Application of pMOS Dosimeters in Radiotherapy

The results of a study on pMOS dosimeters manufactured by Tyndall National Institute, Cork, Ireland and their sensitivity on radiation doses used in radiotherapy are presented. Firstly, we deal with analysis of defect precursors created by ionizing radiation, responsible for increase in fixed and switching traps, which are further responsible for threshold voltage shift as a dosimetric parameter. Secondly, influence of some parameters, such as gate bias during irradiation, gate oxide thickness and photons energies, on threshold voltage shift is presented. Fading of irradiated pMOS dosimeters and possible application of commercial MOSFETs in ionizing radiation dosimetry are also presented

Gamma-ray irradiation and post-irradiation at room and elevated temperature response of pMOS dosimeters with thick gate oxides

Gamma-ray irradiation and post-irradiation response at room and elevated temperature have been studied for radiation sensitive pMOS transistors with gate oxide thickness of 100 and 400 nm, respectively. Their response was followed based on the changes in the threshold voltage shift which was estimated on the basis of transfer characteristics in saturation. The presence of radiation-induced fixed oxide traps and switching traps - which lead to a change in the threshold voltage - was estimated from the sub-threshold I-V curves, using the midgap technique. It was shown that fixed oxide traps have a dominant influence on the change in the threshold voltage shift during gamma-ray irradiation and annealing

Radiation-sensitive field effect transistor response to gamma-ray irradiation

The influence of gate bias during gamma-ray irradiation on the threshold voltage shift of radiation sensitive p-channel MOSFETs determined on the basis of transfer characteristics in saturation has been investigated. It has been shown that for the gate bias during the irradiation of 5 V and 10 V the sensitivity of these transistors can be presented as the threshold voltage shift and the absorbed irradiation dose ratio. On the bases of the subthreshold characteristics and transfer characteristics in saturation using the midgap technique we have determined the densities of radiation induced oxide traps and interface traps responsible for the threshold voltage shift. In addition, the charge pumping technique was used to determine the energy density of true interface traps. It has been shown that radiation-induced oxide traps have dominant role on threshold voltage shift, especially for gate biases during the irradiation of 5 V and 10 V

Application of pMOS dosimeters in low-field mode for radiation dose measurements which could be used in radiotherapy

The pMOS dosimeters sensitivity (Tyndall National Institute, Cork, Ireland) to gamma and X-ray irradiation and 2000 h fading at room temperature is presented. The radiation fields were created using a 60Co source for two dose ranges (1-5 Gy and 10-50 Gy) as well as for X-ray units of a 280 kV spectrum for a single dose range from 0.1 to 1 Gy. Irradiation was performed in low-field mode (no gate bias during irradiation, Virr = 0 V), sensitivity characterized by the threshold voltage shift as a function of the absorbed radiation dose and time after irradiation. Linear dependence between the threshold voltage shift and the absorbed radiation dose was only established for pMOS dosimeters which were irradiated by gamma rays in the dose range of 1 to 5 Gy. Obtained results show that the sensitivity of these components is much higher in case of X-ray radiation than in that of gamma ray radiation. Moreover, the fading of irradiated pMOS dosimeters with X-rays is higher than in the ones irradiated with gamma rays. [Projekat Ministarstva nauke Republike Srbije, br. 171007

Successive gamma-ray irradiation and corresponding post-irradiation annealing of pMOS dosimeters

The paper investigates a possibility of pMOS dosimeter re-use for the measurement of gamma-ray irradiation. The dosimeters were irradiated to the dose of 35 Gy, annealed at room and elevated temperatures, after which they were irradiated again to the same dose value. Changes in the threshold voltage shift during those processes were followed, and it was shown that their re-use depends on a gate polarization during irradiation. For the gate polarization of 5 V during irradiation the pMOS dosimeters can be re-used for measurements of the irradiation dose after annealing without prior calibration. The pMOS dosimeters with the gate polarization during irradiation of 2.5 V can also be re-used for irradiation dose measurements but they require calibration. It is shown that for their re-use it is necessary to anneal the pMOS dosimeter so that the fading is higher than 50%. [Projekat Ministarstva nauke Republike Srbije, br. 171007