Structure of Exotic Nuclei and the Nuclear Force
Transcript of Structure of Exotic Nuclei and the Nuclear Force
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Structure of Exotic Nuclei and the Nuclear Force
Takaharu Otsuka University of Tokyo / RIKEN
Schiffer Fest Argonne
September 20-22, 2006
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But, we did not communicate for a quarter century.
I met John for the first time in 1976 or 77 in Tokyo. We walked together in the Ueno park in an evening. I was a Ph.D. student in Tokyo, but courageous enough to asked to produce more 2-body matrix elements.
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In the meantime, ......
New things have emerged in nuclear physics…..
Let me discuss one of them,shell structure of exotic nuclei.
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Single particle motion in the nucleus
- is determined basically by a potential like a Woods-Saxonwith a spin-orbit splitting term,
- has magic numbers 2, 8, 20, 28, 50, 82 ,…, as Mayer and Jensen have proposed,
- can be basically reproduced by theories (Skyrme or Gogny) consisting of central (finite-range and density-dependent) and 2-body LS forces.
The relative locations among single-particle orbitsdo not change much.
The magic numbers are common for all nuclei, stableor unstable (exotic), while can be broken by deformationoccasionally.
This has been widely conceived for half a century.
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In 1990’s, extensive studies on exotic nuclei have started.
Excess neutrons in halo or skin have more diffuse surface.
A generally conceived idea or hope :
See next 2 pages (LS cartoon)
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ls splitting smaller
exotic nucleus with neutron skin
proton
neutron
ρ
dρ/dr
stable nucleus
Conventional image of ls splitting change
(Scale-type ls quenching)
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From RIA Physics White Paper
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In 1990’s, extensive studies on exotic nuclei have started.
Excess neutrons in halo or skin have more diffuse surface.
A generally conceived idea or hope :
See next 2 pages (LS cartoon)
This mechanism was expected to be a major driving force.
But, in exotic nuclei studied actually, HF results
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8 10 12 14 16 18 20
−20
−10
0
1s1/2
0d5/2
0d3/2
S.P
.E. (
MeV
)
Z
0f7/2
1p3/2
0f5/2
S IIISkM*
Hartree-Fock energies by Skyrme Hartree-Fock
The shell structure remain rather unchanged
-- orbitals shifting together
-- change of potential depth
~ Woods-Saxon.
Neutron Single-Particle Energies at N=20
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In 1990’s, extensive studies on exotic nuclei have started.
Excess neutrons in halo or skin have more diffuse surface.
A generally conceived idea or hope :
See next 2 pages (LS cartoon)
This mechanism was expected to be a major driving force.
But, in exotic nuclei studied actually, HF results
Nothing particular would happen between stability line and dripline ?
Ozawa, Tanihata et al. “discovered” that neutron separation energiesshows strange behavior, and tried to explain it in connection toneutron halo (PRL 84, 5493 (2000)). But, it happens without halo.
Too sad
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A few years later, the quenching of spin-orbitsplitting was studied by John and his companywith John’s favorite toy,transfer reaction.
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1h11/2
1g7/2
Proton orbits 11/2‐
7/2+
exp.
Neutron Number
stillstable nuclei
Ene
rgy
(MeV
)
No mean field theory,(Skyrme, Gogny, RMF)explained.
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Independently, I was trying to see whether or not the shell structure can be changed by the NN interaction even if the dripline is still far away, and if so, how it occurs.
Finally, the key is found to be the tensor force.
Let me jump to this point, skipping some history.
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Tensor Interaction
VT = (τ1τ2) ( [σ1σ2](2) Y(2) (Ω) ) Z(r)
contributes only to S=1 states relative motion
ρ meson (~ π+π) : minor (~1/4) cancellation
π meson : primary source
π σ .σ .
Ref: Osterfeld, Rev. Mod. Phys. 64, 491 (92)
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By using
The standard expression of tensor force may be
, we get
We thus obtain an equivalent expression
Equivalent expressions of tensor force
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Where can we see one pion exchange ?
