The detector gas used in the PPACS is instrument grade isobutane at a pressure
of 4 torr. There are two gas volumes, one for each hemisphere of the detector,
and 10 PPACs are housed within each volume. The circulation and pressure of
the isobutane for each volume is controlled by an independent gas handling
system. A schematic of the gas handling systems is shown in
Figure 10. The pressure difference across of the pressure window is
monitored by a 10 torr MKS 233B differential Baratron gauge, and this pressure
difference is maintained by a MKS 250 controller and associated valve. The
absolute pressure is monitored by a 1000 torr Baratron gauge while the
flow rate is monitored by a MKS 1259 mass flow meter.
