1,106 research outputs found
Dilute ferrimagnetic semiconductors in Fe-substituted spinel ZnGaO
Solid solutions of nominal composition
[ZnGaO][FeO], of the semiconducting spinel
ZnGaO with the ferrimagnetic spinel FeO have been prepared with
= 0.05, 0.10, and 0.15. All samples show evidence for long-range magnetic
ordering with ferromagnetic hysteresis at low temperatures. Magnetization as a
function of field for the = 0.15 sample is S-shaped at temperatures as high
as 200 K. M\"ossbauer spectroscopy on the = 0.15 sample confirms the
presence of Fe, and spontaneous magnetization at 4.2 K. The magnetic
behavior is obtained without greatly affecting the semiconducting properties of
the host; diffuse reflectance optical spectroscopy indicates that Fe
substitution up to = 0.15 does not affect the position of the band edge
absorption. These promising results motivate the possibility of dilute
ferrimagnetic semiconductors which do not require carrier mediation of the
magnetic moment.Comment: 9 pages and 6 figure
Measuring diffuse neutrino fluxes with IceCube
In this paper the sensitivity of a future kilometer-sized neutrino detector
to detect and measure the diffuse flux of high energy neutrinos is evaluated.
Event rates in established detection channels, such as muon events from charged
current muon neutrino interactions or cascade events from electron neutrino and
tau neutrino interactions, are calculated using a detailed Monte Carlo
simulation. Neutrino fluxes as expected from prompt charm decay in the
atmosphere or from astrophysical sources such as Active Galactic Nuclei are
modeled assuming power laws. The ability to measure the normalization and slope
of these spectra is then analyzed.
It is found that the cascade channel generally has a high sensitivity for the
detection and characterization of the diffuse flux, when compared to what is
expected for the upgoing- and downgoing-muon channels. A flux at the level of
the Waxman-Bahcall upper bound should be detectable in all channels separately
while a combination of the information of the different channels will allow
detection of a flux more than one order of magnitude lower. Neutrinos from the
prompt decay of charmed mesons in the atmosphere should be detectable in future
measurements for all but the lowest predictions.Comment: 12 pages, 3 figure
On High-Energy Behavior of Cross Sections in Theories with Large Extra Dimensions
We discuss the high-energy behavior of cross sections in theories with large
extra dimensions and low-scale quantum gravity, addressing two particular
issues: (i) the tension of the D-branes, and (ii) bounds on the cross section
and their relation to approximations in the mode sum over Kaluza-Klein-graviton
exchanges.Comment: 6 pages, late
IceCube Science
We discuss the status of the kilometer-scale neutrino detector IceCube and
its low energy upgrade Deep Core and review its scientific potential for
particle physics. We subsequently appraise IceCube's potential for revealing
the enigmatic sources of cosmic rays. After all, this aspiration set the scale
of the instrument. While only a smoking gun is missing for the case that the
Galactic component of the cosmic ray spectrum originates in supernova remnants,
the origin of the extragalactic component remains as inscrutable as ever. We
speculate on the role of the nearby active galaxies Centaurus A and M87.Comment: 19 pages, 8 figures; Talk at Discrete 08, Valencia, Spai
Optical Properties of Deep Ice at the South Pole - Absorption
We discuss recent measurements of the wavelength-dependent absorption
coefficients in deep South Pole ice. The method uses transit time distributions
of pulses from a variable-frequency laser sent between emitters and receivers
embedded in the ice. At depths of 800 to 1000 m scattering is dominated by
residual air bubbles, whereas absorption occurs both in ice itself and in
insoluble impurities. The absorption coefficient increases approximately
exponentially with wavelength in the measured interval 410 to 610 nm. At the
shortest wavelength our value is about a factor 20 below previous values
obtained for laboratory ice and lake ice; with increasing wavelength the
discrepancy with previous measurements decreases. At around 415 to 500 nm the
experimental uncertainties are small enough for us to resolve an extrinsic
contribution to absorption in ice: submicron dust particles contribute by an
amount that increases with depth and corresponds well with the expected
increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The
laser pulse method allows remote mapping of gross structure in dust
concentration as a function of depth in glacial ice.Comment: 26 pages, LaTex, Accepted for publication in Applied Optics. 9
figures, not included, available on request from [email protected]
Supernova pointing with low- and high-energy neutrino detectors
A future galactic SN can be located several hours before the optical
explosion through the MeV-neutrino burst, exploiting the directionality of
--scattering in a water Cherenkov detector such as Super-Kamiokande. We
study the statistical efficiency of different methods for extracting the SN
direction and identify a simple approach that is nearly optimal, yet
independent of the exact SN neutrino spectra. We use this method to quantify
the increase in the pointing accuracy by the addition of gadolinium to water,
which tags neutrons from the inverse beta decay background. We also study the
dependence of the pointing accuracy on neutrino mixing scenarios and initial
spectra. We find that in the ``worst case'' scenario the pointing accuracy is
at 95% C.L. in the absence of tagging, which improves to
with a tagging efficiency of 95%. At a megaton detector, this accuracy can be
as good as . A TeV-neutrino burst is also expected to be emitted
contemporaneously with the SN optical explosion, which may locate the SN to
