681 research outputs found
Palaeoenvironmental research at Hawelti–Melazo (Tigray, northern Ethiopia) – insights from sedimentological and geomorphological analyses
The sites of Hawelti–Melazo in the Tigray region of the northern Ethiopian Highlands is an archaeological hotspot related to the D'mt kingdom
(ca. 800–400 BCE). The existence of several monumental buildings, which
have been excavated since the 1950s, underline the importance of this area in the Ethio-Sabaean period. We investigated the geomorphological and
geological characteristics of the site and its surroundings and carried out
sedimentological analyses, as well as direct (luminescence) and indirect
(radiocarbon) sediment dating, to reconstruct the palaeoenvironmental
conditions, which we integrated into the wider context of Tigray.
Luminescence dating of feldspar grains from the May Agazin catchment
indicate enhanced fluvial activity in the late Pleistocene, likely connected to the re-occurring monsoon after the Last Glacial Maximum (LGM). The abundance of trap basalt on the Melazo plateau, which provides the basis for the development of fertile soils, and the presumably higher groundwater level during the Ethio-Sabaean Period, provided favourable settlement conditions. The peninsula-like shape of the Melazo plateau was easily accessible only from the east and northeast, while relatively steep scarps enclose the other edges of the plateau. This adds a possible natural protective function to this site.</p
Cosmic antiprotons as a probe for supersymmetric dark matter?
The flux of cosmic ray antiprotons from neutralino annihilations in the
galactic halo is computed for a large sample of models in the MSSM (the Minimal
Supersymmetric extension of the Standard Model). We also revisit the problem of
estimating the background of low-energy cosmic ray induced secondary
antiprotons, taking into account their subsequent interactions (and energy
loss) and the presence of nuclei in the interstellar matter.
We consider a two-zone diffusion model, with and without a galactic wind. We
find that, given the uncertainties in the background predictions, there is no
need for a primary (exotic) component to explain present data. However,
allowing for a signal by playing with the uncertainties in the background
estimate, we discuss the characteristic features of the supersymmetric models
which give a satisfactory description of the data. We point out that in some
cases the optimal kinetic energy to search for a signal from supersymmetric
dark matter is above several GeV, rather than the traditional sub-GeV region.
The large astrophysical uncertainties involved do not, one the other hand,
allow the exclusion of any of the MSSM models we consider, on the basis of
data.
We present besides numerical results also convenient parameterizations of the
antiproton yields of all `basic' two-body final states. We also give examples
of the yield and differential energy spectrum for a set of supersymmetric
models with high rates.
We also remark that it is difficult to put a limit on the antiproton lifetime
from present measurements, since the injection of antiprotons from neutralino
annihilation can compensate the loss from decay.Comment: 22 pages, 11 figures, uses emulateapj.st
Systematic inference of the long-range dependence and heavy-tail distribution parameters of ARFIMA models
Long-Range Dependence (LRD) and heavy-tailed distributions are ubiquitous in natural and socio-economic data. Such data can be self-similar whereby both LRD and heavy-tailed distributions contribute to the self-similarity as measured by the Hurst exponent. Some methods widely used in the physical sciences separately estimate these two parameters, which can lead to estimation bias. Those which do simultaneous estimation are based on frequentist methods such as Whittle’s approximate maximum likelihood estimator. Here we present a new and systematic Bayesian framework for the simultaneous inference of the LRD and heavy-tailed distribution parameters of a parametric ARFIMA model with non-Gaussian innovations. As innovations we use the α-stable and t-distributions which have power law tails. Our algorithm also provides parameter uncertainty estimates. We test our algorithm using synthetic data, and also data from the Geostationary Operational Environmental Satellite system (GOES) solar X-ray time series. These tests show that our algorithm is able to accurately and robustly estimate the LRD and heavy-tailed distribution parameters
Measurement of the cosmic ray hadron spectrum up to 30 TeV at mountain altitude: the primary proton spectrum
The flux of cosmic ray hadrons at the atmospheric depth of 820 g/cm^2 has
been measured by means of the EAS-TOP hadron calorimeter (Campo Imperatore,
National Gran Sasso Laboratories, 2005 m a.s.l.). The hadron spectrum is well
described by a single power law : S(E_h) = (2.25 +- 0.21 +- 0.34(sys))
10^(-7)(E_h/1000)^(-2.79 +- 0.05) m^(-2) s^(-1) sr^(-1) GeV^(-1) over the
energy range 30 GeV-30 TeV. The procedure and the accuracy of the measurement
are discussed. The primary proton spectrum is derived from the data by using
the CORSIKA/QGSJET code to compute the local hadron flux as a function of the
primary proton spectrum and to calculate and subtract the heavy nuclei
contribution (basing on direct measurements). Over a wide energy range E_0 =
0.5-50 TeV its best fit is given by a single power law : S(E_0) = (9.8 +- 1.1
+- 1.6(sys)) 10^(-5) (E_0/1000)^(-2.80 +- 0.06) m^(-2) s^(-1) sr^(-1) GeV^(-1).
