The Challenges of RIB Development

Between radioactive atom production in the target material and delivery to the experimental end station the majority of radioactive atom losses occur in the RIB target/ion source. Target/ion source efficiencies tend to be ~10^-2-10^-4for RIBs we have accelerated at the HRIBF. Particle losses occur during diffusion of the radioactive species through the target material, desorption from the surface of the target material, effusive transport to the ionization region and incomplete ionization in the on-line ion source. Since radioactive species have finite lifetimes, time delays in any of these steps prior to ionization can cause significant particle losses.

At present, RIBs at the HRIBF are first positively ionized by a high efficiency plasma ion source and then charge exchanged in low density vapors to form negative ions for injection into the tandem accelerator. This process tends to have efficiencies ~10^-1 when suitable vapors are selected. An additional loss of ~10^-1 can be expected during stripping in the terminal and transmission through the tandem accelerator.

Given the considerable radioactive atom losses and low initial production rates each target/ion source must therefore be designed to operate with the highest possible efficiency in order to provide enough RIB intensity for physics experiments: at least 10⁵ ions per second is usually desired.

The development and implementation of high efficiency target/ion source systems best suited to the specific chemical nature of each desired radioactive atom has therefore been a principle focus of our beam development efforts.