322 research outputs found
Coplanar emission near the LHC energy range (observed with XREC in the stratosphere)
The alignment of very high energy secondary cosmic rays was observed at both stratospheric and mountain altitudes by several X-ray emulsion chamber experiments. Extensive simulation with CORSIKA demonstrates that such phenomena can be explained by fluctuations with standard physics. However, in the case of two events observed in the stratosphere, specific features contradicts such explanation. According to the properties of those events with a minimal cascading, we explore the hints of new physics which could explain the alignment in terms of relativistic strings and diquark breaking. One description of the consequent coplanar emission expected in colliders is proposed
Fluid and particle dynamics in laser powder bed fusion
In this work, we employ a combination of high-speed imaging and schlieren imaging, as well as multiphysics modelling, to elucidate the effects of the interaction between the laser beam and the powder bed. The formation of denuded areas where the powder was removed during single line and island scans over several layers were imaged for the first time. The inclination of the laser plume was shifted from forwards to backwards by changing power and scan speed, resulting in different denudation regimes with implications to the heat, mass and momentum transfer of the process. As the build progressed, denudation became less severe than for a single powder layer, but the occurrence of sintered and fused powder agglomerates, which were affected by the plume, increased. Schlieren imaging enabled the visualisation of the Ar gas flow, which takes place in the atmosphere above the bed due to the plume, in addition to its interaction with affected particles. Numerical modelling was used to understand and quantify the observed flow behaviour, through the hydrodynamic treatment of the laser plume as a multi-component Ar-Fe plasma. These results promote the characterisation of fluid dynamic phenomena during the laser powder-bed fusion (LPBF) process, which constitutes a key factor in the prevention of defects in additively manufactured parts
STUDIES IN PORPHYRIA
Porphyrin biosynthesis in mammalian skin and in skin obtained from patients with selected types of porphyria has been studied. Cutaneous porphyrinogenesis required the precursor δ-aminolevulinic acid (ALA) which, when added to murine, rat, and human skin in vitro, was rapidly converted to porphyrins. Total porphyrin content was quantitated by fluorescence assay, and spectral studies indicated that more than 80% of the porphyrin produced was protoporphyrin. The majority of skin porphyrinogenesis occurred in epidermis or in epidermal derivatives such as hair roots. Known inducers of hepatic δ-aminolevulinic acid synthetase (ALAS), the rate-limiting enzyme for heme biosynthesis, were not inducers when added to skin in vitro.Skin from patients with acute intermittent porphyria demonstrated a 43% decrease in cutaneous porphyrin production as compared to unaffected normals. This is consistent with the known deficiency of uroporphyrinogen synthetase that has been previously demonstrated in the liver and red blood cells of these patients. Porphyrinogenesis in skin of patients with porphyria cutanea tarda was not different from controls.These studies demonstrate that skin has the enzymatic capacity to synthesize porphyrins from added ALA and that cutaneous porphyrinogenesis from ALA is deficient in patients with acute intermittent porphyria
Diffractive Contribution to the Elasticity and the Nucleonic Flux in the Atmosphere
We calculate the average elasticity considering non-diffractive and single
diffractive interactions and perform an analysis of the cosmic-ray flux by
means of an analytical solution for the nucleonic diffusion equation. We show
that the diffractive contribution is important for the adequate description of
the nucleonic and hadronic fluxes in the atmosphere.Comment: 10 pages, latex, 2 figures (uuencoded PostScript
High energy hadrons in EAS at mountain altitude
An extensive simulation has been carried out to estimate the physical
interpretation of dynamical factors such as , in terms of high
energy interaction features, concentrated in the present analysis on the
average transverse momentum. It appears that the large enhancement observed for
versus primary energy, suggesting in earliest analysis a significant
rise of with energy, is only the result of the limited resolution of the
detectors and remains in agreement with a wide range of models used in
simulations.Comment: 13 pages, 6 PostScript figures, LaTeX Subm. to JPhys
Positron annihilation lifetime spectroscopy at a superconducting electron accelerator
The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear
accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA.
The electron beam is employed for production of several secondary beams including X-rays
from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after
moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation
lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials
science are performed in parallel. The adjustable repetition rate of the continuous-wave
electron beams allows matching of the pulse separation to the positron lifetime in the sample
under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to
perform depth resolved defect spectroscopy and porosity studies especially for thin films
Laser powder bed fusion at sub-atmospheric pressures
The perceived advantages of laser powder bed fusion (PBF) at reduced pressure include a more stable melt pool and reduced porosity. In this study, high-speed imaging was used to investigate the interaction of the laser beam with the powder bed at sub-atmospheric pressures. At atmospheric pressure, the laser plume produces a flow in the ambient atmosphere that entrains particles toward the melt pool. As the pressure decreases, this hydrodynamic entrainment increases but eventually the expansion of the laser plume prevents the particles reaching the melt pool: profiles and cross-sections of the track reveal a drastic reduction in its cross-sectional area. As the pressure decreases further, into the molecular flow regime, particles are only repelled by the plume away from the melt pool. The regime between 1 bar and ∼50 mbar (the threshold pressure at which the penetration depth no longer increases) could provide a window for successful processing but might require a pre-sinter to maintain the integrity of the powder bed. Lower pressures would definitely require a pre-sinter, for which the additional processing time and increase in process complexity might be justified for porosity-critical applications
In vitro study of the deployment performance of 3D printed stents in the diseased artery with the lipid arterial plaques
Atherosclerotic plaque is one of the arterial diseases which builds up in the arterial wall and can be identified by the composition of the plaque. Atherosclerosis causes the narrowing or occlusions of the arterial lumen leading to cardiovascular event. Percutaneous (keyhole) endovascular stenting has become the most common revascularisation method due to its minimum invasive nature and low complication rate. The stents, mostly fabricated by laser machines, have uniform geometries which are not ideal to treat the diseased arteries with lesion-specific properties. In addition, the effect of arterial plaque compositions on the performance of stents is not fully investigated. In this study, the deployment performance of the stents with the varied design, made of the 316L stainless steel and fabricated by additive manufacturing (AM) technology, were investigated. An in vitro experiment was conducted to test the influence of the atherosclerotic plaque compositions at 55% stenosis on the commercial and AM fabricated stents. Two artificial plaques (lipid and calcified) were prepared manually, and their mechanical testing were conducted using an unconfined compression test. Two types of stents, printed and commercial stents, were used to treat the diseased artificial artery, and the data of the pressure and diameter were collected simultaneously when the stent inflation pressure was applied. The results show that the mechanical property of the artificial lipid plaques was very similar to the real lipid plaque that observed from clinical study. From the deployment performance test for these two types of stents, it was observed that as the pressure inside of balloon increases, the diameters measured at the external wall of the artificial artery also increase when the pressure is above 4 atm. Overall, there is a close linear relationship between pressure and arterial wall movement with lipid plaque in both printed and commercial stents though that the stent made of AM technology is less flexible and has lower elastic property than the commercial one
Selective laser melting of AlSi10Mg:Influence of post-processing on the microstructural and tensile properties development
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