Electron detectors coincidence methods

Many experiments (see Section I.B) require the energy analysis and detection of two or more particles with time correlation in other words, coincidence counting techniques must be used. Coincidence methods have long been used in nuclear physics because of the convenient fast detectors that have long been available. The more recent availability of fast, high-gain electron multipliers has created the possibility of coincidence measurements in electron spectroscopy. Various aspects of coincidence measurements have been discussed elsewhere.100 102... 

In the method of delayed coincidences, the coincidence counting rate between detectors responding selectively to genetically related particles, e.g., a jS- or a-par-ticle and a y-ray or an electron or between y-rays, is plotted as a function of a time delay inserted in the electronic circuits used to detect one of them. The development of this method and of improved electronic techniques has led to the use of resolution times of between 10" and 10 sec. We shall not discuss these methods here, for they have been very adequately reported by Bell [i9], 2ff]. Further analysis of the experimental data has made it possible to measure lifetimes of the order of 10 sec. The limit to the method seems to be determined by the scintillating properties of the phosphor and its physical size. 

Using two detectors simultaneously, one has to detect the 122 keV y quanta coming from the Mossbauer source as a start signal. This actually marks the birth of the 14.4 keV Mossbauer excited level (see the decay scheme inO Fig. 25.7). Then, with the other detector, the 14.4 keV y quantum is detected, and the elapsed time measured (stop signal). With this method, Mossbauer spectra can be recorded in any time interval after the nuclear decay of Co. Recording characteristic X-rays following the electron capture, part of the aftereffect events can be filtered off. Due to the coincidence technique, very low activities can only be used, and therefore these measurements are rather time consuming (several weeks or months). 

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