[en] The relative contributions of gamma rays originating in specific fuel pins to the total gamma-ray source strength were calculated as a function of initial gamma-ray energy for a pressurized-water-reactor fuel assembly. Gamma-ray energies used in the calculations ranged from 511 to 2186 keV, the energy range in which most long-lived fission and activation products emit gamma rays. Nondestructive gamma-ray measurement techniques use specific signatures of ₁₃₇Cs or ₁₃₄Cs/₁₃₇Cs and ₁₅₄Eu/₁₃₇Cs isotopic ratios to determine the exposures of irradiated fuel assemblies. Nearly 92% of the measured ₁₃₇Cs signature (661.6 keV) originates in the three outer rows of fuel pins. However, because only 78% of the ₁₅₄Eu signature (1275 keV) originates in these three outer rows, a ratio of ₁₅₄Eu/₁₃₇Cs does not sample exactly the same volume segment of the fuel assembly. Other examples are presented that evaluate the effect of using a collimator at different angular orientations on quantifying the contributions from specific fuel pins to the total source strength