Construction of a Gas Stopping Station at the Nscl

Measurements on nuclei far from stability can and will continue to provide the most stringent tests of nuclear theories. Detailed measurements of masses, half-lives, and decay processes will be possible with thermalized beams of ions from the NSCL s Coupled Cyclotron. Even the most exotic nuclei can be studied if the thermalization and subsequent extraction are rapid (~10 ms). For example, the measurement of masses with high precision is best accomplished using low energy ion beams. Therefore, in order to meet the emerging requirement for thermal energy beams and to study the intricacies associated with stopping and collecting ions from projectile fragment separators, the gas stopping project was initiated as part of the larger LEBIT (Low Energy Beam and Ion Trapping) Project, described elsewhere in the NSCL 2000 Annual Report. The focus of this report is the stopping of the fast radioactive beams in a gas stopping station. The Gas Stopping Station will function as a high pressure, gas-filled radioactive ion source. Beams of exotic nuclei will be produced in the A1900 fragment separator and transported to the N4 vault. The ions will be dispersed on a shaped degrader and refocused onto the entrance window of a gas cell. The Gas Stopping Cell is the key element that is under development. The cell will use approximately one bar of high purity helium gas to stop the intermediate energy ions as they straggle through the entrance window. These exotic nuclei will be thermalized in the helium and extracted from the cell with electric potentials and gas flow through a supersonic nozzle. The ions will be captured in an rf multipole rod structure in a differentially pumped expansion chamber. Then, the ions will be transported at low potential to one of the low-energy beams lines in the N4 vault extension. The group has two goals in the forefront. First, the design and construction will be optimized to perform the stopping and extraction of ions with a high efficiency. Second, in order to conduct interesting physics experiments, the extraction of ions from the gas cell must be completed quickly in order make mass measurements on nuclei that have half-lives on the order of ~10 ms. Until recently, similar efforts involving gas stopping cells were characterized by a clear separation of nuclear physics from the gas stopping or atomic physics. The origin of these efforts can be traced back to the IGISOL work in Jyv skyl [1]. The NSCL s initiative is among several new attempts to stop and collect very energetic ions in relatively high pressure gases. Other gas stopping projects currently underway include: