10,524 research outputs found
A new species of suckermouth catfish (Mochokidae: Chiloglanis) from the Rio Mongo in Equatorial Guinea
No full textSchmidt, Ray C., Barrientos, Christian (2019): A new species of suckermouth catfish (Mochokidae: Chiloglanis) from the Rio Mongo in Equatorial Guinea. Zootaxa 4652 (3): 507-519, DOI: 10.11646/zootaxa.4652.3.
Active X-ray optics for the next generation of X-ray space telescopes
No full textDescribed within is the design, manufacture, metrology and X-ray testing of an active X-ray
prototype intended for the next generation of X-ray telescopes. One of the challenges faced by
the X-ray telescope community is how to combine high resolution and high sensitivity into one
system, as weight limitations place constraints on the optics that can be launched. Therefore the
mandate of the active X-ray prototype is to provide high sensitivity through the ability of the optics
to be nested and to deliver high angular resolution through the active control of the optic’s form.
Piezoelectric unimorph actuators provide the active component: it is intended that they will correct
for figure errors within the optic and therefore increase the angular resolution capability.
The prototype’s design is based upon an ellipsoidal segment which provides point-to-point
focussing of an X-ray source. The prototype itself is composed of an electroformed nickel optic
where the non-reflective surface is populated with 30 piezoelectric actuators and it is the production
of the prototype that is the core of the presented research. Metrology of the actuators’ influence
functions is presented and highlight the prototype’s ability to deform its optic surface by microns.
In addition, the measured influence functions are compared against finite element models and a
distinct similarity between the functions is observed.
The prototype was tested at an X-ray beamline facility in November 2008 and the results
showed the prototype’s ability to correct the optic to achieve an improved angular resolution: from
0.786 arc-minutes to 0.686 arc-minutes in terms of full width half maximum. Finally, difficulties
in the manufacture of the prototype and X-ray testing shall be presented alongside future work in
conclusion to this thesis
Morphometric and molecular variation in mountain catfishes (Amphiliidae:<i>Amphilius</i>) in Guinea, West Africa
No full textFigure 5. Plot of principal component scores for the ventral analysis of 654 Amphilius rheophilus. Specimens examined: Senegal (X) n = 380, Rio Corubal (Δ) n = 140, Konkouré (+) n = 96, Niger (o) n = 10, Loffa (Ʊ) n = 5, Loh (Þ) n = 3, Gambie (v) n = 3, Kaba (q) n = 15, and syntypes (Ɨ) n = 2. Bold plots represent basins where tissue samples were collected. With deformation grids (exaggerated by two).Published as part of Schmidt, Ray C. & Pezold, Frank, 2011, Morphometric and molecular variation in mountain catfishes (Amphiliidae: Amphilius) in Guinea, West Africa, pp. 521-552 in Journal of Natural History 45 (9-10) on page 532, DOI: 10.1080/00222933.2010.534560, http://zenodo.org/record/520510
Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O–CaO–Al2O3–SiO2 glasses : a multinuclear NMR study
No full textThe incorporation mechanisms of fluorine (F) into peralkaline and peraluminous Na2O–CaO aluminosilicate glasses with ∼65 mol% SiO2 (model system for phonolites) were investigated by magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In 19F MAS NMR spectra of the fluorine-bearing peralkaline glasses at least five F sites could be distinguished, while only three of these sites could be found in the corresponding peraluminous glasses, which shows that there are more F incorporation mechanisms in peralkaline than in peraluminous glasses. In the peralkaline glasses containing up to 6.2 mol% F the following F environments were identified: F–Ca(n) at ∼−113 ppm, Si–F–Na(n) or Al–F–Ca(n) at ∼−146 ppm, Al–F–Al at ∼−168 ppm, Al–F–Na(n) at ∼−188 ppm and F–Na(n) at ∼−225 ppm (“n” indicates that the number of atoms is variable or uncertain). F–Ca(n) is the most abundant site which is surprising as Ca is the least common cation in the glasses. The fraction of F–Ca(n) sites increases from 42% to 53% as the F content increases from 1.2 to 6.2 mol%. The addition of up to 16.5 mol% (5.3 wt%) water strongly affects F speciation in peralkaline glasses and results in a decrease in the fraction of F–Al sites compared to F–Ca(n) sites. It seems that hydroxyl groups (OH) and F occupy similar Al environments and that F cannot compete with OH.
In the peraluminous glasses containing up to 18.3 mol% F only three F environments Si–F–Na(n) or Al–F–Ca(n) at ∼−149 ppm, Al–F–Al at ∼−170 ppm and Al–F–Na(n) at ∼−190 ppm are observed. Al–F–Na(n) is the most abundant site with a fraction of 54–61%. The F speciation also changes with the F concentration, with a minimum in Al–F–Na(n) sites between 3.5 and 9.7 mol% F.
