Step background function

HyperLab also fits regions with sharp background steps under the peaks.

This stepped background accounts for a Compton edge. Steps are fitted by one characteristic parameter, the Step Amplitude, which is relative to the amplitude of the Gaussian.

The picture shows a step with relative Amplitude=0.10.

Step's counts may be calculated by the following formula at channel x:

where

	STAmpl:	Step Amplitude relative to that of Gaussian
	Ampl:	Amplitude of Gaussian
	Width:	Gaussian width (Width = FWHM/1.66),

and erfc is the standard complementary error function.