The STAR TPC is very large and secondary electrons from a primary track may drift as much as two meters before reaching the anode plane. So the gas must not attenuate these electrons and it must be kept pure to prevent other modes of electron loss due to attachment on oxygen and water molecules. Typically, the oxygen concentrations should be kept below a few hundred parts per million. This means the gas must be easy to recirculate and clean in order to achieve these stringent standards. Noble gases are good candidates because they are easily cleaned with simple technologies and many pure organic gases are easily handled too. Examples are helium, argon, methane, ethane, and isobutane.
STAR has chosen to run with two gas mixtures: Ar(90%)-Methane(10%)
and He(50%)-Ethane(50%). The noble gas component has a very low affinity
for free electrons while the organic gases quench the propogation of UV
photons throughout the TPC volume. The argon-methane mixture
will be used for the initial running of STAR because it is the least hazardous
of the two. However, the Ar component increases the multiple scattering
of the primary particle relative to He and so the best performance of the
TPC will be achieved with the He mixture.
For the purposes of accurate track reconstruction, it is reasonable to choose an electric field near the peak in the drift velocity curve. This ensures that the drift velocity is saturated and the drift velocity is least sensitive to minor changes in the gas pressure or temperature caused by the local environment. However, the STAR TPC has an automatic drift velocity stabilization feedback loop which works by monitoring the drift of laser tracks in the TPC. Since the origin of these tracks is well known in time and space, it is easy to calculate the actual drift velocity and to apply corrections to the external field to compensate for any time dependent variations in the gas properties. The operating point for the drift velocity must therefore be slightly off the peak in order to provide some slope to the oberserved changes in parameters and to avoid the problem of a double valued solution when the drift velocity is observed to drop.
A brief review of figure one reveals that any reduced field greater than 0.16 V/cm/mm-Hg satisfies these conditions. Transforming to standard temperature and pressure, this means the drift field should be slightly greater than 120 V/cm; which is why the TPC is operated at an average gradient of about 145 V/cm.