Neutron detector under development to monitor spacecraft radiation

It’s no secret that radiation is a great danger to astronauts. Most of the research to date concerns the effects of galactic cosmic rays, but what happens to those particles when they pass into a spacecraft?

A device currently being tested will reveal what kind of neutron energy spectrum astronauts are exposed to from neutrons inside a spacecraft, alerting the occupants 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) technology development team. “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 device that is lightweight and portable that could be transferred from the transport craft to a habitation facility or wherever it is needed. 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.

Primary radiation particles, ranging from infrared photons to galactic cosmic rays, have been measured for years, but neutrons have not been measured adequately particularly at high energy. Instruments used to measure radiation often miss the secondary neutrons, which astronauts are also exposed to. 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, principal staff member at The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., and principal investigator on the project.

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 a response function must be established 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. Approximately 750,000 neutrons were detected by two detector systems over a period of 22 hours, and the data are being analyzed.

“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.

“When traveling outside of the Earth’s atmosphere and its magnetic field, radiation doses to humans increase,” Maurer explained. “Recent measurements from the Mars Odyssey mission show that the total radiation dose is about three times that on the more protected International Space Station.”

The NSBRI, funded by NASA, is a consortium of institutions studying the health risks related to long-duration space flight. The Institute’s research and education projects take place at more than 70 institutions across the United States.