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.