r. kruecken - yale university techniques for doppler-shift lifetime measurements - the yale plunger...

R. Kruecken - Yale University

Techniques forDoppler-Shift Lifetime Measurements

- The Yale Plunger -

Introduction

Magnetic Rotation

The DSAM technique

DSAM across the Pb chain

The RDM technique

The DDCM analysis

RDM in 198Pb

The N.Y.P.D.

Perspectives


Why are lifetimes important?

• additional observable: Ex, J, B,

• measure of absolute matrix element:

• measure of electromagnetic moments:

Example:E2 transitions between rotational states

212

21 i),(

197

1

]!1)! [(2

1) ( 8),(

fE

)36.01(5

3

ndeformatio moment quadrupole

2)K-(I|IK20 EQ 1022.1

20

252t

91

ReZQt


Where lifetimes are important:

• Evolution of collectivity 2(N,Z)

• Test of collective models B(E2) vs [ exp. vs model]

• Test of multiphonon character of states B(E2) of quadr. Vibrational states

• deformation of superdeformed (SD) nuclei Qt

• decay out of superdeformed bands Qt sensitive to mixing between SD and normal defomrmed states

• mixing of coexisting shapes B(E2) sensitive measure

• Test of new phenomena Magnetic Rotation


M1- bands across the Pb chain

A rotational band in 199Pb

• very regular rotational band - but M1• several bands in light Pb isotopes• intensities between 1% and 10%• very large B(M1) values ( ~1-5 W.U.)• very weak quadrupole transitions :

B(M1)/B(E2) ~ 20-40 (n /eb)2

rotational band in nucleus with spherical density distribution !?

R.M. Clark et al., Phys. Rev. Lett. 78,1868 (1997)

Cou

nts

12

5

16

6

21

5

26

8

32

3

37

7 43

0

48

2

53

2

57

3

61

8100 200 300 400 500 600 700 800

4000

2000

Energy [keV]

M1’s

199Pb


G. Baldsiefen et al., Nucl. Phys. A 574, 521 (1994)


The shears mechanism

Symmetry axis

J

R

J

J

Low spinshigh spins

Magnetic moments / B(M1) drop characteristically with increasing spin!!

MagneticMoments

MagneticMoments


Signature of magnetic rotation

Spin is generated by gradually closing of theangle between the large “single-particle” vectors

similar to the closing of the blades of a pair of sissors

Experimental signature:Spin-dependent behavior of the electromagnetictransition probabilities is characteristic:

B M( )12

B(M1) - values should drop with increasing spin

B(M1)

J

Lifetime measurements


The Doppler Shift Attenuation MethodDSAM

• Continuous deceleration of recoil nuclei • Gamma-emission at range of velocities

Target Stopper

Beam

GermaniumDetector

26Mg@137MeV

Gold172,4,6Yb

Energy [keV]

unshiftedmaximum

Doppler-shift

600

400

200

0440 450

< 1ps


Ingredients for DSAM analysis

• Monte-Carlo simulation of stopping

time

velo

city

Fit of spectrum lifetime B(E2) value B(M1) value

Side-feedingassumption

Knownfeeding

• model for population of levels

Q, are effective

parameters

Q, are effective

parameters

=?=?


Uncertainties of DSAM experiments

• Feeding history is uncertain, since not all feeders are observed feeding model

• Gates from above could help but rarely enough statistics

• Little experimental data on stopping powers up to 15-20% systematic uncertainties

• in F() analysis constant Qt assumed

Relative DSAM measurements

• several nuclei populated in same reaction

• similar stopping for these nuclei

• relative lifetimes / Qt have no uncertainties from stopping power

good tool for comparison


B(M

1) [ N

2 ]B

(M1)

[ N

2 ]

T.F. Wang et al., PRL 69, 21 (1992)

M. Neffgen et al., NPA595, 499 (1995)

Energy [MeV]0.0 0.2 0.4 0.0 0.2 0.4

12

10

8

6

420

12

10

8

6

420

12

10

8

6

420

12

10

8

6

420

Previous DSAM results


1000

500

0

400

200

1000

500

0

600

400

200

300

200

100

0600

400

200

0

600

400

200

300

200

100

0600

400

200

0400 410 440 450 460 470 Energy [keV]

403 keV 446 keV 467 keV

130º+145º

90º

35º+50º

Gammasphere -- 186W(18O,xn)198,9PbCollaboration: Berkeley, York, Bonn , LivermoreR.M.Clark et al., Phys. Rev.Lett. 78, 1868 (1997)

DSAM experiment on 193-197Pb

Gammasphere -- 172-6Yb(26Mg,xn)193-7PbCollaboration: Berkeley, York, Bonn , LivermoreR.M. Clark, R. Krücken et al.

DSAM experiment on 198,199Pb

197Pb


Experimental proof of the shears mechanism in Pb nuclei

Rotational frequency [MeV]

B(M

1) [ N

2 ]B

(M1)

[ N

2 ]B

(M1)

[ N

2 ]

What is going on here?

Gammasphere experiment- R.M. Clark, R. Kruecken et al.Calculations by S. Frauendorf


The Recoil Distance Doppler-Shift Method

Eu Es = Eu (1+ v/c cos)

Detector

v ~ 1-2 % cv

u: unshifteds: shifted

d

Target Stopper

Decay Curve

d [m]

Standard Analysis:

Fit with set of exponential functions.

