Adsorption of radon on silver exchanged zeolites at ambient temperatures.

Since more than 100 years, the adsorption of the radioactive noble gas radon (222Rn) is performed on activated charcoal at cryogenic temperatures. There is little-if any-progress in the field of radon adsorption at ambient conditions to facilitate the development of simple and compact radon adsorption systems. We report here on the truly remarkable property of the synthetic silver-exchanged zeolites Ag-ETS-10 and Ag-ZSM-5 to strongly adsorb radon gas at room temperature. 222Rn breakthrough experiments in nitrogen carrier gas have shown that these materials exhibit radon adsorption coefficients exceeding 3000 m3/kg at 293 K, more than two orders of magnitude larger than any noble gas adsorbent known to date. Water vapor and carrier gas type were found to strongly influence radon adsorption, practically qualifying these silver exchanged materials as a new class of radon adsorbents. Our results demonstrate that Ag-ETS-10 and Ag-ZSM-5 are materials that show high affinity towards radon gas at ambient temperatures making them candidate materials for environmental and industrial 222Rn mitigation applications. Adsorption systems based on silver loaded zeolites have the potential to replace activated charcoal as material of choice in many radon related research areas by avoiding the necessity of cryogenic cooling.

Estimation of soil radioactivity-depth profiles using Bayesian inversion of borehole gamma spectrometry data.

Inversion of in situ borehole gamma spectrometry data is a faster and relatively less laborious method for calculating the vertical distribution of radioactivity in soil than conventional soil sampling method. However, the efficiency calculation of a detector for such measurements is a challenging task due to spatial and temporal variation of the soil properties and other measurement parameters. In this study, the sensitivity of different soil characteristics and measurement parameters on simulated efficiencies for a 662 keV photon peak were investigated. In addition, a Bayesian data inversion with a Gaussian process model was used to calculate the activity concentration of 137Cs and its uncertainty considering the sources of uncertainty identified during the sensitivity analysis, including soil density, borehole radius, and the uncertainty in detector position in the borehole. Several soil samples were also collected from the borehole and surrounding area, and 137Cs activity concentration was measured to compare with the inversion results. The calculated 137Cs activity concentrations agree well with those obtained from soil samples. Therefore, it can be concluded that the vertical radioactivity distribution can be calculated using the probabilistic method using in situ gamma spectrometric measurements.

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.

Minimum detectable activity concentration of radio-cesium by a LaBr3(Ce) detector for in situ measurements on the ground-surface and in boreholes.

A 3.81 × 3.81 cm LaBr3(Ce) detector based portable measurement setup has been developed for in situ gamma spectrometric survey of a contaminated site. This system is suitable for above- and below ground surface gamma spectrometric measurements of 137Cs. However, the minimum detectable activity concentration (MDAC), an important parameter of a measurement system, should be estimated for planning purposes of the gamma spectrometric survey. In this study, the MDAC of 137Cs for the measurement setup was investigated. The efficiency of the measurement setups was calculated from Monte Carlo simulations using MCNP code. The numerical model of the different studied set-ups, used in MCNP, performed well for the known cases. The results show that the MDAC varies with the position of the detector with respect to ground surface. A 5-20 min acquisition time, depending on the detector position, can be sufficient to get a MDAC of about 10% of the exemption limit of 137Cs (100 Bq/kg).

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).