Cosmic ray detectors are a common physics experiment and frequently flown on balloons to measure the change in cosmic ray flux with altitude. These experiments are typically performed with off the shelf Geiger–Müller tube based detectors. G-M tubes are typically quite expensive and involve high voltages which can become difficult to manage in the near vacuum experienced with high altitude balloons. A reverse biased photodiode can be used in much the same way as a G-M tube but with lower cost, lower voltage, almost no dead time, and the additional benefit of an output voltage proportional to the radiation energy.
Inspired by a Maxim application note.