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

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Abstract

An in situ borehole gamma logging method using a LaBr3 gamma detector has been developed to characterizea137Cs 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.

Details

Original languageEnglish
Article number106807
Pages (from-to)1-23
Number of pages23
JournalJournal of environmental radioactivity
Volume243
DOIs
Publication statusPublished - 27 Dec 2021

Keywords

  • Radiocesium, Gamma spectrometry, Tikhonov regularization, MCNP, Error-propagation

ID: 7382373