3. DESCRIPTION OF PROGRAM OR FUNCTION
PENELOPE performs Monte Carlo simulation of coupled electron-photon transport in arbitrary materials and complex quadric geometries. A mixed procedure is used for the simulation of electron and positron interactions (elastic scattering, inelastic scattering and bremsstrahlung emission), in which 'hard' events (i.e. those with deflection angle and/or energy loss larger than pre-selected cutoffs) are simulated in a detailed way, while 'soft' interactions are calculated from multiple scattering approaches. Photon interactions (Rayleigh scattering, Compton scattering, photoelectric effect and electron-positron pair production) and positron annihilation are simulated in a detailed way.
The following features are specific to the 2001 and later versions of the code:
1) The simulation parameters (Eabs, C1, C2, Wcc and Wcr) are defined independently for each material. This allows the user to specify regions of interest where the simulation is performed in greater detail.
2) The argument DSMAX of subroutine JUMP specifies the maximum allowed path length between hard interactions of electrons and positrons. By selecting a suitably small value for this parameter,
the user can control the number of hinges in thin regions (which must be larger that 10 or so, to ensure accuracy). This argument is also used for simulating the transport of charged particles in the presence of static external electric and magnetic fields. DSMAX has no effect when set equal to a very large number (e.g. 10**35).
3) The structure of the common /TRACK/ differs from previous versions. The variable ILB(5), which describes the origin of secondary particles, can be used to study partial contributions from particles originated by a given process.
The following features are specific to the 2003 and later versions of the code:
1) The generation of fluorescent radiation (x rays and Auger electrons) resulting from ionization of inner shells by impact of electrons and positrons is described as an independent process. The adopted ionization cross sections were calculated by using the relativistic Born approximation with a generalized oscillator strength model built from the partial photoelectric cross section of the active electron shell. This gives a more accurate description of the generation of characteristic x rays by electron and positron impact.
2) The energies of characteristic x rays are set equal to their experimental values (in previous versions of the code they were calculated from the ionization energies of the active electron shells).
3) The simulation algorithm for electron and positron inelastic collisions has been re-coded to allow more flexibility in the description of the physics and also to improve the simulation speed. Apart from a certain gain in speed (which is higher for heavy elements) this feature is not "visible" from the user's main program.
4) The program TABLES.F, which produces tables of interaction properties (photon mass attenuation coefficients, electron/positron mean free paths, stopping powers, radiation yields, ranges,...) for arbitrary materials, has been included in the distribution package.
The following features have been added to the 2005 version of the code:
1) Version 2005 includes the new program PENMAIN, that works like a blackbox for solving a large variety of problems with quadric geometries. With PENMAIN occasional (and normal) users can solve problems without having to write a single program line.
2) The program TABLES gives access to additional details of the interactions physics, both in a numeric and graphical way, e.g. the energy loss of electrons, or the photon attenuation coefficients in a material.
3) TIMER subroutines have been modified, according to a suggestion from a user from the NEA Data Bank, and are portable now (so far these work correctly with all compilers, under Windows or Linux).
4) Also following a suggestion from a NEA Data Bank user, unit 6 is no longer the standard output unit, except when the program makes a fatal error and stops. Now it is possible to write messages on the screen (like number of simulated showers) at predefined intervals, and consequently, it is possible to follow the program execution.
5) The electron elastic differential cross-sections are now obtained through partial wave analysis (PWA). Before only the total cross-section and the two first momenta of the differential cross-sections were considered..
6) The atomic disexcitation process following the vacancies in the M layer is now described. Before only the X rays and Auger electrons resulting from the vacancies in the K and L layers were described.
7) Some improvements have introduced in the geometry subroutine (PENGEOM). For instance, now it is possible to clone objects.
8) Consequently, some changes had to be made in the geometry viewers. For instance GVIEW allows to visualise materials and shapes (before only materials).
Improvements in the 2006 version compared to version 2005.
1) The correction for the energy dependence of the stopping power and the energy straggling parameter for soft interactions of electrons and positrons has been improved.
2) The sampling algorithm for Rayleigh scattering (which caused the g77 compiler to issue multiple warning messages) has been reformulated and improved.
3) Numerical interpolations, particularly those of elastic-scattering differential cross sections, are more accurate.
4) The structure of several common blocks has been modified to make interaction properties accessible from the program TABLES, and to facilitate future extensions of the interaction database. The names of interaction physics routines have been modified to allow the use of multiple models for each interaction mechanism.
5) The geometry package PENGEOM has been expanded. The present version can handle complex quadric geometries with up to 9,999 surfaces and 5,000 bodies. This extension required slight changes in the FORMAT of the geometry definition files (old files can be adapted with little editing work).
The geometry enclosure is defined as a sphere of 1.0e7 cm radius. This corrects a bug in the 2005 version (which caused inconsistencies when the whole material system was enclosed in a single module and this module was not simply connected).
6) The manual (penelope-2006-NEA.pdf) has been expanded and adapted to the present structure of the code.
Improvements in the 2008 version compared to version 2006.
This new version incorporates substantial changes and additions to the previous versions (1996, 2000, 01, 03, 05 and 06), which aim at improving both reliability and generality of the code system. As for the physics, the ionisation of inner shells by electron and positron impact is now described by using a numerical database of total cross sections for K, L and M shells, which was calculated from the theory described by Bote and Salvat (2008). Also included are photon polarisation effects in Rayleigh and Compton scattering, as well as refinements in the modelling of Rayleigh photon scattering and of inelastic collisions of electrons and positrons. The distribution package includes two examples of main programs: pencyl (for transport in cylindrical geometries), and penmain (for generic quadric geometries). In the present version, these codes are more flexible and allow a number of new options (e.g., extended sources, photon polarisation, and calculation of energy distributions of average fluence in selected bodies). The geometry viewers have also been improved to allow the easy identification of body labels.
Changes in the 2008.1 version:
1) The implementation of Bielajew's random hinge method has been modified. Soft angular deflections are sampled by assuming that the energy of the particle along the step decreases at a constant rate.
2) In the source file 'penelope.f', the RITA subroutines have been modified to allow a more accurate calculation of moments from the interpolating rational function.
3) In PENGEOM, surfaces defined as SURFACE* are never altered. That is, these surfaces are effectively fixed in space. In previous versions, when a SURFACE* was used in the definition of a module, possible translations or rotations of that module affected the surface.
4) A bug in PENGEOM (version 2008 only), which caused inconsistencies for geometries with infinite bodies, has been corrected. Note that now the enclosure is assumed to have a finite volume.
5) A (short) list of errata of the 2008 manual has been included in the .\doc directory.
Changes in the 2011 version:
The 2011 version contains improvements in the description of inelastic collisions of electrons and positrons. The geometry package has been reformulated to permit the description of nanometric structures. Programs for the calculation of mass energy-absorption coefficients for photons, and linear energy transfers for charged particles have been included in the distribution package. The manual has also been revised, and a new chapter on radiation fields and dosimetry has been added.