After the 1941 expedition to Colorado, Rossi decided that the question of whether mesotrons decay had been answered. However, he was not satisfied with the precision with which the lifetime had been determined, for existing estimates depended on mesotron mass, which was not accurately known. To perform a more direct measurement, he designed apparatus to measure the time interval between the arrival of a mesotron in an absorber, where it stopped, and the emission of an electron when the mesotron decayed. To assist, he obtained the help of graduate student Norris Nereson. At the heart of their experiment, was a "chronometer", which was an electronic circuit that produced a pulse whose height was accurately proportional to the time interval, and which could be recorded by photographing an oscilloscope trace.