Measurement of Thermal Neutrons in Aircraft

The TM thermal neutron instrument was designed and fabricated to measure thermal neutrons in aircraft at flight altitudes. We have presented the results of TM measurements on several flights on NASA aircraft. We have estimated the absolute neutron intensity based on a Monte Carlo calculation of the detector efficiency, which agrees well with the manufacturer's sensitivity. The results of our measurements of thermal neutrons in these flights show a strong altitude and latitude dependence of the thermal neutron flux. This dependence can be partially understood in terms of the altitude and latitude dependence of the cosmic-ray induced high-energy neutron flux. Since the thermalization process depends on the geometry of the environment around the detector, we expect that different aircraft will have different thermal neutron intensities even at the same altitude and latitude. In addition, the measured thermal neutron may depend on the proximity of the detector to the fuel, the passenger/luggage loading, and where the detector is located on the aircraft. Additional simulations will be required to assess the effect environmental effects, such as fuel, passenger/ luggage, detector location, etc. The results of these measurements are intended to provide thermal neutron rates for a limited number of aircraft types to be utilized as guidance for the avionics community. To actually predict the SEE rate due to thermal neutrons, it is necessary to know the SEE upset cross section for a particular device to thermal neutrons. This cross section can be measured at the low-energy neutron source at the LANSCE at the Lujan Center or other well-characterized thermal neutron sources. Knowing this cross section and the thermal neutron flux at a particular altitude and latitude, we can predict the upset rate in aircraft due to thermal neutrons.