Abstract
A hot-cavity surface ionization source and a hot-cavity laser ion source are evaluated in terms of ionization efficiencies for generating ion beams of U and Th. The work is motivated by the need for more efficient ion sources for detecting ultratrace U and Th impurities in a copper matrix by mass spectrometry techniques such as accelerator mass spectrometry (AMS). The performances of the ion sources are characterized using uranyl nitrate and thorium nitrate sample materials and sample sizes of 20 - 40 g of U or Th. For the surface source, the dominant ion beams observed are UO+ or ThO+ and ionization efficiencies of 2-4% have been obtained with W and Re cavities. Three-step resonant photoionization of U atoms is studied and an ionization efficiency of 8.7% has been obtained with the laser ion source. The positive ion sources promise more than an order of magnitude more efficient than conventional Cs-sputter negative ion sources used for AMS. In addition, the laser ion source is highly selective and effective in suppressing interfering and ions. Work is in progress to improve the efficiencies of both positive ion sources.
