Simultaneous determination of 226Ra, 233U and 237Np by liquid scintillation spectrometry

A method has been developed for the simultaneous determination of 226Ra, 233U and 237Np by liquid-scintillation (LS) spectrometry. This method consists of the evaluation of the α-spectrum that is composed of the strongly overlapping peaks of 226Ra, 233U, 237Np, 222Rn and 218Po in the energy range of 4.60–6.00 MeV and the single 214Po peak at 7.69 MeV. The α-peaks are analysed by a special peak fit function that considers the deviation of the α-peak at the low energy side from the pure Gaussian shape. First 237Np is determined using its daughter 233Pa by analysing the β-spectrum in the range 150–570 keV. 226Ra follows from the α-spectrum that is measured 6 weeks after sample preparation, i.e., 226Ra is determined from the radioactive equilibrium with its short-lived daughters 222Rn, 218Po and 214Po. Finally the 233U activity results from the fitted spectrum in the range of 4.4–4.8 MeV by subtracting the activity of 226Ra and 237Np. Knowing the exact energy position of the LS-peaks an alternative evaluation consists in the accurate deconvolution of the first three peaks that are formed by 226Ra and 233U (maximum of both at channel 700), 237Np (maxima at channels 700 and 725) and 222Rn (maximum at channel 737). In these two ways 226Ra, 233U and 237Np can be determined in mutual activity ratios of 1:50 with a relative standard deviation of less than 4% for the major activity and 9% for the minor activity.