For peaks whose shape is Gaussian with standard deviation s,
the FWHM is given by 2.35s.
As well as the mean number, the fluctuation in the number of charge
carriers is of interest due to the close connection of this parameter and
the energy resolution of the detector.
The Fano factor is introduced as an adjustment factor to relate the
observed variance to the Poisson theory predicted variance.
 F =

Observed variance
in N


Poisson predicted variance (N) 
Then

R_{statistical limit }=
2.35Ö(F /
N)
Therefore for good energy resolution, the Fano factor should be as
small as possible.
