The ISS (International Space Station) Medical Operations Requirements Document (MORD) establishes the medical support requirements for ionizing radiation exposure, including common dose limits, radiation monitoring, recordkeeping, and management of radiation exposure through As Low As Reasonably Achievable (ALARA) practices through all mission phases. The dynamic, complex, and unique nature of the radiation environment in low Earth orbit is such that radiation health and protection requirements rely upon analytical modeling and continuous measurements of the onboard environment, as well as personal dosimetry that includes analytical assessments of passive dosimeters worn at all times by each crewmember. External radiation detection instruments are necessary to provide near-real-time information about the dynamic radiation environment experienced by crewmembers during EVA (extravehicular activity). Active radiation area monitoring on ISS is necessary to provide continuous information to ground controllers and to the crewmembers for the purpose of maintaining crew exposures below limits and in accordance with ALARA practices.

The IV-TEPC is an active ionizing radiation monitor that measures radiation in near real time using a simulated tissue site. The information generated by the IV-TEPC is used to make operational radiation protection decisions. Additionally, this information is used in risk assessments to estimate the physiological consequences to the crew from radiation exposure during spaceflight.

The IV-TEPC measures the timeresolved lineal energy (y) spectral distributions of the radiation environment internal and external to the ISS. Lineal energy is a measure of energy deposited per mean chord length in a specified volume. Lineal energy is an energy deposition parameter used to interpret the biological significance of absorbed dose from energetic ions, and is used to derive the regulatory quantity’s dose equivalent and effective dose. The lineal energy spectrum varies with position in orbit and with local solar weather conditions.

The IV-TEPC signals an ISS Caution and Warning (C&W) when radiation conditions exceed predefined thresholds. In the event of an alarm, in-flight procedures would direct the crew to relocate to a higher shielded area of the ISS during periods without ground communication. This alarm would also be downlinked to the ground, notifying the mission operations team of elevated environmental conditions.

The IV-TEPC is a single, portable unit comprised internally of radiation detectors, a control and data processor, readout and display electronics, electronic non-volatile memory, 1553 data bus hardware, and power supply.

The IV-TEPC contains tissue equivalent radiation detector(s) that are small, gas-filled spherical proportional counters coupled to signal processing electronics capable of spectroscopy of energy deposition events inside the proportional counter. The detector wall material and gas are equivalent to human tissue with regard to their response to radiation fields encountered in space. The IV-TEPC provides signal conditioning, data manipulation and storage, timing, and transfers real-time telemetry for science and engineering data. The system stores full radiation spectrum measurements and expanded engineering data for extended data download upon ground command.

This work was done by David Swartwout, Perry Piplani, Eddie Semones, Scott Wheeler, Coy Kouba, Kent Dekome, and John Cobarruvias of Johnson Space Center; Fadi Riman, Blaine Miller, William Williams, and David Fuson of Jacobs Technology; Andrew Lee of ERC; and Mario Garcia of UTC. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-25261-1