4. Laser Ion Source Tests Performed at HRIBF
Laser ion sources (LIS) based on resonant photo-ionization have already proved to be of great value at existing ISOL radioactive ion beam facilities for generating useful intensities of isobarically pure radioactive ion beams [1-5]. In these ion sources, ions of a selected isotope are produced via stepwise atomic resonant excitations by two or three laser beams followed by ionization in the last transition. Because each element has its own unique atomic energy levels, the resonant photoionization process can provide elemental selectivity of nearly 100%. A hot-cavity laser ion source with three tunable Ti:Sapphire lasers has been set up at the off-line Ion Source Test Facility of HRIBF, and initial laser ionization experiments have been successfully performed, in collaboration with the Atomic Physics Group of the ORNL Physics Division and the research group led by Dr. Wendt of the University of Mainz. The hot-cavity ion source was modified from a Ta tubular surface ionization source. Ithas the same basic structure as the standard high temperature RIB ion sources employed for on-line operation at the HRIBF. A schematic view of the ion source is shown in Fig. 4-1. The Ti:Sapphire lasers have been developed and provided by the Mainz group . They are pumped by a commercial frequency-doubled Nd:YAG laser operating at 10 kHz repetition rate with a maximum of 60 W average power at 532 nm. The Ti:Sapphire lasers are tunable over the range of 720 to 925 nm. With 2nd and 3rd harmonic generation capabilities, the laser system can provide tunable laser radiation in 360-463 nm (frequency doubling) and 240-308 nm (frequency tripling) spectral region. Fig.4-2 shows a picture of the Ti:Sapphire laser system in operation at HRIBF.
In the initial laser ionization experiments, three-step resonant ionization of Sn, Ni, and Ge has been observed. The excitation schemes used for the three elements are shown in Fig.4-3. A known three-step excitation scheme for Sn, which leads to an autoionization state in the last step , was used in this study. No previous laser ion source work on Ge has been reported. Resonant laser ionization of Ge in a hot-cavity LIS was demonstrated for the first time in this experiment. Ge atoms were excited from the 4p2 3P1 ground state level to the 4p5s 1P1o excited state (l1 = 253.39 nm), and then to the 4p5p 1S0 state (l2 = 909.845 nm). For the last transition, a careful search for autoionization was conducted by scanning the third laser wavelength over the tuning range of the Ti:Sapphire laser. Both Rydberg states and autoionization states were successfully observed in Ge. The best ionization yield was obtained using the autoionization state at 63818.262 cm-1. An extensive search was also conducted to look for autoionization states in Ni. Although no autoionization states were found, a variety of Rydberg states were detected. The ionization of Ni was then accomplished by three-photon resonant excitation to a Rydberg state. Displayed in Fig. 4 are measured mass spectra showing the Ge and Ni ions observed when the laser beams were turned on and the surface ionized Ga ions when the lasers were off. The temperature of the hot-cavity was about 1700-2000° C. No surface ionized Ge and Ni ions were observed when the laser beams were turned off.
Overall ionization efficiencies of 22%, 2.7%, and 3.3% were measured for Sn, Ni, and Ge, respectively, using calibrated samples containing ~1017 atoms. For Sn, the efficiency obtained in this study (22%) is better than the reported off-line and on-line performance at ISOLDE using either dye or Ti:Sapphire lasers . Our results represent the first LIS work reported for Ge and the first efficiency measurement reported for Ni using Ti:Sapphire lasers. The ionization process was not saturated for either Ge or Ni in this work. Thus, improvements for both elements are expected using more efficient excitation schemes. The Rydberg and autoionization states identified in Ge and Ni will be of value in future work. Analysis of these atomic spectroscopic results is in progress.
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