Laser spectroscopy experiments on fission products
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Laser spectroscopy experiments on fission products. Introduction : hyperfine interaction. Principle : use the electronic cloud to probe the nuclear electromagnetic properties.

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Laser spectroscopy experiments on fission products

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Laser spectroscopy experiments on fission products

Introduction : hyperfine interaction

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

Measured quantities : spin I, magnetic moment mI, spectroscopic quadrupole moment Qs, evolution of the mean square charge radius d<r2>c

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


Hyperfine interaction

l=300 nm

1

2

n106 GHz

4GHz

hn04eV

191Ir

3

4

5

n

B

A

Nuclear structure

information

Measurement

Two hyperfine interaction energy terms

mI

YN

A

B

QS

Axial symmetry

3K2-

Nuclear quantities

QS

Q0

b2


Isotope shift

  • Change of nuclear mass between isotopes:

MASSSHIFT

  • Change of the nuclear charge

  • density between isotopes :

VOLUME SHIFT

Measurement

Nuclear quantity

DniAA’

Nuclear droplet model


Nuclear regions explored at ALTO

238U

30 keV

Expected intensities = SPIRAL2 /100

N=82

1+

50 MeV

N=50

target

source

Sn Z=50

Fission

Ni Z=28

e-

g

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

Doubly magic regions 78Ni and 132Sn


Ba

Cs

Xe

Production /s/µA

5 108 – 5 109

Z=50

Sn

108 – 5 108

In

5 107 – 108

Cd

107 – 5 107

5 106 – 107

N=82

106 – 5 106

5 105 – 106

105 – 5 105

N=50

104 – 105

Stable

Sr

Rb

Kr

Z=28

Expected yields at ALTO

Extrapolations from measured yields at PARRNe

Represented yields104pps

minimum yield for the laser set-up we envisage


A “sample” of the physics motivations

Z=56 Ba

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

mid-shell effect

Z=54

Xe

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

b=0.4

b=0.3

b=0.2

d<r2>c

b=0.1

b=0

Shape transition

Sherical shell gap

N=82

N=60

N=50

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

  • The <r2>cvariations 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

<r2>c very rapidly when N 

<r2>c when N 


Illustration of the core polarization effect

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

2d5/2

50

n=2 n=3 n=4

1g9/2

n

40

2p1/2

38

N<50

2p3/2

1f5/2

p

2d5/2

50

1g9/2

n

40

2p1/2

38

N>50

2p3/2

1f5/2

p


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).


Resonant ionization mass spectroscopy system :

COMPLIS

Ionization

Target

Excitation

Excitation

Desorption

Magnet

Emergent beam at 59 kV

Incident beam at 60 kV

Ion detector (MCP)

INJECTOR

Magnet

Ion source (stables)


Characteristics of the COMPLIS set-up

resolution

total

efficiency

10-5-10-6

YAG

pumping

10 Hz

Ionization continuum

desorbed atoms

Ionization zone

1 atome/100

YAG

beam

646,58 nm (rouge)

Dye laser

lambda-physik

graphite

2

323,29 nm (UV)

2

351,7 nm (UV)

ZOOM

tunable

monomode

dye laser

« compulsé »

Ground state

First stage

beam

Ionization

beams

YAG

pumping

10 Hz

a

Ionization volume


Principle of the fast beam collinear laser spectroscopy

dv

dnD=n0

c

n

n

Laser source fixed frequency

Velocity v

n

Velocity v+dv

Frequency in the rest frame of the atoms

The kinematic compression of the velocity distribution

results in a reduction of the residual doppler width

dE=mv dv

Energy spread

velocity spread

Residual doppler width

The hyperfine structure is scanned by a beam energy scan

with U=10-4,  ~50MHz


COLLINEAR laser spectroscopy system

Ion source

Photomultiplier

electrons

Mass

separator

Ellipsoïdal

mirror

Charge-exchange

cell

Separated beam

Retardation

system

RFQ

cooler-buncher

High resolution laser


Efficiency

  • . Transport : 70 %

  • . Neutralization : 80 %

  • . Feeding probability of the selected metastable state : 30%

  • . Spatial overlap between laser beam and ion beam : 5 10- 3

  • . Resonance efficiency : 100%

  • . De-excitation efficiency : 50%

  • . Collection efficiency : : 5 %

  • . Detection efficiency : 90 %

  • TOTAL : ~10-5

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


A few details on the cooler…

grounded

Buffer gas

Ucavity

UHV

UHV

Pulsed cavity

transfert

Ions

Ions

Ekin=e.( UHV-Ucavity )

Ions

Ucavity

UHV

trapping

Longitudinal potential shape

ejection

F. Herfurth NIM A 469 (2001) 254

(ISOLTRAP)

Ion deceleration   10eV


First measurements at ALTO

206 nm

547.7 nm

303.9 nm

422.7 nm

  • 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, ...

N=50


Miroir

ellipsoïdal

Lentilles

d’accélération

ralentissement

Cellule à

échange

de charge

Laser haute

résolution

Coût et main d’œuvre

F. Le Blanc IPN

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

  • Cellule à échange de charge : LAC ou Mainz

  • Détection : 10 k€

  • Lasers et optique : 200 k€

  • Acquisition et commande : 40 k€

  • 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)


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