SOLVENT EXTRACTION LIQUID SCINTILLATION METHOD FOR ASSAY OF URANIUM AND THORIUM IN PHOSPHATE-CONTAINING MATERIAL

A method utilizing solvent extraction coupled with liquid scintillation spectrometry has been developed for the assay of uranium and thorium in fertilizers and phosphate-containing minerals and chemicals. Trioctylphosphine oxide in toluene is used to extract uranium and thorium from a perchloric and nitric acid solution, with phosphate interference being suppressed by the addition of aluminum ion. The uranium and thorium are stripped from this solution, and uranium is separated from the thorium by selective reextraction of uranium into a scintillator with Adogen 364 (tertiary amine) sulfate. The thorium remaining in the aqueous is reextracted into another scintillator with (primary) 1-nonyldecylamine sulfate. Both nuclides are counted separately in a high-resolution liquid scintillation spectrometer. The sensitivity of the counting method is enhanced by the use of pulse-shape rejection of the beta-gamma background. Results indicate a detection threshold of 0. 0038 pCi of uranium (1. 1 part/10**8) with a 1000-min counting time. Reproducibility of plus or minus 2. 5% was found at the 50-ppm level. For thorium detection, thresholds are 4 part/10**1**3 for the same counting time with plus or minus 3. 0% average recovery of **2**3**0Th and 7 part/10**8 of **2**3**2Th.