Differences in the response of two light guide technologies and two readout technologies after an exchange of liquid argon in the dewar

TL;DR

This study compares two light guide and readout technologies for detecting scintillation light in liquid argon, observing significant signal degradation after refilling, likely due to thermal cycling damage or residual xenon contamination.

Contribution

It provides the first comparative analysis of two light guide and readout technologies' responses to liquid argon refilling and thermal cycling effects.

Findings

Significant drop in scintillation signal after dewar refilling.

Degradation likely caused by thermal cycling damage or xenon contamination.

No evidence of damage to MPPCs or ganging boards.

Abstract

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.