The thermalization of neutrons was experimentally measured for the purpose of testing Kushneriuk’s thermalization calculation in a cylindrical shell of moderator surrounding a fuel rod in a thermal reactor. The basis of this study was the measurement of activations for a set of five detectors—55Mn, 197Au, 115In, 239Pu, and 176Lu—inside the central fuel rod and the surrounding moderator at 20-cm hexagonal lattice pitch in the Atomic Energy of Canada Ltd.’s (AECL’s) heavy-water-moderated, natural uranium critical reactor, ZEEP. The neutron flux distribution, calculated by Kushneriuk’s method (first-order and exact solutions), was used in calculating detector activations. When compared, the measured detector activations and those calculated showed good agreement in the moderator, with the exception of points near and inside the fuel rod. The measured detector activations were also compared with the predicted values obtained from the HAMMER code. In this instance, the agreement was generally better than that obtained between measured and calculated values based on Kushneriuk’s method.