ISAC target operation with high proton currents

The TRIUMF-ISAC facility target stations were designed for ISOL target irradiations with up to 100 muA proton beam currents. Since beginning operation in 1998, ISAC irradiation currents have progressively increased from initial values of similar to1 muA to present levels of up to 40 muA on refractory metal foil targets. In addition, refractory carbide targets have operated at currents of up to 15 muA for extended periods. The 1-40 muA operational regime is achieved by tailoring each target to the thermal requirements dictated by material properties such as beam power deposition, thermal conductivity and maximum operating temperature of the target material. The number of heat shields on each target can be varied in order to match the effective emissivity of the target surface for the required radiative power dissipation. Targets of different thickness, surface area and volume have been investigated to study the effect of diffusion and effusion delays on the yield of radioisotopes. For yields of short-lived products, believed to be dominated by diffusion delays, a consistent nonlinear increase of yield as a function of proton current has been observed. The nonlinear dependence is consistent with expectations from radiation enhanced diffusion. (C) 2002 Elsevier Science B.V. All rights reserved.