Pinhole imaging of gamma rays

An approximate calculation is made of the effective area of pinhole collimators allowing for penetration of the metal by gamma -rays of two energies (364 and 140 keV). The particular case of normally-incident rays is treated without approximation. At 364 keV the performance even of heavy alloy (90% tungsten) collimators is dominated by penetration. The effective radius may approach twice the nominal value. These calculations are in fair agreement with experimental data. The sensitivity of a thin NaI(Tl) crystal used with such a collimator is approximately proportional to see zeta , where zeta is the angle of incidence. Allowing for this and the penetration effects, the cos 3 zeta law describing the sensitivity becomes modified, for three commercially-available pinholes of 1/16, 3/32 and 1/8 in. radius, to cos 5.8 zeta , cos 4.6 zeta and cos 4 zeta respectively.