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Radiation detectors are often placed in positions difficult to shield from the effects of terrestrial background. This is particularly true in the case of radiation portal monitor (RPM) systems, as their wide viewing angle and outdoor installations make them susceptible to terrestrial background from the surrounding area. A low background is desired in most cases, especially when attempting to extract a weak signal. The problem of shielding a generalized RPM from terrestrial background is considered, to provide strong guidance to the designers of RPM systems. Various detector and shielding scenarios are modeled with the Monte-Carlo N Particle (MCNP) computer code. Amounts of nominal-density shielding needed to attenuate the terrestrial background to varying degrees are given, along with optimal shielding geometry to be used in areas where natural shielding is limited, and where radiation detection must occur in the presence of natural background. Common shielding solutions such as steel plating are evaluated based on the signal to noise ratio (SNR) and the benefits are weighed against the incremental cost. The greatest SNR increases for a generalized system are gained by shielding the internal detector cavity, direct shielding of the ground between the detectors, and the addition of collimators as long as are possible without changing the detector locations, or unnecessarily reducing the field of view.