The detector gas is supplied by four independent gas systems. Each system recirculates the gas with a compressor at a rate of about 20% detector volume per hour, i.e. 0.6 and 4.0 m³/h for VTPCs and MTPCs, respectively. Flow control is achieved by regulating the TPC over-pressure to 0.50±0.01mbar via frequency modulation of the recirculation pumps.
Fresh gas is mixed and fed into the recirculation system through mass ﬂow controllers which can deliver, in purge mode, the full recirculation rate. In normal operation, the fresh gas is supplied at only 2% detector volume per hour. Nonlinearities and calibration drift of the ﬂow controllers are eliminated by on-line control of gas ampliﬁcation and drift velocity, as the required setting accuracy is beyond the speciﬁcations of the ﬂow regulators. The mixing and monitoring equipment is temperature stabilized to better than 0.1°C. Oxygen is cleaned from the detector gas by ﬁlter columns containing active Cu-granules chosen for use with the CO2 gas mixtures. No absorption or exchange of CO2 with the other gas components or water has been observed. The ﬁlters are regenerated after typically 4–6 months operation periods, using Ar/H2 (93/7) mixture at 200ºC. Fresh ﬁlters absorb water contamination in the gas over periods of some weeks, such that the water content may vary by 10 to 20 ppm.
As mentioned above, the control of gas quality is one of the major issues in the NA49 TPC system. Oxygen and water contaminations are measured with two pairs of O2/H2O sensors which can be switched to different positions in the recirculation systems (B,C,D,G in Fig). Gas purities of 2–4 ppm Oxygen and about 20 ppm water are typically achieved.
Owing to the required dE/dx performance and space resolution in drift direction, and due to the large time constants (50 hours per volume change) involved in the propagation of gas instabilities, the gas mixtures have to be stabilized to a very high level of accuracy. We achieved a relative stability of better than 0.5% in gas gain and a measurement of drift velocity to better than 0.1% absolute and 0.03% relative.
Each of the gas systems has a system of four gas amplitude monitors and one drift velocity monitor. The amplitude monitors measure the signal from 55 Fe photons in a proportional tube, both in the fresh gas input line and in the recirculated gas. Pressure and temperature variations are corrected up to quadratic terms in gas density. Drift velocity is measured in a drift detector using the drift time difference from a pair of Alpha sources at 10cm distance.