Nuclear Isomerism: Probes of Nuclear Structure and Tools for the Future

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Nuclear IsomerismProbes of Nuclear
StructureandTools for the Future
Jennifer Jo Ressler Wright Nuclear Structure
Laboratory Yale University
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What is nuclear structure?
Nuclear structure is the study of nuclear excited
states. The pattern of excited states gives
information on the nucleon-nucleon interaction,
which we dont understand.
atomic structure study of atomic states created
by electronic excitations nuclear structure
study of nuclear states created by
proton and/or neutron excitations
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atomic nuclear comparison
Atomic
Nuclear
Can be described by a shell model, where
electrons fill quantized energy levels.
Can be described by a shell model, where protons
and neutrons separately fill quantized energy
levels.
n, ?, m?, s parity (-1)?
n, ?, m?, s parity (-1)?
Lowest energy levels have maximum S possible
(due to Coulomb) with J L S S?i Ssi or
J Sji S (?i si)
Lowest energy levels have minimum S possible due
to strong force pairing with J Sji S (?i si)
Spin-orbit coupling is weak
Spin-orbit coupling is strong
For 3 electrons in a d orbital
For 3 nucleons in a d orbital
d3/2
d5/2
Energy levels can be calculated by solving
the Schrödinger equation using a central
potential dominated by the nuclear Coulomb field.
Energy levels are not easily calculated nucleons
move and interact within a self-created potential.
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Why is the nucleon-nucleon interaction so hard to
understand?
Nucleon forces do not vary with distance in a
way that can be conveniently described by
mathematical formulas!
Even if we could, each nucleon interacts with a
few of its neighbors with approximately equal
forces many body problem
For electrons, consider each electrons
interaction with the nucleus, then add other
electrons as perturbations 2-body problem
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From the Periodic Chart to the Chart of Nuclides
Magic numbers
2, 8, 20, 28, 50, 82, 126,
At
Po
Bi
Pb
Tl
e-
Z
N
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What use is nuclear structure?

• Fundamental knowledge of the strong force
• Formalisms of quantum mechanics (many body
  system)
• Computational modeling and mathematics
• Astrophysics
• Nuclear medicine
• Material science
• Defense weapons (simulations)
• Etc. (art, forensics, geology, )

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Nuclear structure in a nut shell

• The nucleus is a bound system of protons and
  neutrons.
• The nucleus exhibits different shapes and
  excitations.
• Properties are understood on the basis of
• single particle and collective motion.
• Gamma-rays from the decay of an excited nucleus
  give
• information about the arrangement of
  quantum levels
• in a nuclear potential well, and the
  shape of the potential.
• The arrangement of excited levels and shape
  delicately depend
• on the nucleon number and angular momentum.

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Gamma rays and energy levels
Gamma ray pure electromagnetic radiation change
in charge distribution electric moments change
in current magnetic moments
Ji
Emission of a gamma-ray removes Energy
Angular momentum, L? one unit L 1, dipole M1,
E1 two units L 2, quadrupole M2, E2 parity
electric (-1)L magnetic (-1)L1
coincidence
Jf
Selection rule Ji - Jf ? L ? Ji Jf and
0 ? 0 transitions are forbidden only the lowest
multipolarities are probable
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What is an isomer?
nuclear isomer an excited state that does not
decay within ps

• isomers occur for
• large changes in nuclear spin (gt2)
• M2, M3, E4
• small changes in energy
• large changes in structure

