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expfac: exponential modification

An exponential modification of a spectrum.

$\displaystyle M(E) = \begin{array}{ll} 1+Aexp(-fE) & E > E_c\\ [.2cm]1 & E < E_c \end{array}$
where:

par1=$A$ amplitude of effect
par2=$f$ exponential factor
par3=$E_c$ start energy of modification