NASA’s Ames Research Center is set to make local history when it sends its first space hardware to the International Space Station (ISS) next month on the space shuttle Discovery.
The Passive Dosimeter System (PDS) will serve as a flexible and easy-to-use radiation monitor that will be available for use by any researcher. It also will serve as a useful complement to existing dosimetry used for routine ISS operations. The hardware consists of two kinds of radiation detectors (dosimeters) and an electronic “reader.” The dosimeters can be placed anywhere in the ISS to provide an accurate measurement of the radiation at their locations.
“Monitoring radiation exposure is important both to crew health and to future scientific research on the ISS,” said Robert Jackson, PDS payload manager at Ames, in California’s Silicon Valley. “These dosimeters can stay on the station indefinitely, and they will be available to scientists in a variety of fields.”
Understanding the radiation environment on the ISS will 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 something else. The PDS will be part of NASA’s laboratory support equipment and will be available to all ISS partners’ life science investigators.
One type of radiation dosimeter 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 rays 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 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 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 other dosimeters are Plastic Nuclear Track Detectors (PNTDs) — 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. After the detectors are returned to Earth, the plastic is etched to enlarge the craters, which 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.
STS-102, currently targeted for a March 8 launch, will carry the complete set of 48 TLDs and the reader to the ISS, where they will remain 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 by the STS-105 mission for later analysis.
The Hungarian Space Office provided the thermoluminescent detectors, which are a third-generation version of dosimeters that flew 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 PNTDs are provided and analyzed by ERIL Research Company, San Rafael, CA.
Ames’ 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 2 memory cards for the reader.
“This is just the first step in a series of hardware Ames will be sending to ISS to support fundamental biology research,” Jackson said.
Images of the PDS are available at http://amesnews.arc.nasa.gov:8080/releases/2001/01images/thermolum/thermolum.html
