»(Z) = meen free path for neutrons at altitude Z * Cr n(Z) gy = total neutron cross-section in air. Numerical evaluation of this kernel as a function of E and ¢’is underway, and the re- sults will appear tn a supplement to this report to be published at a later date. Estimates have been made of the range of validity of the simple, one-scattering ap~ proximation proposed here. For an altitude of about 100,000 feet, the calculation should rt ° Source (Wt), Detector ' \ f To Ground To Ground Figure 1.1 Coordinate system for scattering calculation. be reliable out tot’! = 3t. This means, for example, that correction can be made for the effect of 10~Mev scattered neutrons at times corresponding to 1-Mev direct neutrons. In an attempt to overcome the inherent limitations of the single scattering approxi~ mation, discussed above, constant liaison has been maintained with other agencies in- terested in this problem. The hope is to develop a “workable” Monte-Carlo type of machine program in evaluating K(E, t'). 13