A temporal method of avoiding the Cerenkov radiation generated in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams.

The output signal of an organic scintillator probe consists of a scintillation signal and Cerenkov and fluorescence radiation (CFR) signal when the probe is exposed to a mega-voltage photon or electron beam. The CFR signal is usually unwanted because it comes from both the scintillator and light guide and so it is not proportional to the absorbed dose in the scintillator. A new organic scintillator detector system has been constructed for absorbed dose measurement in pulsed mega-voltage electron and photon beams that are commonly used in radiotherapy treatment, eliminating most of the CFR signal. The new detector system uses a long decay constant BC-444G (Bicron, Newbury, OH, USA) scintillator which gives a signal that can be time resolved from the prompt CFR signal so that the measured contribution of prompt signal is negligible. The response of the new scintillator detector system was compared with the measurements from a plastic scintillator detector that were corrected for the signal contribution from the CFR, and to appropriately corrected ion chamber measurements showing agreement in the 16 MeV electron beam used.