However, a closer consideration of the elemental composition of WBCs – as already denoted as HCNO-materials – enables the derivation of mass attenuation coefficients on a theoretical basis by means of tabulated data for X-ray appli-cations (Chapter IV–5.2) with particular respect to radiation energy spectra. Accordingly per-formed computations are respectively compared to measured attenuation data. Hence, WBCs are exclusively considered to be a mixture of HCNO-elements inclusive of some minerals dominated by calcium (Ca) on this scale, which depicts the actual geometric level of radiation-matter inter-action in contradiction to macroscopic scale, which provides measuring information of radio-metric investigations. Subsequently, on ele-mental composition basis, the effective atomic number 𝑍eff (cf. MURTY (1965)) is derived in Chapter IV–5.1 providing an illustrative figure, to compare the impact on X-ray attenuation. Finally and beyond elemental composition, there is no influence of any WBC production process-re-lated structural variations such as consolidation on radiation-matter interaction on the sub-micro-scopic scale. Hence, independent from material (coarse) structure, sub-microscopic level exclu-sively involves elemental- and energy-related in-teraction mechanisms of the respective radiation with the investigated matter.
3.6 Conclusion
With the objective to be introduced as an explan-atory beam path model, the concept is not enti-tled to claim precise quantitative description of three-dimensional WBC structures like afore-mentioned models do (refer to Chapter IV–3.3).
However, partly good agreement is found in comparison of relevant details. Moreover, a qualitative illustration of structural aspects on distinct scales relevant for radiation transmission and attenuation is intended considering both WBC composition and processing across the wide range from mat forming of resinated furnish to the cured final panel.
Designated as a central issue, the WBC struc-ture comprising inter- as well as intra-cellular pores, i. e., voids and cell lumina, respectively, is
considered to directly affect transmitted radiation intensity. Since an equal amount of irradiated identical matter is expected to cause equal at-tenuation, transmitted radiation intensity through WBCs considerably depends on consolidation of the porous matter. The densification ratio and re-sulting total porosity affect radiation propagation off the primary beam axis. Moreover, the impact of the WBC structure on effective radiation atten-uation is considered to be quantifiable regarding metrological applications.
Based on practice-oriented objectives, the con-ditions of radiation transmission through WBCs are both
- simple to generalise regarding low-𝑍 ele-mental composition embedded in virtually consistent solid matter with constituents of similar true density and
- complex to model precisely regarding beam path geometry with respect to potential structural members as well as radiation-mat-ter inradiation-mat-teraction considering actual inhomoge-neity.
Radiation-matter interaction and corresponding attenuation related to elemental composition, are subsequently computable, wherefore refer-ence is made to Chapter IV–5.2. On the contrary, radiation propagation, thus, effective attenuation and resulting transmission, are considered to be attributed to structural conditions on the mesoscopic and microscopic scale, particularly porosity, owing to consistent true density of the condensed matter present. Hence, both compo-sition and structure are relevant for radiation at-tenuation on all considered scales below macro-scopic appearance as solid body.
To conclude in short with reference to Figure IV-12, radiometric density measuring results 𝜌 [kg m⁄ 3] or 𝜌A [kg m⁄ 2] based on transmission intensity detection represent the macroscopic conditions of irradiated WBC matter whether fur-nish mat or final panel, whereas on the one hand their mesoscopic and microscopic structures af-fect total efaf-fective radiation transmission and on the other hand actual radiation-matter interaction ensues on sub-microscopic (atomic) scale. Inde-pendent from consolidation ratio of wood furnish,
128 3 Radiation transmission concept through porous composites Section IV
the true densities of relevant constituents pre-sent, i. e., cell-wall tissue, resin, additives, and water, remain constant. Thus, penetrated matter undergoes no alteration in relation to individual densification. Consequently and regardless of apparent structural impact, transmitted radiation is expected to primarily contain information about irradiated mass of matter, i. e., its area density 𝜌A, expecting, in turn, a homogeneous non-porous absorber. Accordingly, the com-puted mean attenuation data via weighted sum of tabulated mass attenuation coefficients 𝜇 𝜌⁄ (𝐸)𝑖 [m2⁄kg], which refer to atomic cross-sections, exclusively yields valid 𝜌A results for consistent bodies and is not applicable for direct densitometry on porous media. Hence, radiation transmission-based raw density evaluation re-quires effective 𝜇 𝜌⁄ (𝐸) values determined with a priori knowledge of the structural conditions.
