Results from the Radiation Assessment Detector on the International Space Station: Part 1, the Charged Particle Detector

We report the results of the first six years of measurements of the energetic particle radiation environment on the International Space Station (ISS) with the Radiation Assessment Detector (ISS-RAD), spanning the period from February 2016 to February 2022. The first RAD was designed and built for MSL, the Mars Science Laboratory rover, also known as Curiosity; it has been operating on Mars since 2012 and is referred to here as MSL-RAD. ISS-RAD combines two sensor heads, one nearly identical to the single MSL-RAD sensor head, the other with greatly enhanced sensitivity to fast neutrons. These two sensor heads are referred to as the Charged Particle Detector (CPD) and Fast Neutron Detector (FND), respectively. Despite its name, the CPD is also capable of measuring high-energy neutrons and $\gamma$-rays, as is MSL-RAD. ISS-RAD was flown to the ISS in December 2015 and was deployed in February 2016, initially in the USLab module. RAD was used as a survey instrument from January 2017 through May 2020, when the instrument was positioned in the USLab and set to a zenith-pointing orientation. The energetic particle environment on the ISS is complex and varies on short time scales owing to the orbit, which has a 51.6${}^{\circ }$ inclination with respect to the equator and has had an altitude in the 400–440 km range in this time period. The ISS moves continuously through the geomagnetic field, the strength of which varies with latitude, longitude, and altitude. The orbit passes through the South Atlantic Anomaly (SAA) several times a day, where magnetically trapped protons and electrons produce large but transient increases in observed fluxes and absorbed dose rates. The environment inside the ISS is affected by the solar cycle, altitude, and the local shielding, which varies between different ISS modules. We report results for charged particle absorbed dose and dose equivalent rates in various positions in the ISS. In an accompanying paper, we report similar results for neutron dose equivalent rates obtained with the ISS-RAD Fast Neutron Detector.