within a few tenths of a degree at a future km high-energy neutrino
telescope. If the SN is not seen in the electromagnetic spectrum, locating it
in the sky through neutrinos is crucial for identifying the Earth matter
effects on SN neutrino oscillations.Comment: 13 pages, 7 figures, Revtex4 format. The final version to be
published in Phys. Rev. D. A few points in the original text are clarifie
Search for Point Sources of High Energy Neutrinos with AMANDA
This paper describes the search for astronomical sources of high-energy
neutrinos using the AMANDA-B10 detector, an array of 302 photomultiplier tubes,
used for the detection of Cherenkov light from upward traveling
neutrino-induced muons, buried deep in ice at the South Pole. The absolute
pointing accuracy and angular resolution were studied by using coincident
events between the AMANDA detector and two independent telescopes on the
surface, the GASP air Cherenkov telescope and the SPASE extensive air shower
array. Using data collected from April to October of 1997 (130.1 days of
livetime), a general survey of the northern hemisphere revealed no
statistically significant excess of events from any direction. The sensitivity
for a flux of muon neutrinos is based on the effective detection area for
through-going muons. Averaged over the Northern sky, the effective detection
area exceeds 10,000 m^2 for E_{mu} ~ 10 TeV. Neutrinos generated in the
atmosphere by cosmic ray interactions were used to verify the predicted
performance of the detector. For a source with a differential energy spectrum
proportional to E_{nu}^{-2} and declination larger than +40 degrees, we obtain
E^2(dN_{nu}/dE) <= 10^{-6}GeVcm^{-2}s^{-1} for an energy threshold of 10 GeV.Comment: 46 pages, 22 figures, 4 tables, submitted to Ap.
Bounds on the cosmogenic neutrino flux
Under the assumption that some part of the observed highest energy cosmic
rays consists of protons originating from cosmological distances, we derive
bounds on the associated flux of neutrinos generated by inelastic processes
with the cosmic microwave background photons. We exploit two methods. First, a
power-like injection spectrum is assumed. Then, a model-independent technique,
based on the inversion of the observed proton flux, is presented. The inferred
lower bound is quite robust. As expected, the upper bound depends on the
unknown composition of the highest energy cosmic rays. Our results represent
benchmarks for all ultrahigh energy neutrino telescopes.Comment: 12 pages, 6 figure
Vascular uptake on 18F-sodium fluoride positron emission tomography:precursor of vascular calcification?
Background: Microcalcifications cannot be identified with the present resolution of CT; however, 18F-sodium fluoride (18F-NaF) positron emission tomography (PET) imaging has been proposed for non-invasive identification of microcalcification. The primary objective of this study was to assess whether 18F-NaF activity can assess the presence and predict the progression of CT detectable vascular calcification. Methods and Results: The data of two longitudinal studies in which patients received a 18F-NaF PET-CT at baseline and after 6 months or 1-year follow-up were used. The target to background ratio (TBR) was measured on PET at baseline and CT calcification was quantified in the femoral arteries at baseline and follow-up. 128 patients were included. A higher TBR at baseline was associated with higher calcification mass at baseline and calcification progression (ÎČÂ =Â 1.006 [1.005-1.007] and ÎČÂ =Â 1.002 [1.002-1.003] in the studies with 6 months and 1-year follow-up, respectively). In areas without calcification at baseline and where calcification developed at follow-up, the TBR was.11â.13 (PÂ <Â .001) higher compared to areas where no calcification developed. Conclusion: The activity of 18F-NaF is related to the amount of calcification and calcification progression. In areas where calcification formation occurred, the TBR was slightly but significantly higher
Very-high energy gamma-ray astronomy: A 23-year success story in high-energy astroparticle physics
Very-high energy (VHE) gamma quanta contribute only a minuscule fraction -
below one per million - to the flux of cosmic rays. Nevertheless, being neutral
particles they are currently the best "messengers" of processes from the
relativistic/ultra-relativistic Universe because they can be extrapolated back
to their origin. The window of VHE gamma rays was opened only in 1989 by the
Whipple collaboration, reporting the observation of TeV gamma rays from the
Crab nebula. After a slow start, this new field of research is now rapidly
expanding with the discovery of more than 150 VHE gamma-ray emitting sources.
Progress is intimately related with the steady improvement of detectors and
rapidly increasing computing power. We give an overview of the early attempts
before and around 1989 and the progress after the pioneering work of the
Whipple collaboration. The main focus of this article is on the development of
experimental techniques for Earth-bound gamma-ray detectors; consequently, more
emphasis is given to those experiments that made an initial breakthrough rather
than to the successors which often had and have a similar (sometimes even
higher) scientific output as the pioneering experiments. The considered energy
threshold is about 30 GeV. At lower energies, observations can presently only
be performed with balloon or satellite-borne detectors. Irrespective of the
stormy experimental progress, the success story could not have been called a
success story without a broad scientific output. Therefore we conclude this
article with a summary of the scientific rationales and main results achieved
over the last two decades.Comment: 45 pages, 38 figures, review prepared for EPJ-H special issue "Cosmic
rays, gamma rays and neutrinos: A survey of 100 years of research
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