Person: Blanco Ramos, Francisco
Universidad Complutense de Madrid
Faculty / Institute
Estructura de la Materia, Física Térmica y Electrónica
Física Atómica, Molecular y Nuclear
Now showing 1 - 10 of 75
PublicationExperimental and theoretical cross sections for positron scattering from the pentane isomers.(American Institute of Physics, 2016-02-28) Chiari, Luca; Zecca, Antonio; Blanco Ramos, Francisco; Brunger, M. J.Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present. (C) 2016 AIP Publishing LLC. PublicationTheoretical and experimental study on electron interactions with chlorobenzene: Shape resonances and differential cross sections(American Institute of Physics, 2016-08-08) Barbosa, Alessandra Souza; Varella, Marcio T. do N.; Sanchez, Sergio d'A.; Ameixa, Joao; Blanco Ramos, Francisco; García, Gustavo; Limao Vieira, Paulo; da Silva, Filipe Ferreira; Bettega, Marcio H. F.In this work, we report theoretical and experimental cross sections for elastic scattering of electrons by chlorobenzene (ClB). The theoretical integral and differential cross sections (DCSs) were obtained with the Schwinger multichannel method implemented with pseudopotentials (SMCPP) and the independent atom method with screening corrected additivity rule (IAM-SCAR). The calculations with the SMCPP method were done in the static-exchange (SE) approximation, for energies above 12 eV, and in the static-exchange plus polarization approximation, for energies up to 12 eV. The calculations with the IAM-SCAR method covered energies up to 500 eV. The experimental differential cross sections were obtained in the high resolution electron energy loss spectrometer VG-SEELS 400, in Lisbon, for electron energies from 8.0 eV to 50 eV and angular range from 7 degrees to 110 degrees. From the present theoretical integral cross section (ICS) we discuss the low-energy shape-resonances present in chlorobenzene and compare our computed resonance spectra with available electron transmission spectroscopy data present in the literature. Since there is no other work in the literature reporting differential cross sections for this molecule, we compare our theoretical and experimental DCSs with experimental data available for the parent molecule benzene. Published by AIP Publishing. PublicationElectron scattering cross sections from nitrobenzene in the energy range 0.4-1000 eV: the role of dipole interactions in measurements and calculations(Royal Society of Chemistry, 2020-06-28) Álvarez, L.; Costa, F.; Lozano, A. I.; Oller, J. C.; Muñoz, A.; Blanco Ramos, Francisco; Limao-Vieira, P.; White, R. D.; Brunger, M. J.; García, G.Absolute total electron scattering cross sections (TCS) for nitrobenzene molecules with impact energies from 0.4 to 1000 eV have been measured by means of two different electron-transmission experimental arrangements. For the lower energies (0.4-250 eV) a magnetically confined electron beam system has been used, while for energies above 100 eV a linear beam transmission technique with high angular resolution allowed accurate measurements up to 1000 eV impact energy. In both cases random uncertainties were maintained below 5-8%. Systematic errors arising from the angular and energy resolution limits of each apparatus are analysed in detail and quantified with the help of our theoretical calculations. Differential elastic and integral elastic, excitation and ionisation as well as momentum transfer cross sections have been calculated, for the whole energy range considered here, by using an independent atom model in combination with the screening corrected additivity rule method including interference effects (IAM-SCARI). Due to the significant permanent dipole moment of nitrobenzene, additional differential and integral rotational excitation cross sections have been calculated in the framework of the Born approximation. If we ignore the rotational excitations, our calculated total cross section agrees well with our experimental results for impact energies above 15 eV. Additionally, they overlap at 10 eV with the low energy Schwinger Multichannel method with Pseudo Potentials (SMCPP) calculation available in the literature (L. S. Maioli and M. H. F. Bettega,J. Chem. Phys., 2017,147, 164305). We find a broad feature in the experimental TCS at around 1.0 eV, which has been related to the formation of the NO(2)(-)anion and assigned to