A CYLISDRICAL or conical hole in a block of high density material is commonly used as a diaphragm for the definition of an x-ray beam or in conjunction with a gamma-ray detector to define the solid angle of radiation which may reach the detector.The diaphragm, however, cannot be considered ill every case t o limit the radiation passing through the hole t o t'hat defined by the geometrical factors, i.e. hole diameter, length and position of source relative to the diaphragm, and shape of the source, since radiation may penetrate through the diaphragm edges or be scattered by the diaphragm wall.I n a detection system such as a collimated scintillation counter, the diameter of the crystal is usually larger than the diameter of the collimating hole.

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A digital computer is used with a ray tracing technique to calculate the response of a multihole focusing collimator to a point source of radiation placed anywhere in the object space. The analysis is applicable to a focusing collimator with circular holes and includes both the geometrical contribution to the response and the effects arising from both edge and septum penetration.The results are presented as the solid angle subtended at the point source by the effective exit pupil of the collimator and may be calculated as a function of the linear absorption coefficient of the collimator material. The method of testing the computer programs used to solve the equations arising from ray tracing is described. The point source response of the 19-hole Picker Type 2107 collimator for 3 different quantum energies is calculated. The results are compared with those previously published. The responses for a single taper hole collimator and for a series of seven-hole collimators differing from each other only in septum thickness, each hole again having the same dimensions as the 2107, are calculated. It is concluded that the major contribution to the count rate with the 2107 collimator from penetrative effects is by edge penetration and not by septum penetration.

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A figure of merit of a collimating system was proposed by Dewey and Sinclair to describe the ability of a detecting system to distinguish a region containing a different concentration of a gamma emitting isotope from that of its surroundings.