properties of eilipecids of revolution. In Figure 1.1, the source’ anddetector lie at the foot of a family of such ellipsoids such that y+ Ps =svt' oa Where: ¥ “4 z = neutron velocity <[~ In Formula 3 and the subsequent definitions the time of arrival, ¢’, is measured in units ta for convenience. a 1 g(cos 6) n(Z) Ag (%) Ap (*) K(Z, ¢') =— f dx I Where: -_ ce’? - x?) b=1 or 228 zt! x =a variable of integration a@ =detector area cos 6 = 2-H x? pe x2 Li's Z=Zp_ a+ mm 2 n(Z) = density of scatterers (oxygen and nitrogen nuclei) at altitude Z Z =H+s ¢’x-1)= Zy + EX H = shot altitude, 2 =H + g (cos 4) = differential scattering cross-section for neutrons. Ag (x) = exp [ - f Q rm tT Ap (¥) = exp | 0 dr, |, (attenuation factor) dry YZ) ; (attenuation factor) 12 (3)