There are about 
electrons inside the beam field in
the phase-space file of a simulated beam.
Since a phase-space file contains particles outside the beam field
as well as contaminant photons, the actual number of particles in a phase-space
file varies depending on the characteristics of the beam.
The two computer codes, PHSP_EXY and PHSP_SPECXY[8], are used to analyze the phase-space files from BEAM to obtain the mean energy and energy spectra of simulated beams at the surface of the phantom. The codes allow the user to select particles according to particle's history by making use of the LATCH feature of BEAM[1].
The spectra and angular distributions of electrons as well as photons
presented in the following figures
are all normalized to 1 for the peak of electron spectra.
The bin size of spectra and
angular distributions is 100 keV and 1
for all beams.
The angle 
is relative to the
z-axis which is the central-axis of the beam.
The figure captions each specify how many electrons are in the bins normalized
to 1 for energy and angular distributions.
One can thereby determine 
, the number of electrons in each bin, and hence
the fractional statistical uncertainty which is given by 
(1 standard deviation).
The dose distributions are all normalized to 100% for the maximum of the total dose which is contributed by all electrons and contaminant photons in the beam. The statistical uncertainties on the depth-dose curves are typically 1% or better.
Table 1 summarizes
characteristics of simulated clinical electron beams
from five accelerators.
The average kinetic energies 
refer to the number averaged inside the irradiated field at the phantom surface.
is the most probable energy inside the field at the surface of the phantom.
Direct electrons are those which do not include electrons scattered from
the beam defining system of a treatment head.
The difference in surface doses between total and that from direct
electrons is indicative of the number and angle of scattered
electrons in the beam.
R
and 
are obtained from calculated central-axis depth-dose
curves. The % surface doses of total (contributed by all particles in
the simulated beam) or direct (contributed only by those electrons that do not hit
any jaws, collimators or applicators) is relative to maximum total dose.
E
for electrons and photons is the maximum energy of electrons (kinetic energy)
and photons in the phase-space file respectively.
The difference between the maximum electron and contaminant photon energy
reflects the total thickness of vacuum exit window, scattering foils,
beam monitor chamber, mirror and air in the beam.
The maximum contaminant photon energy is about the kinetic energy of
the incident electron at the exit vacuum window.
Table 2 presents
the simulation data of clinical electron beams from five medical accelerators.
E
is the incident electron kinetic energy at the exit vacuum window
of an accelerator.
It is adjusted to match the calculated R to that of the measurement.
e
/100 inc e and 
/100 inc e inside field are the
number of electrons and number of
photons inside the field per 100 incident electrons at the exit vacuum window.
These data can be used to estimate the number of incident electrons (at the exit
vacuum window) that the simulation has to run to produce the required number of
electrons at the phantom surface and inside the beam field.
% direct e and % direct e are defined as:
where direct electrons or photons are those that do not hit any jaws, collimators or applicators. The cpu per incident history is machine dependent as well as simulation parameter dependent[1]. The data presented in table 2 are for an SGI Indigo with an R4400 cpu.
It is interesting to note that there is almost no build-up in the component of the central-axis depth-dose curves contributed by electrons scattered from applicators. This is due to the combination of the broad angular distributions and the low-energy spectra of electrons from applicators.