Sci‐Sat AM (1) General‐09: Dose dependence of MOSFET sensitivity for clinical photon energy spectra between 30kV and 60Co

Abstract

Since MOSFETs medical applications in radiotherapy and radiology are gaining popularity, evaluating them under radiation of different energies is of major interest. This study is a characterization of MOSFET sensitivity with regards to total integrated dose. Their sensitivity is expressed by the calibration factor (CFw) and allows the user to associate the reading displayed by the device (mV) to a dose value (cGy). The CFw of seven p‐type, duals bias MOSFETs were measured at several points in time. The radiation sources used were a unit (〈E〉γ: 1.25 MeV), an HDR unit (〈E〉γ: 0.38 MeV) and an orthovoltage unit providing two x‐rays energies respectively of 30 kV (〈E〉γ: 14.8 keV) and 150 kV (〈E〉γ: 70.1 keV). The CFw value diminishes with increasing threshold voltage, especially for low energy radiation. It was stable for irradiations. Decreases of 6%, 5% and 15% were observed respectively for radiation energies of , 150 kV and 30 kV. This behavior is explained by an alteration of the effective field applied on the MOSFET (bias), caused by the accumulation of holes at the interface. It is strongly dependent on the nature of the radiation (LET) and particularly affects low x‐ray energies. Those results are of major interest since, following the company recommendations, the device should be calibrated every 7 000 mV; this recommendation could lead to significantly underestimated doses. A calibration of the device before every use, and multiple measurements to get a mean dose value should compensate for the observed phenomenon.