TRAX data objects

Projectiles

TRAX is formally designed to support electrons, positrons, photons, neutrons and ions as primary particles. At present, however, only electrons and ions are actually treated. Electrons, positrons, photons and neutrons are specified as e-, e+, x and n, respectively.
Throughout TRAX ionic projectiles may be specified as 
<mass><elementsymbol><charge>
Valid specifications are e.g.: Note that they are case-sensitive, that is 
8BE
would be an error, whereas 
8Be
would be OK. When the mass number is omitted, the most abundant nuclide is chosen.
As of TRAX version > 0804, ionic projectiles are no longer builtin but a large collection of projectiles is provided in the PROJ subdirectory and will be automatically loaded as part of the startup procedure.
In addition, projectile files moonshine as a database of atomic targets.
Projectile files are handled by a command, the file format is described in the appendix.

Target materials

Throughout TRAX target materials are identified by name. The actual chemical composition, physical properties and electronic structure must be specified as an external table, to be read before calculating cross sections or running the simulation. The properties of a few materials like H2O and N2 are builtin in versions earlier than and including 0804. For later versions they are no longer built in but a large collection of target materials is provided in the TARG subdirectory and will be automatically loaded as part of the startup procedure.
Note that the proper handling of composed targets requires the presence of atomic data, provided by the projectile data base.
Target files are handled by a command, the file format is described in the appendix.

Chemical species

The (optional) chemical stage of the simulation works with chemical products, specified just as target materials. To distinguish atoms and molecules from radicals, the latter have a trailing . (dot).
At present, chemical species only live within the evaluation function simulating their developmemt after the completion of the physical stage.
Chemical species and properties of a medium (solvent) are defined within a file, the file format is described in the appendix. They are handled by a command,

Interactions and Cross Sections

A couple of interaction cross sections is supported for the various particles, identified by a single character abbreviation. Cross sections can be calculated by various methods. By default empirical approaches are used which means they combine theoretical formulae with fits to experimental data. Cross sections are handled by a command

Interactions can also be specified as filtering options for listmode data output to reduce post-processing of unwanted information. For example, if only energy deposition distributions are desired, elastic events may be excluded from output, thus reducing space and CPU requirements,
For filtering purposes some "pseudo-interactions" are important:

In particular for absorbed dose calculations the inclusion of "cutoff" events is mandatory, otherwise the scoring will have an "energy leak".
Filtering is specified with the run command.

Random numbers

Two types of linear congruential random number generators are implemented. A single data object holds information pertinent to these generators: The PRN generator maxes out at one billion calls, which means it should only be used for short simulations involving a few thousand primary events.
Consequently the L'Ecuyer generator is the default from TRAX version 1808 onwards.

In order to support "trivially parallel" calculations which need different random sequences it is possible to "fast forward" the random generation for a specified number of events and to retrieve the resulting seed value(s).

For special purposes (e.g. program testing), a second independent generator instance is supported.

Random number generators are handled by a command.

Volumes

Particles are tracked through volumes. Volumes are identified by a unique name (and internally by a running number, starting with 1). They have a shape (cylinder, sphere, box) and consist of a material.
For each volume scoring tables exist which keep information like the number of interactions and the accumulated energy deposition. This information can be retrieved with the score command.
For each volume electric and/or magnetic fields can be specified, which affect the charged particle trajectory and energy deposition in addition to the stochastic interaction events.
Volumes are not accessed directly but are combined into a simulation geometry.

Geometry

Before a simulation can be performed the geometry has to be defined. The geometry comprises one or more non-overlapping (!) volumes. Particles which do not hit any volume are discarded from the simulation.
The geometrical setup is provided by an external file, it is handled by a command, the file format is described in the appendix

Source distributions

The simplest source distribution is a point source emitting monoenergetic particles in a fixed direction. Such situations can be handled by a simple command. More complicated scenarios involving energy and/or spatial distributions of emitters must be specified using an external file.
The source geometry is handled by a command, the external file format is described in the appendix

Particles

The particle object collects all relevant information about a particle traversing the volumes of a given simulation. This information comprises e.g.: These quantities cannot be accessed directly, however, they are stored in binary listmode format for later or immediate evaluation.

There are two basic modes these data are collected and filed, so called track mode and volume mode, respectively.
In track mode, each single interaction is recorded (subject to filtering), thus forming a particle track. In volume mode the dissipative interactions are accumulated per volume and per initial projectile event, before they are written. This corresponds to "classical" detector energy deposition simulation.

Listmode data and trackfiles

Sometimes a simulation might require more sophisticated analysis than just total energy loss or ionization numbers within a volume. Typical examples are spatial distributions (radial doses) with varying grid dimensions or analysis of particle correlations. In such cases particle data can be written as events in listmode on a binary trackfile for later analysis with one of the evaluation procedures.
Note the difference between track mode and volume mode, as described in the particle paragraph. Most evaluation procedures expect data in track mode, it is stated specifically where volume mode is required.
Trackfiles are handled by a command. The file and event formats are described in the appendix

Run

Once all preparations ( materials, cross sections, geometry, source ) are completed, the MC simulation may be started. The MC kernel keeps all particles within a stack of varying size. Newly created particles, e.g. after ionization, are added on top, whereas particles with terminated histories are removed. Termination occurs when a particle's flight direction no longer intersects a volume or when its energy drops below a cutoff threshold. This threshold by default equals the lowest limit of the dissipative interaction cross sections. It may, however, be set even lower for each volume of the simulation geometry. This way the diffusion and escape of elastic scattered particles may be accounted for, as well as the energy take-up of charged particles in electric fields. Note, however, that lowering the cutoff level below the dissipative limit will result in a substantial increase of calculation time.
The kernel also records timing information (CPU, elapsed).
In order to reduce the amount of output data it is possible to provide filtering requirements. That is, for each particle type it may be specified which interaction is written to a trackfile. For details see the run command.
Last updated: M.Kraemer
$Id: traxdataobjects.html,v 1.9 2019/03/17 22:57:50 kraemer Exp $