Theory of Emanation Thermal Analysis: XI. Radon diffusion as the probe of microstructure changes in solids

The theoretical background for the use of radon diffusion as a probe of microstructure changes in solids is given. The high sensitivity of the emanation thermal analysis (ETA) in the study of solid state processes especially interactions taking place on surfaces and in the near surface layers is described. The increasing sensitivity of the method towards bulk processes with rising temperature is theoretically shown. The background considerations to be used in the mathematical modeling of temperature dependences of the radon release from solids on heating (i.e. simulated ETA curves) are presented. Various models for radon diffusion and various functions describing the annealing of structure irregularities, which served as diffusion paths for radon, were used in the modeling. It was shown, that ETA is able to characterize microstructure changes in the surface layers of the thickness from several nanometers to several micrometers.This revised version was published online in November 2005 with corrections to the Cover Date.     

Experimental study of radon and thoron diffusion through barriers

The radon chamber and radon calibration set have been modified for investigation of the diffusion coefficients of the barriers for reduction of radon exposure in the dwellings and for application as filters to separate radon and thoron. The volume radon activities have been measured by continuous monitors with scintillation cell or ionisation chamber. The theory on which the experimental determination of a barrier diffusion coefficient is based, is presented. The diffusion ability of radon has been studied for different materials and the results of the measurements are presented and discussed.     

Untersuchung der Reaktivität und Sinterungsaktivität von Eisen(III)-oxid mittels der Diffusion radioaktiver Edelgase

Various methods for introduction of radioactive noble gases into solids are reviewed and also the diffusion of radioactive noble gases in α-iron(III)oxide, labelled by different methods. The HAHN emanation and ion bombardment methods have been used. Different noble gas diffusion processes have been shown to occur during heating the type of which is influenced by the kind of labelling conditions. The values of the activation energy of the noble gas diffusion are compared. Lattice defects including the amorphous state of Fe^IIIoxide are produced by ion bombardment. On the basis of the diffusion behaviour of radon, being studied by the emanation method, the “activity” of Fe^III oxide prepared by heating of different iron salts and sintered at various temperatures has been estimated. It is shown that the emanation method can be used for the study of the sintering of ferric oxide. On the basis of the solid state reaction ZnO + Fe₂O₃ the reactivity of ferric oxide in dependence on its thermal history has been investigated.     

Diffusion Structural Analysis Study of Titania Films Deposited by Sol-Gel Technique on Silica Glass

The diffusion structural analysis (DSA) was used to characterize microstructure changes of hydrous titania gel films under in situ conditions of heating. TG and DTA were used in order to elucidate the processes controlling the formation of anatase film during heating of hydrous titania gel film. The annealing of porosity and near surface structure defects of the dehydrated titania films was indicated by DSA in the temperature range 255–700°C as the decrease of radon release rate. It was demonstrated that the annealing was enhanced on heating in oxygen in comparison with heating in argon. The DSA experimental results were compared with model curves describing the radon diffusion mobility and the annealing of radon diffusion paths.     

Determination of the radon diffusion coefficient and radon exhalation rate in Moroccan quaternary samples using the SSNTD technique

Measurements have been made of radon (²²²Rn), release from diverse quaternary samples collected from different sediment deposits in the Errachidia and Beni-Mellal areas (Morocco). The radon diffusion coefficient as one of some important parameters of radon transport in the soil has been measured using solid state nuclear track detectors (SSNTD). Radon -activity, uranium content and radon exhalation rate have been determined in the studied samples. Uranium concentrations were found to vary from 0.14 to 9.52 ppm whereas the radon exhalation rate varied from 0.003 to 0.145 Bq^.m^-2.h^-1. A positive correlation has been found between radon exhalation rate and uranium content in the studied samples. The average radon diffusion coefficients were found to vary from (1.26±0.09)^.10^-6 m^2.s^-1 to (4.3±0.36)^.10^-6 m^2.s^-1. Furthermore, the correlation between ²²²Rn diffusion coefficient and porosity are also discussed.     

