The cyclotron gas stopper project at the NSCL

Gas stopping is becoming the method of choice for converting beams of rare isotopes produced via projectile fragmentation and in-flight separation into low-energy beams. These beams allow ISOL-type experiments to be conducted on projectile fragmentation products, such as precision mass measurements with traps or laser spectroscopy. Current gas stopping systems for high-energy beams employ a linear gas cell design filled with 0.1-1 bar of helium. While linear gas cells have found success in a variety of experiments, this success is limited due to the space charge effects induced by the ionization of the helium atoms during the stopping process. These space charge effects pose a limit on the maximum incoming beam rate. Furthermore, the extraction time of stopped ions from these devices can exceed 100 ms causing substantial decay losses for very short-lived isotopes. To avoid these limitations, a new type of gas stopper is being developed at the NSCL/MSU. The new system is based on a sector-focused cyclotron magnet with a stopping chamber filled with Helium buffer gas at low pressure. RF-guiding techniques are used to extract the ions. The space charge effects are reduced by the large volume and due to the separation between the region of stopped ions and the region of highest ionization.