Correction of the influence of 235U on the determination of 226Ra activity concentration using the 186 keV gamma peak

The direct determination of 226 Ra through the 186 keV gamma-ray peak presents, in most cases, positive biases due to interference from 235 U. This study aimed to correct this systematic error by evaluating the influence of 235 U counts on the determination of 226 Ra. To achieve this, four high-purity germanium (HPGe) detectors of different types (XtRa, BEGe, and REGe) were employed to measure point and extended sources containing natural uranium and 226 Ra in synthetic cement matrices. The effects of sample thickness and 235 U activity were analyzed, and a correction factor (η) was developed based on the detector used. The η factor was validated using 64 measurements of 26 solid intercomparison samples (soils, construction materials, and sediments) with activity concentrations that ranged between 6.3 Bq kg -1 and 19,050 Bq kg -1 . Results showed that neither matrix thickness nor 235 U activity introduced significant biases. However, a systematic positive bias of +2.94 Bq was identified in the indirect determination of 235 U from the 63 keV gamma peak of 234 Th, leading to an overestimation of +2.21 Bq (3.65% relative error) in the 226 Ra activity. Applying the specific correction factor calculated for each detector (η ranging from 1.068 to 1.114) increased the rate of acceptable results in the validation (according to the E number ≤1 criterion) from 68.8% to 92.2%.