Collinear laser spectroscopy of radioactive isotopes at IGISOL 4 Liam Vormawah

Transcription

1 Collinear laser spectroscopy of radioactive isotopes at IGISOL 4 Liam Vormawah University of Liverpool

2 Introduction Collinear laser spectroscopy Nuclear properties from hyperfine structures Isotope shifts and charge radii IGISOL 4 Spectroscopy of doubly-charged yttrium Future plans

3 Collinear laser spectroscopy Laser beam Photon emitted from ion Ion beam Fast process E 1 2 mv 2 E mv v const. High resolution Spectral lines resolved into their hyperfine structure Model-independent Can test nuclear models Assist with mapping out regions of nuclear chart Excited state Ground state Excitation of electron e - e - Electron de-excites; photon emitted Photon from laser

4 Hyperfine structure Splitting and perturbation of atomic energy levels due to the nucleus Analogous to, and follows on from, fine structure Splitting of fine structure J-states into hyperfine F-states Fine structure: J L S Hyperfine structure: F I J J-states F-states Counts ,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 Relative frequency (MHz)

5 Nuclear properties from hyperfine structure measurements Magnetic dipole moment, μ Accounts for the splitting of the energy levels E B, where is the magnetic moment of the nucleus, and B is the magnetic field generated by the electrons Sensitive to contributions from each valence nucleon Electric quadrupole moment, Q Accounts for the further perturbation of the split levels Measure of the quadrupole deformation of the nucleus (nuclear shape)

6 Isotope shift Difference in frequency between the centroid of a hyperfine structure for different isotopes Two components; mass shift and field shift Mean-square charge radius (nuclear size and shape) obtained from field shift δν r (fm) 50 Counts 0 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12, V (r) Point nucleus Smaller finite nucleus Larger finite nucleus 0 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 Relative frequency (MHz)

7 Extracting charge radii from isotope shifts Isotope shift between two elements, A and A, related to change in mean-square charge radius, δ<r 2 >, by AA' M A' A AA' F F and M are atomic factors for the specific optical transition Need correct F and M factors to extract charge radii Difficult to calculate for most transitions chosen for study Therefore, alternative transitions are investigated r 2 AA'

8 IGISOL 4 Ion Guide Isotope Separator On-Line; 4 th incarnation Cyclotron beam line (from K-30 or K-130) 55 o dipole analyser magnet Beam switchyard Cooler Ion beam to laser setup IGISOL ion guide (Gas cell with thin-foil target) JYFLTRAP* *Not actually part of laser spectroscopy setup

9 The RFQ cooler-buncher Ion accumulation Doppler tuning electrodes Endplate potential 200ms Ion bunch Laser beam 20μs Ion release (Cooler axis) Z Photomultiplier tube

10 In-cooler optical pumping Enhances the population of metastable states Laser light used to pump electrons into metastable state Can now perform spectroscopy from metastable state Spectroscopy from ground state not always the best option! Pumping in cooler Metastable state (Spectroscopy performed from here) Ground state

11 Yttrium charge radii 2.5 N=60 interesting! 2.0 Sudden huge jump in charge radii 1.5 Shape changes from dynamic oblate to rigid prolate δ<r 2 > (fm 2 ) 1.0 Ground states Metastable states Effect largest for yttrium Data taken in 2007 on singlycharged yttrium ions (Y + ) 0.5 Spectra taken on 363nm 5s 2 1 S 0 4d5p 1 P 1 transition complex! A F and M calculations complicated; need more reliable calibration of data

12 Spectroscopy of doubly-charged radioactive yttrium January 2014: first ever attempt at spectroscopy on doublycharged, radioactive yttrium isotopes Attempted to re-calibrate charge radius data from 2007 Spectra of 96,97,98 Y taken on optically pumped 294.6nm 5s 2 S 1/2 5p 2 S 1/2 transition a pure s p transition! Optical pumping required as transition is spectroscopically weak Natural yield of Y 2+ only 10% of total Y yield However, F and M factors easier to calculate Only require three isotopes (two isotope pairs) Obtain two charge radius values all data re-calibrated!

13 δν( 96,98 Y) δν( 96,97 Y) Y ,290 1,790 2,290 2,790 3,290 3,790 4,290 4,790 Counts Y ,290 1,790 2,290 2,790 3,290 3,790 4,290 4, Y ,290 1,790 2,290 2,790 3,290 3,790 4,290 4,790 Relative frequency (MHz)

14 Future plans Insufficient statistics due to time constraints This can be seen from the messy 97 Y spectrum! Therefore, more data needed Second experiment will be performed Hoping to measure 89 Y on the same laser lock as 96,97,98 Y third calibration point!

15 Collaborators University of Liverpool L. J. Vormawah, B. Cheal University of Manchester P. Campbell, A. Dicker, S. Kelly University of Jyväskylä A. Jokinen, I. D. Moore, A. Voss, M. Reponen

16 Thanks for listening! Any questions?