6,246 research outputs found
SPIRAL 2 coupler preparation and RF conditioning
Proc. On LineInternational audienceFive radiofrequency coupler prototypes have been manufactured. Three of them will be mounted in the cryomodules of the SPIRAL 2 superconducting LINAC (LINear ACcelerator). This paper describes the coupler preparation and the first results of their conditioning
A Study of the Formation of Single- and Double-Walled Carbon Nanotubes by a CVD Method
The reduction in H2/CH4 atmosphere of aluminum-iron oxides produces metal particles small enough to catalyze the formation of single-walled carbon nanotubes. Several experiments have been made using the same temperature profile and changing only the maximum temperature (800-1070 °C). Characterizations of the catalyst materials are performed using notably 57Fe Mo¨ssbauer spectroscopy. Electron microscopy and a macroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe3+, R-Fe, ç-Fe-C, Fe3C) is correlated to their location in the material. The nature of the particles responsible for the high-temperature formation of the nanotubes is probably an Fe-C alloy which is, however, found as Fe3C by postreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors the formation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes are mostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formation mechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the first one but that subsequent ones are formed outside. It is also possible that under given experimental conditions, the smallest (<2 nm) catalyst particles preferentially produce double-walled rather than single-walled carbon nanotubes
Pressure-induced amorphization and polyamorphism in one-dimensional single crystal TiO2 nanomaterials
The structural phase transitions of single crystal TiO2-B nanoribbons were
investigated in-situ at high-pressure using the synchrotron X-ray diffraction
and the Raman scattering. Our results have shown a pressure-induced
amorphization (PIA) occurred in TiO2-B nanoribbons upon compression, resulting
in a high density amorphous (HDA) form related to the baddeleyite structure.
Upon decompression, the HDA form transforms to a low density amorphous (LDA)
form while the samples still maintain their pristine nanoribbon shape. HRTEM
imaging reveals that the LDA phase has an {\alpha}-PbO2 structure with short
range order. We propose a homogeneous nucleation mechanism to explain the
pressure-induced amorphous phase transitions in the TiO2-B nanoribbons. Our
study demonstrates for the first time that PIA and polyamorphism occurred in
the one-dimensional (1D) TiO2 nanomaterials and provides a new method for
preparing 1D amorphous nanomaterials from crystalline nanomaterials.Comment: 4 figure
Blunted cardiovascular responses to acute psychological stress predict low behavioral but not self-reported perseverance
Emerging evidence relates attenuated physiological stress reactions to poor behavioral regulation. However, only a small number of behaviors such as impulsivity and risk taking have been explored. Nevertheless, one opportunistic study suggested that blunted reactivity might relate to poor perseverance. The present study examined the relationship between cardiovascular reactivity to acute active psychological stress and self‐reported and behavioral perseverance. Participants (N = 64) completed a self‐report perseverance questionnaire before heart rate (HR) and blood pressure (BP) were measured at rest and in response to 4‐min active (paced auditory serial addition; PASAT) and passive (cold pressor) stress tests. This was followed by an unsolvable Euler puzzle tracing task, with the time spent and number of attempts endeavoring to solve the puzzle recorded as behavioral perseverance measures. Blunted systolic and diastolic BP reactivity to the PASAT was associated with fewer attempts at the impossible puzzle, and lower diastolic BP PASAT reactivity related to less time persevering at the puzzle. Moreover, attenuated diastolic BP and HR PASAT reactivity predicted poorer perseverance at keeping one's hand in the iced water of the cold pressor task. There was no association between reactivity and self‐reported perseverance. These preliminary findings add to the evidence that implicates blunted reactivity as a physiological marker of poor behavioral regulation, and this may indicate why individuals with blunted reactivity are at increased risk of developing negative health outcomes (e.g., obesity and addictions)
New pixelized Micromegas detector with low discharge rate for the COMPASS experiment
New Micromegas (Micro-mesh gaseous detectors) are being developed in view of
the future physics projects planned by the COMPASS collaboration at CERN.
