Isomer Studies as Probes of Nuclear
Structure in Heavy, Neutron-Rich Nuclei
Dr. Paddy Regan
Dept. of Physics
University of Surrey
Guildford, GU2 7XH
e-mail: p.regan@surrey.ac.uk
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).
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.
K-isomers probe both single particle and collective structure.
74Kr,
shape isomer
a-decay to
states in 208Pb.
high-spin adecaying yrast trap
212Po,
High-spin,
yrast-trap
(E3) in 212Fr
E0 (ec) decay
K-isomer in 178Hf
Search for long (>100ms) K-isomers in neutron-rich(ish) A~180 nuclei.
high-K
mid-K
j
low-K
K
Walker and Dracoulis
Nature 399 (1999) p35
K - sel. rule :
  
(Stable beam) fusion limit
makes high-K in neutron
rich hard to synthesise
82
50
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)
126
82
Conditions fulfilled at
A~170-190 rare-earth reg.
High-W single particle
orbitals from eg. i13/2
neutrons couple together to
give energetically favoured
states with high-K (=SWi).
1- excited state, 4hrs
E=123 keV
7- ground state, 4x1010yrs
176Lu
Aim to study decays of long-lived, high-spin isomers around
A~180.
• How do you make high-spins ?
•Fusion-evaporation reactions (not neutron-rich)
•Projectile fragmentation (see MH talk), limit on lifetimes
to <10ms
• Deep-inelastic/binary collisions with heavy (136Xe)
beams ?
• Should be ok, but will need some form of channel
selection, eg. ASEP at GSI.
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
Rolling limit :
Modified from Introductory
Nuclear Physics, Hodgson,
Gadioli and Gadioli Erba,
Oxford Press (2000) p509
LTLF
2
1
= 
7  1   AB AT )1
LMAX
2 R 2
ECM - VC )
=
2

(2 L  1) 2

L
3  MAX

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
A=186
136Xe
@11.4 MeV/u on to
186W target in thermal ion
source (TIS), tape speed 160 s.
A=185
Mass selection achieved using
dipole magnet in GSI Online
mass separator (ASEP).
A=184
Au.v
2 EAu
B =
=
Qe
Qe
Q = 1, E = 60 keV
A=183
See Bruske et al. NIM 186 (1981) p61
A=182
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
Use grow-in curve technique
Gate on electron (b or ec) at
R=Ao(1-exp(t/t))
implantation point of tape drive,
gives ‘clean’ trigger. Use add-back Select cycle length for specific t,
add together multiple tape cycles.
Extract decay lifetime
from grow-in curve
177Tm->177Yb
T1/2=85 s
177Hf isomer 1600 s
177Yb->177Lu
- t
R = I 0 (1 - e )
A=177
160 s
Assumes only
single component in
the grow-in….what
about when there
are 2 components?
7/2[514] . 9/2-[514] ) x
n7/2-[514] . 9/2[624] . 5/2-[512])
= =37/2-
NB. 23/2- b-decay isomer,
known in 177Lu.
K=37/2- isomer in 177Hf (Chu et
al. Phys. Rev. C6 (1972) p2259)
 7/2[404] . 9/2-[514] . 7/2-[523])
n 7/2-[514] . 9/2[624]) = =39/2-
Are there any shape effects ?
The PES suggest that the deformations of the two
states are equal.
177Hf,
K=37/2-
177Lu,
K=39/2-
b
Configuration constrained PES calcs (by F.R.Xu)
Hf (Z=72) has a higher ionisation
potential and vapour temp.
than Lu (Z=71). One would
expect Hf to be hindered in its
release from a TIS
178Hf,
how is
this seen ?
Search for internal 177Lu branch
and decays on top of 177Hf
K=37/2- isomer.
16000 s
1600 s
160 s
‘Evidence’ for 177Lu (=39/2-) b--Decaying Isomer
• K=37/2- T1/2=51 min isomer decay g rays observed in 177Hf.
• Single component grow-in lifetime for Hf isomer not consisent
with well established value.
• Hafnium is refractory, release is suppressed in thermal ion source.
• 178Hf K=8- isomer observed with lifetime of (feeding) 178Lu b-.
• Direct population of 182Hf K=8- isomer not observed.
• Favoured K=39/2- state predicted by blocked Nilsson calcs.
• Calculations predict a simple  7/2-[523] -> n 5/2-[512] allowed
GT decay between 177Lu and 177Hf isomers.
• Candidate for internal transition (or on isomer) found with lifetime
consistent with 2 component fit (few minutes).
Future areas of beta-decaying isomers?
Doubly-mid-shell nucleus, 170Dy
N=104, Z=66 (Np.Nn=352=Maximum!).
Appears to be a correlation between
fn values and NpNn for K=6+ isomers
in A~180 region.
(see P.Walker, J.Phys. G16 (1990) L233)
170Dy
 T1
fn =  2 W
 T1
 2
exp
1
n
 ,


n =  - 
Extrapolation suggests
isomer in 170Dy lives for
hours….could be
beta-decay candidate.
?
N=104 isotones,
K=6+ energy
172Hf, 174Yb, 174Hf, 176Hf,
178Hf, 178W
K=6+ isomers
Xu, Regan, Walker et al
Future Work ?
• Spectroscopy around ‘doubly mid-shell’ using
ISAC 170Dy
• Use (fragmentation) isomers as channel
selection for new neutron rich spectroscopy.