Process management for avionics - Atmospheric radiation effets - Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics systems
The purpose of this PAS is to provide a more precise definition of the threal that thermal
neutrons pose to avionics as a second mechanism for inducing single event upset (SEU) in
microelectronics．There are two main points that will be addressed in this PAS：1 ) a detailed
evaluation of the existing literature on measurements of the thermal flux inside of airliners and
2)an enhanced compilation of the thermal neutron SEU cross section in currently available
SRAM devices(more than 20 different devices)．The net result of the reviews of these two
different sets of data will be two ratios that we consider to be very important for leading to the
ultimate objective of how large a threat is the SEU rate from thermal neutrons compared to
the SEU threat from the high energy neutrons(E>1 0 MeV)．The threat from the high energy
neutrons has been dealt with extensively in the literature and has been addressed by two
standards([2]1 in avioniCS and【1】in microelectronics on the ground)．
The two ratios that this PAS considers to be so important are：1 ) the ratio of the thermal
neutron flux inside an airline rrelative to the flux of high energy(>10 MeV)neutrons inside
the airliner and 2)the ratio of the SEU cross section due to thermal neutrons relative to that
due to high energy neutrons．These ratios are considered to be important because with them．
once we know what the SEU rates are from the high energy neutrons for an avionics box a
topic which has been dealt with extensively，such as【1】，then the additional SEU rate due to
thermal neutrons can be obtained with these ratios．Thus．given the SEU rate from high
energy neutrons，multiplying this by the two ratios gives the SEU rate from the thermal
neutrons．The total SEU rate will be the combination of the SEU rates from both the high
energy and thermal neutrons．