P.M. Gordo

Free-volume studies in polyurethane membranes by positron annihilation spectroscopy

Abstract The free-volume parameters in various urethane/urea membranes obtained by varying the ratio of the structural constituents, polypropylene oxide and polybutadiene, were studied by positron lifetime and Doppler broadening measurements. On bi-soft segment membranes, a correlation was found between the composition of membranes, the normalized free volume, the radii of the holes and gas permeability. However, the correlation is not clear when PU data are also considered, indicating that other features must also play an important role in the permeation mechanism.

Positron lifetime studies in vinyl polymers of medical importance

Abstract Vinyl polymers used as artificial lens implants in ophthalmology were investigated by positron lifetime spectroscopy. The structure of these polymers with free volumes offers the possibility of charging them with anti-inflammatory drugs for sustained release. A correlation between the amount of normalised free volume and the ratio of the methyl methacrylate to ethyl-hexyl-acrylate, used as polymerisation monomers, was found.

Influence of the a-Si:H structural defects studied by positron annihilation on the solar cells characteristics

The influence of the power density, P , on the density and structure of defects of undoped a-Si:H thin films, deposited by r.f.- D PECVD, is studied by the constant photocurrent method, (CPM), and by slow positron beam spectroscopy, respectively.Deep defect density, N , remains approximately constant at 10 cm , typical of device quality material, for P in the range 7-20 16 y3 DD D mWOcm , as calculated from CPM.Out of this range, N increases roughly one order of magnitude for both lower and higher y3 DD

Correlation between a-Si:H surface oxidation process and the performance of MIS structures

In order to correlate the MIS devices performance with different surface oxidation methods, AFM, spectroscopic ellipsometry and infrared spectroscopy measurements were performed in a-Si:H films, before and after surface oxidation, using different Ž.Ž . Ž oxidation techniques and oxides: thermal dry in air , wet in H O and by oxygen plasma, while MIS metal-insulator-semicon22 . ductor devices were characterised by IV curves, under dark and AM1.5 illumination conditions. The a-Si:H films were grown by the PECVD technique, in a modified triode configuration reactor to allow a precise control of the ion bombardment during the film deposition. We found that the growth of a thin layer of oxide by chemical processes on the top of the a-Si:H surface can cause changes on the surface morphology that are reflected in the electrical behaviour of the devices. The oxygen plasma treatment, cause the rearrangement of the surface atoms leading to a change of their morphology and to the improvement of the electrical properties of the surface for a MIS applications. 2001 Elsevier Science B.V. All rights reserved. Ž.

Gas Permeability and Temperature-Dependent Free-Volume Studies in Polyurethane Membranes by Positron Lifetime and Doppler Spectroscopies

Positron annihilation lifetime and Doppler broadening spectroscopies were used to study the free volume parameters in polypropylene oxide/polybutadiene bi-soft segment urea/urethane membranes (PU/PBDO), with PBDO content varying from 0 up to 75 wt % in the temperature range 298–324 K. The gas permeation features appear to be correlated with the free volume sizes determined by the lifetime measurements whereas phase separation of the various soft and hard segments in the membranes is mirrored by both the lifetime and Doppler results.

Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

This is the author accepted manuscript. the final version is available from EDP Sciences via the DOI in this record

Towards very high resolution RPC-PET for small animals

We present imaging results of needle-like and planar 22Na sources obtained with a prototype of a high-acceptance small-animal positron emission tomograph based on resistive plate chambers (RPC-PET). The maximum-likelihood expectation-maximization (MLEM) reconstruction of the acquired data yielded an excellent and stable resolution of 0.4 mm FWHM.

Methods for multiple-telescope beam imaging and guiding in the near-infrared

Atmospheric turbulence and precise measurement of the astrometric baseline vector between any two telescopes are two major challenges in implementing phase-referenced interferometric astrometry and imaging. They limit the performance of a fibre-fed interferometer by degrading the instrument sensitivity and the precision of astrometric measurements and by introducing image reconstruction errors due to inaccurate phases. A multiple-beam acquisition and guiding camera was built to meet these challenges for a recently commissioned four-beam combiner instrument, GRAVITY, at the European Southern Observatory Very Large Telescope Interferometer. For each telescope beam, it measures (a) field tip-tilts by imaging stars in the sky, (b) telescope pupil shifts by imaging pupil reference laser beacons installed on each telescope using a 2x2 lenslet and (c) higher-order aberrations using a 9x9 Shack-Hartmann. The telescope pupils are imaged to provide visual monitoring while observing. These measurements enable active field and pupil guiding by actuating a train of tip-tilt mirrors placed in the pupil and field planes, respectively. The Shack-Hartmann measured quasi-static aberrations are used to focus the auxiliary telescopes and allow the possibility of correcting the non-common path errors between the adaptive optics systems of the unit telescopes and GRAVITY. The guiding stabilizes the light injection into single-mode fibres, increasing sensitivity and reducing the astrometric and image reconstruction errors. The beam guiding enables us to achieve an astrometric error of less than 50 mu as. Here, we report on the data reduction methods and laboratory tests of the multiple-beam acquisition and guiding camera and its performance on-sky.

The GRAVITY/VLTI acquisition camera software

The acquisition camera for the GRAVITY/VLTI instrument implements four functions: a) field imager: science field imaging, tip-tilt; b) pupil tracker: telescope pupil lateral and longitudinal positions; c) pupil imager: telescope pupil imaging and d) aberration sensor: The VLTI beam higher order aberrations measurement. We present the dedicated algorithms that simulate the GRAVITY acquisition camera detector measurements considering the realistic imaging conditions, complemented by the pipeline used to extract the data. The data reduction procedure was tested with real aberrations at the VLTI lab and reconstructed back accurately. The acquisition camera software undertakes the measurements simultaneously for all four AT/UTs in 1 s. The measured parameters are updated in the instrument online database. The data reduction software uses the ESO Common Library for Image Processing (CLIP), integrated in to the ESO VLT software environment.

Integration and testing of the GRAVITY infrared camera for multiple telescope optical beam analysis

The GRAVITY Acquisition Camera was designed to monitor and evaluate the optical beam properties of the four ESO/VLT telescopes simultaneously. The data is used as part of the GRAVITY beam stabilization strategy. Internally the Acquisition Camera has four channels each with: several relay mirrors, imaging lens, H-band filter, a single custom made silica bulk optics (i.e. Beam Analyzer) and an IR detector (HAWAII2-RG). The camera operates in vacuum with operational temperature of: 240k for the folding optics and enclosure, 100K for the Beam Analyzer optics and 80K for the detector. The beam analysis is carried out by the Beam Analyzer, which is a compact assembly of fused silica prisms and lenses that are glued together into a single optical block. The beam analyzer handles the four telescope beams and splits the light from the field mode into the pupil imager, the aberration sensor and the pupil tracker modes. The complex optical alignment and focusing was carried out first at room temperature with visible light, using an optical theodolite/alignment telescope, cross hairs, beam splitter mirrors and optical path compensator. The alignment was validated at cryogenic temperatures. High Strehl ratios were achieved at the first cooldown. In the paper we present the Acquisition Camera as manufactured, focusing key sub-systems and key technical challenges, the room temperature (with visible light) alignment and first IR images acquired in cryogenic operation.