The spatial sensitivity pattern of a gamma‐ray collimator and NaI scintillation detector was explored with small radiation sources of three gamma‐ray energies (0.145 Mev, 1.12 Mev, 1.59 Mev). The collimator used was a cylindrical hole of ½ in. diam through 8 in. of lead. The results are compared with calculated patterns. The effects of penetration of the collimator edges are clearly shown.