PENELOPE 2003 - OECD Nuclear Energy Agency

154 Chapter 5. Constructive quadric geometry
son et al., 1985), egsnrc (Kawrakow and Rogers, 2000), geant3 (Brun et al., 1986)]
have recourse to condensed (class I) or mixed (class II) simulation, where the global
effect of multiple interactions along a path segment of a given length is evaluated using
available multiple scattering theories. To avoid large step lengths that could place the
particle inside a different medium, these condensed procedures require the evaluation
of the distance from the current position to the nearest interface, an operation with a
high computational cost (see e.g. Bielajew, 1995). The mixed procedure implemented
in penelope is, at least computationally, analogous to detailed simulation (it gives a
“jump-and-knock” description of particle tracks). In fact, the structure of penelope’s
tracking algorithm was designed to minimize the influence of the geometry on the transport
physics. This algorithm operates independently of the proximity of interfaces and
only requires knowledge of the material at the current position of the particle. As a
consequence, the geometry package pengeom is directly linkable to penelope. However,
since pengeom does not evaluate the distance to the closest interface, it cannot
be used with condensed simulation codes, such as those mentioned above.
Let us mention, in passing, that in simulations of high-energy photon transport
complex geometries can be handled by means of relatively simple methods, which do
not require control of interface crossings (see e.g. Snyder et al., 1969). Unfortunately,
similar techniques are not applicable to electron and positron transport, mainly because
these particles have much shorter track lengths and, hence, the transport process is
strongly influenced by inhomogeneities of the medium. With the analogue simulation
scheme adopted in penelope, it is necessary to determine when a particle track crosses
an interface, not only for electrons and positrons but also for photons.
pengeom evolved from a subroutine package of the same name provided with the
1996.02.29 version of the penelope code system. This package was aimed at describing
simple structures with a small number of homogeneous bodies limited by quadric surfaces.
Although it was robust and very flexible, its speed deteriorated rapidly when the
number of surfaces increased. The need for developing a more efficient geometry package
became evident when we started to use penelope to simulate radiation transport in
accelerator heads (the description of which requires of the order of 100 surfaces) or in
studies of total body irradiation (the definition of a realistic anthropomorphic phantom
may involve a few hundred surfaces).
With pengeom we can describe any material system consisting of homogeneous
bodies limited by quadric surfaces. To speed up the geometry operations, the bodies
of the material system can be grouped into modules (connected volumes, limited by
quadric surfaces, that contain one or several bodies); modules can in turn form part of
larger modules, and so on. This hierarchic modular structure allows a reduction of the
work of the geometry routines, which becomes more effective when the complexity of
the system increases.
Except for trivial cases, the correctness of the geometry definition is difficult to check
and, moreover, 3D structures with interpenetrating bodies are difficult to visualize.
A pair of programs, named gview2d and gview3d, have been written to display