Internal Radiation Monitoring On Board the International Space Station (ISS) is responsible for gathering, analyzing, and interpreting the internal environment radiation data for the ISS in order to help ensure crew health protection. Crew health before, during and following space flight is essential to overall ISS mission success. All of the partner agencies recognize the importance of crew health to mission success and are dedicated to maintaining the health of all crewmembers throughout all phases of ISS missions.Principal Investigator(s)
National Aeronautics and Space Administration (NASA)Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)Research Benefits
Information PendingISS Expedition Duration:
Information PendingPrevious ISS Missions
Terrestrial radiation guidelines are considered too restrictive for space activities. For space flight activities, NASA has adopted the recommendations of the National Council on Radiation Protection. Based on the Council’s recommendations, NASA has established career limits for ISS crewmembers’ exposure to space radiation. Specifically, the Agency limits individual risk to 3 percent Risk of Exposure-Induced Death (REID) from cancer. The acknowledged risk for the population at large of developing and dying of cancer is 20 out of every 100 people. NASA requires that the increased risks due to space radiation exposure for astronauts will not be more than 3 percent above the estimate for the general population, or no more than 23 out of every 100 people. Each year, a NASA Radiation Specialist generates the Astronaut’s Annual Radiation Exposure Report, which tracks career exposures of astronauts.
The ionizing radiation environment is monitored with passive and active (powered) instruments to document crew exposures, support risk assessments, and to provide data for dose management.
Each crewmember is provided with a personal radiation dosimeter for continuous use during a mission. The personal dosimeter serves as the “dosimeter of record,” fulfilling requirements to monitor radiation exposures. When combined with environmental monitoring and analytical calculations, the personal dosimeter results provide the individual crewmember’s exposure record that is used to track against defined exposure limits.
Area monitors are passive or active detectors placed throughout the ISS habitable volume and compartment to provide additional information about the time-dependent behavior, biological effectiveness (“radiation quality”), and variations of the ambient radiation field.
First Generation Passive and Active Radiation Monitoring Instruments:
Replacement Suite of Active Monitoring Instruments:
Human exploration and development of space without exceeding acceptable risk from exposure to ionizing radiation is one of NASA’s main objectives. Moral, legal, safety and practical considerations require that NASA limit post flight risks incurred by humans living and working in space to "acceptable" levels.Earth Applications
Maintaining crewmember health and safety while on orbit better ensures that crewmember functionality is not jeopardized as a result of time spent on a mission; therefore, allowing them to resume life on Earth unaffected by space travel.
To protect astronauts living and working aboard the ISS from the adverse effects of radiation, the ISS Medical Operations Requirement Document (MORD) establishes radiation health and exposure monitoring requirements. Specifically, the MORD requires the monitoring and measurement of (1) radiation doses absorbed by human tissue, (2) charged particles and neutron radiation inside the ISS, and (3) charged particles outside the ISS during extra-vehicular activities (EVAs). To characterize and quantify the radiation environment inside and outside the ISS, NASA installed three active monitoring instruments between October 2000 and April 2002.Operational Protocols
Passive Radiation Area Dosimetry - Passive dosimeters, capable of measuring time-integrated absorbed dose, are deployed at designated fixed locations within each pressurized module. Knowledge of the spatial distribution of exposure rate is necessary to identify areas that have a relatively high exposure rate (i.e. avoidance areas) and to reconstruct a crewmember’s exposure in the event of personal dosimeter anomalies. Continuous area monitoring is necessary because exposure rates and their distribution throughout the vehicles change with vehicle altitude, attitude, internal vehicle configuration, number and location of modules, position in solar cycle, etc. Passive dosimeters are not affected by power loss to other monitoring instruments.
