Rapid measurements of concentrations of natural uranium in process stream samples via gamma spectrometry at an extraction facility

A new application of gamma spectrometry in the efficient measurement of natural uranium in the process stream at an extraction plant is described here. The inherent nuclear properties of uranium viz. emanation of characteristic gamma rays (185.7 keV) has been exploited for the determination of concentrations ranging from 5 to 450 g l−1 by passive photon counting of 185.7 keV gamma rays from 235U isotope for a maximum of 3–10 min per sample. This technique is totally matrix independent unlike other instrumental analytical techniques like wavelength dispersive X-ray fluorescence spectrometry and UV–vis spectrophotometry. Solution samples of aqueous and organic phase can be directly counted without the requirement of sample preparation. A MINIM-based gamma spectrometer consisting of a multichannel pulse height analyzer and a 3 in. × 3 in. well-type NaI(Tl) scintillation detector with an approximately 2 in. thick lead shield has been employed for the measurements. The results are compared with those obtained by potentiometry and wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). Relative standard deviation of 1–5% has been obtained depending upon the concentration of uranium, which is more than adequate for routine process control samples. This paper also discusses in detail the problems associated with the determination of high concentrations of uranium in using 63 and 93 keV gamma rays emanating from 234Th (t1/2 24 days) the immediate daughter of 238U isotope in samples that have attained secular equilibrium and the limitations of these energies in the routine analysis of freshly extracted uranium.