Isomers decay by electron conversion, gamma
emission, particle emission (p, a, b, b-)
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Near closed shells
(near magic numbers)
2, 8, 20, 28, 50, 82, 126,
spherical structure potential is spherically
symmetric
each nucleon moves in an approximately
spherical containing potential which represents
the average interaction of each nucleon with
all other nucleons
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SphericalShell Model
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Spherical Shell Model
Spin-orbit splitting brings down high-j states
close to low-j states large change in spin gt
isomers These isomers are evidence for the
shell structure of nuclei.
In 82 lt N lt 126 shell,
odd isotopes
13/2 i13/2
M2
9/2- h9/2
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Spherical Shell Model
Coupling between nucleons may also create isomers
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h9/2 ? i13/2
11-
6
E2
E3
4
8
E2
h9/2 ? f7/2
2
E2
different structure gt isomers
0
h9/22
small change in energy gt isomers
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Non-spherical nuclei
deformed structure spherical symmetry is
lost to a first approximation, potential is
axially symmetric b2 measure of extent of
deformation spherical b2 0 large
deformation b2 0.3 0.4
oblate (b2 lt 0) spherical (b2 0) prolate
(b2 gt 0)
Deformed structures dominate when there are many
valence nucleons i.e. proton AND neutron
numbers far from magic
If b2 0.1, Spherical Shell Model states are
still observed.
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Excited energy levels
Spherical nuclei
Deformed nuclei
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208Pb region
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Z 82, N 126
Near N 126, Pt (Z78) and Hg (Z80) are nearly
spherical but at lower neutron numbers a
deformed structure co-exists with the spherical.
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Zgt82
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How are nuclei produced?

• fusion evaporation reactions
• deep inelastic reactions
• fission fragments
• projectile fragmentation

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How do we measure g-rays?
Ge semiconductor

• when a g-ray enters the detector, it disturbs
• the e- in the Ge crystal and the resulting pulse
• of charge is proportional to the initial energy
• of the gamma ray.

Germanium arrays Gammasphere (LBNL)
CLARION (ORNL) YRAST-Ball (WNSL)
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YRAST-Ball
YRAST-Ball Yale-Rochester Array for
SpecTroscopy

• Up to 13 clover detectors
• 9 at 90o
• 4 at 138.5o
• 2-4 LEPS
• Currently
• 9 clovers and 2 LEPS e 2.7

e 3.9
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SASSYER
Small Angle Separator at Yale for Evaporation
Residues
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Wright Nuclear Structure Laboratory, Yale
University

• Focal plane detector systems
• Solar cell array
• Ge detectors isomer array
• Position sensitive PPACs
• Silicon-strip detector
• MTC, b-decay tagging system
• CdZnTe array

• Target area detectors
• YRAST Ball
• NYPD
• Rutherford detectors
• BGO calorimeter
• ICEY Ball

