Conversion of 92MeV/u38Ca/37K projectile fragments into thermalized ion beams

The results of tests for stopping and extraction of energetic, 92 MeV/u, short-lived 38Ca and 37K fragments of 0.5% and 0.1% full-width of momentum spread with the first version of the National Superconducting Cyclotron Laboratory (NSCL) gas cell are reported. The projectile fragments were thermalized in 51 cm of helium at 1 bar and were transported by electric fields and gas flow into an expansion chamber through a supersonic nozzle, guided by an radio frequency quadrupole (RFQ) guide and collected on a metallic wire. The extraction efficiency was measured for different implantation rates, different electrical field distributions inside the gas cell, and different values of beam degrader thicknesses. The extraction measurements were compared with the experimental stopping efficiency and measurement of ionization induced in the helium gas. Mechanisms responsible for ion losses were identified and possible improvements of the gas cell performance were discussed.