The efficiency of a 4" x 5" Nal(Tl) scintillation counter for detecting gamma-rays has been measured by a number of experimental techniques, and the results compared with the efficiencies predicted from total absorption cross sections. The experimental techniques involve coincidence measurements of the cascade gamma-rays from a Co-60 source (1.173 MeV and 1.333 MeV), and from the reaction B¹¹(pγγ)C¹² (4.43 MeV and 11.68 MeV). The Co-60 measurements also lead to knowledge of the absolute strength of the sources. For both cascades it was necessary to know the angular correlations between the radiations; for the reaction B¹¹ (pγγ)C¹² a separate investigation of this correlation was made using a 180 KeV accelerator. Computer programs were written to analyze the experimental data, and to calculate the theoretical efficiency estimates, taking into account the collimator and shield geometry. The results of this work define the efficiency of the scintillation counters to better than 5% for a number of gamma-ray energies and a specific geometry. On the other hand, the efficiencies based on the total number of counts in the observed spectra were 20 percent to 40 percent higher than the theoretical efficiencies. The departures from theory depend on the gamma-ray energy and the geometry of the shielding and collimators in such a way that it is not possible to provide a simple basis for relating theoretical efficiencies to the experimental data. / Science, Faculty of / Physics and Astronomy, Department of / Graduate