PENELOPE 2003 - OECD Nuclear Energy Agency

178 Chapter 6. Structure and operation of the code system
they are mostly intended to illustrate the use of the simulation routines, they do allow
studying many cases of practical interest. The complete program system is written in
fortran77 (ANSI/ISO standard form) and, therefore, it should run on any platform
with a fortran77 or fortran90 compiler.
6.1 penelope
penelope simulates coupled electron-photon transport in arbitrary material systems
consisting of a number of homogeneous regions (bodies) limited by sharp (and passive)
interfaces. Initially, it was devised to simulate the PENetration and Energy LOss of
Positrons and Electrons in matter; photons were introduced later. The adopted interaction
models (chapters 2-4), and the associated databases, allow the simulation of
electron/positron and photon transport in the energy range from 100 eV to 1 GeV.
It should be borne in mind that our approximate interaction models become less
accurate when the energy of the transported radiation decreases. Actually, for energies
below ∼1 keV, the DCSs are not well known, mostly because they are strongly
affected by the state of aggregation. On the other hand, for electrons and positrons,
the trajectory picture ceases to be applicable (because coherent scattering from multiple
centers becomes appreciable) when the de Broglie wavelength, λ B = (150 eV/E) 1/2 Å,
is similar to or greater than the interatomic spacing (∼ 1 Å). Therefore, results from
simulations with penelope (or with any other Monte Carlo trajectory code) for energies
below 1 keV or so, should be considered to have only a qualitative (or, at most,
semi-quantitative) value. We recall also that, for elements with intermediate and high
atomic numbers, secondary characteristic photons with energies less than the M-shell
absorption edge are not simulated by penelope. This sets a lower limit to the energy
range for which the simulation is faithful.
The source file PENELOPE.F (about 8000 lines of fortran code) consists of four
blocks of subprograms, namely, preparatory calculations and I/O routines, interaction
simulation procedures, numerical routines and transport routines. Only the latter are
invoked from the MAIN program. The interaction simulation routines implement the
theory and algorithms described in chapters 2 and 3. Although the interaction routines
are not called from the MAIN program, there are good reasons to have them properly
identified. Firstly, these are the code pieces to be modified to incorporate better physics
(when available) and, secondly, some of these subroutines deliver numerical values of
the DCSs (which can be useful to apply certain variance reduction techniques). To have
these routines organized, we have named them according to the following convention:
• The first letter indicates the particle (E for electrons, P for positrons, G for photons).
• The second and third letters denote the interaction mechanism (EL for elastic, IN for
inelastic, BR for bremsstrahlung, AN for annihilation, RA for Rayleigh, CO for Compton,
PH for photoelectric and PP for pair production).
• The random sampling routines have three-letter names. Auxiliary routines, which
perform specific calculations, have longer names, with the fourth and subsequent letters