Lunar EVA Dosimetry: Small Active Dosimetry System for Lunar Extravehicular Activity Missions: Spacesuit and Tool-Box Applications

The objective of this project is the development of a compact active dosimetry system that can provide real-time radiation monitoring beyond low-Earth orbit (LEO). The dosimetry system would be capable of radiation monitoring during both ambient and solar particle event (SPE) conditions. It is designed to be responsive to the doses, dose rates, and radiation qualities (including secondary neutrons) expected in interplanetary space. This "mini-TEPC (tissue equivalent proportional counter) for deep space" builds on the extensive (and successful) performance history of the much larger tissue-equivalent proportional counters that have been on the Space Shuttle and on the International Space Station (ISS) for two decades, but which are much too large and power intensive to be used beyond LEO. The key innovations of our mini-TEPC are its compact size, low power, and no saturation during large SPEs, while at the same time maintaining the TEPC advantages of tissue equivalence and lineal-energy response. These advances in miniaturization and power are made possible in part by our incorporation of an innovative variance-covariance data collection and processing approach. As astronauts travel beyond LEO and explore asteroids, the Moon, and Mars, there will be a critical need for compact active personal radiation monitoring. Of particular concern is the need for real-time dose-rate measurements during SPEs so that astronauts can seek prompt shelter. SPEs may occur unexpectedly and with highly variable dose rate and can potentially result in high radiation doses to astronauts. The dynamic radiation environment in space requires the development of suitable detection systems that can detect a broad range of dose rates as well as a complex mixture of radiation types.

This project is a joint effort between NASA Ames Research Center (ARC), Colorado State University (CSU), and Texas A&M University (TAMU). Importantly, two separate National Space Biomedical Research Institute (NSBRI) grants support this effort (Radiation Effects-RE01301--Principal Investigator Borak and RE01302--Principal Investigator Straume) and reports from both of these grants should be viewed together to appreciate the total effort. CSU designs and fabricates the TEPC sensors, TAMU designs and builds the pre-amplifier with charge integrator, and ARC designs and builds the miniaturized electronics package with bias voltage power supply and microprocessor and programs the microprocessor to calculate dose-mean lineal energy (yD), quality factor, dose, and dose-equivalent.

In this project, hardware specifications required by NASA for this technology have either been met or exceeded. We have shown that the tissue equivalent sensor has omni-directional response and can detect radiation with linear energy transfer (LET) between 0.2 and 300 keV/µm. We have also shown that the TEPC monitor can detect maximum credible SPEs without saturation, has good precision and time resolution, consumes only 0.75 W of power, and has a mass of only about 250 g. Hence, it is adaptable to either a real-time personal dosimeter or an area monitor. Additional efforts are, however, required to fine-tune software related to yD calculations, which effect precision of the dose-equivalent rate measurements, but not absorbed dose rate measurements of greatest importance during SPEs. Also, there is a need for additional testing to better define the lower limit of detection, which is important during ambient (non-SPE) conditions. It is recommended that follow-on efforts should be undertaken by NASA to advance this proof-of-principle prototype with the aim of answering the fundamental question: Should this mini-TEPC technology approach be part of NASA's suite of detector systems for deep space missions? Additional evaluations of this technology should be performed to obtain a clear answer to this question.

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