Stopping Relativistic Ions in Gas & Extraction, D.J.Morrissey

A broad range of experiments have been performed in which exotic, short-lived nuclei were separated as they flew from the target and were implanted in various solids. In a new approach, the energetic exotic ions can be stopped in a buffer gas and then extracted back into a vacuum at very low energies. When the radioactive ions are thermalized in ultra pure helium gas a large fraction remain in the 1+ or 2+ charge-state depending critically on the purity, pressure, and residence time in the gas. The thermalized radioactive ions in ultra pure helium gas can be extracted in a few tens of milliseconds, that is, before they beta decay by drifting the ions to a supersonic nozzle. Studies with the rare beams from the NSCL have allowed a fully realistic test and demonstration of the technique for a variety of rare, unstable ions. The studies include the effects of the broad momentum distribution of fragments, their range straggling, contamination of the buffer gas, and buffer-gas ionization effects. The prototype system is now routinely used to provide beams of exotic radioactivities for precise mass measurements in a Penning Trap.

The fast radioactive ions are obtained from the A1900, a large projectile fragment separator, at the National Superconducting Cyclotron Laboratory on the campus of Michigan State University. The so-called "gas cell system" was designed with NSCL beam line magnets that are tuned to provide a horizontal dispersion on a “monoenergetic degrader” to compensate for part of the momentum distribution of the A1900 projectile fragments. This NSCL range-compression system is similar to the one originally described by Weick et al. [Nucl.Instrum.Meth B164-165 (2ooo) 168]. A degrader consisting of very flat glass plates (~2 microns variation for thicknesses of millimeters) at the dispersive image along with a glass wedge and a very uniform beryllium window are used to slow the beam before the gas. Numerous measurements have been made of the range distributions for primary beams and secondary ions. Some examples were first published in 2oo4, see below, and experiments continue up to the present. Values of the ranges are in excellent agreement with the most recent range-energy calculations using the ATIMA code as implemented by in LISE++ and with the range distributions predicted by the monte carlo program SRIM2003.

The overall system consists of four differentially pumped chambers: a high pressure gas volume (~1atm x 0.5m) with a guide electric field and a small supersonic exit nozzle, an expansion chamber with an rf-quadrupole ion guide and skimmer, an intermediate pressure chamber traversed by a second rf-quadrupole, a second skimmer and ion guide through a high vacuum chamber. The entire system was completed in the summer of 2oo3. The major results for range compression using a monochromatic wedge system were published in 2oo4, see references below. Dramatic, quantitative results for the extraction efficiency for 38Ca and 37K radioactive ions from the system were obtained early in 2oo4. The overall results are that on the order of one-third to one-half of the incident ions at 100 MeV/A can be stopped in 0.5 m of helium at 1 bar and that up to ~10% of these stopped fragments can be extracted and observed. An important part of these studies was to determine the extraction efficiency as a function of the implantation rate. These results show a dramatic drop in efficiency as the implantation rate increases. Preliminary results of simulations show that the drop in efficiency is due to the effects of a radial outward force from the space charge of the positive ions acting (a) in the drift volume, and (b) near the nozzle.

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djm - last update: 04-Mar-2008