Determination of the Photon Fluency using Monte Carlo Simulation
Abstract
The Photon Fluency at a point is defined as ratio number
of photon generated to ones of them arrive to this point. It was
used to design of local Cobalt-therapy with an additional
entrance for occupational personnel, reducing slightly the
distance which should be walked for the radiotherapist or other
personnel like physicist in medicine or engineer. Mathcad user
subroutine was implemented for verifying at the proposed design,
the most critical situation from point of view radiological
protection using Monte Carlo simulation.
Introduction
The treatments with Cobalt require that radiotherapists
should, who operate the equipment, go in and go out for each beam
of exposition, because they should change parameters of the
Gantry, collimator, Table and others. If there is an entrance,
the Man-hole (MH), near of the Panel of control allow
them to reduce the distance of walking during their work with the
equipment, only it is necessary to assurance a good radiological
protection, such as not to excel absorbed
dose limits for the occupational personnel.
Method
It was elaborated MathCad 13 user subroutine (See Annex
I) for calculating numbers of photons that initially is randomly
generated, suffer the first scattering Compton on the wall of the
local (In case of the Gantry is at 2700) and enter for the MH.
These photons will hit on the internal wall of the MH.
A geometrical and probabilistic analysis is carried out.
This subroutine contains four modules: geometrical data of the
MH, the First Compton scattering on the wall of the local, the
Second Compton scattering on the internal wall of the MH, and for
radiological protection analysis at the entrance of the
MH.
We generate a number of photons from de source, they
will hit with wall of the local, where it may occur three
processes: scattering Compton, Pair Production and Photoelectric.
We determine the amount of then can enter across the MH. It is
used the amount photons, which enter to the MH for simulating the
interactions on the internal wall of the MH, where the photons
may suffer the same interactions, like on the wall of the local,
but with different probabilities.
For the probabilistic analysis it was taken the water"s
parameters in all modules (first and second radiation scattered)
and energy of the photons 1.25 MeV (average energy of 60 Co) and
0.511 MeV (maximum energy of scattered radiation). We did not
consider the Rayleigh interaction for this range of energy
[Bie01]
We are interested for demonstrating that it will be
little the amount of photons at the hall, which will deliver an
equivalent dose lower to the limits for the occupational
personnel.
Figure No 1 Position and inclination
angle of the Man-hole. By this way it is not seen from the
entrance, near of the Panel of control, the zone of reflexion of
the beam on the wall.
MODULE OF GEOMETRICAL DATA OF THE MAN-HOLE
(MH)
All geometrical data are depicted in Figure
No. 1.
When the MH has 900 of inclination, there
is not escape of photons after scattering on the wall of the
local
MODULE OF THE FIRST COMPTON
SACTTERING
Here we defined for the Cobalt-60 energy
(h?=1.25 MeV) the effective atomic number of the concrete of the
wall of the local for, the probability of the photon-wall,
Photoelectric, and Compton interactions.
a) Random axes on the wall of the
local
b) Projection on XZ plane c) Projection
on YZ plane
Figure No. 2 The position of the random axes (a),
concerning to the wall where it is placed the Man-hole, and
parameters of the first scattering on the wall, and projections
on XZ and YZ planes, b) and c) respectively.
For the geometrical analysis it was placed the axes
coordinate randomly just on one point of area of maximum size of
the field, where values X and Y are defined (See Figure No. 2) ),
which will define the angles related to two axes (a11 and a21
with y axe, and ÃY11 and ÃY21 with
z). The randomly way are generate the angles a and
ÃY correspondent to y and z
respectively, as well as a probability between 0 and 1. Then, if
the probability is lesser than and equal of occurring Compton
effect, and the a and ÃY are between a11 and a21, and
ÃY11 and ÃY21 respectively the photon will enter
across the Man-hole.
MODULE OF THE SECOND COMPTON SACTTERING
a) Random axes on the internal wall of
the MH
b) Projection on the YZ
plane
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