Analytical Methods for Discriminating Stardust in Aerogel Capture Media

Comet 81P/Wild 2's serendipitous orbit change to the inner solar system in 1974 offered researchers a rare opportunity to sample cometary material from the Kuiper belt, a repository of material left over from solar system formation {approx}4.6 Gyr ago. NASA's Stardust mission intercepted the comet in January 2004 and returned with material collected from its tail in January 2006. The cometary material, consisting of particles ranging from 10 microns down to <2 nm, was collected in aerogel, a very low density ({approx}3 mg/cm cm3) silica foam, to minimize the effects of deceleration from 6.1 km/s. The entire deceleration track is extracted from the aerogel block as a pyramidal shape known as a keystone which can be mapped using x-ray fluorescence prior to extraction of terminal or intermediate particles for other analyses. One goal of the track mapping is to determine the bulk composition of the cometary material returned. Unfortunately, although the aerogel is predominantly SiO{sub 2}, there are sufficient quantities of trace elements similar to those expected in the cometary material to require sophisticated discrimination techniques in order to decide whether a fluorescence map pixel contains only aerogel or both aerogel and cometary material. We have developed a dual threshold analysis approach for better distinguishing cometary material from aerogel contaminants and have applied it to five Stardust impact tracks and terminal particles. Here, we present aspects of the dual threshold approach and demonstrate its impact on track composition for one track

First X-ray Fluorescence MicroCT Results from Micrometeorites at SSRL

X-ray fluorescence microCT (computed tomography) is a novel technique that allows non-destructive determination of the 3D distribution of chemical elements inside a sample. This is especially important in samples for which sectioning is undesirable either due to the risk of contamination or the requirement for further analysis by different characterization techniques. Developments made by third generation synchrotron facilities and laboratory X-ray focusing systems have made these kinds of measurements more attractive by significantly reducing scan times and beam size. First results from the x-ray fluorescence microCT experiments performed at SSRL beamline 6-2 are reported here. Beamline 6-2 is a 54 pole wiggler that uses a two mirror optical system for focusing the x-rays onto a virtual source slit which is then reimaged with a set of KB mirrors to a (2 x 4) {micro}{sup 2} beam spot. An energy dispersive fluorescence detector is located in plane at 90 degrees to the incident beam to reduce the scattering contribution. A PIN diode located behind the sample simultaneously measures the x-ray attenuation in the sample. Several porous micrometeorite samples were measured and the reconstructed element density distribution including self-absorption correction is presented. Ultimately, this system will be used to analyze particles from the coma of comet Wild-2 and fresh interstellar dust particles both of which were collected during the NASA Stardust mission

Development and validation of novel clinical endpoints in intermediate age-related macular degeneration in MACUSTAR

Background Currently, no validated clinical endpoints for treatment studies exist for intermediate age-related macular degeneration (iAMD). Objective The European MACUSTAR study aims to develop and clinically validate adequate clinical endpoints for future treatment studies in iAMD and to identify early determinants of disease progression to late stage AMD. Material and methods The MACUSTAR study protocol was developed by an international consortium of researchers from academia, the pharmaceutical industry and medical device companies. The MACUSTAR project is funded by the Innovative Medicines Initiative 2 (IMI2) of the European Union. Results The MACUSTAR study consists of a cross-sectional and a longitudinal investigation. A total of 750 subjects with early, intermediate and late AMD as well as control subjects with no signs of AMD will be included with a follow-up period of 3 years. Overall, 20 European study centers are involved. Conclusion The MACUSTAR project will generate large high-quality datasets, which will allow clinical validation of novel endpoints for future interventional trials in iAMD. The aim is that these endpoints will be accepted as suitable for medication approval studies by the regulatory authorities and that understanding of the disease process will be improved

Soft X-ray spectroscopy of porous silicon and potassium halides

We showed that soft X-ray absorption and emission spectroscopy are excellent tools to study the electronic structure of broad band materials. In SXA and SXE the local partial density of states of the unoccupied conduction band and the occupied valence band are mapped, respectively. We applied the methods to porous silicon, a material of enormous interest: for basic science it is of interest because quantum confinement effects can easily be studied, for industry purposes it is the potential of developing optoelectronic materials on the base of silicon that explains the world wide interest at pSi. In the second part of this diploma thesis we applied SXA and SXE spectroscopy to narrow band materials and studied the excitation dynamics and correlation effects. We used core-to-core emission transitions to study the K 3s"-"13d configuration in KF and KI that cannot be reached in absorption processes alone due to the dipole selection rules. We found that the 3d electron is excitonically bound to the 3s core hole and determined the binding energies of the various excitonic states, the exchange splittings, and the crystal field splittings. The importance of the spin-orbit interaction in the 2p"-"13d states manifests itself in the intensity ratio of transitions into final singlet states vs. transitions into final triplet states, that cannot be reached in purely LS-coupled systems. (orig.)96 refs.SIGLEAvailable from TIB Hannover: RA 831(3064) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Soft x-ray spectroscopy of broad band, size confined, and correlated materials

