Dose verification system based on MOS transistor for real-time measurement

This work presents a dosimetry system based on MOSFET sensors for real-time dose monitoring. MOS transistors were biased during irradiation, and the response of lateral, general-purpose 3N163 and CD4007 transistors were characterized with a 15-MV photon beam provided by a linear accelerator. The electronic circuitry to condition the sensor output and the measurement algorithm are described in depth. Due to the real-time measurement mode, the dosimetric parameter (the source voltage) showed drift. This drift depends on the bias voltage applied between the gate and the bulk terminals alternately during irradiation and readout. It can be minimized by applying a 1-V bias voltage for the 3N163 transistor and 0.85 V for the CD4007 during readout. The CD4007 transistor showed an average sensitivity of (7.8 ± 0.4) mV/Gy and the 3N163 an average of (26.4 ± 0.8) mV/Gy. The low uncertainty and acceptable sensitivity yields a resolution of 0.8 and 1.5 cGy for the CD4007 and 3N163 transistors respectively for unirradiated devices, which increases to 3 cGy and 2 cGy after 16 Gy of cumulative dose, respectively.