»(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