The sensitivity of a cylindrical p ‐type silicon detector was studied by means of air and water measurements using different photon beams. A lead filter cap around the diode was used to minimize the dependence of the detector response as a function of the brachytherapy photon energy. The radial dose distribution of a high‐activity 192 Ir source in a brachytherapy phantom was measured by means of the shielded diode and the agreement of these data with theoretical evaluations confirms the method used to compensate diode response in the intermediate energy range. The diode sensitivity was constant over a wide range of dose rates of clinical interest; this allowed one to have a small detector calibrated in terms of absorbed dose in a medium. Theoretical evaluations showed that a single shielding filter around the p ‐type diode is sufficient to obtain accurate dosimetry for 192 Ir, 137 Cs, and 60 Co brachytherapy sources.
Plants, algae, and their derivatives (paper, textiles, etc.) are complex systems that are chiefly composed of a web of cellulose fibers. The arrangement of solvents within the polymeric structure is of great importance since cellulose degradation is strongly influenced by water accessibility and external agents. In this paper we develop a model that is able to deconvolve the scattering contributions of both polymeric structures and solvent clusters trapped along the polymeric fibers. The surface morphology of cellulose fibers and the spatial distribution of water-filled pores and their dimensions have been recovered from small angle neutron scattering and atomic force microscopy data in function with paper degradation. In addition to providing a boost to the effort to preserve cellulose-supported material (included cultural heritage), the relevance of our model resides in the exploitation of a large number of biopolymer networks that are known to share structures similar to that of cellulose.
We show how small- and wide-angle elastic light scattering ( q ~ 0.03-30 μm -1 ) can be used to quantitatively characterize the structure of polymeric gels made of semi-flexible entangled fibers. We applied the technique to the study of fibrin gels grown from the polymerization of fibrinogen (FG) macromolecular monomers following activation by the enzyme thrombin (TH), at different concentrations and under different physical-chemical conditions of the gelling solution. Our findings show that the gel can be imagined as a random network of fibers of size d and density ρ, entangled together to form densely packed blobs of mass fractal dimension D m and average size ξ, which may overlap by a factor η and exhibit a long-range order. Provided that d ≥ 50-100 nm, all of the above parameters can be recovered by the use of a global fitting function developed by us on the basis on the proposed gel model. When the fibers are thinner ( d < ~50 nm), only the fiber mass/length ratio μ ~ ρ d 2 can be retrieved instead of d and ρ . Our data confirm and quantify the major changes in the gel structure that can be obtained by varying either the salt concentration of the solution and/or the molar ratio TH/FG. Gels formed in Tris-HCl 50 mM/NaCl 150 mM, pH 7.4 and TH/FG = 0.01 are characterized by relatively small, fairly branched ( D m ~ 1.4-2.0) fibers with a mass/length ratio independent of concentration. On reducing the TH/FG ratio, the fibers become increasingly thicker, with d ~ 90 nm at TH/FG = 10 -5 . When the salt concentration is reduced to NaCl 100 mM (TH/FG = 0.01) the fibers are less branched ( D m ~ 1.2-1.4), but much thicker, with μ increasing by an order of magnitude. These effects are quantitatively analyzed and discussed.
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