Counting rate losses

Intensity measurements are simplified when a detector always gives one electrical pulse for each x-ray quantum absorbed the detector remains linear so long as this is true. For low intensities, when the rates of incidence upon the detector are low, the Geiger counter fulfills this condition. As this rate increases above (about) 500 counts per second, the number of pulses per second decreases progressively below the number of quanta absorbed per second. This decrease occurs even with electronic circuits that can handle higher counting rates without appreciable losses. 

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. 

In time-gated photon counting, comparatively high photon count rates can be employed count rates as high as 10 MHz are often used. TG has the advantage of virtually no dead-time of the detection electronics ( 1 ns), whereas the dead-time of the TCSPC electronics is usually on the order of 125-350 ns. This causes loss of detected photons, and a reduced actual photon economy of TCSPC at high count rates. 

A correction for dead time loss can be made from the value of the dead time, t. If the true counting rate is n (t = 0), and m is the measured rate, we have... 

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 ... 

For quantitative electron spectrometry the most important disturbance caused by scattering processes is the loss of intensity in the registered counting rate /reg as compared to the undisturbed counting rate /0. The effect can be described by a scattering factor fs defined by... 

The counting rate that can be obtained in electron spectrometry with gaseous species is an important figure to be considered. It depends on two competing factors, the target density in the source volume and the limitations set by the scattering losses of the electrons. Therefore, special attention must be given to the... 

At low count rates the relative uncertainty of the pulser peak area in 1-day measurements remains below 10 3. At counting losses of 30% and at acquisition times of 1 h the relative uncertainty of the counting time does not exceed 1%. To cover the possible influence of systematic effects on the effective duration of the effective acquisition time originating in different counting conditions, a 1% uncertainty is included in the uncertainty budget at count rates exceeding 5000 s1. 

In a typical EEL spectrum, the count rate Ia (area under the excitation edge after background subtraction, for element A) is a product of the incident electron current density, J0, the number of atoms Na of element A per unit area, and oa> the total ionization cross-section per atom for the excitation of the appropriate inner-shell by the incident electrons. However, to preserve good energy resolution, an aperture is placed after the specimen which limits scattering to angles less than P and hence only a fraction of the core loss signal Ia(P) is measured. Moreover, in most... 

Count the sample for a sufficient time period and frequency to collect reliable data for determining the gamma-ray energy, the count rate for characteristic gamma-ray energy peaks, and the rate of decay of that peak. Also count the empty container with the Ge detector and spectrometer to determine whether any radionuclides remain sorbed on container walls and to estimate the fractional loss by sorption for each radionuclide of interest. Note that the counting efficiency for radionuclides retained on the container must be estimated. 

Figure 4.39 Different ways to demonstrate scattering losses in electron spectrometry. The data refer to 50 eV electrons scattered on argon at varying pressures with <7e Scatt. = 10 15 cm2 and along a path of 50 cm length, (a) Dependence of the registered count rate /reg on the pressure p shown on a linear scale (the undisturbed count rate /0 would follow from / = const p with const = 33 x 106 counts/(s Torr) in the example shown), (b) Scattering factor fs as a function of pressure (logarithmic pressure scale), (c) Plot of ln(/reg//0) = In fs as a function of the pressure. For details see main text.

The observed counting rate can be increased to account for coincidence losses by adding to the counting rate a factor , given by... 

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