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12 November 2003

Neutron detector can measure radiation levels

It's the ultimate sensor for astronauts: A neutron detector. Radiation is a great danger to astronauts, and while most of the research to date concerns the effects of galactic cosmic rays, the question remains about what happens to those particles when they pass into a spacecraft.

An instrument under development can reveal what kinds of neutron energy spectrum exposure astronauts go through from neutrons inside a spacecraft. The detectors would alert astronauts when dangerous levels occur.

"When spacecraft travel through a variety of primary high-energy cosmic rays, large vehicles absorb those rays and convert them into neutrons," said Dr. Richard Maurer, a researcher on the National Space Biomedical Research Institute's (NSBRI's) technology development team and principal staff member at The Johns Hopkins University Applied Physics Laboratory, Laurel, Md. "The spacecraft's thick structure, in a sense, multiplies the primary particles so that there are more neutrons trapped inside a craft than the original number of cosmic rays that created them."

The project's goal is to develop a lightweight and portable device that could transfer from the transport craft to a habitation facility. Currently, there is no compact, portable, real-time neutron detector instrument available for use inside a spacecraft or on planetary surfaces.

"These types of measurements would be crucial for exploration missions outside Earth's orbit where there is no protection from Earth's magnetic field," Maurer said.

Researchers have measured primary radiation particles, ranging from infrared photons to galactic cosmic rays, for years, but they have not adequately measured neutrons at high energy. Instruments used to measure radiation often miss the secondary neutrons, which also hit astronauts. Maurer said the estimates of the radiation that astronauts receive from neutrons account for about one-third of the actual total dose.

"Since neutrons do not carry any electrical charge, they are both harder to detect and can penetrate more deeply into a space traveler's body producing an increased risk of cancer, DNA damage, and central nervous system damage," said Maurer.

To make the measurements, the device converts some of the neutron's energy into light or a charge by making it interact with a detector. This process does not capture every neutron or all the energy of the neutrons detected, so researchers have to establish a response function to calibrate the percentage of particles detected and their energies, which are determined by analyzing the amount of light output or charge in the detectors.

The device, consisting of several detector systems that measure both low- and high-energy neutrons, has been through ground-based testing and calibration using radioactive sources and accelerator facilities.

Besides ground-based and aircraft flight tests, Maurer's real-time neutron spectrometer recently executed a successful scientific experiment on a balloon flight at an altitude of 85,000 feet. The small amount of atmosphere remaining at this altitude on Earth is similar to that on the surface of Mars, and the high-energy neutron spectra should be the same. Researchers found 750,000 neutrons through two detector systems over a period of twenty-two hours, and the data is now undergoing analysis.

"An interesting thing about neutrons is that they are not deterred by the typical heavy materials that shield astronauts from charged particles, but rather by things that contain hydrogen, like water," Maurer said. "Future shielding against neutrons could include water, or possibly a room inside an outpost's water supply."

Damage from neutrons can also be a problem for nuclear workers exposed to neutrons, people living at high elevations, and pilots flying at high altitudes on long-haul schedules or on flights over the poles, where the Earth's magnetic field is weak and cosmic rays can readily penetrate.

For more information on this subject go to www.isa.org/measurement.