Determination of 238U contents in ore samples using CR-39-based radon dosimeter—disequilibrium case

Abstract

A series of experiments were performed to determine the 238U contents in U-bearing ore samples. In this context, ore samples were placed in plastic containers with CR-39-based NRPB radon dosimeters installed in them. The containers were then hermetically sealed and the dosimeters were exposed to radon for three weeks to equilibrate 222Rn and 226Ra. After exposure, CR-39 detectors were etched in 25% NaOH at 80 ∘ C for 16 h and counted under an optical microscope. The track densities thus obtained were related to radon concentrations using calibration factor of 2.7 Tracks cm - 2 h - 1 / ( kBq m - 3 ) . From the measured radon concentration values, 226Ra activities were calculated that ranged from 157 to 454 Bq kg - 1 . Using these activity values, assuming secular equilibrium, 238U contents were calculated which ranged from 13 to 37 ppm in the ore samples under study. In order to verify the validity of the assumption of secular equilibrium, the above samples were analyzed with a high-resolution inductively coupled plasma atomic emission spectrometer (ICP-AES). The results obtained from this method showed higher 238U content that ranged from 356 to 2061 ppm which was a clear indication that there is disequilibrium between 226Ra and 238U. Due to the unavailability of the 226Ra standard for ICP-AES, the equilibrium factor was therefore determined using HPGe-based gamma spectrometry technique. Here, specific activity of 226Ra was determined using gamma line at 609.3 keV (46.1%) of 214Bi. Then specific activity of 238U was determined using the gamma line at 143.76 keV (10.5%) of 235U. Calibration factor, defined as the ratio of the specific activities of 226Ra and 238U, was determined. The NRPB dosimeter data were then corrected for the equilibrium factor which resulted in 238U contents that ranged from 229 to 1968 ppm.