An in situ method for the high resolution mapping of 137Cs and estimation of vertical depth penetration in a highly contaminated environment

2D inversion of in situ gamma-ray spectrometric measurements of 137Cs for site characterization

Environmental contamination by radioactive materials can be characterized by in situ gamma surface measurements. During such measurements, the field of view of a gamma detector can be tens of meters wide, resulting in a count rate that integrates the signal over a large measurement support volume/area. The contribution of a specific point to the signal depends on various parameters, such as the height of the detector above the ground surface, the gamma energy and the detector properties, etc. To improve the spatial resolution of the activity concentration, contributions of a radionuclide from nearby areas to the count rate of a single measurement should be disentangled. The experiments described in this paper, deployed 2D inversion of in situ gamma spectrometric measurements using a non-negative least squares-based Tikhonov regularization method. Data were acquired using a portable LaBr3 gamma detector. The detector response as a function of the distance of the radioactive source, required for the inversion process, was simulated using the Monte Carlo N-Particle (MCNP) transport code. The uncertainty on activity concentration was calculated using the Monte Carlo error propagation method. The 2D inversion methodology was first satisfactorily assessed for 133Ba and 137Cs source activity distributions using reference pads. Secondly, this method was applied on a 137Cs contaminated site, making use of above-ground in-situ gamma spectrometry measurements, conducted on a regular grid. The inversion process results were compared with the results from in-situ borehole measurements and laboratory analyses of soil samples. The calculated 137Cs activity concentration levels were compared against the activity concentration value for exemption or clearance of materials which can be applied by default to any amount and any type of solid material. Using the 2D inversion and the Monte Carlo error propagation method, a high spatial resolution classification of the site, in terms of exceeding the exemption limit, could be made. The 137Cs activity concentrations obtained using the inversion process agreed well with the results from the in-situ borehole measurements and those from the soil samples, showing that the 2D inversion is a convenient approach to deconvolute the contribution of radioactive sources from nearby areas within a detector's field of view, and increases the resolution of spatial contamination mapping.

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

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.

Optimization and validation of a LaBr3(Ce) detector model for use in Monte Carlo simulations

A reliable detector model is needed for Monte Carlo efficiency calibration. A LaBr₃(Ce) detector model was optimized and verified using different radioactive sources (²⁴¹Am,¹³³Ba,¹³⁷Cs,⁶⁰Co and¹⁵²Eu) and geometries (point, extended and surface). PENELOPE and MCNP were used for Monte Carlo simulations. A good agreement was observed between simulated and experimental full energy peak efficiencies (FEPE) as their mean relative difference was 2.84% ± 1.93% and 2.79% ± 1.99% for PENELOPE and MCNP simulation, respectively. The differences between simulated FEPEs of two Monte Carlo codes were negligible except for low energies (< 100 keV).

In situ determination of 137Cs inventory in soil using a field-portable scintillation gamma spectrometer-dosimeter

A new empirical method for in situ determination of the inventory of ¹³⁷Cs in soil (A_Cs, kBq m^-2) at grasslands and forests using a field-portable NaI(Tl) scintillation spectrometer-dosimeter was developed. The method is based on evaluation of the ambient dose equivalent build-up factor. The practical implementation of the new method with the spectrometer-dosimeter does not require a priori knowledge of the vertical distribution of ¹³⁷Cs in soil. Moreover, the method allows assessing a value of the mean migration depth of ¹³⁷Cs in soil (Z) in terms of g cm^-2. The 95% confidence interval for the mean value of the conversion coefficients from the ambient dose equivalent build-up factor to A_Cs and to Z is less than 10%. The new method has been developed and verified using published data that where obtained at territories in Russia and Belarus heavily contaminated with ¹³⁷Cs (A_Cs > 37 kBq m^-2) due to the Chernobyl accident. Therefore, the survey of less contaminated areas requires additional validation of this method.

Optimising sampling strategies for emergency response: Soil sampling

The novel approach for optimising soil sampling strategies in areas affected by radionuclides is suggested. Major factors influencing the efficiency of soil sampling strategies, including (number of samples, sampling area size, sampling depth and spatial resolution of the sample sites are examined to provide optimisation of the soil sampling plan. The experimental field studies to validate the suggested approach were performed in 25 sampling units ranging from 1.2 × 1.2 m to 60 × 60 m size. The sampling units were selected on arable farmlands, natural meadow and former agricultural land), as well as coniferous and deciduous forests with contamination density of ¹³⁷Cs ranging from 2.8 kBq·m^-2 to 24.5 MBq·m^-2. The studied areas were contaminated by both the global fallout and the Chernobyl radioactive particles of different types. To determine the values of standard deviation of the log of the soil contamination density of ¹³⁷Cs, 25 to 256 soil samples were collected with an increment of 0.07-10 m within each sampling unit. It was found that the values of standard deviation of the log of the soil contamination density of ¹³⁷Cs were not dependent on the mean contamination density, the type of radioactive deposition and the landscape features. The mean value of standard deviation calculated for all sites studied was estimated as 0.44 ± 0.15 and 0.30 ± 0.10 for the sampling area 0.001 m² (∅37 mm) and 0.005 m² (∅80 mm) at the relative measurement uncertainties lower than 10% (CI = 95%). Concentrations of ¹³⁷Cs in the soil samples were statistically independent when sampling points were situated at a distance larger than 1 m one from each other. A simple method was developed for assessing minimum sample sizes required for estimation of the median or the geometric mean of radionuclide soil contamination with a relative uncertainty set by the user. The approach was also suggested for estimation of the uncertainty of soil contamination for the case of composite samples. The approach was implemented in the Ukrainian national requirements for assessment of quality of the soil.

