Internal standard method in α spectrometric determination of uranium and thorium radioisotopes using instrumental neutron activation analysis

A new approach to α spectrometric determination of the radioisotopes of uranium and thorium was developed for environmental samples based on 238U and 232Th as internal standards. 238U and 232Th were accurately determined in the sample by simultaneous instrumental neutron activation analysis. Other aliquots of the sample were totally dissolved, uranium and thorium separated by anion exchange, and thin sources electroplated for α spectrometry. From the known mass concentrations of 238U and 232Th, their activity concentrations were obtained, and thus from the relative ratios of uranium (234U, 235U, 238U) and thorium (228Th, 230Th, 232Th) radioisotopes in the α spectra, the absolute isotopic activity concentrations were derived. The advantages of the procedure are that neither the chemical recovery of the radiochemical separation nor the counting efficiency of the α spectrometer is required; the use of internal standards eliminates the need for addition of expensive, calibrated, external radioisotopic tracers such as 232U and 229Th. The new approach was tested on some certified environmental reference materials and compared with the classical method using external radioisotopic tracers. It may advantageously be combined with the standard approach to obtain an independent set of data for quality control. 25 refs