Advanced characterization of an optical fibre sensor system based on an MPPC detector for measurement of X-ray radiation in clinical linacs

A reliable, accurate and in-vivo dosimetry system for measuring the radiation dose and profiling the X-ray beam during radiotherapy is reported. Its dynamic range is investigated using an accurately controlled pulsed light emitting diode (LED) system. Highly resolved temporal analog and digital signals were captured from the analog and digital outputs of a multi-pixel photon counter (MPPC) detector when exposed to the LED system. The photon distribution of a low intensity pulsed LED light source was observed and is found to obey a Poisson distribution with changing light intensity. The average number of photons was obtained using the digital MPPC output signals which in turn allowed the appropriate intensity of the light source to be determined for the correct light exposure conditions for the detector. The average analog output voltage over a single 3 μs pulse is determined to indicate the intensity of the detected light. The MPPC detector output analog signal is limited to a narrow range (0.6 V to 1.4 V) to ensure adequate signal detection level (the lower limit) and prevention of entry into saturation (the upper limit) which also corresponds to a digital output signal range (in counts). An average photon number range of 3 to 7 for the digital output signal is established, which leads to the establishment of a unique and constant photon number to average output voltage ratio of 4.64 ± 0.10. Experimental results show that the establishment of this ratio is significant as adherence to it ensures the correct exposure conditions of the MPPC and speeds up the measurement cycle in the clinical setting.