Proportional counter windows for the Bragg crystal spectrometer on AXAF

摘要

One of the instruments being developed for AXAF (Bragg Crystal Spectrometer, BCS) employs a gas flow proportional counter as its primary detector. Until recently, stretched polypropylene (coated with formvar and graphite) was the baseline material for the windows for the BCS counters. Recent improvements in the fabrication of polyimide, however, have led the BCS team to update the instrument design to replace polypropylene with on the order of magnitude 1 $mu thick polyimide, coated on both sides with on the order of magnitude 200 angstroms of aluminum. Aluminum coated polyimide, while having nearly the same x-ray transmission as polypropylene, leaks gas at a significantly lower rate. This property is exploited in the design of the on-board gas replenishment system, leading to a gas subsystem for the low-leak material that is both lighter and simpler, important considerations for a satellite-borne instrument. In addition to working with polyimide, the BCS team has investigated other materials that require further development, but which have a great deal of potential as candidates for x-ray windows. In each case the candidate films are thin enough to transmit x rays with energies below 1 keV, but they leak gas at a rate at least a factor of 10,000 less than the standard, coated polypropylene. Materials that have been examined include diamond (manufactured by Crystallume, Inc.), beryllium, and silicon-enriched silicon nitride.