In preparation for Trinity, Rossi designed instrumentation to record gamma radiation during the chain reaction, whose duration was expected to be approximately 10 nanoseconds. Observations on this time scale were almost beyond the state of the art in 1945, but Rossi designed and built a large cylindrical ionisation chamber whose speed of response was adequate because its coaxial electrodes were separated by a narrow gap of only 1 centimetre (0.39 in).