Determination of soil, sand and ore primordial radionuclide concentrations by full-spectrum analyses of high-purity germanium detector spectra.

The full-spectrum analysis (FSA) method was used to determine primordial activity concentrations (ACs) in soil, sand and ore samples, in conjunction with a HPGe detector. FSA involves the least-squares fitting of sample spectra by linear combinations of (238)U, (232)Th and (40)K standard spectra. The differences between the FSA results and those from traditional windows analyses (using regions-of-interest around selected photopeaks) are less than 10% for all samples except zircon ore, where FSA yielded an unphysical (40)K AC.

Gamma-ray spectrometry of radon in water and the role of radon to representatively sample aquifers.

Measurement of radon in water by gamma-ray spectrometry using a HPGe detector has been investigated to determine aquifer characteristics. The radon activity concentration is determined by taking the weighted average of the concentrations derived from gamma-ray lines associated with (214)Pb and (214)Bi decay. The role of accurate radon data to representatively sample aquifers was also investigated by studying a semi-cased borehole. A simplified physical model describing the change of radon concentration with the pumping time, reproduces the data and predicts the time for representative sampling of the aquifer.

Resorption of cesium radionuclides by various fungi.

Uptake of 134Cs, 137Cs and 40K is determined by gamma-ray spectroscopy for samples of fungi and humus. For these samples also the stable Cs content is determined by atomic absorption spectrophotometry. The fungi resorpted only the alkali metals and none of the other fission products from the Chernobyl accident. The measurements show a large variation in space and time for fungi of the same species in the stable and radioactive cesium content. Compared to this variation the variation between the different species is in general small. Cs content in mycorrhizal fungi is in general higher than in saprophytic fungi, but differences in absorption in saprophytic and mycorrhizal fungi could not be confirmed. The assumption that certain saprophytic fungi mainly grow in either litter, fermentation or humus layer is not supported by this experiment.

Measurements of time-averaged radon-daughter concentrations with passive dosemeters.

With standard dosemeters of the Karlsruhe type it is possible to measure simultaneously time-integrated radon and radon-daughter concentrations. For this purpose the dosemeters were equipped with a lid and a bottom foil. Pre-etching conditions for either type of foil were optimized; under these conditions no effects of plate-out were observed. The method seems to be sensitive to neutral radon daughters only. Possible improvements of the dosemeter geometry are indicated.

Survey of radon concentrations in Dutch dwellings.

A survey of radon concentrations in Dutch dwellings reveals a median value of 24 Bq/m3, while no excessive high values were observed. Correlations between radon concentration and (combinations of) building parameters will be presented and discussed in terms of the various sources of indoor radon.

Experimental set-up for measuring diffusive and advective transport of radon through building materials.

This study describes an approach for measuring and modelling diffusive and advective transport of radon through building materials. The goal of these measurements and model calculations is to improve our understanding concerning the factors influencing the transport of radon through building materials. To reach this goal, a number of experiments have to be conducted. These experiments, including measurements in a large cylinder for creating diffusive and advective transport of radon under controlled, 'dwelling-like' conditions, are described here and the initial results are presented. A better understanding about the transport of radon through building materials will lead to more effective ways to decrease or to prevent the entrance of radon into dwellings.

Moisture dependence of radon transport in concrete: measurements and modeling.

The moisture dependence of the radon-release rate of concrete was measured under well controlled conditions. It was found that the radon-release rate almost linearly increases up to moisture contents of 50 to 60%. At 70 to 80% a maximum was found and for higher moisture contents the radon-release rate decreases very steeply. It is demonstrated that this dependence can be successfully modeled on basis of the multi-phase radon-transport equation in which values for various input parameters (porosity, diffusion coefficient, emanation factor, etc.) were obtained from independent measurements. Furthermore, a concrete structure development model was used to predict at any moment in time the values of input parameters that depend on the evolution of the concrete microstructure. Information on the concrete manufacturing recipe and curing conditions (temperature, relative humidity) was used as input for the concrete structure model. The combined radon transport and concrete structure model supplied sufficient information to assess the influence of relative humidity on the radon source and barrier aspects of concrete. More specifically, the model has been applied to estimate the relative contributions to the radon exhalation rate of a 20-cm-thick concrete slab of radon produced in the concrete slab itself and due to diffusive transport through the slab of radon from soil gas.

