  The MOLLER experiment proposed at the Thomas Jefferson National Accelerator
Facility plans a precision low energy determination of the weak mixing angle
via the measurement of the parity-violating asymmetry in the scattering of high
energy longitudinally polarized electrons from electrons bound in a liquid
hydrogen target (M{\o}ller scattering). A relative measure of the scattering
rate is planned to be obtained by intercepting the M{\o}ller scattered
electrons with a circular array of thin fused silica tiles attached to air
light guides, which facilitate the transport of Cherenkov photons generated
within the tiles to photomultiplier tubes (PMTs). The scattered flux will also
pass through the light guides of downstream tiles, generating additional
Cherenkov as well as scintillation light and is a potential background. In
order to estimate the rate of these backgrounds, a gas-filled tube detector was
designed and deployed in an electron beam at the MAMI facility at Johannes
Gutenberg University, Mainz, Germany. Described in this paper is the design of
a detector to measure separately the scintillation and Cherenkov responses of
gas mixtures from relativistic electrons, the results of studies of several gas
mixtures with comparisons to simulations, and conclusions about the
implications for the design of the MOLLER detector apparatus.
