Bomb development called for large detectors of ionising radiation, whose response is proportional to the energy released in the detector and follows rapid changes in radiation intensity. From the earliest research on radioactivity, radiation had been measured in terms of Ionisation, but existing Ionisation chambers were slow to respond to changes. To address this problem, Rossi and Staub carried out a careful analysis of the pulses that result when individual charged particles create ions within an ionisation chamber. They realised that the high mobility of free electrons removed from ionised atoms means that the pulses produced by single particles can be very brief. With James S. Allen, Rossi found gas mixtures of high electron mobility and low electron attachment. On the basis of these investigations, Allen and Rossi invented the "fast ionisation chamber", which they patented after the war. It was a crucial factor in the success of the Manhattan Project and became widely used in postwar research on particle physics.