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April 17, 2001

Ann Hutchison

Ames Research Center, Moffett Field, CA

(Phone: 650/604-3039 or 650/604-9000)

Release: 01-24AR

NASA Ames Sends Additional Science Hardware to International Space Station

The second set of space hardware from NASA’s Ames Research Center is set to make its trip to the International Space Station (ISS) on the space shuttle Endeavour this month. Launch currently is scheduled for April 19 from Florida’s Kennedy Space Center.

This hardware is the last part of a suite of radiation-measuring dosimeters known as the Passive Dosimeter System (PDS), which will serve as a flexible and easy-to-use radiation monitor inside the ISS. The PDS, which will be available for use by any researcher, also will serve as a useful complement to existing dosimeters used for routine ISS operations. Endeavour will carry the remaining PDS hardware, which includes 12 Plastic Nuclear Track Detector Assemblies (PNTDs).

"The PNTDs provide a unique capability to measure the dose of high energy particles, which can interact with living organisms," said Robert Jackson, PDS payload manager at Ames, in California’s Silicon Valley. "Monitoring radiation exposure is important both to crew health and to future scientific research on the ISS."

Each PNTD assembly consists of PNTD stacks and holders. The stacks are thin sheets of CR 39 polycarbonate plastic with clear Lexan covers for protection. The CR 39 plastic is similar to material used for some eyeglass lenses. The stacks are inserted into a Nomex cloth-and-aluminum holder that attaches to the other type of dosimeters, known as thermoluminescent detectors, or TLDs. The PNTDs will be co-located with the TLDs during dose accumulation. The holder keeps the PNTD stacks aligned with each other.

As heavy charged ions pass through the PNTDs, the surface becomes pitted with tiny craters. After the detectors are returned to Earth, the plastic is etched to enlarge the craters. After counting the craters, technicians will analyze their shapes and sizes with a microscope. This information is used to improve the accuracy of the radiation dose recorded by the other type of dosimeters and to improve the estimate of the radiation’s biological effects. The Plastic Nuclear Track Detectors are important in determining the energy spectrum of the radiation absorbed by the TLDs. ERIL Research Company, San Rafael, CA provided the PNTDs and will analyze them after they are returned to Earth on the STS-105 mission later this year.

Understanding the radiation environment on the ISS should help 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 another factor. The PDS will be part of NASA's laboratory support equipment and will be available to life science investigators from the space station’s international partners.

Space shuttle Discovery carried the first part of the PDS – the TLDs and electronic reader– to the International Space Station in early March. TLDs contain calcium sulfate crystals that absorb energy from incident ionizing radiation. This process steadily increases the energy level of the electrons in the crystal. After an astronaut aboard the ISS places the dosimeter into the electronic reader, a component inside the reader heats the crystals.

As they are heated, the crystals emit a glow of light proportional to the amount of radiation to which they were exposed. A photomultiplier tube in the reader measures this glow. The reader then stores the measured dose on a memory card that can be returned to Earth for further analysis. After the crystals have emitted all the stored energy, they are ready to begin accumulating another dose and the TLD is ready to be reused. The TLDs will remain onboard the ISS indefinitely to support a variety of future life sciences experiments.

The entire Passive Dosimeter System 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.

Ames has led NASA’s efforts to verify and certify the dosimeters for safety, and to package them in one of four transport containers, which resemble soft insulated lunch bags. Three kits, each holding TLDs, a reader and associated power and data cables, were carried to the ISS on STS-102. The upcoming mission will take the final kit, which includes 12 PNTDs and 2 memory cards for the TLD reader.

"The successful use of this suite of radiation monitoring dosimeters will mark the conclusion of its development and its transition to routine use of the system to provide radiation measurements for a variety of experiments in the ISS," Jackson said.



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