Use of a Metal-Nitride-Oxide-Semiconductor as the Detector for a Radiation Dosimeter

A method of radiation dosimetry using a Metal-Nitride-Oxide-Semiconductor (MNOS) transitor as the detector was developed and partially evaluated. An MNOS dosimeter is shown capable of measuring doses from 10 k rads to 4 M rads. The measured electrical characteristics of the MNOS transitor, which are altered by ionizing radiation, can be electrically reset to their pre-irradiation values. Repeatability of observations of threshold voltage versus radiation dosage indicates a precision of ± 1% of measured dose from 200 k rads to 4 M rads (Si). Dosage in rads is obtained by reference to a calibrated Co60 source exposure rate rather than to absorbed dose in the dosimeter. A schematic is given for the circuit tested. Determination of dosage with the system is indirect and requires the use of calibration curves. Each dosimeter must be individually calibrated. Exposure to 2 × 107 rads did not degrade performance. The MNOS transistors eventually failed: probably due to charge migration from the large integrated circuit chip on which they were fabricated. Discrete, non-stepped gage MNOS transistors are recommended.