Beam thermalization in a large gas catcher

Thermalization of fast ions in a buffer gas provides a method to transform the high-energy, exotic beams produced by projectile fragmentation at the National Superconducting Cyclotron Laboratory (NSCL) into low-energy beams. The process includes slowing down the fast exotic beams in solid degraders combined with momentum compression and removal of the remaining kinetic energy by collisions with a buffer gas. The original beam thermalization area for mass measurements at the NSCL was reconfigured to accommodate a new momentum compression beam line, a large radio-frequency (RF) gas catcher constructed by Argonne National Lab and a low-energy beam transport system. A large variety of exotic isotopes produced by projectile fragmentation and selected by the A1900 fragment separator was thermalized in the 1.2 m long gas catcher filled with helium at approximately 100 mbar. The ions were guided to an extraction nozzle with a combination of electrostatic and RF potentials and ejected by the gas flow. A novel RF ion guide was used in a differential pumping system to remove the helium and transport the ions into ultrahigh vacuum. Finally, the ions were accelerated to 30 kV for transport to various experiments. The distribution of the thermalized ions among chemical adducts is one of the operational challenges. The important steps implemented to minimize the production of the chemical adducts in the gas catcher are discussed. The operational status of the facility and some example results from characterization of the gas catcher operation with 37K and 47K beams are presented.