Soil radioactivity-depth profiles from regularized inversion of borehole gamma spectrometry data

• Published in: Journal of environmental radioactivity 2022-03, Vol.243, p.106807-106807, Article 106807
• Main Authors: Hasan, Md Moudud, Rogiers, Bart, Laloy, Eric, Rutten, Jos, Camps, Johan, Vidmar, Tim, Huysmans, Marijke
• Format: Article
• Language: English
• Subjects: Error-propagation; Gamma spectrometry; MCNP; Radiation Monitoring; Radioactivity; Radiocesium; Soil; Spectrometry, Gamma; Tikhonov regularization
• ISSN: 0265-931X; 1879-1700
• DOI: 10.1016/j.jenvrad.2021.106807

An in situ borehole gamma logging method using a LaBr3 gamma detector has been developed to characterize a137Cs contaminated site. The activity-depth distribution of 137Cs 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 a137Cs contaminated pipe, and in most of the cases, the calculated activity of 137Cs was found to be within the acceptable range. The calculated 137Cs activity-depth profiles from in situ measurements were also in good agreement with the ones obtained from soil sample analysis, with an R2 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 R2 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 137Cs activity concentrations above the exemption limit down to depths of 0.50–0.90 m. •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.