Soil radioactivity-depth profiles from regularized inversion of borehole gamma spectrometry data. (March 2022)

• Record Type: Journal Article
• Authors: Hasan, Md Moudud; Rogiers, Bart; Laloy, Eric; Rutten, Jos; Camps, Johan; Vidmar, Tim; Huysmans, Marijke
• Is Part Of: Journal of environmental radioactivity. Volume 243(2022)
• Publication Date: 2022-03
• Subjects: Radiocesium -- Gamma spectrometry -- Tikhonov regularization -- MCNP -- Error-propagation; Radioactivity -- Periodicals; Radiation, Background -- Periodicals; Radioecology -- Periodicals; Radioactive pollution -- Periodicals; Environmental Pollutants -- Periodicals; Radioactive Pollutants -- Periodicals; Radioactivity -- Periodicals; Radioécologie -- Périodiques; Pollution radioactive -- Périodiques; Fond de rayonnement -- Périodiques; 539.752
• DOI: 10.1016/j.jenvrad.2021.106807
• Languages: English
• ISSNs: 0265-931X

Abstract

Abstract: An in situ borehole gamma logging method using a LaBr3 gamma detector has been developed to characterize a 137 Cs contaminated site. The activity-depth distribution of 137 Cs was derived by inversion of the in situ measurement data using two different least squares methods, (i) Least squares optimization (LSO) and (ii) Tikhonov regularization. The regularization parameter (λ) of the Tikhonov regularization method was estimated using three different methods i.e. the L-curve, Generalized Cross Validation (GCV) and a prior information based method (PIBM). The considered inversion method variants were first validated for a 137 Cs contaminated pipe, and in most of the cases, the calculated activity of 137 Cs was found to be within the acceptable range. The calculated 137 Cs activity-depth profiles from in situ measurements were also in good agreement with the ones obtained from soil sample analysis, with an R 2 ranging from 0.76 to 0.82. The GCV method for estimating λ appeared to perform better than the two other methods in terms of R 2 and root mean squared error (RMSE). The L-curve method resulted in higher RMSE than the other Tikhonov regularization methods. Instability was observed in the activity concentration depth profile obtained from the LSO method. Therefore, we recommend the Tikhonov regularization with GCV for estimating λ for estimating the activity concentration-depth profile. The site studied showed 137 Cs activity concentrations above the exemption limit down to depths of 0.50–0.90 m. Highlights: Borehole gamma logging is a useful method to collect radioactivity-depth profiles. Data inversion can be used to calculate the depth distribution of activity concentration. A good agreement between in situ - and soil sample measurements is observed. GCV method performs better in estimating smoothness of radioactivity-depth profiles.