1,714 research outputs found
A Method to Obtain a Maxwell–Boltzmann Neutron Spectrum at kT = 30 keV for Nuclear Astrophysics Studies
AbstractA method to shape the neutron energy spectrum at low-energy accelerators is proposed by modification of the initial proton energy distribution. A first application to the superconductive RFQ of the SPES project at Laboratori Nazionali di Legnaro is investigated for the production of a Maxwell–Boltzmann neutron spectrum at kT = 30 keV via the 7Li(p, n)7Be reaction. Concept, solutions and calculations for a setup consisting of a proton energy shaper and a lithium target are presented. It is found that a power dentisity of 3 kW cm−2 could be sustained by the lithium target and a forward-directed neutron flux higher than 1010 s−1 at the sample position could be obtained. In the framework of the SPES project the construction of a LEgnaro NeutrOn Source (LENOS) for Astrophysics and for validation of integral nuclear data is proposed, suited for activation studies on stable and unstable isotopes
Reversible Graphene decoupling by NaCl photo-dissociation
We describe the reversible intercalation of Na under graphene on Ir(111) by
photo-dissociation of a previously adsorbed NaCl overlayer. After room
temperature evaporation, NaCl adsorbs on top of graphene forming a bilayer.
With a combination of electron diffraction and photoemission techniques we
demonstrate that the NaCl overlayer dissociates upon a short exposure to an
X-ray beam. As a result, chlorine desorbs while sodium intercalates under the
graphene, inducing an electronic decoupling from the underlying metal. Low
energy electron diffraction shows the disappearance of the moir\'e pattern when
Na intercalates between graphene and iridium. Analysis of the Na 2p core-level
by X-ray photoelectron spectroscopy shows a chemical change from NaCl to
metallic buried Na at the graphene/Ir interface. The intercalation-decoupling
process leads to a n-doped graphene due to the charge transfer from the Na, as
revealed by constant energy angle resolved X-ray photoemission maps. Moreover,
the process is reversible by a mild annealing of the samples without damaging
the graphene
Extreme cycles. The center of a Leavitt path algebra
In this paper we introduce new techniques in order to deepen into the structure of a Leavitt path algebra with the aim of giving a description of the center. Extreme cycles appear for the first time; they concentrate the purely infinite part of a Leavitt path algebra and, jointly with the line points and vertices in cycles without exits, are the key ingredients in order to determine the center of a Leavitt path algebra. Our work will rely on our previous approach to the center of a prime Leavitt path algebra
On the kinematics of the neutron star low mass X-ray binary Cen X-4
We present the first determination of the proper motion of the neutron star
low mass X-ray binary {Cen X-4} measured from relative astrometry of the
secondary star using optical images at different epochs. We determine the
Galactic space velocity components of the system and find them to be
significantly different from the mean values that characterize the kinematics
of stars belonging to the halo, and the thin and the thick disc of the Galaxy.
The high metallicity of the secondary star of the system rules out a halo
origin and indicates that the system probably originated in the Galactic disc.
A statistical analysis of the galactocentric motion revealed that this binary
moves in a highly eccentric () orbit with an inclination of
to the Galactic plane. The large Galactic space velocity
components strongly support that a high natal kick as a result of a supernova
explosion could have propelled the system into such an orbit from a birth place
in the Galactic disc. The high Li abundance in the secondary, comparable to
that of stars in star forming regions and young stellar clusters like the
Pleiades, may suggest a relatively recent formation of the system. Following
the orbit backwards in time, we found that the system could have been in the
inner regions of the Galactic disc 100--200 Myr ago. The neutron star
might have formed at that moment. However, we cannot rule out the possibility
that the system formed at a much earlier time if a Li production mechanism
exists in this LMXB.Comment: 6 pages, 4 figures, accepted for publication in A&
Observational - relation for Sct stars using eclipsing binaries and space photometry
Delta Scuti ( Sct) stars are intermediate-mass pulsators, whose
intrinsic oscillations have been studied for decades. However, modelling their
pulsations remains a real theoretical challenge, thereby even hampering the
precise determination of global stellar parameters. In this work, we used space
photometry observations of eclipsing binaries with a Sct component to
obtain reliable physical parameters and oscillation frequencies. Using that
information, we derived an observational scaling relation between the stellar
mean density and a frequency pattern in the oscillation spectrum. This pattern
is analogous to the solar-like large separation but in the low order regime. We
also show that this relation is independent of the rotation rate. These
findings open the possibility of accurately characterizing this type of
pulsator and validate the frequency pattern as a new observable for
Sct stars.Comment: 11 pages, including 2 pages of appendix, 2 figures, 2 tables,
accepted for publication in ApJ
Wireless Sensor Network based on OCDMA for closed environments
