Laser spectroscopy experiments on fission products

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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 

7. 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

8. 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).

9. 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)

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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)