Determination of the Photon Fluency using Monte Carlo Simulation
Enviado por Terman Frómeta Castillo
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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