New Techniques of Laser Spectroscopy on Exotic Isotopes of Gallium and Francium Thomas John Procter School of Physics and Astronomy 2013 CERN-THESIS-2013-040 30/04/2013 A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences 2 Contents List of Figures 7 List of Tables 11 Abstract 13 Declaration 15 Copyright Statement 17 Acknowledgements 19 1 Laser Spectroscopy in Nuclear Physics 21 1.1 Introduction . 21 1.2 This Work . 23 2 Theory of Laser Spectroscopy 25 2.1 Hyperfine Structure . 25 2.1.1 Magnetic Dipole Moment . 28 2.1.2 Hyperfine Anomaly . 29 2.1.3 Electric Quadrupole Moment . 29 2.1.4 Spin Determination . 30 2.1.5 Angular Distribution of Fluorescence Photons . 31 2.2 Isotope Shift . 32 2.2.1 Mass Shift . 32 2.2.2 Field Shift . 33 2.2.3 Atomic Factors . 33 2.2.4 King Plot Method . 34 2.3 Experimental Considerations . 35 2.3.1 Photon Absorption . 35 2.3.2 Atomic Linewidths . 35 2.3.3 Power Broadening . 36 2.3.4 Doppler Broadening . 36 2.3.5 Experimental Lineshapes . 36 3 Laser Spectroscopy on Exotic Isotopes 39 3.1 Methods of Production . 39 3.1.1 Isotope Separation On-Line . 39 3.1.2 In-Flight Separation . 40 3 Contents 3.2 The ISOLDE Facility . 41 3.2.1 ISOLDE at CERN . 41 3.2.2 Production of Radioactive Isotopes . 43 3.2.3 Ion Cooling and Bunching . 45 3.3 Laser Spectroscopy Techniques . 46 3.3.1 Collinear Laser Spectroscopy . 46 3.3.2 Resonant Ionisation Spectroscopy . 48 4 The COLLAPS Beam Line 51 4.1 Beam Line Components . 51 4.2 Hyperfine Structure Measurements via Doppler Tuning . 54 4.2.1 Calibration of Scanning Voltages . 55 4.3 Cooling/Bunching with ISCOOL . 56 4.4 Experimental Procedure . 57 5 Laser Spectroscopy of Gallium Using COLLAPS 59 5.1 Previous Studies on the Neutron-Rich Gallium Isotopes . 59 5.2 Motivation for the Neutron-Deficient Gallium Isotopes . 62 5.3 Experimental Setup at ISOLDE . 65 5.3.1 ISOLDE Target . 65 5.3.2 RILIS Ion Production . 66 5.3.3 ISCOOL . 66 5.3.4 Atomic Transition . 68 5.4 Experimental Data . 69 5.4.1 Analysis Technique . 69 5.4.2 Lineshape of Optical Transitions . 71 5.5 Hyperfine Structure Results . 73 5.5.1 70Ga................................ 73 5.5.2 68Ga................................ 76 5.5.3 64Ga and 66Ga........................... 78 5.5.4 63Ga................................ 80 5.5.5 Nuclear Moments of 63Ga and 70Ga . 82 5.5.6 Shell-Model Calculations of 63Ga . 83 5.6 Isotope Shift Analysis . 84 5.6.1 Extracting the Changes in Mean-Square Charge Radii . 84 5.6.2 Experimental Errors . 87 5.6.3 Interpretation of the Changes in Mean-Square Charge Radii . 88 5.7 Conclusion and Outlook . 90 6 The CRIS Beam Line 93 6.1 A Brief History of CRIS . 93 6.2 CRIS at ISOLDE . 94 6.3 Components of CRIS . 96 6.3.1 Laser Setup . 96 6.3.2 Charge-Exchange Cell . 97 6.3.3 Ion Detection Setup . 99 6.3.4 Ultra-High Vacuum Interaction Region . 102 6.3.5 Hyperfine Structure Scanning Methods . 103 4 Contents 6.3.6 Hardware Control . 104 6.3.7 Decay Spectroscopy Station . 106 6.3.8 Off-Line Ion Source . 107 6.3.9 Electrostatic Ion Optics and Faraday Cups . 108 6.4 Summary . 115 7 Laser Spectroscopy of Francium Using CRIS 117 7.1 Motivation for the Radioactive Francium Isotopes . 117 7.2 Experimental Setup at ISOLDE . 118 7.2.1 ISOLDE Target . 118 7.2.2 Ionisation Process . 119 7.2.3 Experimental Procedure . 123 7.3 Experimental Analysis . 124 7.3.1 Experimental Efficiency . 124 7.3.2 Components of Experimental Efficiency . 126 7.3.3 Background Rate . 130 7.3.4 Study of Experimental Error from the Analysis of 221Fr . 131 2 207;211;220;221 7.3.5 A(7s S1=2) values and Isotope Shifts of Fr . 136 2 2 7.3.6 Atomic Factors for the 7s S1=2 ! 8p P3=2 Transition . 139 7.3.7 Changes in Mean-Square Charge Radii . 