One pion exchange ~ Tensor force
The atomic nucleus is bound due to meson exchange. (Yukawa 1935)
First-order tensor force effect in spectroscopymanifestation of pions in nuclei
Multiple pion exchangesstrong effective central forces in NN interaction(as represented by σ meson, etc.)nuclear binding
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Intuitive Picture
wave function of relative motion
large relative momentum small relative momentum
deuteron attractive repulsive
spin of nucleon
TO et al., Phys. Rev. Lett. 95, 232502 (2005)
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j>
j<
j’<proton
neutron
j’>
Monopole Interaction of the Tensor Force
Identity for tensor monopole interaction
(2j> +1) vm,T + (2j< +1) vm,T = 0( j’ j>) ( j’ j<)
vm,T : monopole strength for isospin T
TO et al., Phys. Rev. Lett. 95, 232502 (2005)
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protons in g9/2
Lines : π + ρ meson tensor
Points : exp. level
Changes of N=51 neutron effective single‐particle energies from Zr to Sn
repulsion between g7/2 and h11/2N=51 isotones
shown relative to d5/2Zr Sn
Federman-Pittelmechanism
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Mean-field models(Skyrme or Gogny)do not reproduce thisreduction.
Tensor force effect due to vacancies ofproton d3/2 in 4718Ar29 :650 (keV) by π+ρ mesonexchange.
f 5/2
f 7/2
Neutron single-particle energies
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Change of proton single‐particle energies due to the tensor force (π + ρ meson exchange)
calculation only
f5/2
f7/2
proton neutron
g9/2
neutrons in g9/2
taken from GXPF1 interaction
Low-lying 2+ levels in Ni,M. Sawicka et al., Phys. Rev. C68, 044304 (03)
68Ni 78Ni
Tensor monopole
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Single‐particle levels of 132Sn core
Weakening of Z=64 submagic structure for N~90
64
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Weakening of Z=64 submagic structure for N~90
8 neutrons in 2f7/2reduces the Z=64 gap to the half value
8 protons in 1g7/2pushes up 1h9/2by ~1 MeV
1h9/2
64
2d3/2
2d5/2
Proton collectivityenhanced at Z~64
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Implementation of tensor interaction into mean field calculations
Gogny interaction (J. Decharge and D. Gogny, 1980)
(1+σσ+ττ+σσττ) (Gauss1 + Gauss2) + Density Dep.finite range zero range
Tensor interaction is addedAll parameters are readjustedNuclear matter properties reproducedwith improvement of imcompressibility
Gogny-Tokyo interaction - 2 (GT2)
Successful descriptions of various properties withD1S interaction (J.F. Berger et al., Nucl. Phys. A428, 23c (84))
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Triplet-Even potential due to the Tensor force
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Two Gogny(-type) interactions : D1S and GT2
16
20
The same change as in the shell model with SDPF-M int.
w/o tensor with tensor
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Otsuka et al. Phys. Rev. Lett. 87, 082502 (2001)σσττ central
Tensor
Tensor interaction is the primary origin ofthe p-n j>-j< coupling.
0
4
8
j>= l +1/2
j<= l −1/2
1s1/2
0d3/2
1s1/2
0d3/224O168
30Si1614
(a) (b)
(c) (d)
proton neutronp n
pn
τστσ
ES
PE
(MeV
) pf shellpf shell
N=20N=16
} }
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Phys. Rev. C 41, 1147 (1990), Warburton, Brown and Becker
Island of Inversion : region of intruder ground states
Only 9 nuclei in the original model (1990)
By now, we know that the magic gap is small also
here…
Island of Inversion is being changed …
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Single-particle energies of exotic Ni isotopes
w/o tensor with tensor
N=28
Z=28
relevance toR-process
s.p.e.’sbinding energies
TO, Matsuo, Abe, Phys. Rev. Lett. in press (2006)
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Proton
Neutron
One-body mean potential
Vautherin - Brink
2-body LS force
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Wave functions of f7/2 and f5/2 and derivatives of densities
proton neutron
f5/2
f7/2
derivatives ofdensities
N=40N=50
Peaks of derivatives do notget lower, but move outwards.
