laser spectroscopy experiments on fission products

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TAS Workshop LPC Caen, March 30-31, 2004 D. Verney

Laser spectroscopy experiments on fission products

Principle : use the electronic cloud to probe the nuclear electromagnetic properties

Measured quantities : spin I, magnetic moment I, spectroscopic

quadrupole moment Qs, evolution of the mean square charge radius <r2>c

Introduction : hyperfine interaction

Physics case (part of)

Physics of medium mass nuclei produced by fission

Laser spectroscopy systems

Resonant Ionisation spectroscopy (RIS) : COMPLIS

Collinear Spectroscopy after beam cooling : future laser system at ALTO

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Hyperfine interaction

=300 nm

106 GHz

h4eV

6

0

10E h

IJ

H 0 NIA

B )0(JJ e QS

2Q0

Axial symmetry

JIF

191Ir

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Two hyperfine interaction energy termsMeasurement

Nuclear quantities3)1)(2I(I

)1I(IK3

2

Nuclear structureinformation

QS

3K2-

A B

4

3

2 1

4GHz

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AA'AA'Vol F

• Change of the nuclear charge density between isotopes :

VOLUME SHIFT

AA'iVol

AA'iM i

AA’

2cr

2 21

2

AA'

• Change of nuclear mass between isotopes:

AA'

A'-AMM iSiN

AA'iM

MASS SHIFT

Measurement Nuclear quantity

Nuclear droplet model

Isotope shift

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Ni Z=28

N=50

neutron rich nuclei produced by fission at ALTO (Orsay) and then at SPIRAL2 (GANIL)

Fission

Nuclear regions explored at ALTO

N=82

Sn Z=50

Doubly magic regions 78Ni and 132Sn

e-

238U

target source

30 keV

1+

50 MeV

Expected intensities = SPIRAL2 /100

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Production /s/µA

Stable

104 – 105

105 – 5 105

5 105 – 106

106 – 5 106

5 106 – 107

107 – 5 107

5 107 – 108

108 – 5 108

5 108 – 5 109

Expected yields at ALTO

Extrapolations from measured yields at PARRNe

Represented yields 104pps

minimum yield for the laser set-up we envisage

Z=28

Z=50

N=50

N=82

Kr

RbSr

CdIn

Sn

XeCsBa

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Z=56 Ba

N=82

Z=54

Xe

mid-shell effect

A “sample” of the physics motivations

The evolution of the charge distribution is very sensitive to the structural changes

<r2>c

N=60N=50

=0.4

=0.3

=0.2

=0.1

=0

Rb (Z=37) C. Thibault Nucl. Phys. A367, 1 (1981)

Sr (Z=38) F. Buchinger Phys. Rev. C 41, 2883 (1990)

Sherical shell gap

<r2>c when N

<r2>c very rapidly when N

Shape transition

The <r2>c variations reflect both the change in volume and departures from spherical symmetry, the origins of which can be :

-rigid deformation (rotor behaviour)

-Zero point quadrupolar vibrations (or more generally dynamical effects)

-Core polarization

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Origin : monopole part of the neutron-proton interaction importance of the radial part of the orbital wave functions

Illustration of the core polarization effect

n=2 n=3 n=4

N<50

N>50

2d5/2

1g9/2

2p1/2

2p3/2

1f5/2

50

2d5/2

1g9/2

2p1/2

2p3/2

1f5/2

50

40

38

40

38

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Illustration of the “dynamical” effects

Recent results from

the COMPLIS measurements

on tin

F. Le Blanc et al. to be published in Phys Lett B

Theoretical Data

NL3 : G.A Lalazissis et al., At. Data and Nucl. Data Tables 71 (1999)1.Gogny : M. Girod and S. Péru, Private comm. (2001)SLy4 and SLy7 : P. Bonche and J. Meyer, Private comm. (2002).

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Resonant ionization mass spectroscopy system :COMPLIS

Desorption

Excitation

Ionization

Excitation

Target

Ion detector (MCP)

Magnet

Incident beam at 60 kV Emergent beam at 59 kV

Ion source (stables)Magnet

INJECTOR

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graphite YAGbeam

dtca

resolution

MHz250

a

Ionization volume

First stagebeamIonization

beams

Ground state

Ionization continuum

351,7 nm (UV)

323,29 nm (UV)

646,58 nm (rouge)

Ionization zonedesorbed atoms

Dye laserlambda-physik

2

tunablemonomodedye laser

« compulsé »

YAGpumping10 Hz

2

1 atome/100

totalefficiency

10-5-10-6

YAGpumping10 Hz

Characteristics of the COMPLIS set-up

ZOOM

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Principle of the fast beam collinear laser spectroscopy

E=mv vEnergy spread

velocity spread

The kinematic compression of the velocity distribution

results in a reduction of the residual doppler width

D=0vcResidual

doppler width

Velocity v

Velocity v+v

Laser source fixed frequency

Frequency in the rest frame of the atoms

The hyperfine structure is scanned by a beam energy scan

with U=10-4, ~50 MHz

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COLLINEAR laser spectroscopy system

electrons

Ion source

Mass separator

Photomultiplier

Retardation system

Charge-exchange cell

Ellipsoïdalmirror

High resolution laser

Separated beam

RFQ cooler-buncher

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A. Transport : 70 %B. Neutralization : 80 % C. Feeding probability of the selected metastable state : 30%D. Spatial overlap between laser beam and ion beam : 5 10- 3

E. Resonance efficiency : 100%F. De-excitation efficiency : 50%

G. Collection efficiency : : 5 %

H. Detection efficiency : 90 %

TOTAL : ~10-5

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Efficiency

but : signal/noise ratio strongly increased by the use of the cooler buncher

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A few details on the cooler…

Ion deceleration 10eV

F. Herfurth NIM A 469 (2001) 254(ISOLTRAP)

Pulsed cavity

UcavityUHV

Ions

UHVUcavity

transfert

Ekin=e.( UHV-Ucavity )

Buffer gas

Ions

grounded UHV

Longitudinal potential shape

Ions

trapping

ejection

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•Ag (Z=47) : from A=111 to A=123 (or further from the stability line depending on the effective productions) complete the measurements on this isotopic chain on the right side of the valley of stability•Transition : Z.Phys. A274 (1975)79.

• Ge (Z=32) : from A=77 to A=83 N=50 crossing

•then, les Br, As and Ga towards Ni, Sb, I, ...

547.7 nm

206 nm

422.7 nm

303.9 nm

First measurements at ALTO

N=50

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Coût et main d’œuvreCoût et main d’œuvre

Lentilles d’accélérationralentissement

Cellule àéchange de charge

Miroirellipsoïdal

Laser hauterésolution

1. Ligne de faisceau, éléments d’optique ionique et pompage : 50 k€

2. Cellule à échange de charge : LAC ou Mainz

3. Détection : 10 k€

4. Lasers et optique : 200 k€

5. Acquisition et commande : 40 k€

6. Total : 300 k€

Durée du montage et de la mise au point : 2 ans à 2 chercheurs plein temps plus aide service technique (construire l’acquisition et réaliser la ligne de faisceau)

F. Le Blanc IPN

laser spectroscopy experiments on fission products

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