Detector Response and Energy-Threshold Optimization for Imaging with Cosmic-Ray Muon-Induced Secondary Radiation

Imaging with cosmic-ray muon-induced secondary radiation complements muon transmission radiography and muon scattering tomography by exploiting the electrons and bremsstrahlung photons created along the muon track inside the target. Because these secondaries are produced predominantly at low energies, the performance of such a system depends critically on parameters that are usually treated as routine: the uniformity of the response of the surrounding scintillator array and the energy threshold applied during acquisition. This contribution reports the instrumental characterization of the muon camera (MUCA) setup operated at the University of Novi Sad, comprising a five-layer Close Cathode Chamber tracker and four large-area plastic scintillation detectors. Over a continuous run of about 24 h, the combined tracking efficiency was about 0.96 and stable at the percent level. Single-detector and joint spectra recorded with 137Cs and a bulk KCl source were used to equalize the high voltage across the array, and the acquisition threshold was set to approximately 230 keV. A direct comparison of images of a copper cylinder reconstructed with a 250 keV and with a 1 MeV analysis cut shows that the conventional MeV-level threshold discards the majority of the target-related signal. The corresponding limitation of the geometrical acceptance is also discussed. The design of the COMIS upgrade, which extends the scintillator coverage to the forward region and introduces a compact multi-channel readout, is outlined together with a first map of the light collection uniformity of its large-area scintillators.