Efficient Gas Stopping

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Gas stoppers are the tool of choice for slowing down beams of short-lived rare isotopes produced by in-flight fragmentation and separation. These devices can make use of the advantages of in-flight separation techniques used at NSCL and several other leading laboratories in the world—fast, efficient and chemistry-independent separation of isotopes. The properties of ions at rest can be studied in ion traps, for example at the LEBIT facility at NSCL, where high-precision mass measurements are successfully carried out with a linear gas stopper. Gas stoppers are also the first step in preparing re-accelerated beams for low-energy experiments.

The basic principle of gas stopping is simple. After appropriate slowing down the fast fragments in solid degraders the ions are finally stopped in a chamber filled with helium gas. Remaining singly or doubly-charged, ions are guided out of the gas cell using electric and/or magnetic fields and gas flow and then prepared into a low-emittance, low-energy ion-beam by means of RF ion guiding techniques. In order to maximize the benefit of the gas stopping approach, the following requirements have to be fulfilled:

  • Short extraction times. In order to minimize decay losses the extraction time should be comparable or shorter than the shortest half-life of the ions to be studied. Extraction times of 10 ms or less are desirable.
  • Efficient stopping and extraction at high rates. The efficiency of a gas stopper (the fraction of stopped and subsequently extracted ions compared to the number of injected ions) is a key performance benchmark for any facility based on the gas stopper concept. Next-generation facilities will provide beam intensities of up to 109/s, many orders of magnitude higher than available at present fragmentation facilities.
  • Applicability to all fragment beams. In order to be universal, the gas stopper needs to be able to handle beams of isotopes with largely different charges Z and neutron-to-proton ratios.

The cyclotron gas stopper is a new scheme under development at NSCL that promises to overcome some of the performance limitation of linear gas stoppers.