AbstractID: 9264 Title: MOSFET Behavior Under Irradiation by Gamma Rays: Results from
a Monte Carlo Study
Tiny volume and high sensitivity of MOSFET dosimeters make them excellent tools to measure entrance skin dose during irradiations from
linear accelerators to radiological imaging systems. Recent version of this dosimeter shows an angular dependence of less than 3%. For low
energy irradiation, non-water equivalence has been reported. Simulations were performed to analyze the behavior of particles in its sensitive
volume. The GEANT4 simulation toolkit was chosen for its flexibility and ability to consider particles of energy as low as 1 keV. Validity
for medical application was carefully tested. Simulations with photons ranging from 10 keV to 25 MeV were done to characterize angular
dependence, dose deposition and out of field measurements. Thomson and Nielsen Company provided details regarding MOSFET geometry
and materials. For irradiation at megavoltage energies, the dose deposited in the dosimeter agreed well with the dose in the medium. In a
depth dose curve for 5 MeV photons, the average absolute difference in the first ten centimeters after the buildup is 0.7% (σ = 0.6%).
However the MOSFET overresponds at low energy because of the photoelectric effect (2 times more energy is deposited in silicon than in
water at 100 keV). In out of field measurements the number of electrons in the sensitive area decreases exponentially with distance from the
beam. From the data obtained, confidence in the measurement of dose deposition was gained for high-energy irradiation. Given the
important difference with water at low-energy, great care must be taken during calibration at kilovoltage energies.