Choosing the Gas

There are many factors that must be considered in choosing the best gas for the TPC.   Gas purity, multiple scattering, drift velocity, cost, and safety are just a few of the more important issues.

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.

Choosing the Drift Field

The STAR TPC uses an externally applied electric field to drift the electrons from any point in the TPC to the anode & pad plane.  The rate of drift is proportional to the applied field, but it isn't linear and it depends on the gas composition (see figure one). 

The figure is from Sauli's 1977 Cern  lectures.  Drift velocity is shown plotted versus the reduced electric field (ie. electric field / gas pressure ).  STAR uses P10 gas ... 90% Argon, 10% Methane.

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.

Page created by Jim Thomas, send comments to
Last modified on March 31st, 1998