The electron-beam-plasma ion source as source of negative fluorine

Abstract

Radioactive ion beams (RIBs) of short-lived isotopes of fluorine are in demand for investigating astrophysical phenomena related to the hot CNO cycle and rp processes responsible for stellar nucleosynthesis. Since negative ion beams are required for injection into tandem electrostatic accelerators, such as the 25 MV tandem accelerator used for post acceleration of RIBs for the Holifield radioactive ion beam facility research program at the Oak Ridge National Laboratory, efficient, direct-formation F− ion sources are highly desirable. We have conceived and evaluated a direct extraction F− source for potential RIB applications which is predicated on the reverse polarity operation of a positive electron-beam-plasma target/ion source (EBPTIS) while simultaneously feeding fluorine-rich compounds and Cs vapor into the source. The source is found to operate in two separately distinct temperature regimes for the generation of F−: (1) a high cathode temperature regime or plasma mode and (2) a lower cathode temperature regime or surface ionization mode. For the latter mode of operation, net efficiencies of η=0.2% were attained for the EBPTIS; delay times, τ, attributable to the transport of F and fluoride compounds from the target to the ionization chamber of the source, typically, were found to be τ∼60 s. Brief descriptions of the EBPTIS and experimental techniques used in the studies, as well as net efficiency and effusive flow data for the negative EBPTIS, are presented in this article.