The United States Department of Energy Grand Junction Office (DOE−GJO) recently developed and deployed a passive total-count γ-ray logging system to monitor subsurface plumes of nuclear waste at the DOE Hanford Site in the state of Washington. High count rate effects for the three thallium-activated sodium iodide (NaI(Tl)) detectors were investigated because zones of intense radiation are frequently encountered. Total count rates and “filtered” count rates were determined from γ-ray spectra collected by logging DOE−GJO natural-uranium-bearing calibration standards. Each filtered rate was calculated by dividing the counts in the part of the spectrum above 570 keV by the counting time. Filtered count rates were linear in relation to uranium concentration, even for high uranium concentrations, but total count rates for high uranium concentrations fell below the values predicted by extrapolating the count-rate-versus-uranium-concentration graphs that were nearly linear at lower concentrations. The physics of low energy photon absorption explains why the total count rate did not rise in proportion to the uranium concentration. Low energy photons contribute significantly to the total count and, within calibration standard media, the rate of creation of such photons is proportional to the uranium concentration. However, the low energy photon flux does not grow linearly with the uranium concentration because of the Z effect: An increase in the Z (average atomic number) of a medium is accompanied by a large increase in the fraction of low energy photons absorbed in the medium. Elevated Z values are obviously associated with elevated uranium concentrations because uranium has a high atomic number. Logging systems with high-purity germanium (HPGe) detectors that are also operated by DOE−GJO to assess nuclear waste at the Hanford Site are calibrated using DOE uranium-bearing standards. With spectra associated with uranium activities higher than about 2000 pCi/g, line intensities for γ-rays with energies below about 500 keV are not directly proportional to source concentration, and are not used for calibration.