Mass attenuation coefficients of H2O, PbO, and cellulose were determined using 59.5 keV gamma energies with CdTe and NaI (Tl) detectors at five different source-sample distances. The pencil beam method was used to measure the mass attenuation coefficients. All experimental mass attenuation coefficients were compared with simulated results. A linear relationship between source-sample distance and mass attenuation coefficients was determined by regression analysis. The correlation coefficients were 0.99, 0.96, and 0.98 for cellulose, H2O, and PbO using a NaI (Tl) detector, while the values were 0.98, 0.99, and 0.99 with the Cd(Te) detector. If the beams were emitted as a narrow beam instead of as a pencil beam, a constant relationship instead of a linear increase was observed for the mass attenuation coefficients as a function of distance. Consequently, the narrow beam method was superior to the pencil beam approach.