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Dual study prompt and delayed
30Si (148 MeV) 184W ? 210Ra 4n
SASSYER
Recoil TOF 800 ns
prompt g-rays
delayed g-rays
t
Isomer Decay Tagging
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Prompt g-ray spectra
209,210,211Ra
Ra x-rays
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Delayed g-ray spectra
1.7(2) ms
t1/2 1.7(2) ms
8
6
577
750
210Ra
4
4
Counts/keV
773
600
2
603
0
210Ra
Energy, keV
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Correlated spectra
t1/2
8
6
577
750
4
4
773
600
2
603
0
210Ra
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4 feeding
prompt g-ray spectrum
t1/2 1.7(2) ms
8
6
577
750
4
4
773
600
2
603
0
210Ra
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Comparisons to isotones
3.0
11-
11-
11-
10
10
10
1.7 ms
ph9/22
8
2.0
473 ns
8
6
8
6
212 ns
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Energy, MeV
4
4
4
4
4
4
2
1.0
2
2
2
0.0
0
0
0
208Rn
210Ra
206Po
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Comparisons to isotopes
6 protons above Z82 h9/26 h9/22 0, 8
2(h9/22) 10, .. h9/2f7/2
8 h9/2i13/2 11-
17
230 ns
4.0
16
14
285 ns
12,13
12
13-
12
12-
12
3.0
10
11-
11-
11-
11
333 ns
850 ns
10
10
Energy, MeV
8
8
8
1.7 ms
8
8
2.0
11 ms
6
67 ms
6
6
4
4
4
2
4
1.0
2
2
0
0
0
0
212Ra
214Ra
210Ra
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8 isomer half-life
124
122
126
212Ra
214Ra
210Ra
? t1/2 decreases
88
10.9 ms
1.7 ms
67 ms
More neutron degrees of freedom low spin states
are not pure ph9/22
208Rn
210Rn
212Rn
86
473 ns
644 ns
910 ns
More proton degrees of freedom 8 of ph9/22
mixes with ph9/2 x pff/2
206Po
208Po
210Po
84
350 ns
212 ns
99 ns
? t1/2 increases
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Are there other isomers?
126
126
Probably.
3p1/2
(3p1/2)
2f5/2
(2f5/2)
206Po 9- (ni13/2 x nf5/2) 1.0 ms
3p3/2
(3p3/2)
114
1i13/2
1i13/2
10
1h9/2
2f7/2
9-
2f7/2
1h9/2
82
82
p
n
ph9/22
8
isomers in Po and Rn
11- (ph9/2 x pi13/2)
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Neutron-rich nuclei
NZ
large neutron excess magic numbers no longer
magic! New shell structure what are the new
magic numbers? Radioactive Ion Beams
(RIA?) Isomers are excellent probes!
Z
N
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Far from closed shells
collective motion gt I J R rotational
bands
I
R
J
j
s
symmetry axis
l
L
W
K
E (1/2I) I(I1) K2
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Nilsson model
?
?
K ?1 ?2
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Deformed odd-mass A80 nuclei dominated by
4225/2 and 3013/2- configurations Z
39 N 39 79Y 5/2
77Sr 5/2 81Y 5/2, 3/2- 113 keV Z 41 N
41 83Nb 5/2 81Zr 3/2-, 5/2 X keV 79Sr
3/2-, 5/2 177 keV
80Y Z 39, N 41
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What happens if we combine a deformed odd-proton
and odd-neutron?
For p4225/2 ? n3013/2- expect 4- ground
state, 1- excited state
1-
K 1
4-
K 4
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80Y rotational bands
D. Bucurescu et al., Z. Phys. A 352, 361 (1995).
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80Y isomers
4.7 s isomer 19 beta decay 81 gamma decay
(229 keV) suggested to be M3 1- 4- decay
to ground state M3 nature confirmed 4.7 ms
isomer gamma decay (83 keV) suggested to be E1
1-
4-
J. Döring, H. Schatz, A. Aprahamian et al., PRC
57, 1159 (1998).
p4225/2 ? n3013/2-
A. Piechaczek, E. F. Zganjar et al., PRC 61,
047306 (2000).
C. Chandler, P. H. Regan et al., PRC 61, 044309
(2000).
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80Zr beta decay
Beta decay Gamow-Teller
4.7 ms
4.7 ms
3
2
83 keV
E1
83 keV
E1
4-
1-
Either scenario feeds the ms isomer
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rp-process nucleosynthesis
H. Schatz et al., Phys. Rep. 294, 167
(1998). X-ray burst gt 10 reaction flow
P
P
P
P
P
P
P
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X-ray burst
From H. Schatz, MSU
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Astrophysical implications of 80Zr t1/2
Nb
X
X-ray burst cools while 80Zr decays proton
capture on 80Y becomes more difficult.
6.85 s
Zr
Y
Sr
Rb
S
Kr
40 41 42 43 44
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80Zr set-up
Oak Ridge National Laboratory
RMS
MTC
195 MeV 58Ni on 500 mg/cm2 24Mg
0
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RMS Recoil Mass Spectrometer
M/Q separator or mass separator
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80Zr half-life
TAC Start scintillator (b) Stop LOAX (low
energy g)
80Zr t1/2 4.1(8) s
J. J. Ressler, W. B. Walters, M. Wiescher, A.
