PENELOPE 2003 - OECD Nuclear Energy Agency

Chapter 6
Structure and operation of the code
system
In this chapter we describe the structure of the penelope code system and its operation.
The kernel of the system is the fortran77 subroutine package penelope, which
performs “analogue” simulation of electron-photon showers (i.e. the simulated showers
are intended to be replicas of actual showers) in infinite (unbounded) media of various
compositions. Photon histories are generated by using the detailed simulation method
(see section 1.4), i.e. all interaction events are simulated in chronological succession.
The generation of electron and positron tracks is performed by using the mixed procedure
described in chapter 4. Secondary particles emitted with initial energy larger
than the absorption energy –see below– are stored, and simulated after completion of
each primary track. Secondary particles are produced in direct interactions (hard inelastic
collisions, hard bremsstrahlung emission, positron annihilation, Compton scattering,
photoelectric absorption and pair production) and as fluorescent radiation (characteristic
x rays and Auger electrons). penelope simulates fluorescent radiation that results
from vacancies produced in K shells and L subshells by photoelectric absorption and
Compton scattering of photons and by electron/positron impact. The relaxation of
these vacancies is followed until the K and L shells are filled up, i.e. until the vacancies
have migrated to M and outer shells.
Being a subroutine package, penelope cannot operate by itself. The user must
provide a steering MAIN program for her/his particular problem. Nevertheless, this
MAIN program is normally fairly simple, since it only has to control the evolution of
the tracks simulated by penelope and keep score of relevant quantities. penelope is
devised to do the largest part of the simulation work. It allows the user to write her
or his own simulation program, with arbitrary geometry and scoring, without previous
knowledge of the intricate theoretical aspects of scattering and transport theories. In
the case of material systems with quadric geometries, the geometrical operations can be
done automatically by using the package pengeom (see chapter 5). The distribution
package also includes various examples of MAIN programs for simple geometries (slab
and cylindrical) and for general quadric geometries with limited scoring. Although