The EGS4 user code BEAM[1] is used to simulate accelerators and obtain detailed information about each particle emerging from an accelerator. The position, energy, angle and a complete history of where a particle has been are stored in a ``phase-space'' data file. The phase-space data files are used repeatedly for analyzing beam or as input to the EGS4 user code DOSXYZ[5] to calculate the dose distributions in a water phantom.
By using the variable LATCH[1] which records each particle's complete history in the beam simulation we are able to obtain not only the energy spectra of all electrons and contaminant photons but also the spectra of electrons and photons scattered from collimators or applicators of the beam defining system.
The DOSXYZ code has been modified to read a phase-space file produced by the BEAM simulation as an incident beam and to make use of the variable LATCH to calculate dose components contributed by scattered electrons and contaminant photons in the beam.
The beam field size studied here is
for beams with energies below 22 MeV and
or 
for those with energies above 22 MeV.
The SSD is 100 cm and 110 cm for energies 5 - 40 MeV and 50 MeV, respectively.
The monoenergetic electron energies at the exit vacuum window are
adjusted to match R
, the depth at which the dose falls to 50%
of dose maximum on the measured
central-axis depth-dose curve in a water phantom.
The spectra presented in the following are for incident monoenergetic beams and any width in the incident beam energy spectrum at the exit vacuum window of an accelerator would add to the breadth of the final spectra [6].
In the beam simulation the energy cutoffs for particle transport are set to ECUT = 0.700 MeV, AE = 0.521 MeV (total energy) and PCUT = AP = 0.010 MeV to ensure adequate energy-loss straggling. In the dose calculations the energy cutoffs for particle transport are set to ECUT = AE = 0.700 MeV (total energy) and PCUT = AP = 0.010 MeV. In all cases, the PRESTA electron transport algorithm[7] is employed with ESTEPE = 0.01 or 0.04.