

N=80 isotonic chain, 10+ isomers(h11/2)-2

I=10+

Structure of 8+ final state changes from 134Xe -> 136Ba ?

Isomer decayalso depends on structureof final state

N=80, (h11/2)-210+ isomers

136Sb

135Te

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

Online-Mass Separation Technique

Select by massSelect by decay times

Lifetimes from grow-in curve

Surrey/GSI/Liverpool,136Xe+Tanat

A=184

A=185

A=186

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 Onlinemass separator (ASEP).

keVEQQeEAu

QevAuB

60 ,1

2.

Z selection by tape speed to remove activity.

See Bruske et al. NIM 186 (1981) p61

S. Al Garni, PhD thesis, Surrey (2002) 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

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

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

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).

e.g., Diehl et al., Atron. Astrophys 97, 181 (1993); Publications of the Astr. Society of the Pacific 110:637 (1999)

26Al isomer and the rp-process

5+

0+

3+

1+

0228

417

1058

2070 2+

t1/2=6st1/2=0.7My

Thermal photonsequilibrate isomer and ground state populations for T>5x109K. see Runkle et al., Astr.J. 556, 970 (2001)

T<4x108K not in equilibrium, must be modelled as separate isotopes in rp-process path.Jose et al., Astr. J. 520, 347(1999)

rp-process can bypass 26Al ground state, decays via isomer

Mg)(Al)(Si),(Al),(Mg 26026262524

pp

gs26226gs262525 Mg)MeV8.1(Mg)(Al),(Mg)(Al

xEp

Yield of 1.8 MeV s from 26Algs decay (e.g., relative to 22Na decay)gives insight into T and where rp-process forming 26Al occurs.

How do you produce and measure (high-energy) isomers ?

• Produce via nuclear reaction e.g., fusion-evaporation, deep-inelastic, projectile fragmentation…..

• Isomeric targets ? (see A.Tonchev NIM paper).

• Isomeric ‘beams’– Measure, depending on lifetime using

• 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: Measure mass differences from ground state using e.g. ion traps, coupled

cyclotrons etc.

In-beam, electronic technique (t)eg, PHR, G.D. Dracoulis 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

94Zr+16O-> 110Cd*->103Pd+3n

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.(See Filip Kondev’s talk)

136Sm, 138Gd Isomers orginally seen in fusion-evap (ANU data)A.M.Bruce et al. Phys.Rev. C50 (1994) p480and C55 (1997) p620

Heaviest odd-odd,N=Z gammas, isobaric analog states ? N=Z=43; 86Tc, C. Chandler et al. Phys. Rev. C61 (2000) 044309

~500 86Tc in ~ 1 week

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

iitot

i

fii

qq

effimp

tot

b

ttG

TOFTOF

FGbNN

R

1

expexp1

1

expF

, )1(

0

2

2

22

1

11

Isomeric Ratio Calculations

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

2

3/22

22

2)1(exp

ratio,isomer predicts model off-cut sharp

13

3210178.0

, 2

)1(exp2

12

fJ jth

p

ppf

ffjj

JJdJPR

AAAA

A

JJJP

m

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

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

max31blfmax

31tlf

max

3/13/1

0

221

max

1

172

1

172

fragments. twoebetween th mom. ang. relative the

and , intosplit is limit, mode rolling In the

25.12

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where, approach,closest of distance by thegiven ismax. issection -cross DIC the whereangle The

. and 219.0

is mom. ang. peripheral max. y theclassicall-Semi

l

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ll

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l

lll

fmAAEeZZd

dgrazing

AAAAVERl

B

T

T

B

blftlf

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k

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TBCMCM

Bock et al., Nukleonika22 (1977) 529