Monitoring the tagging efficiency of the Low-Energy Muon beamline through background analysis: Insights into the long-term performance of ultrathin carbon foils

TL;DR

This paper presents a background analysis method to monitor muon tagging efficiency at the Low-Energy Muon beamline, ensuring long-term stability and performance of ultrathin carbon foils used in LE-μSR measurements.

Contribution

The study introduces a novel background-based method for monitoring tagging efficiency that does not require additional detectors, validated over several years at PSI's beamline.

Findings

Method reliably tracks tagging efficiency over 6 years

Foil contamination affects tagging efficiency

Laser cleaning restores foil performance

Abstract

The efficient tagging of individual muons, along with the stability of the involved ultrathin carbon foil, is critical for ensuring fast, reliable and reproducible low-energy muon spin rotation (LE-$\mathrm{\mu}$SR) measurements. At the Paul Scherrer Institute's Low-Energy Muon (LEM) beamline, we developed a method to monitor the tagging efficiency of the beamline using routinely collected muon decay histograms. This method leverages background comparison before and after the arrival time of the muons at the sample to extract the tagging efficiency, eliminating the need for additional detectors or measurements. By analyzing data collected between 2018 and 2024, we confirm the method's reliability and validate its results using independent reference measurements. Furthermore, we establish a correlation between the tagging efficiency and foil thickness by investigating the impact of contamination and outgassing on the carbon foil, as well as the restoration effects of laser cleaning. The findings underscore the importance of monitoring the carbon foil's condition to maintain consistent beamline performance and reproducibility. This method offers a practical approach for detector efficiency monitoring, applicable to other beamlines employing similar setups.