Calculation of temperature dependence of radon emanation due to alpha recoil

Many experimental and modeling studies have examined several factors affecting radon emanation. Of these factors, the effect of temperature shown in earlier experiments has not been discussed with model calculations. In the present study, radon emanation fractions were calculated for various temperatures, namely different air densities, using a simple model into which the emanation processes originating from alpha recoil were incorporated. As a result, the slightly negative correlation that the radon emanation fraction decreased with increasing temperature was observed within a certain range of grain size, while temperature had no effect within the other range. Considering the current knowledge of processes through which radon emanation occurs, this result would be expected to be qualitatively reasonable. However, the result is not similar to all previous experimental results showing the positive correlation. These papers occasionally explain that the positive correlation is attributable to the adsorption of radon on solids during its transfer among grains. Since the general definition of radon emanation includes no transfer process after its release from a grain, their explanation could not be well established. The discrepancy between the calculated and measured data may suggest the needs of review and improvement of experimental and/or modeling techniques.     

Use of radiometric emanation method in the characterization of anthropogenic glass analogue for vitrification of nuclear waste

Anthropogenic analogues can be used as one of the sources of information about long-term behaviour of engineered barriers, used for geological disposal of radioactive waste. The radiometric emanation method, based on the measurement of radon release from the solid samples, was used in the study of the alteration of anthropogenic glass analogues for radioactive vitrified waste. The samples were labeled at their surface by a solution containing ²²⁸Th and ²²⁴Ra,serving as source of ²²⁰Rn nuclides. The radon ²²⁰Rn nuclides were introduced in the uranium glass samples due to the recoil energy of the α-decay of ²²⁸Th and ²²⁴Ra nuclides. The man-made uranium glass samples from a locality in the Czech Republic were used. The measurement, based on the release of radon atoms from the samples, was applied to characterize the radon diffusion permeability and microstructure changes of weathered and non-weathered uranium glass samples during heat treatments. The radon diffusion characteristics and microstructure stability of the uranium glass analogue samples were evaluated by using a mathematical model.     

Industrial commercial respirator filter as indoor radon monitor

This paper presents a method for measuring indoor radon concentrations using a commercially available air-purifying respirator filter as a component of the radon monitor. The filter used was Survivair’s NIOSH (National Institute for Occupational Health and Safety)-approved 100800 model. The method is based on the diffusion of radon gas into the activated carbon of the filter and the measurement of the radioactive daughters resulting from the radon decay. The photopeaks of the ²¹⁴Bi daughter gamma rays (0.609 MeV) were analyzed with a Hyper-Pure Germanium (HPGe) detector and a multichannel system. A monotonically increasing and very close to linear response relation between the integrated area under the ²¹⁴Bi photopeak and the radon concentration of the activated carbon was found. A well-defined relation held for radon levels ranging from 15 to 4,700 Bq/m³. This procedure results in highly reproducible and reliable measurements of indoor radon levels. Interesting applications include the investigation of radiological accidents involving radon and the retrospective measuring of indoor radon concentrations by analyzing the filters of the respirators worn by personnel working during the relevant period.     

Radon determination by activated charcoal adsorption and liquid scintillation measurement

A passive diffusion method for the determination of radon concentration has been optimised and calibrated. The device consists of a scintillation vial containing activated charcoal, a diffusion barrier and a desiccant agent. The response to diverse atmospheric humidity and variable exposure intervals was studied. The result is a detector independent of atmospheric humidity till 7 days of exposure. The method was compared with electret detectors (US EPA) with very satisfactory results. The advantages of this method are its simplicity, low cost, low detection limit, the total automatization of the measurement and its total independence of humidity to measure in a wide range of radon concentrations.     

State of radon in crystalline polymers

The state of radon in crystalline polymers, polyethylene and polypropylene, has been studied by methods of macro- and microauto-radiography. Radon-222 was introduced into the samples by diffusion from the gas phase at various temperatures and time intervals. Effects on the state of radon of such factors as density and polymer crystallinity, extent of macromolecular chain branching, spherulite radius, content of different admixtures, -ray photon irradiation dose or irradiation with accelerated electrons, etc., were investigated. In pure polymers, radon was found to be in an atomically dispersed state. Radon forms thermally stable accumulations at the surface as well as in the bulk of the material due to admixtures, gas bubbles and crazes. The probable effect of radon's tendency to form accumulations on the kinetics of its diffusion in crystalline polymers is discussed.     

Spatial and vertical distribution of radon and thoron in a typical Indian dwelling

Radon and thoron have been identified as potential radiological health hazard and the dose estimation due to their exposure is an important task. Understanding their behavior in indoor environment helps in calculating the inhalation doses due to them. Present study aims at the distribution of radon and thoron concentrations in a typical Indian dwelling. Solid state nuclear track detectors are employed in the study. The concentration of radon is found to be invariant in indoor environment. The thoron concentration is found to decrease exponentially as a function of distance from the source (wall/floor). Solution of one dimensional diffusion equation is used for regression fittings for thoron variation, from which the diffusion constants and the exhalation rates were calculated. The diffusion constants varied from 0.00195 to 0.00540 m² s⁻¹.     

