Geometry File

The required geometry file is created by the user. Three namelist commands are available, &initial_coordinates, &block, and &cylinder. Since the regions can only be of block type or cylindrical type, the commands &block and &cylinder can't be mixed in the same geometry file. The fields for the first command are:

&initial_coordinates
   double x_cm = 0.0
   double y_cm = 0.0
   double z_cm = 0.0
   double xp = 0.0
   double yp = 0.0
&end

• Defaults are shown on the right hand side of ``=''. The word ``double'' in the above syntax definition isn't part of the actual command and should be omitted in a command. It is shown here only to specify the data ytpe.
• x_cm,y_cm,z_cm -- Amount by which to change the coordinates of all particles defined in all input files. Units are in centimeters.
• xp,yp -- Amount by which to change the slope with respect to the $z$-axis of all particles defined in all input files.

The &block command defines a block-like region of material:

&block
   STRING label = NULL
   STRING material = NULL
   double x_lower_cm = 0.0
   double x_upper_cm = 0.0
   double y_lower_cm = 0.0
   double y_upper_cm = 0.0
   double z_lower_cm = 0.0
   double z_upper_cm = 0.0
   double Bx = 0.0;
   double By = 0.0;
   double Bz = 0.0;
   double Ex = 0.0;
   double Ey = 0.0;
   double Ez = 0.0;
   long x_slices = 1;
   long y_slices = 1;
   long z_slices = 1;
   STRING outputfile = NULL;
   double photonEnergyCutoff = 0.0;        
   double leptonEnergyCutoff = 0.0;        
   long split = 1;        
   double minimumWeight = 0.0;
   double maximumDeltaE = 0.02;
&end

• label -- An optional label identifying the region. The label value will appear in the output file.
• material -- A string specifying the material in the region. This material can be an element, a mixture, or a compound, and must be defined in the cross-section file.

  The material string value is converted to uppercase and passed to EGS4 for identification of data arrays in the cross-section data file defined by environment variable EGS4_CROSSSECTION (see section 4.1). If this cross-section file exists, and contains the required information on the specified element, mixture, or compound, the relevant data is read into EGS4 subroutine arrays. Otherwise, an error message is printed out, and execution stops.

  In order to track through empty space, the empty space must be defined as a region of pre-set material ``vacuum''. Otherwise, a particle entering an undefined region of space will be discarded, and optionally recorded in a coordinate file (see command line arguments).

• {x,y,z}_lower_cm,{x,y,z}_upper_cm -- Boundaries of the block region in units of cm (the units used here is to be consistent with the units used by EGS4). If a later &block definition overlaps with the volume of one defined earlier, then the first block takes precedence. Smaller objects such as targets where the shower process usually starts should be defined first so that they can be nested inside larger regions.

• Bx,By,Bz -- Uniform magnetic field in entire region. Only one component of B is allowed in one region. Each tracking segment in a region with a magnetic field is converted into an arc of a circle approximated by shorter line segments joined by a small angle, with the possibility of interaction with matter at each vertex.

• Ex,Ey,Ez -- Electric field in region. Not implemented yet.

• {x,y,z}_slices -- Number of equal divisions of the region along a particular direction. Each sub-region becomes a new region with the same label, material, magnetic field. This usually doesn't affect the Monte Carlo simulation. However, for regions with dimensions smaller than the mean free path of a particle, the larger number of steps may improve the simulation of Coulomb scattering. Also, in the regions summary file each subdivision of a region is considered as a separate region with the same material and label values. This is a convenient way to determine absorbed dose as a function of position.

  The slice option does not work with the outputfile specification. Therefore slices should be left equal to 1 if an output file is specified.

• outputfile -- Creates an ouput file which records coordinates of particles entering the region. Once in the output region, the particles are discarded from further tracking. Therefore the region with the output file is often the ``last'' region of the geometry.

• photonEnergyCutoff, leptonEnergyCutoff -- Modifies the energy cut-off of photon and lepton transport for the particular region. The energy cut-off is defined as the minimum energy at which the particle can no longer move forward and is absorbed on the spot. The cut-off energy cannot be made smaller than the default value found in the cross-section data file. Using a larger value than the default values may reduce simulation time at the expense of reduced accuracy. The user must make his own determination.

• split -- When greater than one, allows particle splitting when particle enters the region, in order to improve statistics for that region. The number of particles is increased by the value of split. At the same time the weighting of the particles are reduced by the same factor, thus unaffecting the energy deposited in the region.

• minimumWeight -- Specifies the minimum weighting allowed when particle splitting is enabled. This prevents a runaway situation for some geometries where potentially a very large number of particles are created from a single particle.

• maximumDeltaE -- Specifies the maximum relative energy loss a particle may suffer when it is tracked through a single step. This parameter forces the reduction of the path length such that the relative energy loss is approximately equal to the value given.

The &cylinder command is similar to the &block command:

&cylinder
   STRING label = NULL
   STRING material = NULL
   double r_lower_cm = 0.0
   double r_upper_cm = 0.0
   double z_lower_cm = 0.0
   double z_upper_cm = 0.0
   double Bx = 0.0
   double By = 0.0;
   double Bz = 0.0;
   double Ex = 0.0;
   double Ey = 0.0;
   double Ez = 0.0;
   long r_slices = 1
   long z_slices = 1
   STRING outputfile = NULL
   double photonEnergyCutoff = 0.0;        
   double leptonEnergyCutoff = 0.0;
   long split = 1;        
   double minimumWeight = 0.0;
   double maximumDeltaE = 0.02;
&end

The only difference with the &block command is the absence of the dimension variables in $y$, and the replacement of the variables in $x$ with variables in $r$. The cylinder defined are concentric and are centered on the $z$-axis.

The &block and &cylinder commands can't be mixed in the same geometry file. If the first material definition command is of one type, then the rest must be of the same type.