112,564 research outputs found
Neutral particle Mass Spectrometry with Nanomechanical Systems
Current approaches to Mass Spectrometry (MS) require ionization of the
analytes of interest. For high-mass species, the resulting charge state
distribution can be complex and difficult to interpret correctly. In this
article, using a setup comprising both conventional time-of-flight MS (TOF-MS)
and Nano-Electro-Mechanical-Systems-based MS (NEMS-MS) in situ, we show
directly that NEMS-MS analysis is insensitive to charge state: the spectrum
consists of a single peak whatever the species charge state, making it
significantly clearer than existing MS analysis. In subsequent tests, all
charged particles are electrostatically removed from the beam, and unlike
TOF-MS, NEMS-MS can still measure masses. This demonstrates the possibility to
measure mass spectra for neutral particles. Thus, it is possible to envisage
MS-based studies of analytes that are incompatible with current ionization
techniques and the way is now open for the development of cutting edge system
architectures with unique analytical capability
Precision Measurements of the W-Boson Mass
The Standard Model of electroweak interactions has had great success in
describing the observed data over the last three decades. The precision of
experimental measurements affords tests of the Standard Model at the quantum
loop level beyond leading order. Despite this great success it is important to
continue confronting experimental measurements with the Standard Model
predictions as any deviation would signal new physics. As a fundamental
parameter of the Standard Model, the mass of the W-boson, M_W, is of particular
importance. Aside from being an important test of the SM itself, a precision
measurement of M_W can be used to constrain the mass of the Higgs boson, M_H.
In this article we review the principal experimental techniques for determining
M_W and discuss their combination into a single precision M_W measurement,
which is then used to yield constraints on M_H. We conclude by briefly
discussing future prospects for precision measurements of the W-boson mass.Comment: 37 pages, 13 figures, LaTex, to be published in volume 50 of Annual
Review of Nuclear and Particle Scienc
Nuclear effects in atomic transitions
Atomic electrons are sensitive to the properties of the nucleus they are
bound to, such as nuclear mass, charge distribution, spin, magnetization
distribution, or even excited level scheme. These nuclear parameters are
reflected in the atomic transition energies. A very precise determination of
atomic spectra may thus reveal information about the nucleus, otherwise hardly
accessible via nuclear physics experiments. This work reviews theoretical and
experimental aspects of the nuclear effects that can be identified in atomic
structure data. An introduction to the theory of isotope shifts and hyperfine
splitting of atomic spectra is given, together with an overview of the typical
experimental techniques used in high-precision atomic spectroscopy. More exotic
effects at the borderline between atomic and nuclear physics, such as parity
violation in atomic transitions due to the weak interaction, or nuclear
polarization and nuclear excitation by electron capture, are also addressed.Comment: review article, 53 pages, 14 figure
Interpretation of transverse tune spectra in a heavy-ion synchrotron at high intensities
Two different tune measurement systems have been installed in the GSI
heavy-ion synchrotron SIS-18. Tune spectra are obtained with high accuracy
using these fast and sensitive systems. Besides the machine tune, the spectra
contain information about the intensity dependent coherent tune shift and the
incoherent space charge tune shift. The space charge tune shift is derived from
a fit of the observed shifted positions of the synchrotron satellites to an
analytic expression for the head-tail eigenmodes with space charge.
Furthermore, the chromaticity is extracted from the measured head-tail mode
structure. The results of the measurements provide experimental evidence of the
importance of space charge effects and head-tail modes for the interpretation
of transverse beam signals at high intensity
Decay-assisted collinear resonance ionization spectroscopy: Application to neutron-deficient francium
This paper reports on the hyperfine-structure and radioactive-decay studies
of the neutron-deficient francium isotopes Fr performed with the
Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the ISOLDE
facility, CERN. The high resolution innate to collinear laser spectroscopy is
combined with the high efficiency of ion detection to provide a
highly-sensitive technique to probe the hyperfine structure of exotic isotopes.
The technique of decay-assisted laser spectroscopy is presented, whereby the
isomeric ion beam is deflected to a decay spectroscopy station for alpha-decay
tagging of the hyperfine components. Here, we present the first
hyperfine-structure measurements of the neutron-deficient francium isotopes
Fr, in addition to the identification of the low-lying states of
Fr performed at the CRIS experiment.Comment: Accepted for publication with Physical Review
Ultrafast generation and decay of a surface metal
Band bending at semiconductor surfaces induced by chemical doping or electric
fields can create metallic surfaces with properties not found in the bulk, such
as high electron mobility, magnetism or superconductivity. Optical generation
of such metallic surfaces via BB on ultrafast timescales would facilitate a
drastic manipulation of the conduction, magnetic and optical properties of
semiconductors for high-speed electronics. Here, we demonstrate the ultrafast
generation of a metal at the (10-10) surface of ZnO upon photoexcitation.
Compared to hitherto known ultrafast photoinduced semiconductor-to-metal
transitions that occur in the bulk of inorganic semiconductors, the
metallization of the ZnO surface is launched by 3-4 orders of magnitude lower
photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, we
show that the phase transition is caused by photoinduced downward surface band
bending due to photodepletion of donor-type deep surface defects. At low photon
flux, surface-confined excitons are formed. Above a critical exciton density, a
Mott transition occurs, leading to a partially filled metallic band below the
equilibrium Fermi energy. This process is in analogy to chemical doping of
semiconductor surfaces. The discovered mechanism is not material-specific and
presents a general route for controlling metallicity confined to semiconductor
interfaces on ultrafast timescales
All-particle cosmic ray energy spectrum measured with 26 IceTop stations
We report on a measurement of the cosmic ray energy spectrum with the IceTop
air shower array, the surface component of the IceCube Neutrino Observatory at
the South Pole. The data used in this analysis were taken between June and
October, 2007, with 26 surface stations operational at that time, corresponding
to about one third of the final array. The fiducial area used in this analysis
was 0.122 km^2. The analysis investigated the energy spectrum from 1 to 100 PeV
measured for three different zenith angle ranges between 0{\deg} and 46{\deg}.
Because of the isotropy of cosmic rays in this energy range the spectra from
all zenith angle intervals have to agree. The cosmic-ray energy spectrum was
determined under different assumptions on the primary mass composition. Good
agreement of spectra in the three zenith angle ranges was found for the
assumption of pure proton and a simple two-component model. For zenith angles
{\theta} < 30{\deg}, where the mass dependence is smallest, the knee in the
cosmic ray energy spectrum was observed between 3.5 and 4.32 PeV, depending on
composition assumption. Spectral indices above the knee range from -3.08 to
-3.11 depending on primary mass composition assumption. Moreover, an indication
of a flattening of the spectrum above 22 PeV were observed.Comment: 38 pages, 17 figure
Relative affinity constants by electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry: calmodulin binding to peptide analogs of myosin light chain kinase
Synthetic RS20 peptide and a set of its point-mutated peptide analogs have been used to analyze the interactions between calmodulin (CaM) and the CaM-binding sequence of smooth-muscle myosin light chain kinase both in the presence and the absence of Ca2+. Particular peptides, which were expected to have different binding strengths, were chosen to address the effects of electrostatic and bulky mutations on the binding affinity of the RS20 sequence. Relative affinity constants for protein/ligand interactions have been determined using electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry. The results evidence the importance of electrostatic forces in interactions between CaM and targets, particularly in the presence of Ca2+, and the role of hydrophobic forces in contributing additional stability to the complexes both in the presence and the absence of Ca2+
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