Momentum compression for the FRIB fragment separator validated

FRIB will produce rare isotopes for experiments when the high-intensity primary beam reacts with a production target at the entrance of the Advanced Rare Isotope Separator (ARIS). In this process many different rare-isotope species will be produced. The main task of ARIS will be to deliver as many particles as possible of the one or more species of interest to the experiment while suppressing all other species that would otherwise come along as contaminants.

Many interesting isotopes will emerge from the target with broad momentum distributions. In order to transmit these to the experiments efficiently, FRIB has developed a scheme to compress their momentum widths. The rare isotope beam will be sent through a wedge-shaped piece of aluminum where the faster particles will get slowed down more than the slower particles, such that an incoming momentum width of e.g. ± 3 percent will be compressed to ± 1 percent, all while maintaining the achromaticity that enables a mass selection at the end of the preseparator.

A collaboration of researchers from FRIB, the National Superconducting Cyclotron Laboratory, Argonne National Laboratory, RIKEN, Gesellschaft für Schwerionenforschung (GSI), and Bucknell University tested this scheme experimentally in the A1900 fragment separator at NSCL. A special beam optics was developed for the A1900 and a special “wedge” was installed at the separator’s main dispersive image. The beam-optical properties were characterized in tests and then the momentum compression was tested with fragments from an 82Se primary beam. The momentum width before and after compression were determined. Preliminary results from the ongoing analysis for the isotope 63Cu show that the intended momentum compression was achieved, validating a key feature of the FRIB facility.