Interstitial brachytherapy is a cancer treatment in which radioactive material is placed closely to the target tissue of the affected site using an afterloader (HDR-brachytherapy) or manually (LDR- and PDR-brachytherapy). For HDR-brachytherapy, the accuracy of this placement is calibrated using an external reference system that locates the radioactive material according to the radiation levels measured at locations around the source. At each of these locations, a scintillator produces light when irradiated by the radioactive material. This light is proportional to the level of radiation at each location. The light produced by each scintillator is converted to an electrical signal that is proportional to the light and the radiation level at each location. The radioactive material is located according to the plurality of electrical signals.