for the finite extension of the source, it was necessary to perform tne followings numerical integration: x 7 a K= J, ° Krdr J Ts rar where r, is the radius of tne extended source, Tne determination of K is somewhat simplified by the fact that X is constant over part of the ranse of r, K = a”/l6(z = b/2)? ~- i.e.: (osr <2/2) ~ where a is the collimator diameter, b the collimator thickness and z the distance from the center of the collimator to the source. When r is larger than a/2 and smaller than az/b, K is some function of r determined grephically by intervolation and derived from Fig. 9 of Mather's paper. For values of r equal to or larger than az/b, K vanishes. To make the reduction factor X, enersy dependent, i.e.,to include penetration effects, an approximation? is used. Penetration is a function of gamma-ray energy and the increased averture due to increased energy is approximately the same as the geometrical aperture of an opaque collimator two mean-free-patns less tnick. For the particular geometry involved in the present experiment, the values of the machine parameters are as follows: _ 13