Active Radiation Area Monitoring - Active radiation area monitors provide continuous information to ground controllers for tracking cumulative crew exposures during missions, identifying areas within the vehicle to avoid due to high dose rates, identifying low dose rate areas to use as “storm shelters,” and alerting to enhanced radiation environment conditions. Measurements from various advanced active area monitors are needed to reduce the uncertainty in final calculated crew risk assessments and to support operational practices through verification of numerical ISS shielding model needed to convert the absorbed dose, provided by personal dosimeters, to the regulatory-required equivalent dose. Radiation particles’ Linear Energy Transfer (LET) spectra inside a vehicle depends on many factors, including the geographic location, local vehicle mass distribution, space weather conditions, position in the solar cycle, etc. Spectrometers which provide time-resolved measurements are required to resolve the LET spectra into contributions from radiation source terms (i.e., GCR, trapped protons, solar particle event protons, etc). Because the LET spectrum is strongly dependent on local shielding, LET spectrometers need to be portable and capable of reaching most locations inside the vehicle.
Internal Time-resolved Charged-Particle Monitoring - Measurement of the time-resolved energy-and direction-dependent distribution of charged particles inside the vehicle provides the most accurate radiation source term for computing organ-level exposure and the resulting risk. All other physical quantities (such as LET spectra and absorbed dose) are not singular, and therefore result in ambiguity and hence increased uncertainty in estimates of crew health risk. The measured charged particle energy spectra also provides definitive benchmarks for validating analytical models used to compute the radiation environment inside the vehicle.
Neutron Monitoring - Radiation monitoring instruments should provide the capability to characterize the neutron contribution to crew exposures. Results from scientific research demonstrate that secondary neutrons may contribute 10-30% of the total radiation effective dose received by astronauts inside a space vehicle such as ISS. Since neutrons represent an important fraction of the crew’s effective dose, it is necessary that this contribution be monitored for accurate reporting (as required by agency regulations) and accurate risk assessment determination. Neutrons can be monitored directly through neutron spectroscopy. However, because of the technical difficulties inherent in performing such measurements in the mixed neutron-charged particle environment behind spacecraft shielding, measurements designed to accurately measure the contribution of neutrons to the dose and dose equivalent can be used as a surrogate for direct neutron spectroscopy.
Inspection of the auroral precipitation of energetic ions and electrons maps produced hourly by the US National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellite (POES) constellation show that ISS passes through the precipitating auroral electrons several times every day. The question of whether or not flight through the same kind of environment that produces charging and the occasional recoverable anomaly on Defense Meteorological Satellite Program (DMSP) constitutes a risk or hazard for ISS or ISS EVA crew remains open. During the first two years of flight (during the current solar maximum), no ISS equipment anomalies have been reported that correlate with geomagnetic storms or flight through either the diffuse or visible auroras. The ISS crews have reported flying through visible Aurora Australis on at least two occasions (Koontz et al. 2004).
The ISS is performing well within expectations with respect to total ionizing dose (TID) degradation and single event effects (SEE) impacts on electromechanical (EEE) parts and avionics performance. Until recently, ISS has been flying at altitudes between 350 and 400 km during solar maximum, well below the 500 km specified for the worst-case radiation design environment. TID accumulated to date is well below the performance degradation threshold for EEE parts. Ionizing radiation dose measurements, made within the habitable volume with thermoluminescent dosimeters and crew personal dosimeters, range from 5 to 12 μ Gy (0.5 to 1.2 milli rads) per hour, depending on location in the habitable volume, corresponding to an annual dose range of 44 to 105 milli Gy (4.4 to 10.5 rads). The variation in TID with location in the habitable volume is largely a result of variations in effective shielding mass with location.
No destructive SEE events of any kind have been observed during the first two years of flight. Only one ISS vehicle equipment item fault that may be uniquely attributed to SEE with a reasonable level of confidence has been observed.
Detailed consideration of the effects of both the natural and induced ionizing radiation environment during ISS design, development, and flight operations has produced a safe,
efficient manned space platform that is largely immune to deleterious effects of the LEO ionizing radiation environment. However, model estimates show the need for more work directed to development of a practical understanding of secondary particle production in massive structural shielding for single event effect (SEE) design and verification. Total dose estimates using computer aided design (CAD) based shielding mass distributions functions and the Shieldose Code provided a reasonable accurate estimate of accumulated dose in Grays internal to the ISS pressurized elements, albeit as a result of using worst-on-worst case assumptions (500 km altitude x 2) that compensate for ignoring both galactic cosmic rays (GCRs) and secondary particle production in massive structural shielding (Koontz et al. 2005).