140 7.4 Conclusion and Outlook . 141 Appendix A Publications 143 References 145 Final word count: 28 462 5 6 List of Figures 2.1 Precession of I and J about the coupled angular momentum, F ... 26 2.2 Example hyperfine structure of a spectral line in 69Ga . 27 3.1 Diagrams of the ISOL and In-Flight beam production methods . 40 3.2 Layout of the ISOLDE facility at CERN . 41 3.3 Image of the ISOLDE facility and location within the CERN acceler- ator complex . 42 3.4 Schematic drawing of a surface ion source . 44 3.5 Photo of ISCOOL . 45 3.6 Diagram of the electrode potentials in ISCOOL . 46 3.7 Diagrams of the collinear fluorescence and resonant ionisation spec- troscopy methods . 47 4.1 Schematic diagram of the COLLAPS beam line . 52 4.2 Diagrams of the COLLAPS light collection region . 53 4.3 Diagram of the COLLAPS voltage scanning system . 54 4.4 Calibration of the voltage scanning system . 55 4.5 Variation of the amplification factor of the voltage scanning system . 56 4.6 Diagrams of the COLLAPS timing triggers . 58 5.1 Shell model orbital occupations of the protons and neutrons within the gallium isotopes . 60 5.2 Previously measured optical spectra for the odd-A 67-81Ga isotopes . 61 5.3 Previously measured optical spectra for the even-A 72−80Ga isotopes . 63 5.4 Excitation energies and matter radii of the gallium isotopes . 64 + + + 5.5 21 and 4 =2 energies in zinc and germanium . 65 5.6 ZrO2 target yields . 66 5.7 The RILIS ionisation scheme for gallium . 67 5.8 A ToF measurement of 69Ga bunches from ISCOOL . 67 5.9 The atomic transition chosen in gallium for fluorescence detection . 68 5.10 Measured hyperfine structure of 69Ga . 70 5.11 Optimised fits of the gallium spectra using different lineshapes . 72 5.12 Measured hyperfine structure of 70Ga . 74 5.13 Hyperfine structure fits of 70Ga for different hyperfine coefficient values 75 5.14 Simulated hyperfine structure of 68Ga using known moment values . 76 5.15 Measured hyperfine structure of 68Ga . 77 2 68 70 2 5.16 χ values for fits of Ga and Ga with varying A(5s S1=2) values . 78 5.17 Measured single component of 64Ga................... 79 7 List of Figures 5.18 Measured single component of 66Ga................... 79 5.19 Measured hyperfine structure of 63Ga . 80 2 2 5.20 A(5s S1=2)=A(4p P3=2) ratio values of the gallium isotopes . 81 5.21 Changes in mean-square charge radii of the gallium isotopes, calcu- lated using different KMS values . 86 5.22 Changes in mean-square charge radii of the gallium isotopes using the initial and final KMS value . 87 5.23 Changes in mean-square charge radii of the gallium isotopes, plotted alongside neighbouring isotope chains . 89 6.1 3D drawing of the CRIS beam line . 94 6.2 Schematic drawing of the CRIS beam line . 95 6.3 Diagram of the CRIS laser locations within ISOLDE . 97 6.4 Technical drawing of the CEC . 98 6.5 Schematic drawing of the CEC . 98 6.6 Technical drawing of the MCP and dynode plate . 100 6.7 Circuit diagram of the MCP electronics . 101 6.8 Screenshot of the MCP detection oscilloscope . 101 6.9 Differential pumping of the interaction region from the CEC . 103 6.10 Drifting effects of an ion beam with voltages applied to the CEC . 105 6.11 Infrastructure of the control of the main components used for data collecting with CRIS . 106 6.12 Layout of the implantation site in the DSS . 107 6.13 Diagram of the off-line ion source . 107 6.14 3D drawing of the CRIS beam line . 108 6.15 3D drawing of the vertical steerers . 109 6.16 Schematic drawing of the quadrupole triplet . 109 6.17 3D drawing of the bending plates . 110 6.18 3D drawing of the quadrupole doublet . 111 6.19 3D drawing of the ion kicker . 112 6.20 3D drawing of the deflector plates . ..