Though radiation-matter interaction ensues on atomic level, measuring information is attributed to macroscopic level of the inhomogeneous body with considerable impact of structural con-ditions on mesoscopic and microscopic scales below. The latter are characterised by alterna-tions along the beam path of (moist) air and var-ious solid constituents, i. e., radiation is consid-ered not to constantly travel through condensed matter, which facilitates potentially free radiation propagation off the primary beam axis, where ra-diation photons or corpuscles undergo mini-mised likelihood of interaction within pores and voids.
Finally, the developed conceptual beam path model illustrates actual transmission conditions of WBC whether furnish mat or final panel obvi-ously not fulfilling good-architecture conditions (i. e., monochromatic narrow-beam, refer to Chapter II–1.3). However, in an ideal case, radi-ation attenuradi-ation is described by exponential in-tensity diminution following the well-known Beer’s law according to eq. (II-10) and eq. (II-11), respectively, where initial intensity 𝐼0 [a. u.] is ex-ponentially attenuated yielding transmitted inten-sity 𝐼T [a. u.] in dependence of mass attenuation coefficient 𝜇 𝜌⁄ (𝐸) [m2⁄kg], transmission dis-tance 𝑡 [m], and raw density 𝜌 [kg m⁄ 3] or area density 𝜌A [kg m⁄ 2], respectively. In applied radi-ometric WBC investigations by means of X-rays
or other forms of radiation, the conditions for full validity of Beer’s law cannot be reasonably met in any case leading to biased exponential con-text. Subsequently, the explanatory beam path model is revisited and refined in Chapter IV–6.5, where particular consequences of WBC struc-ture and composition as well as further related effects on quantitative measurements are dis-cussed.
4 X-ray measurements
4.1 Sampling and sample preparation in general
All performed X-ray measurements require a more or less similar specimen cutting and prep-aration. The number of specimens, however, de-pends on the respective measuring series. Note here, sample sizes are considered to be rather small due to the exploratory character of this study. However, material manufacturing and general specimen preparation were already de-scribed in the respective sections in Chapter IV–
1 considering the individual material types lab-made furnish mats and homogenous fibreboards as well as customary industrial panels. Before and after cutting, all material was stored at standard conditions 20 °C and 65 % RH (unless otherwise stated) as primarily applied for the X-ray measurements. Additionally, SL particles were stored at dry (OD) and moist (20/83) condi-tions. The individual conditioning to constant mass was ensured prior to each measuring se-ries. In the case of any transport to external measuring facilities, the material was vacuum-sealed to avoid changes in EMC. The actual 𝑀𝐶 is evaluated after conditioning and summarised in Table IV-5 complete with the further properties thickness 𝑡, raw density 𝜌, and area density 𝜌A, which are furthermore discussed with descrip-tion of their determinadescrip-tion in Chapter IV–1.5. For measurements with regard to area density 𝜌A de-termination on furnish mats (Fmat, SLmat, and CLmat), not total real-size mats like in the indus-trial production but sufficiently large specimens (174 mm diameter) are utilised as manufactured according to Chapter IV–1.1. The like applies to the lab-made and particularly the customary in-dustrial panels (labMDF, Chapter IV–1.2 and indMDF, IV–1.3, respectively). All indMDF spec-imens were randomly cut from the panels with the required dimensions. In due consideration of the individual X-ray beam geometries as well as detector sizes, it was verified that the respec-tively employed specimens sufficiently cover the core beam and a relevant surrounding area in or-der to obtain appropriate radiation transmission
and scattering conditions like on real-size mate-rial. Given the nondestructive nature of X-ray measurements, most of the sample sets could be employed for numerous measurements on the different devices and setups or as repetition for verification of observations before they were partly utilised for the destructive material analy-ses (Chapter IV–2) or RDP determination via ref-erence method in terms of the round robin test (Chapter IV–4.3.1). Eventually, specific details on sampling and sample preparation are pro-vided at the beginning of each respective result presentation chapter.
4.2 Methods
4.2.1 Overview
The following subchapters point out the em-ployed methods for X-ray measurements with particular purposes. However, basically two X-ray devices with either Ag (Chapter IV–4.2.2) or W (Chapter IV–4.2.3) as tube-target material were utilised for manifold applications, where the latter was furthermore varied in its application-oriented setup. Regardless of control and data acquisition software as well as sample manipu-lation, descriptions rather focus on radiation-re-lated components and their parameters to keep reproducibility of results and related implications.
