EVA Radiation Monitoring: A Study of Radiation Doses Experienced by Astronauts in EVA (EVARM) characterized the radiation doses experienced by crewmembers during extravehicular (spacewalk) activities. The data determined which parts of the human body are exposed to the highest radiation levels so that routine dosage monitoring in future missions can be done on the appropriate parts of the human body.Principal Investigator(s)
Canadian Space Agency (CSA), Ottawa, Ontario, Canada
Johnson Space Center, Flight Research Management Office, Houston, TX, United States
Canadian Space Agency (CSA)Sponsoring Organization
Information PendingResearch Benefits
Information PendingISS Expedition Duration:
December 2001 - May 2003Expeditions Assigned
4,5,6Previous ISS Missions
Experiments using MOSFET, a Russian dosimeter similar to EVARM, were conducted in the 1990s on the Russian biosatellites BION-10 and BION-11 as well as on Mir.
A Study of Radiation Doses Experienced by Astronauts in EVA (EVA radiation monitoring (EVARM)) was designed to quantify the radiological dose received by astronauts while performing EVAs at the ISS. Extravehicular mobility units (EMUs, or spacesuits), worn by spacewalking astronauts, provide less shielding from radiation than the spacecraft. This means that
spacewalkers are exposed to higher radiation levels during EVAs than at other times on orbit. When planning EVAs, teams take into account mission
parameters, estimated duration, ISS altitude and inclination plus information on space weather conditions (e.g., solar activity, geomagnetic field conditions, proton flux) anticipated for that day.
In addition to specific lifetime radiation limits, medical standards specify that radiation doses achieved by astronauts should be as low as reasonably achievable (ALARA). To create new and improved shielding for EVAs, researchers must know the type and flux of radiation inside the EMU. EVARM investigates the dose received by different parts of the body (skin, eyes, blood-forming organs) during an EVA by measuring dose rate, based on the time and position of EVAs as compared to the orbit, altitude, and attitude of the ISS.
As part of EVARM, spacewalkers wore dosimeters placed in small pockets along the EMU undergarments. Two dosimeters were placed either inside the thermal comfort undergarment or the liquid-cooled ventilation garment, one dosimeter was placed around the calf to measure absorbed dose to skin, and another dosimeter was worn above the eye inside the communications carrier assembly. EVARM used tiny metal oxide semiconductor field effect transistor (MOSFET) dosimeters, a 0.04-in2 silicon chip placed on a badge made of aluminum. When an MOSFET is exposed to ionizing radiation, a positive charge builds up on the silicon surface, creating a negative shift in threshold voltage. Measurements were taken by comparing the change in threshold voltage with the radiation dose, which was recorded using a photodiode. New dosimeters were worn by the crew during each EVA.
A complete understanding of the space radiation environment and the potential radiation doses astronauts receive on various parts of their bodies allows space agencies worldwide to plan mission activities such as EVAs, with crew long-term health in mind. EVARM and other space radiation research provide the data necessary to create models and issue recommendations for space radiation protection.Earth Applications
Shielding and countermeasures developed for the space program can also be used on Earth to protect people who work in high-radiation areas.
The EVARM experiment includes twelve dosimeter badges-three for each crewmember plus a spare set. The crew will wear the dosimeters during all EVAs. The Reader unit, which turns the measurements into usable data, is powered by the HRF-1 via a 28 Vdc cable. When not in use, the dosimeter badges are stored in the lid of the Reader which is stowed in the HRF-1. Each MOSFET dosimeter is powered by ten 30 Vdc batteries.
In addition to ground-based training on the placement of dosimeters and data transfer, the crew will also take a self-guided, computer-based refresher course while on Station. The dosimeters and Reader unit are calibrated on the ground and should not require additional calibration while on Station.
The Reader unit must be powered-up for 15 minutes before use. The badges are inserted into their appropriate slots in the Reader unit before an EVA to determine a baseline for each crew member. The Reader unit is powered-down for the duration of the EVA and then repowered at the conclusion of the EVA. The post-EVA data is transferred from the badges to the Reader unit. The Reader unit translates the dosimeter information into usable data and then transfers that data to the HRF-1 laptop. The HRF-1 stores the data until it can be downlinked to the Canadian Space Agency's Payload Mission Support Center in Saint-Hubert, Quebec. The crew also takes monthly background radiation readings which allows the investigator to compare the dose received during an EVA with the general radiation environment inside the Station. The information, when available, is downloaded once a week.
For the EVARM investigation, ten complete sets of data were collected between February and November of 2002. These badges were compared to radiation monitors already on the ISS as well as, the European Space Agency's Space Environment Information System (SPENVIS).
The results from EVARM have shown that EVA doses are elevated from those inside the ISS, but not significantly. In addition, this time period recorded doses during a time of increased geomagnetic activity (October/November 2002). It was determined that during this event doses to EVA participants were increased due to elevated levels of electrons in Earth orbit. These electrons are easily shielded by spacecraft materials and thus not measured inside the ISS. Fortunately, proper positioning of the spacecraft can dramatically reduce the radiation field encountered during EVA missions. A significant finding was that a single detector placed at the astronaut's torso was not sufficient to accurately determine organ doses. Results show that the MOSFET detectors are accurate as compared with other monitoring equipment; however, the use of this battery device may present problems in the EVA environment. (Evans et al. 2009)
Badhwar GD. The radiation environment in low-Earth-orbit. Radiation Research. 1997; 148: S3-S10.
Lewis BJ, Bennett LG, Green AR, McCall MJ, Ellaschuk B, Butler A, Pierre MC. Galactic and solar radiation exposure to aircrew during a solar cycle. Radiation Protection Dosimetry. 2002; 102(3): 207-227.
Measurements (NCRP) N. Guidance on Radiation Received in Space Activities: Recommendations of the National Council on Radiation Protection and Measurements. Bethesda, MD: Guidance on Radiation Received in Space Activities: Recommendations of the National Council on Radiation Protection and Measurements; 1989.
N. Radiation and the International Space Station: Recommendations to Reduce Risk. Washington, DC. Washington, DC: Radiation and the International Space Station: Recommendations to Reduce Risk; 2000.
Badhwar GD, Watts J, Cleghorn TF. Radiation dose from reentrant electrons. Radiation Measurements. 2001; 33(3): 369-372.
Kiefer J. Space radiation research in the new millenium--From where we come and where we go. Physica Medica: European Journal of Medical Physics. 2001; 17(Suppl 1): 1-4.
Thomson I. EVA dosimetry in manned spacecraft. Mutation Research - Fundamental and Molecular Mechanisms of Mutagenesis. 1999; 430(2): 203-209.