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Reiner Krücken - Yale University
Reiner KrückenWright Nuclear Structure Laboratory
Yale University
• Why do we measure lifetimes ?
• The recoil-distance method (RDM)
• Recent achievements with the RDM
• Perspectives for RDM experiments
* using transfer reactions
* in fission fragments* using Coulex of the RIBs* using reactions with RIBs
Precision lifetime measurements using
the Recoil Distance Method
Reiner Krücken - Yale University
Why measure lifetimes?
• our standard spectroscopic observables: E , I , W(), P Ex , B , J
• lifetimes and g-factors provide additional sensitive nuclear structure information
• direct measure of multipole moments:
— E2 quadrupole moment Q0
deformation of nucleus• measure of shape/configuration mixing
Absolute transition matrix-elements give clues onamount of mixing and shapes involved
2121 i),( ),( fE
before after
a + bb + a
Reiner Krücken - Yale University
Nuclear structure with lifetimes:
• Evolution of collectivity 2(N,Z)
•Test of collective models B(E2) vs. [ exp. vs. model]
• Test of multi-phonon character of states B(E2) of quadr. vibrational states
• deformation of superdeformed (SD) nuclei Qt
• mixing of coexisting shapes B(E2) is a sensitive measure
• decay out of superdeformed bands Qt sensitive to mixing between SD
and normal deformed states
• Test of new phenomena Magnetic Rotation
Reiner Krücken - Yale University
The Recoil Distance Doppler-Shift Method
Eu Es = Eu (1+ v/c cos)
Detector
v ~ 1-2 % cin fusion reactions
v
u: unshifteds: shifted
d
Target Stopper
Decay Curve
d [m]
Standard Analysis:
2-fit with set of exponential functions.
Feeding behavior asinput of fit.
No feedback of fit results.
1 - 1000ps
= ?
Reiner Krücken - Yale University
Example: 126Ba experiment with GASP
shifted unshifted shifted unshifted
2+ 4+ Spectra for backward detectors
100Mo(30Si,4n)126Ba
A. Dewald et al.PRC 54 (97) R2219
Decay curves for 4+, 6+, and 8+ states
Reiner Krücken - Yale University
dt
td
ttt
dtthdttdtdt
td
dttit
ttdt
td
fi
hfhi hfi
i
fi
h th
tii
t
i
tifi
hh
hiii
IIbI
nnn
nI
nnn
fff
f
)(
)()(1
)(
)( )( )(
)()( :Stoppeak ofArea
)()( )(
The Differential Decay Curve Method
} Lh Li
Lifetime value for each flight time tf
A. Dewald et al., Z. Phys. A334 (1989) 163
Reiner Krücken - Yale University
RDM in 198PbR. Krücken et al,
PRC 58, R 1876 (98)
-curve
Difference of unshiftedintensities
Slope of shiftedintensity
vdts
uu
dtddB
dAdBd
/|)(
)()()(
)()( dAdB uu
vdts
dt
ddB/|
)(
= 0.70 (6) ps
A
B
Gate
=?198Pb(3) (21-) -> (20-)
216keV
A. Dewald et al., Z. Phys. A334 (1989) 163
The Differential Decay Curve Method
in coincidence with feeding transitions
Reiner Krücken - Yale University
Gammasphere
Reiner Krücken - Yale University
Th
e N
. Y. P
. D. d
esig
n
Inch
wor
mM
otor
Fee
db
ack
-P
iezo
m-g
auge
-hea
d
Mov
ing
inn
er t
ub
efo
r ta
rget
pos
itio
nin
g
Des
ign
by
A. D
ewal
d, U
niv
. of
Köl
n, G
erm
any
Tar
get
Sto
pp
er
Reiner Krücken - Yale University
The N.Y.P.D.(New Yale Plunger Device)
• based on Köln design by A. Dewald
• designed for large -ray arrays like Gammasphere, YRAST Ball
• stable mechanical guidance for moving target foils remain parallel at all times
• distance measurement using capacitance (since heat expansion prevents mechanical measurement)
• LabView based feed-back system to stabilize distances in beam to better than 0.1 m (Jeff Cooper)
• possible combination with Rochester PPAC, CHICO and other charged particle detectors
• operational January 1999
Reiner Krücken - Yale University
YRASTBALL
Reiner Krücken - Yale University
Achievements of the RDM (biased view!)
With current plunger design and multi-detector arrays it was possible to reach new benchmarks for RDM experiments:
• lifetimes in weakly populated (~1-10%) bands:– superdeformed bands in A~130/190 regions– 10-30% Qt 5-10%– investigation of decay-out mechanism– limited by statistics and stability of distance
• sub-picosecond lifetimes in M1 bands– test of shears mechanism in 198Pb– limited by statistics and stability of distance
• precision lifetimes of low-spin collective states– experiments in A~130 region with GASP – 2% achieved– sufficient statistics but limited by systematic errors
ND
SD
Spin 10: Decay out by mixing of cold superdeformed and hot spherical states
Spin ~ 40: Cold superdeformed and spherical states are well separated
0 10 20 30 40
Links
Observed links and lifetime measurements have shown that:• SD and ND states mix only weakly (1-4%)• Decay of SD band is statistical :
• B(E1) ~ 8 x 10-6 W.U.• B(E2) < 5 x 10-2 W.U.• B(M1) < 5 x 10-4 W.U.
