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Monitoring the tagging efficiency of the low-energy muon beamline through background analysis: Insights into the long-term performance of ultrathin carbon foils

Gianluca Janka1,*, Herman Ishchenko1,2, Zaher Salman1, Andreas Suter1, and Thomas Prokscha1,†

  • *Contact author: gianluca.janka@psi.ch
  • †Contact author: thomas.prokscha@psi.ch
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Phys. Rev. Accel. Beams 28, 082802 – Published 22 August, 2025

DOI: https://doi.org/10.1103/21tp-bccj

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 measurements. At the Paul Scherrer Institute’s low-energy muon 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.

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Phys. Rev. Accel. Beams 28, 082802– Published 22 August, 2025
  • Received 26 February 2025
  • Accepted 30 July 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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