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Energy and integrated dose dependence of MOSFET dosimeter sensitivity for irradiation energies between 30 kV and Co 60
Author(s) -
Lavallée MarieClaude,
Gingras Luc,
Beaulieu Luc
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2349301
Subject(s) - dosimeter , dosimetry , mosfet , materials science , calibration , optoelectronics , voltage , irradiation , sensitivity (control systems) , threshold voltage , radiation , optics , transistor , nuclear medicine , physics , medicine , electronic engineering , nuclear physics , quantum mechanics , engineering
Since metal‐oxide‐semiconductor field effect transistors (MOSFETs) medical applications in radiotherapy and radiology are gaining popularity, evaluating them under radiation of different energies is of major interest. This study aims at a characterization of MOSFET sensitivity with regard to total integrated dose. Sensitivity is expressed by the water calibration factor ( CF w ) and allows the user to associate the voltage difference reading displayed by the device to a dose value in water at the MOSFET location. TheCF wof seven p ‐type dual‐bias MOSFETs were measured for several accumulated doses. The radiation sources used were aCo60unit ( ⟨ E ⟩ γ : 1.25 MeV ) , anIr192high dose rate unit ( ⟨ E ⟩ γ : 380 keV ) , and an orthovoltage unit providing two x‐ray energy spectra for tube voltages of 30 kV ( ⟨ E ⟩ γ : 14.8 KeV ) and 150 kV ( ⟨ E ⟩ γ : 70.1 keV ) . TheCF wvalue diminishes with increasing threshold voltage, especially for low‐energy radiation. It was stable forCo60irradiations, while it decreased 6%, 5%, and 15% for beam energies ofIr192 , 150 kV , and 30 kV , respectively. The decrease rate is higher for the first half of the device lifetime. This behavior is explained by an alteration of the effective electric field applied to the MOSFET during irradiation, caused by the accumulation of holes at the Si – SiO 2interface. It is strongly dependent on the nature of the radiation, and particularly affects low x‐ray energies. A frequent calibration of the device for this radiation type is essential in order to achieve adequate measurement accuracy, especially in low‐energy applications, such as superficial therapy, brachytherapy, and diagnostic and interventional radiology.