Counting using pulser

Instrumentation. The steady-state fluorescence spectra were measured with Perkin-Elmer MPF-44B fluorescence spectrophotometer. The single-photon counting instrument for fluorescence lifetime measurements was assembled in-house from components obtained from EG G ORTEC. A PRA-510B light pulser filled with gas was used as the excitation source. Instrument response function was obtained with DuPont Ludox scatter solution at the excitation wavelength. 

The measurements were performed on three detectors. Two detectors, a Ge(Li) and a p-type, were connected to PGT 386 amplifiers. One of the amplifiers operated with a shaping time of 4 ps and the base-line restorer threshold set to VAR. With the second detector connected to the amplifier operating at 3 ps two sets of measurements were made one with the base-line restorer threshold set to AUTO and the other with the base-line restorer threshold set to VAR. The thresholds of the base-line restorers set to VAR were adjusted manually to minimize the tails of the pulser peak at low count rates. The third detector was a low-energy detector connected to an OR-TEC 573 amplifier operating with a shaping time of 6 ps and in the Gaussian mode. This amplifier was equipped with an automatic base-line restorer. The third detector was used in order to see how different amplifiers influence the performance of the pulser peak method. 

The simplest way to determine the overall dead time is to measure it using a pulser. The pulser signals should be connected to the preamplifier of the detector, using a pulse amplitude greater than that from any expected capture photon. From the known rate of the pulser and the recorded number of pulser counts, the number lost due to the dead time can be determined. The best performance is obtained with counting a radioactive source or a random time pulser together with the sample (Knoll 2000). 

Although, at first sight, a pulser does seem to be a very direct way of correcting for all pulse losses in the pulseprocessing system, at high count rate, where correction for losses is most important, there are the most difficulties. I am aware of laboratories where the pulser method is used... 

Apart from the normal routine checks, one would have to assure oneself that the electronic system was working satisfactorily - at high count rate, there is the additional burden of confirming that counting losses are being adequately accounted for. There are procedures that have been widely used for many years. In 1990, Gehrke proposed the particular procedure below, which is now enshrined in the US standards ANSI N41.14 (revised). It is a test of the precision of automatic or semi-automatic dead time correction of whatever type - by a measured correction factor, by PUR and ETC circuits, by the pulser method, by the virtual pulser, or by any combination of these. The procedure is as follows ... 

A normal electronic pulser (a fixed interval pulser) does not give pulses randomly distributed with respect to time, therefore it is important that the pulser does not contribute significantly to the total spectrum. To this end, the ratio of the injected pulses to the total number of pulses should not exceed 0.1 if dead-time losses are to be corrected without error.This restriction arises because injected pulses cannot interact with each other but only with detector pulses however, detector pulses can interact with each other such that more detector pulses are lost than injected pulses. The difference becomes negligible if the injected pulses do not significantly alter the overall count rate. The whole effect can be avoided if a random-interval pulse generator is used when accurate correction can be made over a much wider range of count rates and relative pulse rates. 

F is the pulser frequency (50 Hz) and P is the number of measured counts under the pulser peak. Strijckmans (42) shows that under the conditions used this method results in a negligible systematic error. 

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