An improved method for determination of heat production with gamma-ray scintillation spectrometry

Gamma-ray scintillation spectrometry is often the method of choice for determination of the heat-producing elements (HPE's) uranium, thorium and potassium. This article updates the procedure, describing modem equipment and proposing revised measurement calculations and error analysis. New equations for calculating HPE concentrations and applicable errors from gamma-ray spectra allow for standards that contain all three HPE's, rather than requiring that there be only one element in each of three standards. The equations also account automatically for all peak interferences and quantify the error effects of varying counting times. Errors obtained from repeated analyses and distributions of paired analyses compare favorably with those obtained from the equations. The possibility and potential magnitude of secular disequilibrium can be evaluated simultaneously with an analysis by utilizing the low-energy part of the gamma-ray spectrum (< 0.123 MeV). The factor that is calculated to test for secular disequilibrium also provides a first-order correction for its effects. Repeated analysis of a single sample crushed to varying degrees provides a test of the effects of net sample density on gamma-ray determinations. Uranium and thorium show no variation with sample density in the range studied, but potassium does show a variation, a result that has been observed before. This effect has been explained as a result of source attenuation due to potassium determination utilizing the lowest-energy gamma-rays (1.461 MeV), which should be most susceptible to source absorption. However, in the analyses presented here the energy range utilized for thorium is lower (0.916-0.969 MeV), but no source absorption effects are observed. The effect of the potassium variation observed on determination of heat production is negligible, however, and corrections are inadvisable.