Thorium and Uranium Prospection

Prospecting measurements using a portable gamma-ray spectrometer with NaI(Tl) or BrilLanCe detectors provide a fast and safe method for in-situ determination of thorium and/or uranium contents in rock or other materials. The quantitative spectrum analysis with SODIGAM yields reliable data on the Th and U contents.

The spectrum below was measured from a rock sample containing low concentrations of uranium and thorium in an activity ratio of approximately 4 : 1.

Figure 1: NaI(Tl) spectrum of a U/Th sample (744 seconds measuring time)

Many of the overlapping peaks in the spectrum are well suitable for quantitative determination of uranium and thorium- The spectrum was measured in a well-defined, calibrated beaker geometry and analysed with the SODIGAM program.

The deconvolution analysis of a multiplet is shown in the following figure.

Figure 2:   SODIGAM fit of the multiplet around 238.6 keV (channel 91.6)

The peak around 238.6 keV  (channel 91) is from ²¹²Pb, a daughter of ²³²Th and
the peak around 295.2 keV (channel 110) is from ²¹⁴Pb, a daughter of ²³⁸U. 

For optimised analysis, a fast energy calibration is made from peaks in the spectrum and several regions of the spectrum are automatically analysed using batch-files.

In the following SODIGAM printout table the peak assignment to nuclides and quantitative activity calculation is shown for peaks that were analysed in this prospecting task.
 

The peak at 186 keV is assigned to ²²⁶Ra, however, its area actually contains shares from ²²⁶Ra (57.56%) and ²³⁵U (42.44%). Percentage shares were calculated with the assumption of natural uranium that has not undergone enrichment processes.

The correspondingly corrected activity of the ²²⁶Ra-share is A_Ra-226 = 6763.3 ± 384.0 Bq/kg.
The activity of ²²²Rn and progeny is somewhat lower than the ²²⁶Ra activity because part of the noble gas ²²²Rn is released in the decay of ²²⁶Ra by alpha-recoil and due to its long halflife it can emanate from the sample.

²³⁸U and ²³²Th contents in a sample is quantitatively determined with SODIGAM even after short measuring times, with high precision and small uncertainties.