Feeding behavior asinput of fit.

No feedback of fit results.

1 - 1000ps


The Differential Decay Curve Method

dt

td

ttt

dtthdttdtdt

td

dttit

ttdt

td

fi

hfhfi

i

fi

h th

tii

t

i

tifi

hh

hiii

III

nnn

nI

nnn

fff

f

)(

)()(1

)(

)( )( )(

)()( :Stoppeak of Area

)()( )(

dt

td

ttt

dtthdttdtdt

td

dttit

ttdt

td

fi

hfhfi

i

fi

h th

tii

t

i

tifi

hh

hiii

III

nnn

nI

nnn

fff

f

)(

)()(1

)(

)( )( )(

)()( :Stoppeak of Area

)()( )(

} Lh Li

Lifetime value for each flight time tf

A. Dewald et al., Z. Phys. A334 (1989) 163


Advantages of the DDCM

• lifetime is only determined from observables

• lifetime is determined for each distance

(d) is sensitive to systematic errors

• with gates from above one selects a certain decay path

no sidefeeding

feeding history does not enter analysis as external parameter (it is automatically included)


Energy [keV]

RDM Experiment an 197,8Pb

Gammasphere, Köln Plunger, 154Sm(48Ca,xn)197,8PbCollaboration: Berkeley, Köln, LivermoreR. Krücken, R.M. Clark et al.

200 250 300 350

2000

1000

2000

1000

2000

1000

2000

1000

1 m

25 m

11 m

4.5 m

198Pb (3)


20-,21- decay curves


DDCM in coincidence

A.Dewald et al, Z. Phys. A334 (1989) 163

-curve

Difference of unshiftedintensities

Slope of shiftedintensity

vdts

uu

dtddB

dAdBd

/|)(

)()()(

)()( dAdB uu

vdts

dt

ddB/|

)(

= 0.70 (6) ps

A

B

Gate

=?


B(M1) values near the band head of ashears band in 198Pb

Rotational frequeny [MeV]

R.M. Clark et al., Phys. Rev. C50, 84 (1994)

B(M

1) [ N

2 ]

Old DSAMRDM

RDM

RDM

New DSAM

New DSAM

B(M

1) [ N

2 ]


R. Kruecken, R.M. Clark et al.

10

5

00.0 0.2 0.4 0.6

OldRDM

New RDM

NewDSAM


Technical requirements for the RDM

• minimize material around target for coincidence measurements with multi-detector system

• flat, clean and stretched foils roughness, dirt limit shortest distance

• accurate parallel positioning limit for shortest distance

• continuous distance measurement in beam capacitance method

• precision mechanics to keep relation distance capacitance reliable

• precision position measurement to calibrate capacitance measurement

• feedback mechanism to correct for thermal expansions piezo-crystal for corrections

• good heat conductivity to keep thermal expansions at their minimum


The N.Y.P.D.(New Yale Plunger Device)

• based on Cologne design by A. Dewald

• designed for large -ray array like Gammasphere, Euroball, Yrastball

• stable mechanical guidance for moving target foils remain parallel

• distance measurement using capacitance

• LabView based feed-back system stabilizing distances in beam to better than 0.1 m (Jeff Cooper)

• possible combination with Rochester PPAC, CHICO

• operational summer 1998


Th

e N

. Y

. P

. D

. d

esig

n

Inch

wo

rmF

eed

bac

k-P

iezo

m-g

aug

e-h

ead

Mo

vin

g in

ner

tu

be

for

targ

et p

osi

tio

nin

g

Des

ign

by

A. D

ewal

d, U

niv

. of

Kö

ln


Plunger Picture


Yrastball picture


Future perspectives with theN.Y.P.D.

• Lifetimes of A~110 neutron rich nuclei via heavy ion induced fission

• Deformation of neutron rich nuclei around A~190 via deep inelastic or transfer reactions

• The backbending phenomenon in shears bands

• Lifetimes of (multi-)phonon states in nuclei

• Evolution of collectivity in the light actinides

• Test of the Q-phonon picture of the IBA

• Precision lifetimes for model tests


Lifetimes of A~110 neutron rich nuclei via heavy ion induced fission

Detector

v ~ 3-4 % cv

Target Stopper

• Little lifetime information for 4+ and above

• 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


The backbending phenomenon in shears bandsB

(M1)

[ N

2 ]


0.0 0.2 0.4 0.6 0.8

1210 8 6 4 2 0

Sp

in []

197Pb (2)


Deformation of neutron rich nuclei aroundA~190 via deep inelastic or transfer reactions

Most basic experimental observables tofollow shape evolution:

• E(2+)

• R4/2 = E(4+) / E(2+)

• B(E2, 2+ 0+)

Hg

Pt

Os

W

Hf

Yb

Er

V. Zamfir


Summary

• Lifetimes are important observables of nuclear structure

Techniques:• DSAM for short lifetimes (< 1ps) (but some systematic problems involved)• relative DSAM is very powerful• RDM for lifetimes 1~1000 ps• DDCM analysis reduces systematic errors• N.Y.P.D. is a new exciting device

Physics:• Proof of Magnetic Rotation from lifetimes• Towards the “terra incognita”: - fission fragments - heavy rare earth nuclei via transfer / DI• Sensitive tests of nuclear models (Shell model as well as collective models)

r. kruecken - yale university techniques for doppler-shift lifetime measurements - the yale plunger...

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