  A low mass time expansion chamber (TEC) has been developed to measure
distributions of position and angle of the TRIUMF low energy surface muon beam
used for the TWIST experiment. The experiment is a high precision measurement
of muon decay and is dominated by systematic uncertainties, including the
stability, reproducibility, and characterization of the beam. The distributions
measured by two TEC modules are one essential ingredient of an accurate
simulation of TWIST. The uncertainties, which are extracted through comparisons
of data and simulation, must be known to assess potential systematic
uncertainties of the TWIST results. The design criteria, construction,
alignment, calibration, and operation of the TEC system are discussed,
including experiences from initial beam studies. A brief description of the use
of TEC data in the TWIST simulation is also included.