Measurements of radon-daughter concentrations in and around dwellings in the northern part of The Netherlands; a search for the influences of building materials, construction and ventilation.

The concentration of radon daughters has been determined in and around 80 dwellings located in the northern part of the Netherlands by using a one-filter method. Median values of 2.0 and 0.4 mWL were measured for the indoor and outdoor concentrations, respectively. The average outdoor concentration was about an order of magnitude higher for wind directions between SE and SW than for SW-NW. On the average, dwellings with double-pane windows and/or concrete floors were found to have significantly higher radon concentrations than those with single-pane windows and/or wooden floors. For the living room of a particular dwelling 18 measurements were carried out. The data for this dwelling indicate a linear relation between the concentration indoors and outdoors with a slope of 3.8 +/- 2.0. This unexpected behaviour is thought to be related to ventilation via the crawl space. Measurements of ventilation patterns and measurements of radon concentrations in the living room and the crawl space are consistent with this picture.

Radionuclide concentrations in the northern part of The Netherlands after the Chernobyl reactor accident.

Concentrations of radionuclides originating from the Chernobyl reactor accident were measured as a function of time in air, rainwater, grass, cow's milk, vegetables and dust by means of high-resolution gamma-ray spectroscopy. Special attention was paid to grass and milk originating from the same meadows. Also, milk of cows temporarily kept inside after the accident was monitored until a few days after their release from the stables. Activity ratios in various types of samples and the implication of the sheltering measures for cows are discussed.

Diffusive transport of radon in a homogeneous column of dry sand.

To validate a model for radon transport in soil, measurements of diffusive radon transport under well-defined and controlled conditions have been made in a homogeneous column of dry sand with an air-filled volume on top. This volume simulates a crawl space. The measurements concern time-dependent diffusive transport with several different crawl-space heights and steady-state diffusive transport with different crawl-space ventilation rates. This study forms part of an extensive validation which will also cover more complex situations (e.g., advective transport, presence of pore-water). Properties of the sand that are presently being used in the radon transport model were determined from independent measurements. The comparison between model calculations and experimental results showed a good correspondence with maximum deviations of less than 10%.

Combined diffusive and advective transport of radon in a homogeneous column of dry sand.

To validate a model for radon transport in soil, measurements of combined advective and diffusive radon transport under well-defined and controlled conditions have been made in a homogeneous column of dry sand with and without an air-filled volume on top. This volume simulates a crawl space. The measurements concern steady-state as well as time-dependent combined advective and diffusive transport and are performed in the framework of a systematic validation study which will also cover more complex situations (e.g., presence of pore-water). The experimental data were compared with results of a two-dimensional numerical model based on the finite-difference approach of the differential equation describing radon transport. Validations of the numerical model with analytical solutions were carried out for one-dimensional steady-state combined diffusive and advective transport, and for two-dimensional time-dependent diffusive transport. The results of the measurements with combined advective and diffusive transport are in good agreement with (numerical) model calculations with maximum deviations <10%.

Foil coverage of a crawl-space floor: measurements and modeling of radon entry.

The mitigative impact of covering the floor of a crawl space with a membrane has been studied under well-defined and controlled conditions. The measurements have been done with a homogeneous column of dry sand covered with a sheet of polyethylene foil. An air-filled volume on top of the column simulates a crawl space. The experiments mainly concern long-term measurements of the crawl-space radon concentration in combination with steady-state diffusive and combined advective and diffusive transport through the sand column and crawl space. The experimental data are analyzed with both simplified mass-balance models for radon entry into the crawl space and with a two-dimensional numerical model based on a finite-difference approach. In all experiments the influence of atmospheric pressure variations is clearly present. For most experiments the agreement between calculations, which make use of independently measured transport parameters for both sand and foil, and measurements is within 10%. However, the discrepancy is larger for experiments with continuous advective transport from the crawl space to the sand. With undamaged foil, the calculations overestimate the measurements by 20%. Reversely, with an opening in the center of the foil, the calculations underestimate the measurements by 20-40%. The results show that under controlled conditions radon transport from a foil-covered sand column into a crawl-space can be described within 40% on basis of separately measured parameters.