Position-type ls quenching
Scale-type ls quenchingTO, Matsuo, Abe, Phys. Rev. Lett. in press (2006)
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Contributions of Kinetic+Central, 2-body LS, and Tensorcomponents to the change of f7/2 – f5/2 gapin going from N=40 to N=50 (g9/2 occupancy)
Kin+Cent and LS : almost the same among three calculations
Tensor : largest effect
TO, Matsuo, Abe, Phys. Rev. Lett. in press (2006)
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ls splitting smaller
exotic nucleus with neutron skin
proton
neutron
ρ
dρ/dr
stable nucleus
Conventional image of ls splitting change
?(Scale-type ls quenching)
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D1S
GT2
RIBF region
post-RIBF region
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0
2
1
N=64 82
Neutrons into 1h11/2 , …
Neutronsinto 1h9/2
N=94 104
Central
LS
Tensor
1h11/2
1g7/2
Proton orbit
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New magic numbers N=32 and 34 :
Their appearance and disappearance
Frontier of RI-beam experiments(currently impossible)
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34 3434
Monopole effect of tensor force (f7/2 occupancy)
Effective single-particle energies of Ca, Ti and Cr isotopescalculated by GXPF1B interaction
Gap = 2 MeV at N=32 and 3 MeV at N=34
Gap = 2 MeV at N=32 and 1.5 MeV at N=34
48Ca
32 32 32
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Exotic Ca Isotopes : N = 32 and 34 magic numbers ?
52Ca 54Ca
51Ca 53Ca GXPF1B int.:p3/2-p1/2part refinedfrom GXPF1 int.
(G-matrixproblem)
2+ 2+?
Some exp.levels : priv. com.
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R.V.F. Janssenset al. PLB546
(2002) 55
54Ti
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GXPF1B vs. G-matrix (Bonn-C) KB3G vs. G-matrix (Bonn-C)
each point = a 2-body matrix element
G-matrix (MeV)
Comparison with G-matrix + polarization correction
G-matrix (MeV)
GXPF
1B (M
eV)
KB3G
(MeV
)
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Summary
-drives j> or j< levels in a specific and robust wayintuitive picture many cases expected from p-shell to superheavies
- is the dominant origin of shell evolution
Shell evolution due to 2-body LS interactions
John’s Sb (Sn) experiment first testexcluding other possibilities
connection to exp. in RIBF, …., hopefully “RIA”
Shell evolution due to tensor interactions
Nuclear shell evolves in unique ways as compared to other physical systems, particularly in exotic nuclei(note that ΔN must be >10 to see this RNB ! ).
Naïve picture (scale-type) may not be relevantReal phenomena may be of position-type ls splitting decreases
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consistent with Chiral Perturbation (Weinberg)
Central forces : basic bindingComplex origins : Multiple meson exchange,
hard core (quarks, QCD may be needed)3-body forces
Tensor force : amusing changes in single particle propertiesSimpler origin
dominated by one pion exchange (of course ρ, higher order …)
Remark on forces
Physics opened by upcoming RNB machines can be connected to QCD
New principle for further shell-model and mean-field studies
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Collaborators
D. Abe TokyoT. Matsuo Hitachi Ltd.
T. Suzuki Nihon U.R. Fujimoto U. TokyoM. Honma U. AizuH. Grawe GSIY. Akaishi KEK
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END
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Monopole interactionafter subtraction of tensor part
same l
almost constant
T=0
GXPF1 - Tensor between same l
GXPF1 = shell model interaction for pf-shell (G-matrix + fit)
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Stancu, Brink and Flocard, Phys. Lett. 68B, 108 (1977) Zero-range spin-momentum tensor coupling term
large relative momentum small relative momentum
deuteron attractive repulsive
This is not be a good approximation to the tensor force itself,but may simulate the monopole effect of the tensor shown below, picking up differences in relative momenta.
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Phys. Rev. Lett. 94, 022501 (2005), G. Neyens, et al.
Strasbourgunmixed USD (only sd shell)Tokyo
MCSM
31Mg19
1/2+ state comes down by 2 MeV from USD result by strong mixing between sd and pf shell configurationsdue to narrower N=20 gap
fine details to be refined (odd-A nuclei are difficult)
2.5 MeV
0.5 MeV
Outside the originalIsland of Inversion
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Proton effective single-particle levels(relative to d3/2)
f7/2
neutrons in f7/2
d5/2
d3/2
proton neutron
π + ρ meson tensor
exp.Cottle and Kemper, Phys. Rev. C58, 3761 (98)
Tensor monopole
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34 3434
Monopole effect of tensor force
Effective single-particle energies of Ca, Ti and Cr isotopes
Gap = 2 MeV at N=32 and 3 MeV at N=34
Gap = 2 MeV at N=32 and 1.5 MeV at N=34
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The shell structure is the basis of quantum many-body systems and also reflects the underlying mechanism to make the system bound.
