next up previous contents index
Next: PSPC Gas Flow Up: 3.2 Operation Previous: Rate-Gain Effect

Gas Control System


For a stable operation of the PSPC, the detector gas must be replenished at a flow rate of several cm tex2html_wrap_inline16755 min tex2html_wrap_inline16757 . There are several reasons for this constant flow. The first is to maintain the relative composition of the detector gas mixture (65% Ar, 20% Xe, 15% CH4 ),   which would otherwise be changed by the differential gas diffusion through the thin polypropylene/lexan window. The second reason is to prevent gas poisoning by polymerization products of the quench gas as well as outgasing from glue and plastic. In order to maintain a temperature-independent gas gain, the detector gas is density  rather than pressure controlled. Otherwise, a one degree temperature change would result in a 5% gain shift.

The basic components of the gas supply system are the gas tanks  (three tanks of 12 l each with an initial pressure of 200 bar) with pressure regulators, two redundant solenoid valves for the detector pressure regulation, and two artificial leaks (2.5 cm tex2html_wrap_inline16759  min tex2html_wrap_inline16761 and 3.8 cm tex2html_wrap_inline16763  min tex2html_wrap_inline16765 ) at the outlet of each detector. Sealed reference volumes are integrated in the detector housings to guarantee a good thermal coupling between detector and reference gas. They are filled with detector gas at the nominal operating pressure (1465 mbar at 22 tex2html_wrap_inline16767  C). The gas control unit maintains the detector pressure at the pressure of the reference volume, yielding a density control  (i.e., when the temperature goes up, the regulating pressure goes up accordingly, and vice-versa). The control unit is a bistable regulator with a selectable duty cycle (1s - 60s).

If you have problems/suggestions please send mail to rosat_svc@mpe-garching.mpg.de