We don’t know why [stoppi71] needs to do gamma spectroscopy. We only know that he has made one, including a high-voltage power supply, a photomultiplier tube, and–what else–an Arduino. You also need a scintillation crystal to convert the gamma rays to visible light for the tube to pick up.
He started out using an open source multichannel analyzer (MCA) called Theremino. This connects through a sound card and runs on a PC. However, he wanted to roll his own and did so with some simple circuitry and an Arduino.
The tube detects very faint light in the crystal so they need to be in a light-tight enclosure. The Arduino needs a little help to read the pulses from the tube in the form of a simple circuit. The circuit acts as a peak detector and the peaks last only about twenty microseconds. The post doesn’t have the schematic for the peak detector but the video, seen below, does.