8,666 research outputs found

    Structural Studies of Nonionic Dodecanol Ethoxylates at the Oil–Water Interface: Effect of Increasing Head Group Size

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    The conformation of charged surfactants at the oil-water interface was recently reported. With the aim to assess the role of the head group size on the conformation of the adsorbed layer, we have extended these studies to a series of non-ionic dodecanol ethoxylate surfactants (C12En, ethylene oxide units n from 6 to 12). The study was performed using neutron reflectometry to enable maximum sensitivity to buried interfaces. Similarly to charged surfactants, the interface was found to be broader and rougher compared to the air-water interface. Irrespectively of the head group size, the tail group region was found to assume a staggered conformation. The conformations of the head group were found to be significantly different compared to the air-water interface, moving from a globular to an almost fully extended conformation at the oil-water interface. The stretching of the head groups is attributed to the presence of some hexadecane oil molecules, which may penetrate all the way to this region. It is proposed here that the presence of the oil, which can efficiently solvate the surfactant tail groups, plays a key role in the conformation of the adsorbed layer and is responsible for the broadening of the interface

    Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field

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    Starting from the observation that kilohertz Quasi Period Oscillations (kHz QPO) occur in a very narrow range of X-ray luminosities in neutron star low mass X-ray binaries, we try to link the kHz QPO observability to variations of the neutron star magnetospheric radius, in response to changing mass inflow rate. At low luminosities, the drop off of kHz QPOs activity may be explained by the onset of the centrifugal barrier, when the magnetospheric radius reaches the corotation radius. At the opposite side, at higher luminosities, the magnetospheric radius may reach the neutron star and the vanishing of the magnetosphere may led to the stopping of the kHz QPOs activity. If we apply these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1) and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits naturally apply in the framework of beat frequency models but can also work in the case of general relativistic models.Comment: 4 pages (emulateapj macro), 1 figure. Accepted for publication in ApJ Letter

    The quiescent X-ray emission of three transient X-ray pulsars

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    We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U 0115+63 and V 0332+53. These observations allowed us to study these transients at the faintest flux levels thus far. Spectra are remarkably different from the ones obtained at luminosities a factor >10 higher, testifying that the quiescent emission mechanism is different. Pulsations were not detected in any of the sources, indicating that accretion of matter down to the neutron star surface has ceased. We conclude that the quiescent emission of the three X-ray transients likely originates from accretion onto the magnetospheric boundary in the propeller regime and/or from deep crustal heating resulting from pycnonuclear reactions during the outbursts.Comment: Accepted for publication on ApJ (5 pages and 2 figures

    Characterization of the lutetium-yttrium orthosilicate scintillating crystals for the CMS experiment

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    The Compact Muon Solenoid (CMS) detector is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High-Luminosity LHC starting in 2027. In particular, a new timing detector, the Mip Timing Detector (MTD) will measure minimum ionizing particles with a time resolution of 30–50 ps. The technology selected for the central part of the detector, the Barrel Timing Layer, consists of scintillating crystals of lutetium yttrium orthosilicate doped with cerium (LYSO:Ce) read out with Silicon Photo-Multipliers. A study of the performances of LYSO samples from different producers has been performed last year in Rome; in this report the experimental setup is described and the optical measurements are presented
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