Energy and integrated dose dependence of MOSFET dosimeter sensitivity for irradiation energies between 30 kV and {sup 60}Co
- Departement de Radio-Oncologie et Centre de Recherche en Cancerologie, CHUQ Pavillon L'Hotel-Dieu de Quebec, 11 Cote du Palais, Quebec, Quebec, G1R 2J6 (Canada) and Departement de Physique, de Genie Physique et d'Optique, Universite Laval, Quebec, Quebec, G1K 7P4 (Canada)
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{sub 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. The CF{sub w} of seven p-type dual-bias MOSFETs were measured for several accumulated doses. The radiation sources used were a {sup 60}Co unit (<E>{sub {gamma}}:1.25 MeV), an {sup 192}Ir high dose rate unit (<E>{sub {gamma}}:380 keV), and an orthovoltage unit providing two x-ray energy spectra for tube voltages of 30 kV(<E>{sub {gamma}}:14.8 KeV) and 150 kV(<E>{sub {gamma}}:70.1 keV). The CF{sub w} value diminishes with increasing threshold voltage, especially for low-energy radiation. It was stable for {sup 60}Co irradiations, while it decreased 6%, 5%, and 15% for beam energies of {sup 192}Ir, 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{sub 2} interface. 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.
- OSTI ID:
- 20853589
- Journal Information:
- Medical Physics, Vol. 33, Issue 10; Other Information: DOI: 10.1118/1.2349301; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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