CERN Program Library Long Writeup W5013 - CERNLIB ...

Geant 3.16 GEANT User’s Guide PHYS221
Origin : G.N.Patrick, L.Urbán Submitted: 26.09.83
Revision : Revised: 16.12.93
Documentation :
Simulation of Compton scattering
1 Subroutines
CALL GCOMP
GCOMP generates the Compton scattering of a photon on an atomic electron. It uses the random number
techniques of Butcher and Messell [10] to sample the scattered photon energy according to the Klein-
Nishina formula [16].
The interaction produces one electron, which is put in the /GCKING/ common block for further tracking.
Tracking of the scattered photon will continue, with direction and energy changed by the interaction. All
input/output information is through GEANT common blocks.
Input: via COMMON /GCTRACK/
Output: via COMMONs /GCTRAK/ and /GCKING/
Compton scattering is selected in GEANT by the input data record COMP. When Compton scattering is selected,
GCOMP is called automatically from the GEANT photon tracking routine GTGAMA.
2 Method
For a complete account of the Monte Carlo methods used the interested user is referred to the publications of
Butcher and Messel [10], Messel and Crawford [17] and Ford and Nelson [12]. Only the basic formalism
is outlined here.
The quantum mechanical Klein-Nishina differential cross-section is:
Φ(E,E ′ )= X 0nπr 2 0 m e
E 2
[ ] [ 1
ɛ + ɛ 1 − ɛ ]
sin2 θ
1+ɛ 2
where, E = energy of the incident photon
E ′ = energy of the scattered photon
ɛ = E ′ /E
m e = electron mass
n = electron density
r 0 = classical electron radius
X 0 = radiation length
Assuming an elastic collision, the scattering angle θ is defined by the Compton formula:
m e
E ′ = E
m e + E(1 − cos θ)
Using the combined “composition and rejection” Monte Carlo methods described in chapter PHYS211, we
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