Gas Flow Calculations and Measurements for a Gas Stopping Cell

Along with several other nuclear physics laboratories around the world, the NSCL has undertaken the challenge of stopping a radioactive ion beam in order to make low-energy beams for experiments that require nearly thermal beam energies (precision mass measurements, laser spectroscopy, etc.). As part of the LEBIT (Low Energy Beam and Ion Trapping) Project at the NSCL, a gas stopping station is being constructed in the N4 vault. It will serve as a high-pressure, gas filled radioactive ion source for low energy beams. The most critical component of the gas stopping station will be the Gas Stopping Cell. It is here that the NSCL s intermediate energy radioactive ion beam will be stopped in a high purity, high-pressure gas. In this cell, the ions will be thermalized and then extracted without re-ionization. Through the use of rf multipoles and other ion optical devices, the beam will be transported to one of several low-energy beam lines for nuclear physics research. Intermediate energy beams of exotic nuclei will first encounter the gas cell s thick (several mm) silicon or beryllium window (Figure 1). Passing through this window, the ions will lost most of their energy and straggle into the high-pressure (~1 bar) helium gas, where they will slow to thermal energies. Using a combination of electric potentials and gas flow, the ions will be extracted from the gas cell through a supersonic nozzle and continue downstream.