Thermal Neutron Flux Characterization at Aircraft Altitudes with the TinMan Detector

Recently, concerns have been raised about thermal neutrons causing single event effects in semiconductor devices. For example, if B-10 is present in a semiconductor device, thermal neutron capture reactions can produce several MeV alpha particles. The charge that these alpha particles deposit in a sensitive volume can cause a failure. The number of single event effects from thermal neutrons is the product of the thermal neutron intensity times the thermal neutron failure cross section. This paper presents the status of the thermal neutron flux measurements with the TinMan thermal neutron detector at aircraft altitudes. Thermal neutrons are produced in an airplane when high energy-neutrons, created by the interaction of cosmic rays in the atmosphere, are thermalized in the fuel, the passengers and the airplane itself. Therefore, they are very environment dependent. Airplanes are a particular concern because at aircraft altitudes, the cosmic-ray neutron flux is approximately 300 times the sea-level neutron flux. Therefore, the understanding of the thermal neutron intensity at aircraft altitude is critical for understanding the risks of thermal neutrons to avionics systems. We have designed and built TinMan to measure the thermal neutron environment in prototypical aircraft. The TinMan detector consists of two He-3 ionization chambers. One of the 3He ionization chamber is wrapped in a thin layer of cadmium. Because the cadmium shielding attenuates the thermal neutrons, the difference in count rates between these two detectors reflects the number of thermal neutrons detected. We have calibrated the detector prior to the flight, acquired data during several flights in a NASA ER-2 airplane and plan to fly the detector on other NASA experimental aircrafts, which are more prototypical of a large commercial airplane. This paper will present the status of these measurements.