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 γ-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∘ 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.

Results from the Radiation Assessment Detector on the International Space Station, Part 2: The fast neutron detector

We report the results of the first six years of measurements of so-called fast neutrons on the International Space Station (ISS) with the Radiation Assessment Detector (ISS-RAD), spanning the period from February 2016 to February 2022. ISS-RAD combines two sensor heads, one nearly identical to the single sensor head in the Mars Science Laboratory RAD (MSL-RAD). The latter is described in a companion article to this one. The novel sensor is the FND, or fast neutron detector, designed to measure neutrons with energies in the range from 200 keV to about 8 MeV. ISS-RAD was deployed in February 2016 in the USLAB module, and then served as a survey instrument from March 2017 until May 2020. Data were acquired in Node3, the Japanese Pressurized Module, Columbus, and Node2. At the conclusion of the survey portion of RAD's planned 10-year campaign on ISS, the instrument was stationed in the USLAB; current plans call for it to remain there indefinitely. The radiation environment on the ISS consists of a complex mix of charged and neutral particles that varies on short time scales owing to the Station's orbit. Neutral particles, and neutrons in particular, are of concern from a radiation protection viewpoint, because they are both highly penetrating (since they do not lose energy via direct ionization) and, at some energies, have high biological effectiveness. Neutrons are copiously produced by GCRs and other incident energetic particles when they undergo nuclear interactions in shielding. As different ISS modules have varying amounts of shielding, they also have varying neutron environments. We report results for neutron fluences and dose equivalent rates in various positions in the ISS.

Results from the Radiation Assessment Detector on the International Space Station: Part 3, combined results from the CPD and FND

The energetic particle radiation environment on the International Space Station (ISS) includes both charged and neutral particles. Here, we make use of the unique capabilities of the Radiation Assessment Detector (ISS-RAD) to measure both of these components simultaneously. The Charged Particle Detector (CPD) is, despite its name, capable of measuring neutrons in the energy range from about 4 MeV to a few hundred MeV. Combined with data from the Fast Neutron Detector (FND) in the 0.2 to 8 MeV range, we present the first broad-spectrum measurements of the neutron environments in various locations within the ISS since an early Bonner-Ball experiment that was conducted before the Station was fully constructed. The data presented here span the time period from February 2016 to February 2022. In addition to presenting broad-spectrum neutron fluence measurements, we show correlations of the measured neutron dose equivalent with charged-particle dose rates. The ratio of charged-particle dose to neutron dose equivalent is found to be relatively stable within the ISS.

The importance of time-resolved personal Dosimetry in space: The ISS Crew Active Dosimeter

Monitoring space radiation is of vital importance for risk reduction strategies in human space exploration. Radiation protection programs on Earth and in space rely on personal and area radiation monitoring instruments. Crew worn radiation detectors are crucial for successful crew radiation protection programs since they measure what each crewmember experiences in different shielding configurations within the space habitable volume. The Space Radiation Analysis Group at NASA Johnson Space Center investigated several compact, low power, real-time instruments for personal dosimetry. Following these feasibility studies, the Crew Active Dosimeter (CAD) has been chosen as a replacement for the legacy crew passive radiation detectors. The CAD device, based on direct ion storage technology, was developed by Mirion Dosimetry Services to meet the specified NASA design requirements for the International Space Station (ISS) and Artemis programs. After a successful Technology demonstration on ISS, the CAD has been implemented for ISS Crew operations since 2020. The current paper provides an overview of the CAD development, ISS results and comparison with the ISS Radiation Assessment Detector (RAD) and the Radiation Environment Monitor 2 (REM2) instruments.

Detection of DNA damage by space radiation in human fibroblasts flown on the International Space Station

Although charged particles in space have been detected with radiation detectors on board spacecraft since the discovery of the Van Allen Belts, reports on the effects of direct exposure to space radiation in biological systems have been limited. Measurement of biological effects of space radiation is challenging due to the low dose and low dose rate nature of the radiation environment, and due to the difficulty in distinguishing the radiation effects from microgravity and other space environmental factors. In astronauts, only a few changes, such as increased chromosome aberrations in their lymphocytes and early onset of cataracts, are attributed primarily to their exposure to space radiation. In this study, cultured human fibroblasts were flown on the International Space Station (ISS). Cells were kept at 37°C in space for 14 days before being fixed for analysis of DNA damage with the γ-H2AX assay. The 3-dimensional γ-H2AX foci were captured with a laser confocal microscope. Quantitative analysis revealed several foci that were larger and displayed a track pattern only in the Day 14 flight samples. To confirm that the foci data from the flight study was actually induced from space radiation exposure, cultured human fibroblasts were exposed to low dose rate γ rays at 37°C. Cells exposed to chronic γ rays showed similar foci size distribution in comparison to the non-exposed controls. The cells were also exposed to low- and high-LET protons, and high-LET Fe ions on the ground. Our results suggest that in G1 human fibroblasts under the normal culture condition, only a small fraction of large size foci can be attributed to high-LET radiation in space.

A semiconductor radiation imaging pixel detector for space radiation dosimetry

Progress in the development of high-performance semiconductor radiation imaging pixel detectors based on technologies developed for use in high-energy physics applications has enabled the development of a completely new generation of compact low-power active dosimeters and area monitors for use in space radiation environments. Such detectors can provide real-time information concerning radiation exposure, along with detailed analysis of the individual particles incident on the active medium. Recent results from the deployment of detectors based on the Timepix from the CERN-based Medipix2 Collaboration on the International Space Station (ISS) are reviewed, along with a glimpse of developments to come. Preliminary results from Orion MPCV Exploration Flight Test 1 are also presented.