The aim of this work is to investigate the temporal stability of a polyacrylamide gelatin hydrogel used for 3D monomer/polymer gel dosimetry techniques involving different methods of analysis. Long-term instabilities for a similar gel have recently been reported, but differ markedly from those described in this work. Two kinds of long-term instabilities are described. One affects the slope of the dose-R2 plot and is related to post-irradiation polymerization of the comonomer/polymer aggregates. It is observed that post-irradiation polymerization only lasts 12 hours after irradiation. The other instability affects the intercept of the dose-R2 plot, lasts for up to 30 days and is related to the gelation process of gelatin. Further studies were performed on gelatin gels of varying compositions to obtain a better understanding of the molecular mechanism that causes the instability due to gelation. The studies included observations of the spin-spin and spin-lattice relaxation rates in combination with diffusion measurements and optical measurements. It is shown that the heating history during the manufacture of the gel affects the absolute R2 value of the gel but not its variation. The findings presented in this study may help in producing more stable and reproducible monomer/polymer gel dosimeters.
We have measured for the first time the external quantum efficiency (EQE) of silver clusters containing zeolites (henceforth referred to as silver-clusters zeolite composites). These materials, fabricated by silver cation exchange followed by a thermal autoreduction process, have EQEs up to 69%. Because of their unique spectral features such as large Stokes shift and high EQE, these materials could be potentially used as phosphors for the fabrication of fluorescent lamps and as wavelength convertors in solar cells. An EQE comparison between less pure commercial silverloaded zeolites and self-synthesized silver-zeolites showed the importance of the chemical and optical purity of the starting host material. Besides this, the zeolite topology and silver content play an important role on the luminescent performance of such materials. The ability to reliably measure the EQE enabled us to further optimize the synthesis of silver-zeolite composites. A new reduction−oxidation cycle is demonstrated not only to improve the luminescent performance of the silver-zeolite composites but also to enhance their water stability.
The determination of the long-term memory colours of objects has been the subject of investigation for many years. Colour acceptance boundaries have been determined from the visual assessments of objects under variable illumination or by presenting manipulated images of objects on a calibrated computer display. However, a systematic and quantitative rating of the colour of real objects with respect to memory colour is not available at this moment. In this article, nine familiar real objects with colours distributed around the hue circle were positioned in a specially designed LED illumination box. For each object, approximately hundred real illumination spectra were synthesized in a random order keeping the luminance of the object approximately constant. Observers were asked to rate, on a five-point scale, the similarity of the perceived object colour to their idea of what the object looked like in reality. By avoiding specular reflections, the observer was unable to identify any clues as to the colour of the illumination. For each object, similarity ratings showed a good intraobserver and interobserver agreement. The ratings of all the observers were pooled and successfully modeled in IPT colour space by a bivariate Gaussian distribution. It was found that the chromaticity corresponding to the highest rating tended to be shifted toward higher chroma in comparison with the chromaticity calculated under D65 illumination. The bivariate distributions could be very useful in applications where the quantitative evaluation of the colour appearance of an object stimulus is required, such as in the evaluation of the colour rendering capabilities of a light source.
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