Note, some components cannot be further spec-ified owing to their origin from a bilateral re-search project and to keep proprietary data. Ac-cordingly, brand and manufacturer names as well as detailed pictures are partly omitted.
Notwithstanding basically similar transmission setups, Ag- and W-target device differ tremen-dously from each other considering components and interconnection. Accordingly, the same ap-plies to raw data processing and evaluation.
However, equivalent output of measuring results is eventually obtained regardless of device-spe-cific procedures.
130 4 X-ray measurements Section IV
Both devices were furthermore part of a round robin test (Chapter IV–4.2.5) with several cus-tomary measuring devices involved for raw den-sity profile (RDP) determination. Demanding ver-ification of radiometric raw density measuring re-sults in the case of RDP determination required the development of a gravimetric reference method as pointed out in Chapter IV–4.2.4. Be-yond actual measurements, X-ray spectra deter-mination (Chapter IV–4.2.6) was performed by means of simulation methods with explicit con-sideration of the respective measuring parame-ters.
4.2.2 Ag-target device
Notwithstanding particular modifications, the first of the two utilised X-ray devices is equivalent to the Itrax Multiscanner (without XRF unit) by Cox Analytical Systems, Mölndal, Sweden as de-scribed by COX (2016) and hitherto likewise re-ferred to as Itrax Woodscanner. Besides the aforementioned regular tree-ring analysis appli-cations, this device was initially employed for WBC investigations by GRUCHOT (2009) with ad-ditional specimen-modifying installations. How-ever, SOLBRIG (2009) finalised, evaluated, and optimised his adaptions for RDP determination, which is likewise reported by SOLBRIG et al.
(2010). For a detailed description with compre-hensive focus on control and data acquisition software, reference is made to the very same.
Hereinafter, the particularly adapted Itrax device was exclusively applied for investigations re-garding vertical RDP determination on 50 × 50 mm² specimens (as exemplarily shown in Figure IV-19 with the round robin test sam-ples). Figure IV-13 shows the internal space with the main radiation-related components of the de-vice and Table IV-18 compiles selected specifi-cations. Here, a water-cooled glass diffraction X-ray tube with long fine focus on the Ag target serves as radiation source. Moreover, the Ag-tar-get device particularly features capillary optics made of quartz glass for flat-beam collimation (refer to Chapter II–1.3). Detailed specifications of the actually involved polycapillary optics are, however, not available. Since device setup was
initially not designed for variable pre-filter instal-lation, a respective additional fastener was mounted directly after beam emission from capil-lary optics. Subsequent impact on beam geom-etry in terms of more distinct divergence can, however, not be excluded. Nevertheless, due to primary application for RDP measurement, the parallel alignment of the flat beam toward speci-men plane was carefully maintained. To this end, the capillary optics gimbal mounting provides two translational and three rotational degrees of freedom as illustrated in Figure IV-13. The rota-tion around the beam axis (Z via C) was further-more horizontally aligned toward the detector slit aperture. Moreover, sophisticated beam align-ment facilitates precise spatial resolution of the specimen in terms of measuring values true to thickness position. Therefore, poor beam align-ment potentially causes non-parallel and layer-crossing radiation transmission through the specimen, which obviously emerges as surface raw density decrement, where actually no dis-tinct gradient is present. Beyond manipulation, stability is likewise considered. In dependence of temperature, thus also of operating time, varia-tions in beam alignment were observed via the detector signal of radiation intensity. The regular variations are primarily not attributed to focal spot drift but are rather caused by thermal ex-pansion of the tube housing with mounted capil-lary optics and further related components. Con-sequently, in addition to so-called tube warm-up in terms of gradually high-voltage and current in-crement lasting up to 30 min, a two-hour device warm-up at regular high-voltage settings was consistently performed in advance of each measuring cycle.
Notwithstanding preliminary trials with parame-ter and setup variations, presented results will be limited to regular Itrax device settings referring to SOLBRIG (2009) at 𝑈a,nom= 55 kV and 𝐼a,nom= 40 mA without pre-filter unless otherwise stated.
The utilised upper tube power limit enables max-imum photon flux yield, which is required by the employed detector. For the purpose of at least one energetic variation, 1.5 mm Al was applied as additional pre-filter likewise shown in Figure IV-13 (e). For the two exclusive configurations of Ag-target device, the correspondingly derived