Estimating the Aging Rate for the Cosmic Ray Veto Detector of the Mu2e Experiment at Fermilab

Abstract

The Mu2e experiment at Fermilab will search for the neutrinoless conversion of a muon to an electron in the presence of an atomic nucleus. An observation of Mu2e conversion is a clear sign of new physics. In order to achieve the expected single event sensitivity, the Mu2e detector apparatus has been designed to carefully mitigate backgrounds while selecting for the conversion signature of a monoenergetic 105 MeV/c electron emerging from the muon stopping target. Even in the case of no observation, the Mu2e experiment will improve the current upper limit for the muon conversion-to-capture ratio by four orders of magnitude.

Cosmic ray induced backgrounds present the largest background mode contribution for Mu2e. The Cosmic Ray Veto (CRV) serves as an active shield against cosmic rays by surrounding the bulk of the Mu2e detector apparatus with four layers of scintillator, arranged in long bars. An overall efficiency of 99.99% is required for the CRV to reduce the total number of cosmic ray-induced background events to below one event for the duration of the experiment.

Plastic-based scintillating detectors can degrade over time and, as a result, generally produce less light in response to the same amount of energy deposition when cosmic particles pass through it. By measuring the light yield of the CRV over time, an aging rate may be estimated for the CRV and the impacts to detector response as a result of aging can be anticipated and mitigated if necessary. Cosmic ray data collected with the Wideband test bench was used to estimate an aging rate for the Mu2e CRV, in the first study of its kind, and the estimated aging rate was determined to be 8.06% +/- 0.33% per year in units of photoelectron yield in the channel lost annually.

The estimated aging rate prompted an independent investigation into the base components used to produce CRV counters to determine if one component could be responsible for most of the overall aging in the CRV. For this dissertation, titanium dioxide samples which are used in the reflective outer cladding for the CRV counters were monitored over time for decrease in surface reflectivity. It was observed that titanium dioxide lost surface reflectivity at an average rate on the order of 0.5% per year. Using this average rate in conjunction with simulations of the Mu2e CRV counters predicts roughly a 4% drop in light yield in a given CRV channel.