Diffusive transport of radon in a column of moisturized sand.

Diffusive transport of radon in a moisturized sand column was investigated under well-defined and controlled conditions using a laboratory facility that consists of a cylindrical vessel (height and diameter 2 m) filled with sand. Equilibrium pore-gas radon-concentration profiles in the sand column were measured as a function of depth for several different levels of the ground-water table. The measurements were performed in the framework of a systematic validation study that also covered experiments with dry sand. The experimental data were compared with results of model calculations. Most parameters in the transport model were obtained from separate experiments on small sand samples. In this manner, the radon diffusion coefficient and emanation coefficient were determined as functions of pore-water content, as well as the water-retention characteristics of the sand. In general, the experimental data are reasonably well reproduced by the calculations with differences of 10%-40%. However, with respect to earlier results with dry sand indicating differences of generally smaller than 10%, the agreement between experiment and model for moisturized sand is clearly worse. The main part of the disagreement probably results from inhomegeneities in the sand column that are not accounted for in the transport model.

Radon penetration of concrete slab cracks, joints, pipe penetrations, and sealants.

Radon movement through 12 test slabs with different cracks, pipe penetrations, cold joints, masonry blocks, sealants, and tensile stresses characterized the importance of these anomalous structural domains. Diffusive and advective radon transport were measured with steady-state air pressure differences controlled throughout the deltaP = 0 to 60 Pa range. Diffusion coefficients (deltaP = 0) initially averaged 6.5 x 10(-8) m2 s(-1) among nine slabs with only 8% standard deviation, but increased due to drying by 0.16% per day over a 2-y period to an average of 2.0 x 10(-7) m2 s(-1). An asphalt coating reduced diffusion sixfold but an acrylic surface sealant had no effect. Diffusion was 42 times higher in solid masonry blocks than in concrete and was not affected by small cracks. Advective transport (deltaP < or = 60 Pa) was negligible for the slabs (10(-16) m2 permeability), pipe penetrations, and caulked gaps, but was significant for cracks, disturbed pipe penetrations, cold joints, masonry blocks, and concrete under tensile stress. Crack areas calculated to be as small as 10(-7) m2 significantly increased radon advection. Algebraic expressions predict air velocity and effective crack width from enhanced radon transport and air pressures. Masonry blocks, open cracks, and slab cold joints enhance radon penetration but stressed slabs, undisturbed pipe penetrations, and sealed cracks may not.

Monte Carlo simulations as a feasibility tool for non-metallic land-mine detection by thermal-neutron backscattering.

The use of Monte Carlo simulations is presented for modelling a simplified land-mine detector system with thermal neutron backscattering (TNB) analysis based on a 252Cf-neutron source. Different aspects and a variety of external conditions, related to localisation and identification of a buried object have been investigated. In particular, the influence of moisture in a formation has been assessed, as moisture can be a serious interference for hydrogen as an indicator for explosives. The results of sensitivity calculations confirm that land-mine detection methods, based on an analysis of TNB may be applicable in homogeneous formations with low porosity provided that pore-water remains <5% by weight. In dry limestone, the TNT-based explosives can be well distinguished from other hydrogen-rich materials, except wood. However, in dry siliciclastic sands TNT explosives and wood are distinguishable.

Natural radionuclides as tracers of the dispersal of dredge spoil dumped at sea.

