| 521. |
- Svensson, Tomas, et al.
(författare)
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Laser absorption spectroscopy of water vapor confined in nanoporous alumina: wall collision line broadening and gas diffusion dynamics.
- 2010
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Ingår i: Optics Express. - 1094-4087. ; 18:16, s. 16460-16473
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate high-resolution tunable diode laser absorption spectroscopy (TDLAS) of water vapor confined in nanoporous alumina. Strong multiple light scattering results in long photon pathlengths (1 m through a 6 mm sample). We report on strong line broadening due to frequent wall collisions (gas-surface interactions). For the water vapor line at 935.685 nm, the HWHM of confined molecules are about 4.3 GHz as compared to 2.9 GHz for free molecules (atmospheric pressure). Gas diffusion is also investigated, and in contrast to molecular oxygen (that moves rapidly in and out of the alumina), the exchange of water vapor is found very slow.
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| 522. |
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| 523. |
- Svensson, Tomas, et al.
(författare)
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Pore size assessment based on wall collision broadening of spectral lines of confined gas : Experiments on strongly scattering nanoporous ceramics with fine-tuned pore sizes
- 2013
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Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. ; 110:2, s. 147-154
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Konferensbidrag (refereegranskat)abstract
- Wall collision broadening of absorption lines of gases confined in porous media is a recently opened domain of high-resolution spectroscopy. Here, we present an experimental investigation of its application for pore size assessment. We report on the manufacturing of nanoporous zirconia ceramics with well-defined pore sizes fine-tuned from 50 to 150 nm. The resulting pore structure is characterized using mercury intrusion porosimetry, and the optical properties of these strongly scattering materials are measured using femtosecond photon time-of-flight spectroscopy (transport mean free paths found to be tuned from 2.3 to 1.2 μm as the pore size increase). Wall collision line broadening is studied by performing near-infrared (760 nm) high-resolution diode laser spectroscopy of confined oxygen molecules. A simple method for quantitative estimation of the pore size is outlined and shown to produce results in agreement with mercury intrusion porosimetry. At the same time, the need for improved understanding of wall collision broadening is emphasized.
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| 524. |
- Svensson, Tomas, et al.
(författare)
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VCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solids
- 2008
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 90:2, s. 345-354
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Tidskriftsartikel (refereegranskat)abstract
- We present a minimalistic and °exible single-beam instrumen- tation based for sensitive tunable diode laser absorption spectroscopy (TD- LAS) and its use in structural analysis of highly scattering pharmaceu- tical solids. By utilising a vertical-cavity surface emitting laser (VCSEL) for sensing of molecular oxygen dispersed in tablets, we address structural properties such as porosity. Experiments involve working with unknown pathlengths, severe backscattering and di®use light. These unusual experi- mental conditions has lead to the use of the term gas in scattering media absorption spectroscopy (GASMAS). By employing fully digital wavelength modulation spectroscopy and coherent sampling, system sensitivity in am- bient air experiments reaches the 10¡7 range. Oxygen absorption exhibited by our tablets, being in°uenced by both sample porosity and scattering, were in the range 8 £ 10¡5 to 2 £ 10¡3, and corresponds to 2-50 mm of pathlength through ambient air (Leq). The day-to-day reproducibility was on average 1.8% (0.3 mm Leq), being limited by mechanical positioning. This is the ¯rst time sub-millimetre sensitivity is reached in GASMAS. We also demonstrate measurements on gas transport on a 1 s timescale. By employing pulsed illumination and time-correlated single photon counting, we reveal that GASMAS exhibits excellent correlation with time-domain photon migration. In addition, we introduce an optical measure of porosity by relating oxygen absorption to average photon time-of-°ight. Finally, the simplicity, robustness and low cost of this novel TDLAS instrumentation provides industrial potential.
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| 525. |
- Tillman, C, et al.
(författare)
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Elemental biological imaging by differential absorption with a laser-produced x-ray source
- 1996
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Ingår i: Journal of the Optical Society of America B: Optical Physics. - 0740-3224. ; 13:1, s. 209-215
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the novel application of hard x rays emitted by a laser-produced plasma for differential imaging of elements. An x-ray-emitting laser-produced plasma, obtained by the focusing of radiation from a 10-Hz terawatt laser, is used for biological imaging. The x-ray source can be arranged to yield characteristic x-ray emission lines with photon energies that bridge the K absorption edge of a chosen atomic species. One can obtain element-specific radiographs by recording transillumination images for different target materials on digital image plates and by subsequently subtracting or dividing the images. Successful phantom and experimental animal imaging are performed utilizing tantalum and gadolinium as target materials for the terawatt laser and gadolinium as the imaged contrast agent.
