Jan Hovind

On-line monitoring of hygroscopicity and dimensional changes of wood during thermal modification by means of neutron imaging methods

Abstract European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] have been thermally modified in a combined air-steam atmosphere between 70°C and 150°C and pressures up to 4 bar, and the changes of dimensions and moisture contents (MC) of the samples were studied in-situ by means of neutron imaging (NI). This non-invasive testing method offers unique opportunities. NI is highly sensitive for hydrogen and thus well suited for monitoring the MC changes, although some metals (e.g., Al), indispensable for sample environments under high temperature and pressure, are practically transparent to neutrons. The results show that the modification induced changes of MC and dimensions both in radial and tangential direction can well be determined by NI. Dimensional changes from water saturation to oven-dry state, the sorption isotherms, and the differential swelling were observed. Additionally, the sorption behaviour at 20°C was investigated after thermal modification and colour measurements were carried out before and after the thermal treatment.

Water and entrapped air redistribution in heterogeneous sand sample: Quantitative neutron imaging of the process

Saturated flow in soil with the occurrence of preferential flow often exhibits temporal changes of saturated hydraulic conductivity even during the time scale of a single infiltration event. These effects, observed in a number of experiments done mainly on heterogeneous soils, are often attributed to the changing distribution of water and air in the sample. We have measured the variation of the flow rates during the steady state stage of the constant head ponded infiltration experiment conducted on a packed sample composed of three different grades of sand. The experiment was monitored by quantitative neutron imaging, which provided information about the spatial distribution of water in the sample. Measurements were taken during (i) the initial stages of infiltration by neutron radiography and (ii) during the steady state flow by neutron tomography. A gradual decrease of the hydraulic conductivity has been observed during the first 4 h of the infiltration event. A series of neutron tomography images taken during the quasi-steady state stage showed the trapping of air bubbles in coarser sand. Furthermore, the water content in the coarse sand decreased even more while the water content in the embedded fine sand blocks gradually increased. The experimental results support the hypothesis that the effect of the gradual hydraulic conductivity decrease is caused by entrapped air redistribution and the build up of bubbles in preferential pathways. The trapped air thus restricts the preferential flow pathways and causes lower hydraulic conductivity.

Rotation axis demultiplexer enabling simultaneous computed tomography of multiple samples.

Graphical abstract

Development of neutron imaging quantitative data treatment to assess conservation products in cultural heritage

AbstractDistribution, penetration depth and amount of new mineralogical phases formed after the interaction between an inorganic treatment and a matrix are key factors for the evaluation of the conservation treatment behaviour. Nowadays, the conventional analytical methodologies, such as vibrational spectroscopies, scanning electron microscopy and X-ray diffraction, provide only qualitative and spot information. Here, we report, for the first time, the proof of concept of a methodology based on neutron imaging able to achieve quantitative data useful to assess the formation of calcium oxalate in a porous carbonatic stone treated with ammonium oxalate. Starting from the neutron attenuation coefficient of Noto stone-treated specimens, the concentrations of newly formed calcium oxalate and the diffusion coefficient have been calculated for both sound and decayed substrates. These outcomes have been also used for a comparative study between different treatment modalities. Graphical abstractHorizontal slice at 300 mm depth and CaOx molar density profile by NEUTRA output

Neutron imaging and digital volume correlation to analyse the coupled hydro-mechanics of geomaterials

A new approach to characterise the evolution and coupling of deformation and fluid flow in geomaterials is presented. The method exploits some key features of neutrons, namely penetration of dense materials used for triaxial pressure cells, sensitivity to hydrogen and the possibility to distinguish hydrogen from its isotope deuterium (in normal water, H2O, and heavy water, D2O, respectively). Illustration of the approach is provided with results from a combined fluid flow/triaxial compression test on a cemented sand specimen performed in-situ (i.e., acquiring images during loading) at a neutron imaging station. Quantitative analysis of neutron tomography images acquired at different stages of deformation is made by Digital Volume Correlation to provide full 3D strain fields that highlight the evolution of localised deformation features. Spatio-temporal tracking of the effect of the evolution of the permeability in the sample was possible by neutron radiographies acquired during pressure driven flow of H2O into the sample saturated with D2O. By exploiting the H2O/D2O neutron transmission contrast and similarities of their flow behaviour, the tracking of the H2O/D2O front can be considered as an indicator of the permeability of the sample that is correlated to the measured evolution of the deformation. (Less)

A QUANTITATIVE EVALUATION OF THE WATER DISTRIBUTION IN A SOIL SAMPLE USING NEUTRON IMAGING

This paper presents an empirical method by Kang et al. recently proposed for correcting two-dimensional neutron radiography for water quantification in soil. The method was tested on data from neutron imaging of the water infiltration in a soil sample. The raw data were affected by neutron scattering and by beam hardening artefacts. Two strategies for identifying the correction parameters are proposed in this paper. The method has been further developed for the case of three-dimensional neutron tomography. In a related experiment, neutron imaging is used to record ponded-infiltration experiments in two artificial soil samples. Radiograms, i.e., two-dimensional projections of the sample, were acquired during infiltration. A calculation was made of the amount of water and its distribution within the radiograms, in the form of two-dimensional water thickness maps. Tomograms were reconstructed from the corrected and uncorrected water thickness maps to obtain the 3D spatial distribution of the water content within the sample. Without the correction, the beam hardening and the scattering effects overestimated the water content values close to the perimeter of the sample, and at the same time underestimated the values close to the centre of the sample. The total water content of the entire sample was the same in both cases. The empirical correction method presented in this study is a relatively accurate, rapid and simple way to obtain the quantitatively determined water content from two-dimensional and three-dimensional neutron images. However, an independent method for measuring the total water volume in the sample is needed in order to identify the correction parameters.