Imaging UK railway tunnels using a mobile muon tomography system

Cosmic ray muon radiography utilizes highly-penetrating cosmic ray muons to image the density profile of an object of interest. Recently we have performed the first field trial of a portable cosmic ray tracking system developed for imaging the overburden in UK railway tunnels. Using only short duration scans (c. 30 minutes), the muon rate detected as a function of position was found to correlate well with calculations of the expected flux derived from manual surface height measurements. Since access to the surface of a tunnel may not always be possible, a method was developed to infer surface height from muon rate data alone. Comparisons between measured and expected muon rates were capable of identifying three open shafts in a tunnel with an extremely high statistical significance. In addition to this, two unknown density anomalies were identified away from these open shafts, which were later confirmed by railway authorities to be areas requiring further investigation. In this talk we will report on this initial field trial, before discussing algorithm and detector developments made to image these identified density anomalies with a higher resolution in a subsequent field trial.