  In this investigation the response to the scintillation light generated by
through-going cosmic muons in liquid argon (LAr) was measured by two light
guide technologies and two readout technologies after five weeks of running in
the TallBo dewar at Fermilab. The response was remeasured after the dewar was
drained of LAr, refilled, and then run again for an additional four weeks.
After the dewar was refilled, there was clear evidence that the scintillation
signal had dropped significantly. The two light guide technologies were
developed at Indiana University and MIT/Fermilab. The two readout technologies
were boards that passively or actively ganged 12 Hamamatsu MPPCs. Two possible
explanations were identified for the degraded signal: the response of the two
light guide technologies degraded due to damage caused by thermal cycling,
and/or unknown differences in the trace residual Xe contamination in the fills
of LAr led to the observed drop in scintillation light. Neither absorption nor
quenching by N2, O2, and H2O contamination can account for the degradation.
Neither the individual Hamamatsu MPPCs nor the passive/active ganging boards
appear to have been affected by the thermal cycling. The path length
distributions of the cosmics traversing the dewar appear quite similar in both
event samples.
