Neutron Bubble Chamber in the Stratosphere

June 23, 2015: For nearly two years, Spaceweather.com and the students of Earth to Sky Calculus have been using helium balloons to monitor X-rays and gamma-rays in the stratosphere. Their successful program has detected multiple radiation events in response to solar storms. On June 18th they added something new. For the first time, the group’s Space Weather Payload included a bubble chamber for measuring neutrons. The device flew 109,012 feet above California’s Sierra Nevada mountain range, then parachuted back to Earth, landing near Death Valley National Park. The results: Neutrons were detected.

Circled above are some of the bubbles that formed as neutrons passed through the chamber. By counting the bubbles, it is possible to estimate the total dose of neutron radiation absorbed during the 3 hour flight. The answer is 600 microRads (energy range 200 keV – 15 MeV). Interestingly, this is almost the same dose detected by onboard X-ray and gamma-ray sensors: 620 microRads (energy range 10 keV to 20 MeV). Lesson: if you only count X-rays and gamma-rays, then you are missing at least half of the radiation in the atmosphere. Neutrons matter, too.

Indeed, neutrons are a very important form of cosmic radiation, providing much of the biologically effective radiation dose at altitudes of interest to aviation and space tourism.  Low-energy neutrons also cause single-event upsets in aircraft avionics, especially devices that contain Boron 10. Adding a neutron sensor to the Space Weather Payload allows the students to monitor this type of radiation at altitudes ranging from ground level to 120,000 feet.


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