Production of 10-80 keV negative heavy ions by charge exchange in Na vapour

As a continuation of previous work, a systematic experimental investigation of equilibrium charge-state fractions in a sodium vapour target has been carried out for several incident 10-80 keV ions with atomic numbers in the range from 3 to 79. Particular attention has been paid to the production of negative ions for use in tandem accelerators. In the energy interval investigated, the maximum negative fractions Fsub(-1 infinity) were: 1.4%(Li), 3%(Be), 11%(B), 43%(C), 32%(O), 0.5%(Na), <10-6(Mg), 13%(Al), 17%(Cl), 0.4%(Ca), 6%(Fe), and 66%(Au). These maximum Fsub(-1 infinity) values showed a nearly monotonical increase with the electron affinity of the projectile element, thus allowing the results to be extrapolated to other elements. It is concluded that in the energy interval of interest to tandem-accelerator applications, sodium vapour is a more efficient electron-donor target than magnesium vapour for projectiles of low electron affinity. (Auth.)