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| Passive Dosimeter System |
| Overview |
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Monitoring radiation exposure in space is important both to crew health and to future scientific research on the International Space Station. The STS-102 mission will transport the Passive Dosimeter System (PDS) to the ISS, where it will serve as a flexible and easy-to-use radiation monitor that will be available for use by any experimenter. The PDS hardware consists of two kinds of radiation dosimeters and an electronic "reader." The dosimeters can be placed anywhere in the ISS to provide an accurate point measurement of the radiation at their locations. One of the radiation dosimeters is a thermoluminescent detector, or TLD. Each TLD, which resembles a fat fountain pen, contains calcium sulfate crystals inside an evacuated glass bulb. These crystals absorb energy from incident ionizing radiation (protons, neutrons, electrons, heavy charged particles, gamma and x-rays) as the radiation passes through them. This process results in a steady increase in the energy level of the electrons in the crystal. To read the accumulated radiation dose, an astronaut aboard the ISS removes the crystal-containing dosimeter from its measurement location and places it into the electronic reader. A component inside the reader heats the crystals. As the crystals are heated they emit a glow of light that is proportional to the amount of radiation they have been exposed to. This glow is measured by a photomultiplier tube in the reader. The reader stores the measured dose on a memory card that can be returned to Earth for further analysis. After the crystals have emitted all their stored energy, they are ready to begin accumulating a dose again and the TLD is ready to be reused. The other dosimeter is a set of Plastic Nuclear Track Detectors (PNTDs). The PNTDs are thin sheets of plastic, similar to the material used for some eyeglass lenses. As heavy charged ions pass through the PNTDs, the surface becomes pitted with tiny craters. When the PNTDs are subsequently returned to Earth, the plastic is etched to enlarge the craters. Then the craters are counted and their shapes and sizes are analyzed using a microscope. This information is used to improve the accuracy of the radiation dose the TLDs have recorded and to improve the estimate of the biological effects of the radiation. The complete set of 48 TLDs and the reader will be carried into orbit on STS-102. They will remain on the ISS indefinitely to support a variety of future scientific experiments. A set of 12 PNTDs will be carried to the ISS on STS-100 later this year. The combined dosimeters will be used to measure radiation as part of the DOSMAP experiment, which is being conducted by the NASA Human Research Facility on the ISS. The exposed PNTDs will be returned to Earth for analysis by the STS-105 mission. Understanding the radiation environment on the ISS will be useful in helping scientists explain experimental results that otherwise might be unaccounted for. The radiation measurements can help scientists determine whether a given effect is due to microgravity, radiation or something else. The Passive Dosimeter System is the first ISS hardware from NASA's Ames Research Center, Moffett Field, Calif. The Hungarian Space Office provided the thermoluminescent detectors, which are a third-generation version of dosimeters that flew previously on the Russian space stations Salyut 7 and Mir, and on the Space Shuttle. The Hungarian Space Office also provided the compact radiation reader, which is smaller than a typical shoebox. The Plastic Nuclear Track Detectors are provided and analyzed by ERIL Research Company in San Rafael, CA. Ames' role has led efforts to verify and certify the dosimeters for safety, and to package them in one of four transport containers, which resemble insulated lunch bags. One container holds a reader and 12 TLDs with associated power and data cables. Two additional kits each hold 18 TLDs. The final kit holds 12 PNTDs and two memory cards for the reader. |
Editorial/Technical Comments: ShuttlePresskit