The validity of the CORSIKA/QGSJET code for such application has been checked
using the EAS-TOP and KASCADE experimental data by reproducing the ratio of the
measured hadron fluxes at the two experimental depths (820 and 1030 g/cm^2
respectively) at better than 10% in the considered energy range.Comment: 16 pages, 9 figures, accepted for publication in Astroparticle
Physic
Attributing Effects of Aqueous C60 Nano-Aggregates to Tetrahydrofuran Decomposition Products in Larval Zebrafish by Assessment of Gene Expression
Simulation of Atmospheric Muon and Neutrino Fluxes with CORSIKA
The fluxes of atmospheric muons and neutrinos are calculated by a three
dimensional Monte Carlo simulation with the air shower code CORSIKA using the
hadronic interaction models DPMJET, VENUS, GHEISHA, and UrQMD. For the
simulation of low energy primary particles the original CORSIKA has been
extended by a parametrization of the solar modulation and a microscopic
calculation of the directional dependence of the geomagnetic cut-off functions.
An accurate description for the geography of the Earth has been included by a
digital elevation model, tables for the local magnetic field in the atmosphere,
and various atmospheric models for different geographic latitudes and annual
seasons. CORSIKA is used to calculate atmospheric muon fluxes for different
locations and the neutrino fluxes for Kamioka. The results of CORSIKA for the
muon fluxes are verified by an extensive comparison with recent measurements.
The obtained neutrino fluxes are compared with other calculations and the
influence of the hadronic interaction model, the geomagnetic cut-off and the
local magnetic field on the neutrino fluxes is investigated.Comment: revtex, 19 pages, 19 Postscript figures, submitted to Phys. Rev.
Radiation damage of electronic components in space environment
The PAMELA apparatus is dedicated to study cosmic rays on board of a satellite mission scheduled to start at the beginning of 2004. All the electronics components of such a mission have to be chosen carefully, because no replacement is possible after launch. Irradiation tests have been performed in order to study effects of highly ionizing particles on chips and to evaluate thresholds for Single Event Upset and Latch-up. The first effect, observed in digital components, is a radiation-induced change of state in a memory cell and gives rise to loss of the stored information. The second one, present also in analog components, happens when a parasitic conduction channel opens through the chip: this can fuse the component unless a protection circuit limits the current flow. Estimates of on-orbit fluxes and results of dedicated beam tests are reported
Time dependence of the electron and positron components of the cosmic radiation measured by the PAMELA experiment between July 2006 and December 2015
Cosmic-ray electrons and positrons are a unique probe of the propagation of
cosmic rays as well as of the nature and distribution of particle sources in
our Galaxy. Recent measurements of these particles are challenging our basic
understanding of the mechanisms of production, acceleration and propagation of
cosmic rays. Particularly striking are the differences between the low energy
results collected by the space-borne PAMELA and AMS-02 experiments and older
measurements pointing to sign-charge dependence of the solar modulation of
cosmic-ray spectra. The PAMELA experiment has been measuring the time variation
of the positron and electron intensity at Earth from July 2006 to December 2015
covering the period for the minimum of solar cycle 23 (2006-2009) till the
middle of the maximum of solar cycle 24, through the polarity reversal of the
heliospheric magnetic field which took place between 2013 and 2014. The
positron to electron ratio measured in this time period clearly shows a
sign-charge dependence of the solar modulation introduced by particle drifts.
These results provide the first clear and continuous observation of how drift
effects on solar modulation have unfolded with time from solar minimum to solar
maximum and their dependence on the particle rigidity and the cyclic polarity
of the solar magnetic field.Comment: 11 pages, 2 figure
Time dependence of the e^- flux measured by PAMELA during the July 2006 - December 2009 solar minimum
Precision measurements of the electron component in the cosmic radiation
provide important information about the origin and propagation of cosmic rays
in the Galaxy not accessible from the study of the cosmic-ray nuclear
components due to their differing diffusion and energy-loss processes. However,
when measured near Earth, the effects of propagation and modulation of galactic
cosmic rays in the heliosphere, particularly significant for energies up to at
least 30 GeV, must be properly taken into account. In this paper the electron
(e^-) spectra measured by PAMELA down to 70 MeV from July 2006 to December 2009
over six-months time intervals are presented. Fluxes are compared with a
state-of-the-art three-dimensional model of solar modulation that reproduces
the observations remarkably well.Comment: 40 pages, 18 figures, 1 tabl
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