Fluorine has only a small effect on the 23Na and 29Si MAS NMR spectra. 27Al MAS NMR spectra of the peralkaline glasses show only four-coordinated Al while in the peraluminous glasses ∼5% of the Al was found to be five-coordinated. The amount of five-coordinate Al does not change with increasing F content, but the environment of the five-coordinate Al becomes more symmetric with increasing F
Study of precipitation reactions by X-ray microscopy: CaCO3 precipitation and the effect of polycarboxylates
No full textA fundamental understanding of precipitation reactions is based on information about the structural evolution of the system on all length scales and starting at the beginning of the reaction. We introduce X-ray microscopy as an additional analytical tool in order to achieve this understanding. X-ray microscopy combines high resolution (down to 40 mo) and experiments at ambient pressure. The latter point allows the study of colloidal systems within the water phase and the performance of time-resolved microscopy. Taking CaCO3 precipitation as a model system, it is shown how further knowledge can be obtained on the dynamics of structure formation. We discuss the following points: formation and recrystallization of precursors; transient stabilization of CaCO3 nanoparticles by polycarboxylates; dependence of the structure evolution (stabilization or recrystallization) on the polymer concentration
X-ray polarization in relativistic jets
No full textWe investigate the polarization properties of Comptonized X-rays from relativistic jets in active galactic nuclei (AGN) using Monte Carlo simulations. We consider three scenarios commonly proposed for the observed X-ray emission in AGN: Compton scattering of blackbody photons emitted from an accretion disc; scattering of cosmic microwave background (CMB) photons and self-Comptonization of intrinsically polarized synchrotron photons emitted by jet electrons. Our simulations show that for Comptonization of disc and CMB photons, the degree of polarization of the scattered photons increases with the viewing inclination angle with respect to the jet axis. In both cases, the maximum linear polarization is ≈20 per cent. In the case of synchrotron self-Comptonization (SSC), we find that the resulting X-ray polarization depends strongly on the seed synchrotron photon injection site, with typical fractional polarizations P≈ 10–20 per cent when synchrotron emission is localized near the jet base, while P≈ 20–70 per cent for the case of uniform emission throughout the jet. These results indicate that X-ray polarimetry may be capable of providing unique clues to identify the location of particle acceleration sites in relativistic jets. In particular, if synchrotron photons are emitted quasi-uniformly throughout a jet, then the observed degree of X-ray polarization may be sufficiently different for each of the competing X-ray emission mechanisms (synchrotron, SSC or external Comptonization) to determine which is the dominant process. However, X-ray polarimetry alone is unlikely to be able to distinguish between disc and CMB Comptonization
Bis(arylimido) molybdenum(VI) amidinate and guanidinate complexes; Molecular structures of [(ArN)(2)MoMe{N(Cy)C[N(i-Pr)(2)]N(Cy)}] (Ar=2,6-i-Pr2C6H3; Cy = cyclohexyl) and [(2,6-i-Pr2C6H3N)(2)MoCl2]center dot[NH=C(C6H5)CH(SiMe3)(2)]
No full textThe reaction of [(ArN)(2)MoCl2]. DME (Ar = 2,6-i-Pr6C6H3) (1) with lithium amidinates or guanidinates resulted in molybdenum(VI) complexes [(ArN)(2)-MoCl(N(R-1)C(R-2)N(R-1))] (R-1 = Cy (cyclohexyl), R-2 = Me (2); R-1 = Cy, R-2 = N(i-Pr)(2) (3); R-1 = Cy, R-2 = N(SiMe3)(2) (4); R-1 = SiMe3, R-2 = C6H5 (5)) with five coordinated molybdenum atoms. Methylation of these compounds was exemplified by the reactions of 2 and 3 with MeLi affording the corresponding methylates [(ArN)(2)MoMe(N(R-1)C(R-2)N(R-1))] (R-1 = Cy, R-2 = Me (6); R-1 = Cy, R-2 = N(i-Pr)(2) (7)). The analogous reaction of 1 with bulky [N(SiMe3)C(C6H5)-C(SiMe3)(2)]Li . THF did not give the corresponding metathesis product, but a Schiff base adduct [(ArN)(2)MoCl2]. [NH=C(C6H5)CH(SiMe3)(2)] (8) in low yield. The molecular structures of 7 and 8 are established by the X-ray single crystal structural analysis
FIGURE 7. Distichodus mbiniensis, a in Cast netting new species: Integrative taxonomy of Distichodus notospilus (Characiformes: Distichodontidae) discovers new species and overlooked areas of endemism in Central Africa
No full textFIGURE 7. Distichodus mbiniensis, a new species, holotype, USNM 451317, ALC, 99.9 mm SL, voucher EqGui2017_0127, Equatorial Guinea, Wele-Nzas, Rio Ntoro at Aconibe—Acurenam Rd., 668 m elev, 1.29091° N, 10.9174° E; Photograph by S. Raredon.Published as part of Schmidt, Ray C., Knobloch, Elise C. & Barrientos, Christian, 2021, Cast netting new species: Integrative taxonomy of Distichodus notospilus (Characiformes: Distichodontidae) discovers new species and overlooked areas of endemism in Central Africa, pp. 291-313 in Zootaxa 4952 (2) on page 307, DOI: 10.11646/zootaxa.4952.2.5, http://zenodo.org/record/467407
FIGURE 1 in Cast netting new species: Integrative taxonomy of Distichodus notospilus (Characiformes: Distichodontidae) discovers new species and overlooked areas of endemism in Central Africa
No full textFIGURE 1. Localities of Distichodus notospilus populations included in the analysis. Holotype locality for Distichodus microps sp. nov. (triangle) and Distichodus mbiniensis sp. nov. (star), and localities of paratypes of D. mbiniensis (square). Localities of specimens included in the molecular analyses (closed circle) and all localities sampled during the 2017 expeditions (open circle). White line denote boundaries between drainages.Published as part of Schmidt, Ray C., Knobloch, Elise C. & Barrientos, Christian, 2021, Cast netting new species: Integrative taxonomy of Distichodus notospilus (Characiformes: Distichodontidae) discovers new species and overlooked areas of endemism in Central Africa, pp. 291-313 in Zootaxa 4952 (2) on page 296, DOI: 10.11646/zootaxa.4952.2.5, http://zenodo.org/record/467407
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