Response of 3″ × 3″ NaI(Tl) detectors to terrestrial gamma radiation

Abstract A 3″ × 3″ NaI(Tl) detector placed in the air or water above a geologic formation (rock or soil) is considered, and a model is established for the emission, transport, and detection of the natural gamma radiation from the formation. Scattered flux components are evaluated by means of a double- P 1 polynomial expansion technique. It is shown that the rate of detection is given by the product of the scalar number flux and the average angular counting cross section for the scintillation crystal. The continuous part of the energy deposition spectrum is derived from parametrized expressions by Berger and Seltzer, whereas photofractions are taken from an evaluation by Snyder. From measurements in water on large concrete slabs containing evenly dispersed radioactive minerals it is demonstrated that the spectrum calculation method is accurate to within 10%. It is also shown that the total count rate above a counting threshold of 0.37 MeV is proportional to the exposure rate (dose rate) at the point of detection.