Rapid in situ assessment of radiocesium wood contamination using field gamma-ray spectroscopy to optimise felling

The Chernobyl nuclear power meltdown that took place in 1986 has left a radioactive contamination legacy that currently severely limits the economic potential of impacted regions including the Polessie State Radioecology Reserve in Southern Belarus. Extensive areas of forested land could potentially become economically viable for firewood and building materials if radioactive contamination, notably ¹³⁷Cs, could be characterised faster, whilst closely adhering to regulatory limits. Currently, laboursome tree coring and unreliable transfer factors derived from limited soil sampling data are routinely employed in felling decision making, which has financial repercussions owed to the large amounts of waste produced and unnecessary transportation costs. In this study, it is demonstrated that a combination of targeted mobile gamma-ray spectrometry and a newly developed, lead shielded, in situ gamma-ray spectrometry method can significantly speed up the process of characterisation of ¹³⁷Cs wood activity in the field. For the in situ method, Monte Carlo calibration routines were developed alongside spectral processing procedures to unfold spectra collected in the field allowing for separation of ground and tree spectral components. Isolated contributions from the tree could then be converted to activity. The method was validated at a test facility and then demonstrated at three separate sites with differing contamination levels. This technique showed that single trees could be measured within approximately 20% of the activity compared to conventional tree core data. However, some discrepancies were found which were attributed to under sampling using the tree corer and low count rates at the lowest activity site, prompting the need for further data collection to optimise the method. It was concluded that this real-time approach could be a valuable tool for management of contaminated forested areas, releasing valuable timber and ultimately reducing the risk associated with living and working in these areas.

Integration of Ground- Penetrating Radar and Gamma-Ray Detectors for Nonintrusive Characterisation of Buried Radioactive Objects

The characterisation of buried radioactive wastes is challenging because they are not readily accessible. Therefore, this study reports on the development of a method for integrating ground-penetrating radar (GPR) and gamma-ray detector measurements for nonintrusive characterisation of buried radioactive objects. The method makes use of the density relationship between soil permittivity models and the flux measured by gamma ray detectors to estimate the soil density, depth and radius of a disk-shaped buried radioactive object simultaneously. The method was validated using numerical simulations with experimentally-validated gamma-ray detector and GPR antenna models. The results showed that the method can simultaneously retrieve the soil density, depth and radius of disk-shaped radioactive objects buried in soil of varying conditions with a relative error of less than 10%. This result will enable the development of an integrated GPR and gamma ray detector tool for rapid characterisation of buried radioactive objects encountered during monitoring and decontamination of nuclear sites and facilities.

Nonintrusive Depth Estimation of Buried Radioactive Wastes Using Ground Penetrating Radar and a Gamma Ray Detector

This study reports on the combination of data from a ground penetrating radar (GPR) and a gamma ray detector for nonintrusive depth estimation of buried radioactive sources. The use of the GPR was to enable the estimation of the material density required for the calculation of the depth of the source from the radiation data. Four different models for bulk density estimation were analysed using three materials, namely: sand, gravel and soil. The results showed that the GPR was able to estimate the bulk density of the three materials with an average error of 4.5%. The density estimates were then used together with gamma ray measurements to successfully estimate the depth of a 658 kBq ceasium-137 radioactive source buried in each of the three materials investigated. However, a linear correction factor needs to be applied to the depth estimates due to the deviation of the estimated depth from the measured depth as the depth increases. This new application of GPR will further extend the possible fields of application of this ubiquitous geophysical tool.

Reconstructing the deposition environment and long-term fate of Chernobyl 137Cs at the floodplain scale through mobile gamma spectrometry

Cs-137 is considered to be the most significant anthropogenic contributor to human dose and presents a particularly difficult remediation challenge after a dispersal following nuclear incident. The Chernobyl Nuclear Power Plant meltdown in April 1986 represents the largest nuclear accident in history and released over 80 PBq of ¹³⁷Cs into the environment. As a result, much of the land in close proximity to Chernobyl, which includes the Polessie State Radioecology Reserve in Belarus, remains highly contaminated with ¹³⁷Cs to such an extent they remain uninhabitable. Whilst there is a broad scale understanding of the depositional patterns within and beyond the exclusion zone, detailed mapping of the distribution is often limited. New developments in mobile gamma spectrometry provide the opportunity to map the fallout of ¹³⁷Cs and begin to reconstruct the depositional environment and the long-term behaviour of ¹³⁷Cs in the environment. Here, full gamma spectrum analysis using algorithms based on the peak-valley ratio derived from Monte Carlo simulations are used to estimate the total ¹³⁷Cs deposition and its depth distribution in the soil. The results revealed a pattern of ¹³⁷Cs distribution consistent with the deposition occurring at a time of flooding, which is validated by review of satellite imagery acquired at similar times of the year. The results were also consistent with systematic burial of the fallout ¹³⁷Cs by annual flooding events. These results were validated by sediment cores collected along a transect across the flood plain. The true merit of the approach was confirmed by exposing new insights into the spatial distribution and long term fate of ¹³⁷Cs across the floodplain. Such systematic patterns of behaviour are likely to be fundamental to the understanding of the radioecological behaviour of ¹³⁷Cs whilst also providing a tracer for quantifying the ecological controls on sediment movement and deposition at a landscape scale.