Fast photomultiplier tube gating technique for time-resolved fluorescence measurements

摘要

Abstract: A nanosecond gating technique applied to conventional, 'off the shelf', photomultiplier tubes (PMTs) is described. This technique is most suitable for detection and analysis of fluorescence, induced by pulsed lasers. A high voltage square wave generator is applied to 3 dynodes of the PMT. The result is a fast time-gated optical detector with a high dynamic range of approximately 80 dB. The active state of the gate, i.e. the pulsewidth of the applied square wave, sets the integration time in accordance to the decay time of the fluorescence signal. It can be selected by the user from less than 2 ns to several hundred nanoseconds. The pulse generator gets its energy from a transmission line, which is charged by a DC voltage of 650 Volts. This determines the magnitude of the pulsed voltage, which is applied to the 3 PMT stages. Twice the electrical length of the transmission line gives the pulse width. The pulse is initiated by a fast electrical switch with switching times of approximately 700 ps and a low time jitter of only 200 ps related to the trigger input. A distinct separation of the fluorescence and superposing background, e.g. scattered laser light, is achieve due to fast pulse transitions. Output signals from the PMT are processed by a fast integration. This can reduce the effects of electrical cross talk of the gating pulse into the signal line to sufficiently low values. !1