TRAX Data objects
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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.
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 already builtin.
A collection of target data files can be found in the
TARG
subdirectory of the installation.
Target files are handled by a command,
the file format is described in the appendix
A couple of interaction cross sections is supported for
the various particles, identified by a single character abbreviation.
- electrons
-
l
elastic scattering
-
x
excitation
-
i
ionization
- positrons
- photons
-
f
photoelectric effect
-
c
compton effect
- ions
-
x
excitation
-
i
ionization
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 some "pseudo-interactions" are important:
-
k
cutoff at lower energy limit
-
o
outside, particle doesn't hit any more volume surface
-
e
electric or magnetic field step
-
y
boundary crossing
In particular for absorbed dose calculations the inclusion of "cutoff"
events is mandatory, otherwise you will have an "energy leak".
Two types of linear congruential random number generators are implemented.
A single data object holds information pertinent to these generation:
- the last seed value(s)
- how many times the generator was called
- the type of generator used:
- a simple linear congruential one with a period of about 2**30
- the L'Ecuyer generator using two seed values with much longer period
In order to support parallel calculations it is possible to "fast forward"
the random generation for a specified number of events and to retrieve
the resulting seed value(s).
Random numbers are handled by a command.
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 separately but are combined
into a simulation 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
The most simple 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 file format is described in the appendix
The particle object collects all relevant information of a
particle traversing the
volumes of a given simulation.
This information comprises e.g.:
- particle type
- the primary event number
- a unique particle identifier number
- a unique interaction sequence number
- the last interaction type
- the source volume number of the primary particle
- current location, time and direction of flight
- current energy and last suffered energy loss
These quantities cannot be accessed directly,
however, they can be 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.
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.
Sometimes a simulation requires 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,
data, 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
Once all preparations
(
materials,
cross sections,
geometry,
source
)
have been 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 it's energy drops below a cutoff threshold.
This threshold is by default equal to 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 CPU 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.
| GSI Biophysics
| TRAX long write-up
|
Last updated: 22-Nov-2002, M.Kraemer@gsi.de