A high efficiency, kinetic-ejection negative ion source for RIB generation

Chemically active radioactive species, diffused from RIB target materials, often arrive at the ionization chamber of the source in a variety of molecular forms. Because of the low probability for simultaneously dissociating and efficiently ionizing the individual atomic constituents of molecules with conventional hot-cathode electron-impact ion sources, the species of interest are often distributed in several mass channels in the form of molecular side-band beams and consequently, their intensities are diluted. The sputter negative ion beam generation technique offers an efficient means for simultaneously dissociating and ionizing highly electronegative atomic species present in molecular carriers. We have incorporated these principles in the design and fabrication of a kinetic ejection negative ion source and evaluated its potential for generating 17,18F- beams for the Holifield Radioactive Ion Beam Facility astrophysics research program. The source utilizes Cs+ beams to bombard condensable fluorine compounds that emanate from a target material, such as Al2O3, and are transported to the cooled inner surface of a conical-geometry cathode where they are adsorbed. The energetic Cs+ beams efficiently dissociate these molecules and sputter their constituents. Since the work functions of cesiated surfaces are low, highly electronegative species such as fluorine are efficiently ionized in the sputter-injection process. Measured efficiencies for ionizing atomic fluorine, dissociated from condensable compounds that are formed by reactions of SF6 with fibrous Al2O3 material, exceed 6.5%. In this report, we describe the mechanical design features and principles of operation, and present emittance, F- yield and ionization efficiency data derived from off-line, experimental evaluation of the source