An approximate calculation is made of the effective area of pinhole collimators allowng for pmetrctlon of the metal by y.raj.s of two energies (364 and 140 km-), The particular cas? of normally-incident rays is treated without approximatlon. At 364 key the pprformance even of heavy alloy tunggten) collimator, is clommated h)-penetration. The effectlve radius may approach twice the nominal 1-alue. These calculatloni are in fair agreement with experimental data,The +msitivitI-of a thin SaI(TI) crystal user1 with Yuch a collimator is approximately px~portional to sec 5, where 5 is the angle of incidence. Allowing for this and the prnrtration effects, the ~0 ~~5 h x v describing the sensitivity becomes modified, for three c~~mmerciall~-a~-ailahIe pinholr~s of 1 16. 3 ' 3 2 and 1,'s in. ratlin-, t o co.j.*<, cos4.65 and COCA 5 rwpectively.
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