The following topics were dealt with: soft X-ray spectroscopy, wide band gap semiconductors, electronic structure, atomic clusters, nanocrystals, X-ray photoemission, X-ray scattering, polymorphism, crystal structure, electronic density of states203 refs.Available from TIB Hannover: RA 831(3544) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Study of the L-2,L-3 edges of 3d transition metals by X-ray absorption spectroscopy

WOS: 000261693900103In the soft X-rays energy region, near edge X-ray absorption fine structure (NEXAFS) spectra are generally recorded by monitoring yield signals of secondary particles. These secondary particles, electrons or fluorescence photons, follow from the decay of the core hole excited in the primary absorption process. In general the yield signals are, to a good approximation, proportional to the absorption coefficient. However, in several applications it would be desirable to measure the absorption coefficient quantitatively. To derive the absorption coefficient quantitatively from a yield spectrum, one needs to know the escape depth of the emitted electrons (Ne) of the applied yield technique in the material of interest. Since this quantity is difficult to calculate, it is unknown for most materials. In this paper we present the first results of our systematic investigation of the total electron-yield (TEY) escape depth of the 3d transition metals (Fe, Co and Cu). In addition our results gave important information on the variation of the TEY escape depth with the filling of the 3d band. (C) 2008 Elsevier B.V. All rights reserved.Department of Energy, Office of Basic Energy Science; SSRL University of Cukurova; DOE Cooperative Research ProgramThe authors express their thanks to Curtis Troxel for his support at the Stanford Synchrotron Radiation Laboratory (SSRL), where the NEXAFS experiments have been carried out. SSRL is supported by the Department of Energy, Office of Basic Energy Science. Support by SSRL University of Cukurova and the DOE Cooperative Research Program for SESAME project is acknowledged by Y.U

Recovering the elemental composition of comet Wild 2 dust in five Stardust impact tracks and terminal particles in aerogel

The elemental (non-volatile) composition of five Stardust impact tracks and terminal particles left from capture of comet 81P/Wild 2 dust were mapped in a synchrotron X-ray scanning microprobe with full fluorescence spectra at each pixel. Because aerogel includes background levels of several elements of interest, we employ a novel "dual threshold" approach to discriminate against background contaminants: an upper threshold, above which a spectrum contains cometary material plus aerogel and a lower threshold below which it contains only aerogel. The difference between normalized cometary-plus-background and background-only spectra is attributable to cometary material. The few spectra in-between are discarded since misallocation is detrimental: cometary material incorrectly placed in the background spectrum is later subtracted from the cometary spectrum, doubling the loss of reportable cometary material. This approach improves accuracy of composition quantification. We present the refined whole impact track and terminal particle elemental abundances for the five impact tracks. One track shows mass increases in Cr and Mn (1.4x), Cu, As and K (2x), Zn (4x), and total mass (13%) by dual thresholds compared to a single threshold. Major elements Fe and Ni are not significantly affected. The additional Cr arises from cometary material containing little Fe. We exclude Au intermixed with cometary material because it is found to be a localized surface contaminant carried by comet dust into an impact track. The dual threshold technique can be used in other situations where elements of interest in a small sample embedded in a matrix are also present in the matrix itself.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Recovering the elemental composition of comet Wild 2 dust in five Stardust impact tracks and terminal particles in aerogel

The elemental (non-volatile) composition of five Stardust impact tracks and terminal particles left from capture of comet 81P/Wild 2 dust were mapped in a synchrotron X-ray scanning microprobe with full fluorescence spectra at each pixel. Because aerogel includes background levels of several elements of interest, we employ a novel "dual threshold" approach to discriminate against background contaminants: an upper threshold, above which a spectrum contains cometary material plus aerogel and a lower threshold below which it contains only aerogel. The difference between normalized cometary-plus-background and background-only spectra is attributable to cometary material. The few spectra in-between are discarded since misallocation is detrimental: cometary material incorrectly placed in the background spectrum is later subtracted from the cometary spectrum, doubling the loss of reportable cometary material. This approach improves accuracy of composition quantification. We present the refined whole impact track and terminal particle elemental abundances for the five impact tracks. One track shows mass increases in Cr and Mn (1.4x), Cu, As and K (2x), Zn (4x), and total mass (13%) by dual thresholds compared to a single threshold. Major elements Fe and Ni are not significantly affected. The additional Cr arises from cometary material containing little Fe. We exclude Au intermixed with cometary material because it is found to be a localized surface contaminant carried by comet dust into an impact track. The dual threshold technique can be used in other situations where elements of interest in a small sample embedded in a matrix are also present in the matrix itself.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202