If the shell is created by the potential source at the Center (like Coulomb potential of Hydrogen-like atoms), the shell structure does not change much.
But, the most relevant point today is that John Schiffer likes single-particle motion.
However, if the shell structure is made by the particles themselves, the shell may evolve lively as the number of particles changes. Is this the case with exotic nuclei ?
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Among many possible issues, e.g. loose binding effect, on the shell evolution, I would like to focus on the following two points.
- As N increases from the β stability line, the spin-orbit splitting becomes smaller due tothe change of the density distribution.
Is this true ? If so, how does it happen ?
- More recently, variations of the shell structure off the β stability linebut still far away from the drip linehave emerged due to the tensor force.
How does the tensor force workin old and new problems ?
How do these two effects compare in their sizes ?
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• Structure of neutron-rich Si isotopes– 42Si: a candidate for a new magic nucleus
→ to be deformed due to the shell evolution by the tensor force– Quite low 3/2-
1 state is predicted for 41Si: 0.24 MeV (TS2) vs. 1.12 MeV (MK).
0
1
2
Ex (
MeV
)
22 24 26 28N
0
200
400
600
B(E
2) (
e2 fm4 )
MKTS1
TS2
-100 0 100
Q0 (fm
2)
-2
0
2
4
6
8
ener
gy (
MeV
)
42Si
MK
TS1
TS2
Constrained HF calculationin the shell-model space
About 1.5 protons are excitedacross the Z=14 shell gap.
Si (Z=14) isotopes
(ep=1.2e, en=0.45e)
Poster byUtsuno et al.
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The following points were raised as new aspects of shell evolution in exotic nuclei :
- As N increases from the β stability line, mean potential has a more diffuse surface, and the spin-orbitsplitting becomes smaller.
More recently, variations of the shell structure off the β stabilityline but still far away from the drip line have emerged. Such variations appears to be due to the spin-isospin component of the NN interaction, particularly the tensor.
- In dripline nuclei, extremely loosely bound orbits, particularlys-orbit, remain bound by tunnel effect (neutron halo).
- Also near the drip line, coupling to continuum may produce new phenomena, e.g., those like BCS/BEC (pairing dynamics).
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Phys. Rev. Lett. 94, 162501 (2005), V. Tripathi et al.
3/2+, 5/2+ ?(from log ft )
only state below2.8 MeV by USD(large log ft )
4 additional statesof intruder config.by SDPF-M int.(MCSM calc.)zero BR
Lowest states are intruders (> 50%)
29Na18
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Level scheme of 28Ne
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Federman and Pittel, Phys. Lett. B 69, 385 (1977) - Overlap of radial wave functions is emphasized -
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Tensor potential tensor
no s-wave to s-wave
coupling
differences in short distance :irrelevant
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Basic ideas to fix GT2 parameters
1. Include tensor follow meson exchange resultsbut cut out singularity at short distance
2. Imcompressibility
3. T=1 mean field is made more repulsive, while the pairingis made stronger
See the result for oxygen isotopes
4. Parameters fitted by B.E.’s of 16O, 40,48Ca, (56Ni,) 132Sn, 208Pb,and by single-particle structure of neutron rich oxygen
Far from perfection, particularly in pairing correlation
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0 1 2
−10
0
kF[fm−1]
E/A
[MeV
]D1SGT1
GT2
Nuclear matter property
E/A
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GT2
i 1 2μ 0.7 1.2
V0 -210 -25
Vσ -1040 129
Vτ -781 120
Vστ 700 -65
W0 160
t3 1400
x3 1
D1S
i 1 2μ 0.7 1.2
V0 -512.94 -3.52
Vσ 300.60 -25.76
Vτ 557.37 -25.42
Vστ -349.40 55.98
W0 130
t3 1390.6
x3 1
Parameters
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Potential in each spin-isospin channel
V10
V01
V00
V11