Space radiation measurements were made on the International Space Station (ISS) with the Bulgarian made Liulin-E094 instrument, which contains 4 Mobile Dosimetry Units (MDU), and the NASA Tissue Equivalent Proportional Counter (TEPC) during the time period May 11–July 26, 2001. Four MDUs were placed at fixed locations: MDU #1 in the ISS ‘‘Unity’’ Node-1 and MDU #2–#4 were located in the US Laboratory module. The MDU #2 and the TEPC were located in the US Laboratory module Human Research Facility (rack #1, port side). Liulin-E094 doses in ISS in May 2001 at altitude about 400 km correspond closely with the data published in the NASA ‘‘Spaceflight radiation health program’’ and with model predictions. Comparison of the Liulin-E094 and TEPC doses shows differences, which are in the range of few to 50% in dependence of the differences of their shielding. Liulin-E094 trapped proton directional dependency in the South Atlantic Anomaly (SAA) is shown by strong differences in the doses and fluxes on ascending and descending parts of orbits for the location of MDU #2 in ISS. The enhanced doses at ascending parts of the orbits are explained by different shielding generated by the different geometry against the predominating eastward drifting protons in the SAA region (Dachev et al. 2006).
At the present, the best active dosimeters used for radiation linear energy transfer (LET) are the tissue equivalent proportional counter (TEPC) and silicon detectors; the best passive dosimeters are the thermoluminescence dosimeters (TLDs) or optically stimulated luminescence dosimeters (OSLDs) for low LET, and CR-39 plastic nuclear track detectors (PNTDs) for high LET. TEPC, CR-39 PNTDs, TLDs and OSLDs dosimeters were all used to investigate the radiation environment for ISS Expedition 12, STS-112 and STS-114, and proven to be successful and consistent in measuring the LET spectra and all radiation quantities with excellent agreement among different detector types. The sensitivity fading of Cr-39 detectors for long time exposures was observed and the method of “internal LET calibration using GCR iron peak” was developed to correct for the sensitivity fading giving final CR-39 results that are consistent with those measured by TEPC and TLDs/OSLDs. This study indicates the LET spectrum method using CR-39 PNTDs, the LET calibration for CR-39 detectors and the combination method for results measured by passive dosimeters are reliable and will be continually used in future space missions (Zhou et al. 2007).
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NASA Image: ISS009-E-06471 (05 Dec. 2004) --- EXPRESS Rack 5 with the Intravehicular Charged Particle Directional Spectrometer (IV-CPDS) (gold box in left field of view) and the Tissue Equivalent Proportional Counter (TEPC) Radiation Detector (gold cylinder) and upper storage compartments visible. EXPRESS Rack 5 is in the Destiny / U.S. Laboratory. Image taken during Expedition 9.
NASA Image: ISS015-E-12111 (15 Jun. 2007) --- View of the Tissue Equivalent Proportional Counter (TEPC) Radiation Detector (gold cylinder) and the TEPC Spectrometer (gold box) in the U.S. Laboratory/Destiny during Expedition 15. The TEPC monitors radiation doses at the cellular level.
NASA Image: ISS002-E-6593 (15 Aug. 2001) View of Mobile Dosimetery Unit (MDU) 1, the Human Research Facility (HRF) DOSMAP Nuclear Track Detector Package (NTDP) 1and TLD A0103 attached to a starboard rack in the Destiny laboratory module of the International Space Station (ISS).
NASA Image: ISS026-E-033449 (14 Mar. 2011) --- View of Radiation Area Monitor (RAM), Dosimeter, ULF5/NOD3FD5, SEZ33111519-313, Serial Number (S/N): 2446, in the Node 3. Photo was taken during Expedition 26.