The delayed-coincidence method using

Principle of the technique. The delayed-coincidence technique, which is well known in nuclear physics, was first applied to the measurement of the lifetimes of excited atoms by Heron et al, (1954, 1956). However, it was not widely used until Bennett (1961) improved the method 

In this technique a sample of atoms or molecules is excited to some level k, usually by a pulsed electron gun. 

The excitation is then cut off instantaneously at time t=0. The probability of detecting a photon on the decay transition from level k to level i in a time interval between t and t+At after the removal of the excitation is given by 

The emitted photons are detected by a photomultiplier. Each photo-electron produces a pulse of charge at the anode and the number of pulses received in a given time interval is counted by means of a fast scaler. A semilogarithmic plot of the number of cotmts accumulated as a function of the delay time t enables the radiative lifetime tj to be obtained. 

2 X 10 s duration to the grid of the electron gun, which was normally held negative. The electron gun produced a current of 100 pA and was pulsed at a repetition rate of 10 kHz. The accelerating voltage was adjusted between 30 and 100 volts to give the maximum intensity for the line 

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