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| 526. |
- Tillman, Carl, et al.
(författare)
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High-resolution spectroscopy of laser-produced plasmas in the photon energy range above 10 keV.
- 1997
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Ingår i: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - 0167-5087. ; 394:3, s. 387-396
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Tidskriftsartikel (refereegranskat)abstract
- X-rays from a laser-produced plasma were analysed using single-photon counting germanium detectors. Special attention was given to the extremely high photon flux, requiring either a long source-detector distance and narrow collimated measurement geometry in order to avoid severe pile-up of pulses in the spectrometer, or the use of a Compton scattering geometry. With the use of these techniques, individual characteristic K-lines were resolved for the first time. Efficiencies for the conversion of the laser light into X-rays were determined, being up to 10−4 for Bremsstrahlung, and up to 10−5 for the characteristic K-line emission. Electron temperatures of not, vert, similar100 keV were assessed.
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| 527. |
- Tillman, C, et al.
(författare)
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Imaging Using Hard X-rays From A Laser-produced Plasma
- 1995
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Ingår i: Applied Physics B. - 0946-2171. ; 61:4, s. 333-338
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Tidskriftsartikel (refereegranskat)abstract
- Imaging of technical and biological objects using hard X-rays from a laser-produced plasma source is demonstrated. Magnification radiography and single-shot imaging of biological samples are feasible with the source, which utilised focused radiation from a short-pulse terawatt laser. Differential imaging with element specificity and a new projection geometry for X-ray radiography are proposed.
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| 528. |
- Tillman, Carl, et al.
(författare)
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Survival of mammalian cells exposed to ultrahigh dose rates from a laser-produced plasma x-ray source
- 1999
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Ingår i: Radiology. - 1527-1315. ; 213:3, s. 860-865
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: To determine whether intense laser-produced x rays have an increased radiation hazard. MATERIALS AND METHODS: Mammalian cells were exposed to x rays from a laser-produced plasma that produced ultrahigh peak absorbed dose rates, up to a factor of 10(10) higher than those produced by conventional x rays used in imaging. The cell survival was studied as a function of the absorbed dose. The survival of mammalian cells exposed to high peak absorbed dose rates with laser-produced x rays was compared with the survival of cells exposed to standard absorbed dose rates with conventional x-ray sources. Comparative survival studies were performed by using a conventional x-ray tube and a cobalt 60 source. The absorbed doses in the irradiation field were measured with thermoluminescent dosimeters. RESULTS: Cell survival following irradiation by filtered, laser-produced x rays with a high dose rate was not markedly different from the survival following irradiation by conventional sources. There was, however, a notable difference between the survival after exposure to filtered, laser-produced x rays and the survival after exposure to unfiltered laser-produced x rays. CONCLUSION: Exposure to filtered, laser-produced x rays with a high dose rate does not lead to increased harm to mammalian cells exposed in vitro compared with the harm from exposure to x rays from conventional sources, which indicates that the use of high-power laser facilities for medical imaging is justified
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| 529. |
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| 530. |
- Tylewicz, Urszula, et al.
(författare)
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Gas in Scattering Media Absorption Spectroscopy (GASMAS) Detected Persistent Vacuum in Apple Tissue After Vacuum Impregnation
- 2012
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Ingår i: Food Biophysics. - : Springer Science and Business Media LLC. - 1557-1866 .- 1557-1858. ; 7:1, s. 28-34
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure and the capillary pressure of the pore space are important variables for better understanding of the complex phenomena occurring during vacuum impregnation (VI) of plant tissues. In this study, we used GASMAS (Gas in Scattering Media Absorption Spectroscopy) of oxygen to, non-destructively, measure the dynamics of the internal pressure in apple pieces after restoration of the atmospheric pressure. Apple pieces were impregnated with isotonic sucrose solution (18% w/v) at different reduced pressures (15, 30, 45 kPa (abs.)). After restoration of the atmospheric pressure, the pressure of the remaining pore space gas could remain as low as 50 kPa (abs) and rise slowly toward ambient over a time scale of hours. Both the residual vacuum and the timescale of pressure equilibration with ambient varied with applied vacuum level and apple variety. It is proposed that at least a part of the pore space of apples may be hydrophobic, giving rise to a negative Laplace pressure, and thus the convective flow of impregnating solution is arrested at a mechanical equilibrium where internal pressure is lower than external pressure. Further pressure equilibration can then only be achieved either by gas diffusion in gas phase, or by gradual wetting of the pores.
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