Count rate maximum

Optimum resolution, i. e. low full width at half-maximum (FWHM), is a trade-off between high count rate, i. e. low dead-time, and good spectral resolution. 

The cause of this difficulty therefore resides within the counter itself. The difficulty is described by saying that the Geiger counter has a dead time, by which is meant the time interval after a pulse during which the counter cannot respond to a later pulse. This interval, which is usually well below 0.5 millisecond, limits the useful maximum counting rate of the detector. The cause of the dead time is the slowness with which the positive-ion space charge (2.5) leaves the central wire under the influence of the electric field. This reduction in observed counting rate is known as the coincidence loss. 

Detector Resolution Efficiency Maximum count rate in single-pulse technique (kHz)... 

A novel variation of the above technique can be used in some cases. By choosing an appropriate source so that the maximum resonance in the paramagnetic state occurs at or near zero relative velocity, one eliminates the need of a Mossbauer spectrometer completely, and the transition temperature can be determined by measuring the count rate transmitted through a stationary absorber and emitted by a stationary source as a function of temperature—see, for example. Refs. 18, 20. 

What is the maximum allowed counting rate with a scintillation detector with a 0.25-p,s dead time if you can only tolerate a 3% counting loss ... 

Since for a given measuring time nlrut It, and acc /e2i> where /el is the electron beam current, it can be seen that when acc /Jtrue the statistical error is optimal and independent of ItV Second, the coincidence rate is restricted by the maximum allowable singles count rates in either channel (—106— 107 cps, as discussed earlier). 

Finally, an estimate of the counting rate / p(max) at the maximum of the photoline can be derived. Approximating the measured photoline with a Gaussian distribution of fwhm = A exp one derives from equ. (2.28c)... 

The formula for the self-absorption factor is exact for gamma rays (see Experiment 3) but approximate for beta particles. That it is applicable at all is due to the near-linear decrease of the logarithm of the count rate with absorber thickness of a beta-particle group (see Figure 2.6 in the Radioanalytical Chemistry textbook). The obvious deviation is that this relation ends at the range of the maximum-energy beta particle, whereas it continues indefinitely for gamma rays. 

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