This report describes the radiation environment in the atmosphere, and an empirical model used to calculate absorbed dose rate within this environment as a function of altitude and geomagnetic location. This study complements the student education/outreach portion of a recent NASA EPSCoR grant that measures absorbed dose rate from cosmic radiation at altitudes up to approximately 100k feet using inexpensive ionization chambers flown on standard weather balloons. A simple computer program called Atmospheric Ionizing Radiation (AIR) verifies the results from such flights. This empirical model calculates absorbed dose rate based on the following considerations:
- Galactic cosmic ray (GCR) spectrum
- Ionization rate
- Neutron contribution
- Geomagnetic cutoff
- Geomagnetic field
- Solar cycle phase
The AIR model was modified to make use of neutron count rates and vertical cutoff rigidities provided by the Climax Neutron Monitor in Colorado, the nearest operating neutron monitor to Stillwater, Oklahoma. In this report, we describe this model and show comparisons with actual balloon flight data.
The radiation environment within the Earth’s atmosphere is complex and changes with altitude, geomagnetic position, and time. The radiation environment is dominated by cosmic radiation at all but the lowest altitudes. This is not due to primary cosmic radiation, but cosmic ray secondaries produced through nuclear interactions. These secondary particles are the result of air showers or particle cascades initiated when primary cosmic rays interact with the constituent nuclei of the atmosphere. An understanding of the radiation environment at high altitudes can be obtained by considering the sources of cosmic radiation and the different physical processes that affect this radiation. For high altitudes one must first consider propagation of this radiation field through the Earth’s magnetosphere before taking into account its propagation in the atmosphere.
For the most part, life is shielded against this radiation by approximately 1033 g/cm2 of air, which is comparable to a water depth of roughly 10 m. On the ground, cosmic rays contribute less than 10% of the total absorbed dose rate of natural background radiation. While not a health issue on the ground, exposure to cosmic radiation may be a health concern at altitudes greater than 30k ft. At these altitudes, airline pilots and flight attendants are considered radiation workers since they can accumulate a biologically-weighted absorbed dose of approximately 9 mSv/yr, assuming 600 hr/yr exposure time. This is compared to approximately 0.3 mSv/yr for the average person at sea level. Prolonged exposure to high altitude cosmic radiation may increase the risk of fatal cancer, as well as the development of cataracts in the eyes. Exposure to ionizing radiation can also adversely affect aircraft electronics (avionics). In addition to practical concerns, atmospheric ionizing radiation is part of our environment and thus intrinsically interesting.