A digital version of Rossi's coincidence circuit

After more than a century since their discovery, cosmic rays remain a very active and fascinating research field and the coincidence method developed in those early studies is still a fundamental tool in experimental nuclear and particle physics. In this paper, Bruno Rossi's coincidence circuit, originally developed in 1930, is replaced by digital signal analysis of the outputs of three modern Geiger–Müller (GM) tubes. These tubes are conceptually similar to their historical counterparts, and their outputs are digitized using a computer sound card. The relation of coincidence rate to the thickness of overlying lead shielding, known as Rossi's curve, is reconstructed and compared with original historical data, the first experimental evidence of the electromagnetic showers. The digital implementation presented in this paper provides an effective and cheap educational tool for studying cosmic rays through signal coincidences using GM tubes and for approaching and deepening concepts that are still important in current physics research, like resolving time, and true and random coincidences. The total cost of the apparatus, including three detectors and the sound card, is around 1000 €.