Measurement of radon emanation of drainage layer media by liquid scintillation counting

Slab-on-ground is a typical base floor construction type in Finland. The drainage layer between the slab and soil is a layer of sand, gravel or crushed stone. This layer has a minimum thickness of 200 mm and is sometimes even 600 mm thick, and thus may be a significant contributor to indoor air radon. In order to investigate radon emanation from the drainage layer material, a simple laboratory test was developed. Many organic solvents have high Ostwald coefficients for radon, i.e., the ratio of the volume of gas absorbed to the volume of the absorbing liquid, which enables direct absorption of radon into a liquid scintillation cocktail. Here, we first present equations relating to the processes of gas transfer in emanation measurement by direct absorption into liquid scintillation cocktails. In order to optimize the method for emanation measurement, four liquid scintillation cocktails were assessed for their ability to absorb radon from air. A simple apparatus consisting of a closed glass container holding an open liquid scintillation vial was designed and the diffusion/absorption rate and Ostwald coefficient were determined for a selected cocktail. Finally, a simple test was developed based on this work.     

Radon Dose and Aerosols

The equilibrium factor value (F) was measured in the NRPB radon chamber and the corresponding track density ratio (r = D/D ₀) of bare (D) and diffusion (D ₀) LR-115 nuclear track detectors was determined, as well as the regression equation F(r). Experiments with LR-115 nuclear track detectors and aerosol sources (burning candle and cigarette) were carried out in the Osijek University radon chamber and afterwards an empirical relationship between the equilibrium factor and aerosol concentration was derived. For the purpose of radon dose equivalent assessment, procedures for determining the unattached fraction of radon progeny were introduced using two nuclear track detectors.     

Hydration of tricalciumsilicate (Ca3SiO5) investigated by emanation thermal analysis

The method of emanation thermal analysis (ETA), based on the measurement of radon release from samples, has been used in the investigation of the tricalcium-silicate (Ca₃SiO₅) to characterize the microstructure development during the sample hydration. Results of the ETA made it possible to obtain the diffusion structural diagnostics of the material under in situ conditions of it is hydration. The influence of temperature and surface area of the tricalcium-silicate sample on kinetics of the sample hydration was characterized. Computer modeling of time dependences of radon release rate during hydration of tricalcium-silicate was carried out. A good agreement of the numerical model with the experimental results of the radon release was found.     

A chromatographic analysis of hindered diffusion of saccharides in silica gels

Summary The chromatographic technique as a means for experimental studies of diffusion in porous solids has been demonstrated. In this paper we report the effective diffusion coefficients which we have chromatographically obtained for two dextran samples and three low molecular weight solutes. The chromatographic method of measuring diffusional resistances in porous solids has the advantages of rapidity and simplicity. This method also provides information on the axial dispersion and distribution of a solute between the moving phase and the solvent held stationary in the pore. The elution curves of a packed column in pulse response experiments were analyzed by the method of moments to give the parameters of interest. The results indicate that pore diffusion of the solutes was appreciably restricted in comparison with diffusion in bulk solution. The degree of hindrance depended on the size of the diffusing substances.     

Optimization of time resolution and detection limit for online measurements of 222Rn in water

The online measurements of radon in flowing water with high temporal resolution and a lower limit of detection of some Bq/l is of growing interest in environmental research and earth sciences. Promising new fields of application in hydrogeology are the study of exchange and mixing processes and the monitoring of pumping procedures before and during groundwater sampling. A suitable, simple method has been proposed by Surbeck based on the separation of air and water by a diffusion membrane. Process parameters influencing the temporal resolution as well as the radon detection efficiency have been studied. Considering these results a new instrument has been developed enabling online radon-in-water measurement with time resolution of about one minute.     

Quantum chemical contributions on the reactivity of solids

The concept of reactivity is reviewed. Various reactivity indices are discussed. Different methods for the simulation of solids are systematically described. Applications for the reactivity of solids can be subdivided into three categories: surfaces, interfaces and bulk. Examples for the categories are presented with a special emphasis on diffusion processes in the bulk.