Several major upgrades compared to present detectors are being studied:
detectors standing five times higher luminosity with hadron beams, detection of
beam particles (flux up to a few hundred of kHz/mm^{2}, 10 times larger than
for the present Micromegas detectors) with pixelized read-out in the central
part, light and integrated electronics, and improved robustness. Two solutions
of reduction of discharge impact have been studied, with Micromegas detectors
using resistive layers and using an additional GEM foil. Performance of such
detectors has also been measured. A large size prototypes with nominal active
area and pixelized read-out has been produced and installed at COMPASS in 2010.
In 2011 prototypes featuring an additional GEM foil, as well as an resistive
prototype, are installed at COMPASS and preliminary results from those
detectors presented very good performance. We present here the project and
report on its status, in particular the performance of large size prototypes
with an additional GEM foil.Comment: 11 pages, 5 figures, proceedings to the Micro-Pattern Gaseous
Detectors conference (MPGD2011), 29-31 August 2011, Kobe, Japa
Measurement of Strange Quark Contributions to the Nucleon's Form Factors at Q^2=0.230 (GeV/c)^2
We report on a measurement of the parity-violating asymmetry in the
scattering of longitudinally polarized electrons on unpolarized protons at a
of 0.230 (GeV/c)^2 and a scattering angle of \theta_e = 30^o - 40^o.
Using a large acceptance fast PbF_2 calorimeter with a solid angle of
\Delta\Omega = 0.62 sr the A4 experiment is the first parity violation
experiment to count individual scattering events. The measured asymmetry is
A_{phys} =(-5.44 +- 0.54_{stat} +- 0.27_{\rm sys}) 10^{-6}. The Standard Model
expectation assuming no strangeness contributions to the vector form factors is
. The difference is a direct measurement of the
strangeness contribution to the vector form factors of the proton. The
extracted value is G^s_E + 0.225 G^s_M = 0.039 +- 0.034 or F^s_1 + 0.130 F^s_2
= 0.032 +- 0.028.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Letters on Dec 11, 200
Femtometer Toroidal Structures in Nuclei
The two-nucleon density distributions in states with isospin , spin
=1 and projection =0 and 1 are studied in H, He,
Li and O. The equidensity surfaces for =0 distributions are
found to be toroidal in shape, while those of =1 have dumbbell shapes
at large density. The dumbbell shapes are generated by rotating tori. The
toroidal shapes indicate that the tensor correlations have near maximal
strength at fm in all these nuclei. They provide new insights and simple
explanations of the structure and electromagnetic form factors of the deuteron,
the quasi-deuteron model, and the , and =2 (-wave)
components in He, He and Li. The toroidal distribution has a
maximum-density diameter of 1 fm and a half-maximum density thickness of
0.9 fm. Many realistic models of nuclear forces predict these values,
which are supported by the observed electromagnetic form factors of the
deuteron, and also predicted by classical Skyrme effective Lagrangians, related
to QCD in the limit of infinite colors. Due to the rather small size of this
structure, it could have a revealing relation to certain aspects of QCD.Comment: 35 pages in REVTeX, 25 PostScript figure
Modulation of the nucleation rate pre-exponential in a low-temperature Ising system
A metastable lattice gas with nearest-neighbor interactions and
continuous-time dynamics is studied using a generalized Becker-Doring approach
in the multidimensional space of cluster configurations. The pre-exponential of
the metastable state lifetime (inverse of nucleation rate) is found to exhibit
distinct peaks at integer values of the inverse supersaturation. Peaks are
unobservable (infinitely narrow) in the strict limit T->0, but become
detectable and eventually dominate at higher temperatures.Comment: 4 pages, 2 Postscript figures, LaTeX, submitted to Phys. Rev. Lett.
Changes: updated references, re-written section around eqs.(5),(6), typos,
minor wording changes in conclusion and other parts of text (mostly in
response to referees' comments). Paper resubmitted to PR
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