An infrared optical wireless system is presented, consisting on autonomous remote nodes communicating with a central node. The network is designed for telecommand/telemetry purposes, comprising a large number of nodes at a low data rate. Simultaneous access is granted by using CDMA techniques, and an appropriate selection of the code family can also keep power consumption to a minimu
LED Jitter-Induced Limitation Effects in the Baud Rate of VLC
In this paper we show that the effect of jitter due to driver and LED is the limiting factor in the baud rate in L-PPM formats for VLC systems
OptEEmAL: Decision-Support Tool for the Design of Energy Retrofitting Projects at District Level
Designing energy retrofitting actions poses an elevated number of problems, as the definition of the baseline, selection of indicators to measure performance, modelling, setting objectives, etc. This is time-consuming and it can result in a number of inaccuracies, leading to inadequate decisions. While these problems are present at building level, they are multiplied at district level, where there are complex interactions to analyse, simulate and improve. OptEEmAL proposes a solution as a decision-support tool for the design of energy retrofitting projects at district level. Based on specific input data (IFC(s), CityGML, etc.), the platform will automatically simulate the baseline scenario and launch an optimisation process where a series of Energy Conservation Measures (ECMs) will be applied to this scenario. Its performance will be evaluated through a holistic set of indicators to obtain the best combination of ECMs that complies with user's objectives. A great reduction in time and higher accuracy in the models are experienced, since they are automatically created and checked. A subjective problem is transformed into a mathematical problem; it simplifies it and ensures a more robust decision-making. This paper will present a case where the platform has been tested.This research work has been partially funded by the European Commission though the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No 680676. All related information to the project is available at https://www.opteemal-project.eu
The stellar content, metallicity and ionization structure of HII regions
Observations of infrared fine-structure lines provide direct information on
the metallicity and ionization structure of HII regions and indirectly on the
hardness of the radiation field ionizing these nebulae. We have analyzed a
sample of Galactic and Magellanic Cloud HII regions observed by the Infrared
Space Observatory (ISO) to examine the interplay between stellar content,
metallicity and the ionization structure of HII regions. The observed
[SIV]10.5/[SIII]18.7 mum and [NeIII]15.5/[NeII]12.8 mum line ratios are shown
to be highly correlated over more than two orders of magnitude. We have
compared the observed line ratios to the results of photoionization models
using different stellar energy distributions. The derived characteristics of
the ionizing star depend critically on the adopted stellar model as well as the
(stellar) metallicity. We have compared the stellar effective temperatures
derived from these model studies for a few well-studied HII regions with
published direct spectroscopic determinations of the spectral type of the
ionizing stars. This comparison supports our interpretation that stellar and
nebular metallicity influences the observed infrared ionic line ratios. We can
explain the observed increase in degree of ionization, as traced by the
[SIV]\[SIII] and [NeIII]\[NeII] line ratios, by the hardening of the radiation
field due to the decrease of metallicity. The implications of our results for
the determination of the ages of starbursts in starburst galaxies are assessed.Comment: 9 pages; accepted for publication in A&A; figure 3 modifie
Status of the LEgnaro NeutrOn Source facility (LENOS)
Abstract LENOS is a new facility under development at Laboratori Nazionali di Legnaro (LNL). It is based on a new technic for neutron beam shaping in accelerator based neutron sources. The main advantage of this method is to be able to shape the primary charged-particle beam to a defined energy distribution that, impinging on a neutron producing target, generates the desired neutron spectra at the sample position. Together with the proton energy distribution, other degrees of freedom are used to obtain the desired neutron energy spectra, e.g. the angular distribution of produced neutrons, the nuclear reactions used for the neutron spectra production, and the convolution of neutron spectra coming from different target materials. The main advantage of this new approach is the good control over the energy and spatial distribution of the produced neutron spectrum avoiding most of the problems due to neutron moderation, since it is easier to work with charged particles than with neutrons. The goal of the LENOS facility is to obtain a Maxwell-Boltzmann neutron energy spectrum with tunable temperature and a high neutron flux at sample position by using the 7Li(p,n) reaction. To maximize the neutron flux a very narrow primary proton beam has to be used, so the target has to remove a very high specific power. Currently available lithium targets are inadequate to sustain the high specific power that needs to be dissipated in the LENOS facility. A dedicated target based on micro-channel geometry and liquid metal cooling has been developed and tested. This contribution describes the status of the LENOS facility
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