Decay Out Of Superdeformed Bands at A~190
Spin
En
ergy
[M
eV] 10
5
Lifetimes{
deformation
deformation
194Pb
Reiner Krücken - Yale University
Magnetic Rotation in Pb isotopes
Prediction:Magnetic moments / B(M1) drop characteristically
with increasing spin!!
J
Symmetry axis
R J
J
Low spinshigh spins
R.M. Clark et al., PRL 78, 1868 (1997)125
166
215 268
323
377 430
482
532
573
618
Energy [keV]
M1’s
199Pb
B(M
1) [ N
2 ]
Rotational frequency [MeV]
10
5
0
0.0 0.2 0.4 0.6
New RDM
NewDSAM
R. Krücken,R.M. Clarket al.,PRC 58,R1876 (1998)
198Pb
Reiner Krücken - Yale University
Precision Lifetimes in the A~130 region
Spin
Qt(I
I-
2)/ Q
t(2
0)
1.2
1.0
0.8
1.2
1.0
0.8
1.2
1.0
0.8
4 6 8 10 4 6 8 10
124Ba 122Xe
128Ba
126Ba
132Nd
126Xe
Ratio of transition quadrupole moment shows shape changes with increasing spin.
For some cases an impressive accuracy was achieved!
P. von Brentano et al. GASP II
Reiner Krücken - Yale University
Most basic experimental observables tofollow shape evolution:
E(2+)
R4/2 = E(4+) / E(2+)
B(E2, 2+ 0+)
Lifetimes in neutron rich nuclei at A~190
Hg
Pt
Os
W
Hf
Yb
Er
3662.971.94
4282.49
4232.50
4162.50
1552.70
4132.43
4052.671.371632.683.95
1922.562.98
2662.532.60
1323.093.62
1273.153.81
1063.24
773.31
843.31
983.29
1073.26
1433.09
2192.75
300
4702.36
E(2+)E(4)/E(2)
B(E2)
80
78
76
74
72
70
68
104 106 108 110 112 114 116 118 120 122 124
RDM experiments using transfer/ deep inelastic reactionsinvolving the plunger and heavy-ion detectors
Reiner Krücken - Yale University
Lifetimes in neutron rich nuclei at A~100
Kr 86
Sr 88
Zr 92 94 96
Mo 96 98 100
Ru 100 102 104
Pd 104 106 108 110
Cd 108 110 112 114 116
stable
lifetime 21+ and 41
+ & others non-yrast states
lifetime 21+ and non-yrast spin 0,2 states
lifetime 21+ only
208Pb + 18O at 91 MeV (symmetric)252Cf spontaneous fission (asymmetric)
Current situation in even-even nuclei around A~100
Reiner Krücken - Yale University
Lifetimes of A~100 neutron rich nuclei via heavy ion induced fission
Detector
v ~ 3-4 % cv
Target Stopper
• Little lifetime information for 4+ and above
• Shape coexistence in Sr and Zr isotopes
• Transitional region from Mo-Cd
• Claims of octupole correlations in Mo
• Claims of triaxiallity in 108,110Ru
new territory for RDM experiments
Solar cells,PPAC
Reiner Krücken - Yale University
104Zr beam
106 p/sec
Lifetimes of exotic beams at an ISOL facility(Inverse Kinematics Coulomb Excitation)
Detector
v1 ~ 7.5 % c
CoulexTarget
Retardationfoil
Ring of Plasticcounters
v1
1mg/cm2
27Al2 mg/cm2
27Al
v2 ~ 6 % c
104Zr
140 keV
312 keV
474 keV
0+
4+
2+
6+
474 495 501
324 327
324 327
Coulex on 2nd foil
Coulex on 1st foil
Gate onmaximalshifted
partonly
Coulex on 1st foil
v2
Gate
Reiner Krücken - Yale University
Estimate of Statistics
106 104Zr nuclei per second with 300 MeV energy
incident on 1 mg/cm2 27Al foil (v/c = 7.3%)
Cross-sections for 104Zr levels:
2+ 1000 mb4+ 150 mb6+ 25 mb8+ 2 mb
coincidence Gammasphere (.09) GRETA (.4) (24 hrs) (6 hrs) (6+ | 4+) 30000 26000 (8+ | 4+, 6+) 2400 2100
This should allow about 2% error for (4+) !!
Very sensitive experiment even with low intensity beams!
Reiner Krücken - Yale University
Nuclear Structure near the proton drip-line
• drip-line nuclei are difficult to reach with stable beams and targets
• neutron deficient radioactive ion beams will allow new reactions with higher yields
• Lifetime measurements will be possible for N=Z nuclei such as 80Zr search for effects of proton-neutron pairing
• precision tests of shell model calculations around N=Z with accurate transition matrix elements