Abstract: A method and a system for a TOF-PET scanner, the TOF-PET scanner including a scintillation chamber and a detection system comprising detection modules that surround the scintillation chamber. The method includes, for each detection module being calibrated, determining distributions of lifetimes of positrons based on differences of the times of registration of annihilation quanta and of the times of registration of the de-excitation quanta assigned to the common events and registered by the module being calibrated; extracting, from the determined distributions of lifetimes of positrons, a distribution of lifetimes of positrons with annihilation by para-positronium; and determining a time delay constant based on the extracted distributions of lifetimes of positrons with annihilation by para-positronium.