Method

We have parameterised the total cross-section of -pair production by photon as:

$\sigma (Z,E$_γ) = Z(Z+1) &sp;(F₁(X) + F₂(X) Z +F₃(X)/Z) &quad;(barn/atom)

where

$X$	=	$ln(E$γ/m) ,
$m$	=	electron mass,
$E$γ	=	photon energy

and

$F$i(X)

=

$\sum$n=05cnXn

The parameters $c$_n were taken from a least-square fit to the data [] and can be found in the DATA statement in the function GPRSGGZE which computes the formula () (in barn/atom).

This parameterisation gives a good description of the data in the range:

$.$

$1\le Z\le 100$

$1.5\; \hspace{1em}MeV\le E$γ≤100  GeV

This mean free path is tabulated at initialisation as a function of the medium and of the energy by the routine GPRSGA.

PHYS211