ct with oxygen and nitrogen in the atmosphere and produce a 2.3 x 0.6 um 1.3 X 0.6 um 0.75 X 0.35 um Feedback Resistance (k2) Figure 6.4. Predicted SEU threshold LET VS. feedback resistance from 3D mixed-level simulations of a 0.6-um, 5-V technology and a 0.35-um, 3.3-V technology. Note the requirement for extremely large resistors as the fabrication technology scales. II-59 cosmic ray shower of daughter products [1,307-309]. Note that the primary galactic cosmic rays are SO energetic that some of the daughter products can reach all the way through the atmosphere to ground level, equivalent to passing through more than 13 feet of concrete [308]. A diagram of a cosmic ray shower is shown in Figure 6.5. The daughter products primarily responsible for causing upsets in high-altitude and terrestrial electronics are neutrons and protons [307]. The fluxes of neutrons and protons have similar characteristics with respect to energy and altitude variation, with both populations extending to energies greater than 1 GeV. Both neutrons and protons show a maximum flux at an altitude of 55,000-60,000 feet (17-18 km), with the sea-level flux being several hundred times lower than at aircraft altitudes [307]. A plot of the altitude variation of the neutron flux is shown in Figure 6.6a. The neutron flux also varies as a function of latitude, as shown in Figure 6.6b. The neutron flux is highest at the poles, because the primary galactic cosmic rays can penetrate furthest into the atmosphere there due to reduced geomagnetic rigidity [1]. Models have been produced that give the variation of atmospheric neutron flux as a function of altitude, latitude, and solar activity [310]. Note that the atmospheric neutron flux varies with solar activity due to its dependence on the incident galactic cosmic ray flux. Further details on the terrestrial and high-altitude radiation environment can be found in [307,309] and references therein. Cosmic rays 1600/m -S cascade to sea level Primaries disappear Low energy ~25 km deflected 100/cm -S at 120