An MOS Dosimeter for Use in Space

This paper describes a four-sensor, integrating dosimeter that is designed for use aboard earth-orbiting satellites. The radiation-detecting sensors utilized are radiation-soft, PMOS transistors. The dosimetric parameter employed is the shift in threshold voltage, ¿VT. This voltage shift is related to the dose absorbed, D, in the SiO2 film of the transistor. The relationship between ¿VT and D is determined with a calibrated Co-60 gamma-ray source. The design of the dosimeter minimizes the differences in charge recombination between the calibrating radiation, Co-60 gamma rays, and the space radiation environment, which consists mostly of energetic electrons and protons. Each of the four transistors that are incorporated into the Spacecraft MOS Dosimeter is beneath a different thickness of aluminum in order to obtain depth-dose information. The circuit that applies the required biases to the transistor gates and also provides the needed analog voltage data to the spacecraft telemetry system consists mostly of radiation-hard operational amplifiers and stable, precision resistors. This circuit has been tested to 150 krad with 7-MeV electrons, and there was no observable degradation in performance.