Let 72 and 73 be the attenuation factors in zones 2 and 3. In zone 3 the flux should remain finite at infinity. It must have the form

u3(r) = a3 e-73r. (10.5)

At radius R2, one writes the continuity of neutron flux and current, and obtains a relation between the coefficients a2 and b2 which appear in

and

u2(r) = a2 e-‘2r(1 +C e2’l(r-Rl)). (10.10)

Note that, as expected, b2 cancels out for large values of R2 and, also, if the properties of medium 3 are the same as those of medium 2.

The value of a2 is obtained from the value of the neutron current at

r = Rp.

Defining

one obtains