Monitoring large (underwater) surfaces, with rapidly varying composition, requires a sampling density which exceeds the capabilities of standard techniques. These techniques involve sample collection and a number of treatments and measurements in the laboratory; both steps are laborious, tedious and costly. This paper presents an in situ method in which a detector system is trailed over the surface and measures continuously the gamma rays emitted by the natural radionuclides in the sediment. Since each sediment component has its own characteristic set of activity-concentration values (radiometric fingerprint), the composition of the sediment can be deduced quantitatively. This paper shows the application of this technique for monitoring the dispersal of dredge spoil from Rotterdam harbour, dumped in the North Sea. In addition to a qualitative picture of dredge spoil dispersal, a mass balance equation has been used to quantitatively assess the dredge spoil transport with time.

Gamma-ray spectrometry intra- and interlaboratory test of the Dutch protocol.

This paper presents the results of a test programme to validate the Dutch pre-normative protocol NVN 5623 'Radioactivity measurements: Determination of the activity of gamma-ray emitting nuclides in a counting sample by means of semi-conductor gamma-spectrometry'. Batches of two reference materials, water and sand, both spiked with radionuclides (133Ba, 134Cs, 137Cs, 40K, 57Co and 60Co) were used in an intra- and interlaboratory test. For the water and sand, 10 and 9 laboratories, respectively, agreed to analyse three samples of each reference material strictly according to the pre-normative protocol. The interlaboratory test programme showed that for both reference materials the values for the reproducibility standard deviation were 1.5-6 times larger than those for the repeatability standard deviation. This extra variation is caused by the parameter 'laboratory'. For both reference materials the repeatability standard deviation was found in the range 0.9-2.0% of the mean activity concentration; the reproducibility standard deviation ranged from 2.5-6.0%. Bias of the test method was found for 134Cs (-6.8%) and 133Ba (-2.3%) in case of the spiked water and for 134Cs (-7.8%) in case of the spiked sand. This bias is attributed to no or insufficient correction for coincidence summing. In general, the interlaboratory test showed satisfactory agreement between laboratories. In the light of regulations on radioactivity in materials it is concluded that for measurement of this activity the Dutch protocol may serve as a blueprint for an (European Community) protocol.

MCNP modelling of scintillation-detector gamma-ray spectra from natural radionuclides.

gamma-ray spectra of natural radionuclides are simulated for a BGO detector in a borehole geometry using the Monte Carlo code MCNP. All gamma-ray emissions of the decay of 40K and the series of 232Th and 238U are used to describe the source. A procedure is proposed which excludes the time-consuming electron tracking in less relevant areas of the geometry. The simulated gamma-ray spectra are benchmarked against laboratory data.

Full-spectrum analysis of natural gamma-ray spectra.

In this paper, a new system to measure natural gamma-radiation in situ will be presented. This system combines a high-efficiency BGO scintillation detector with full-spectrum data analysis (FSA). This technique uses the (nearly) full spectral shape and the so-called 'standard spectra' to calculate the activity concentrations of 40K, 232Th and 238U present in a geological matrix (sediment, rock, etc.). We describe the FSA and the determination of the standard spectra. Standard spectra are constructed for various geometries and a comparison in intensity and shape will be made. The performance of such a system has been compared to a more traditional system, consisting of a NaI detector in combination with the 'windows' analysis. For count rates typically encountered in field experiments, the same accuracy is obtained 10-20 times faster using the new system. This allows for shorter integration times and hence shorter measurements or a better spatial resolution. The applicability of such a system will be illustrated via an example of an airborne experiment in which the new system produced results comparable to those of much larger traditional systems. This paper will conclude with a discussion of the current status of the system and an outlook for future research.

Cluster analysis of radionuclide concentrations in beach sand.

This paper presents a method in which natural radionuclide concentrations of beach sand minerals are traced along a stretch of coast by cluster analysis. This analysis yields two groups of mineral deposit with different origins. The method deviates from standard methods of following dispersal of radionuclides in the environment, which are usually based on the construction of lines of equal concentrations. The paper focuses on the methodology of quantitatively correlating activity concentrations of natural radionuclides in two groups of minerals. The methodology is widely applicable, but is demonstrated for natural radioactivity in beach sands along the coast of South West Australia.