Determination of 226Ra, 228Ra and 210Pb in NORM products from oil and gas exploration: Problems in activity underestimation due to the presence of metals and self-absorption of photons

Typical calibration of solid environmental samples for the determination of 226Ra, 228Ra and 210Pb entails the use of standard reference materials which have a very similar matrix. However, TENORM samples from the oil and gas exploration contain unusually high amounts of calcium, strontium and barium which can severely attenuate the photons of 210Pb and 226Ra with their characteristic 46.1 keV and 186.2 keV gamma-rays, respectively and to some extent 228Ra with the characteristic gamma-rays of 911.2 keV and 969.0 keV. We used neutron activation analysis to evaluate the content of TENORM for calcium, barium and strontium and then used a software program SELABS to determine the self-absorption. Our results confirm that even in Petrie® containers with small dimensions the 210Pb can be underestimated by almost by a factor of four while 226Ra can be underestimated by 5%. The 228Ra activities are virtually unaffected due to the higher energy gamma-rays. However, the implications for TENORM studies that employ large Marinelli® containers having sample sizes between 0.25 and 1.0 L may be severely compromised by the presence of high Z elements in elevated concentrations. The usual spectral interferences on 226Ra, 228Ra and 210Pb coming from other radionuclides in the 234U, 235U and 238U decay chains are virtually nonexistent due the very high activity levels of 226Ra, 228Ra and 210Pb in the tens of thousands of Bq/kg.