Reiner Krücken - Yale University Reiner Krücken Wright Nuclear Structure Laboratory Yale University Why do we measure lifetimes ? The recoil-distance method

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


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


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


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

= ?


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


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


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


Gammasphere


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


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


YRASTBALL


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


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


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


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


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


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


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


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!


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

Documents

Reiner Krücken - Yale University Reiner Krücken Wright Nuclear Structure Laboratory Yale University Why do we measure lifetimes ? The recoil-distance method