Tail background function
According to the original HYPERMET concept, a slowly decreasing exponential background tailing component may also be used under the low energy side of the peaks, in order to compensate detector surface effects.
Nowadays, however, this phenomenon seems to be less significant with the most common detectors, therefore when you perform a peak evalu­ation with “Default settings for moderate count rate, regular peaks” configuration, tail will be disabled.
As tail also descends from a convolution of an exponential and a Gaussian, it is very similar to Left skew, but has much less intensity and greater slope. Important difference, however is that the tail is component of the background, while skew is a part of the peak.
Therefore counts under the skew is added to the peak area, while the area under the tail only contributes to the sum of background and peak functions, which helps the nonlinear fitter to match the measured counts. The picture shows the case of Tail Amplitude=0.01 and Tail Slope=2.5 parameters.
Tail counts may be calculated by the following formula at channel x:
where
     
TLAmpl:
Tail Amplitude relative to that of Gaussian (may vary from 0 to 0.01)
TLSlope:
Tail Slope (2.5 – 50)
Ampl:
Amplitude of Gaussian
Pos:
Position of the Gaussian centroid
Width:
Gaussian width (Width = FWHM/1.66),
and erfc is the standard complementary error function.
Note: If you observe significant scattering of Chi values in the peak fitting residual under the majority of the singlet peaks, and this may not be resolved by adding new peaks, it is possible that the background tail component should be enabled. This may be done either by manually enabling the component in the Peak evaluation algorithm settings window, or selecting another algorithm setting, where tail has been enabled previously. This is, however, may designate that your measurement set-up is improperly set or became noisy.
 
 
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