Modeling gan dosimetric response to therapeutic photon beam radiation

Introduction The use of Gallium Nitride (GaN), a direct-gap semiconductor, has been recently pro-posed as a radioluminescent transducer for real-time in-vivo dosimetric applications [1]. GaN has the following advantages: a lower e–h pair creation energy than conventional scintillators, a high prompt radioluminescence yield resulting from the high probability of band edge radiative recombination. Nevertheless, it shares the common drawback of silicon diodes or MOSFET dosimeters with an over response relative to water for low energy scattered photons. The aim of our study is to compensate for this dependence through modelling, which may lead us to improve the accuracy of GaN-based dosimetric systems. Our modeling work presented here is focused on GaN dosimetric response for therapeutic photo beam irradiation from medical linacs. Experimental The dosimetric response of a GaN transducer was first modeled by combining large cavity theory and the Spencer– Attix small cavity theory respectively for the low and high energy components of the local spectrum according the approach proposed in [2]. The local spectra were calculated from fluence pencil kernels, and Monte Carlo simulations were performed to determine some key parameters included into the model. Results and discussion The developed model was used to compute TMR curves of a implantable GaN-based probe. A good agreement with measurements is achieved which validates the modeling work. Furthermore, we used the model to calculate GaN response factor at different depths in water for 6 MV photon beam. The obtained results (not shown here) confirm that GaN response factor is proportional to the square field aperture (with a proportionality coefficient which varies with depth). This linear relationship was experimentally observed in clinical conditions [3].