Aprahamian, et al. Phys. Rev. Lett. 84, 2104
(00).
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Astrophysical implications of 80Zr t1/2
Nb
X
X-ray burst remains warm while 80Zr decays
proton capture on 80Y becomes possible.
4.1 s
Zr
Y
Sr
Does 1- beta decaying isomer play a role?
Rb
S
Kr
40 41 42 43 44
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IDT experiment set-up
Oak Ridge National Laboratory
85 MeV 28Si on 300 mg/cm2 54Fe
Recoil TOF 3.5 ms
prompt g-rays
delayed g-ray
t1/2 4.7 ms
83 keV
Isomer Decay Tagging
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CLARION Clover Array for Radioactive ION beams
11 clovers 2.5 efficiency for 1.3 MeV
g-rays 5 at 90o 4 at 132o 2 at 155o
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IDT results
J. J. Ressler et al., PRC 63, 067303 (01).
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E 312 keV
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But why is this state isomeric?
(7)
731
324
623
(6)
299
537
(5)
238
431
(4)
193
(3)
336
143
2
4.7 ms
83 keV
E1
1-
4.7 s
M3
229 keV
4-
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2
p4225/2 ? n4311/2
Compare with N41 79Sr ½ band similar moment
of inertia very deformed
gt 2 state may be a shape isomer
1- is not pure 4137/2 ? 3035/2-,
4225/2 ? 3123/2-, 4313/2 ? 3013/2-,
4313/2 ? 3035/2-, 4311/2 ? 3013/2-
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K-isomers
Expect K-isomers in regions where high-?
orbitals of deformed nuclei are near the Fermi
surface
high-K
4-qp
n5147/2- x n6249/2 x p4047/2 x p5149/2-
K 16
16
2.4 MeV
2-qp
64 n5147/2- x n6249/2 36 p4047/2 x
p5149/2-
K 8
8
1.1 MeV
K 0
0
0
178Hf
low-K
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K-isomers
Z 74, N 104 Ta/Hf region A 180 near
stability long-lived isomers (sec years) Z
66, N 104 Dy, A 170 neutron
dripline unstable beams Z 66, N 74 A
130, 140 proton dripline ms isomers
IDT Heavy elements No (Z 102) and Z110 have
been reported
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A170-190
proton rich
neutron rich
Need unstable beams (NgtZ) or alternative
production mode (not fusion evaporation)
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Non-structure interests
Astrophysics most elements are produced in
neutron-capture processes s-process
proceeds via successive neutron captures and beta
decays slow neutron capture rate Rcapture ltlt
Rbeta r-process proceeds via successive
neutron captures and beta decays fast neutron
capture rate Rcapture gt Rbeta
Hf Lu Yb
3.7 hours
1-
4 x 1010 years
7-
176Lu
104 105 106
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Non-structure interests
Potential energy source controlled triggering of
isomer decay
40 20 keV
2.4 MeV, t½ 31 years
178Hf
observed Phys. Rev. Lett. 82, 695
(1999). refuted Phys. Rev. Lett. 87, 072503
(2001).
g-ray lasers 43,44,45,46Sc, 58Co, 57Fe,
63Ni, 65,67Zn, 74Ga, 69,73,75,77Ge, 76As,
77,79Se, 77,79,83Kr, 83Rb, 90,92Nb, 99Mo, 105Ru,
100,105Rh, 107Pd, 103,107,109,110,111,116,118,120A
g, 116,119In, 109Cd, 115Sn, 118,120,122,126Sb,
120,122I, 125Xe, 134,140Cs, 137,138La, 140Nd,
141,152,154Eu, 153,157Gd, 157,169,171Er, 165,167Tm
, 172,173,177Lu, 173,175Hf, 177,181,183Ta,
179,180Re, 181Os, 187Pt, 189Au,
207,209,210Po, 206Bi, 243Cm Bio-engines??
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Chart of Nuclides
Exotic proton rich nuclei
Z
Neutron rich nuclei very little known!
N
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Summary of isomer studies presented
Spherical 210Ra probe spherical
structure tool correlate states across the
ms-isomer Deformed 80Y probe deformed
structure tool 80Zr beta decay and to
correlate states across the ms-isomer Understandi
ng why certain states are isomeric is important!
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THANKS!
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Is a recoil/mass separator necessary?
Not always
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Other techniques

• thick target experiments
• -- recoil is stopped
• -- short-lived (ns) isomers
• beam pulsing
• -- beam on/off regularly
• -- short-lived isomers
• c) Moving tape collector
• d) fission gt isomers

target
beam
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Comparisons to isotopes
6 protons above Z82 h9/26 h9/22 0, 8
2(h9/22) 10, .. h9/2f7/2
8 h9/2i13/2 11-
17
230 ns
4.0
16
14
285 ns
12,13
12
13-
12
12-
12
3.0
10
11-
11-
11-
11
333 ns
850 ns
10
10
Energy, MeV
8
8
8
2.2 ms
8
8
2.0
11 ms
6
67 ms
6
6
4
4
4
2
4
1.0
2
2
2
0
0
0
0
212Ra
214Ra
210Ra
67
Temp