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COTS MOS Dosimetry on the MeMOSat Board, Results After 2.5 Years in Orbit

We present the results after 2.5 years in or-bit of Total Ionizing Dose (TID) measurements done using Metal Oxide Semiconductor (MOS) dosimeters on the MeMOSat board. The MeMOSat board was launched on July 19th 2014 at the BugSat-1 "Tita" microsatellite developed by Satellogic to stay at LEO. We used as dosimeters p-channel Commercial Off The Shelf (COTS) MOS transistors with gate oxides of 250~nm. Before launch, a subset of transistors with similar drain current to voltage (I-V)curves where selected from a group of 100 devices. The temperature dependence of the (I-V) curves was studied to find the minimum temperature coefficient biasing point. Then, a calibration subgroup of sensors was irradiated using a $^{60}$Co gamma source to study their response to TID, showing responsivities of $\sim$75~mV/krad when the sensors are irradiated without gate bias. Also, the post irradiation response of the sensors was monitored, in order to include a correction for low dose rate irradiations, yielding 30~mV/krad. A biasing and reading circuit was developed in order to allow the reading of up to 4 sensors.The threshold voltage was monitored during different periods of the mission. After 2.5 years in orbit,the threshold voltage of the sensor mounted on the MeMOSat Board had a V$_\mathrm{T}$ shift of approximately 35~mV corresponds to a dose of 1.2~krads.