the pi*(b(1)) resonance, according to previous mass spectra available in the literature. Other local maxima in the TCSs are found at 4.0 +/- 0.2 and 5.0 +/- 0.2 eV and are assigned to core excited resonances leading to the formation of the NO(2)(-)and O(2)(-)anions, respectively. Finally, for energies below 10 eV, differences found between the present measurements, the SMCPP calculation and our previous data for non-polar benzene have revealed the importance of accurately calculating the rotational excitation contribution to the TCS before comparing theoretical and experimental data. This comparison suggests that our dipole-Born calculation for nitrobenzene overestimates the magnitude of the rotational excitation cross sections below 10 eV. PublicationModeling crosstalk and afterpulsing in silicon photomultipliers(Elsevier science BV, 2015-07-01) Rosado Vélez, Jaime; Aranda, V. M.; Blanco Ramos, Francisco; Arqueros Martínez, FernandoAn experimental method to characterize the crosstalk and afterpulsing in silicon photomultipliers has been developed and applied to two detectors fabricated by Hamamatsu. An analytical model of optical crosstalk that we presented in a previous publication has been compared with new measurements, confirming our results. Progresses on a statistical model to describe afterpulsing and delayed crosstalk are also shown and compared with preliminary experimental data. (C) 2014 Elsevier B.V. All rights reserved. PublicationUltrahigh Energy Neutrinos at the Pierre Auger Observatory(Hindawi Publishing Corporation, 2013) Arganda, E.; Arqueros Martínez, Fernando; Blanco Ramos, Francisco; García Pinto, Diego; Minaya Flores, Ignacio Andrés; Ortiz Ramis, Montserrat; Rosado Vélez, Jaime; Vázquez Peñas, José RamónThe observation of ultrahigh energy neutrinos (UHE nu s) has become a priority in experimental astroparticle physics. UHE nu s can be detected with a variety of techniques. In particular, neutrinos can interact in the atmosphere (downward-going nu) or in the Earth crust (Earth-skimming nu), producing air showers that can be observed with arrays of detectors at the ground. With the surface detector array of the Pierre Auger Observatory we can detect these types of cascades. The distinguishing signature for neutrino events is the presence of very inclined showers produced close to the ground (i.e., after having traversed a large amount of atmosphere). In this work we review the procedure and criteria established to search for UHE nu s in the data collected with the ground array of the Pierre Auger Observatory. This includes Earth-skimming as well as downward-going neutrinos. No neutrino candidates have been found, which allows us to place competitive limits to the diffuse flux of UHE nu s in the EeV range and above. PublicationA search for anisotropy in the arrival directions of ultra high energy cosmic rays recorded at the Pierre Auger Observatory(IOP Publishing, 2012-04) Arganda, E.; Arqueros Martínez, Fernando; Blanco Ramos, Francisco; García Pinto, Diego; Minaya Flores, Ignacio Andrés; Ortiz Ramis, Montserrat; Rosado Vélez, Jaime; Vázquez Peñas, José RamónObservations of cosmic rays arrival directions made with the Pierre Auger Observatory have previously provided evidence of anisotropy at the 99% CL using the correlation of ultra high energy cosmic rays (UHECRs) with objects drawn from the Veron-Cetty Veron catalog. In this paper we report on the use of three catalog independent methods to search for anisotropy. The 2pt-L, 2pt+ and 3pt methods, each giving a different measure of self-clustering in arrival directions, were tested on mock cosmic ray data sets to study the impacts of sample size and magnetic smearing on their results, accounting for both angular and energy resolutions. If the sources of UHECRs follow the same large scale structure as ordinary galaxies in the local Universe and if UHECRs are deflected no more than a few degrees, a study of mock maps suggests that these three method can efficiently respond to the resulting anisotropy with a P-value = 1.0% or smaller with data sets as few as 100 events. using data taken from January 1, 2004 to July 31, 2010 we examined the 20, 30, ... , 110 highest energy events with a corresponding minimum energy threshold of about 49.3 EeV. The minimum P-values found were 13.5% using the 2pt-L method, 1.0% using the 2pt+ method and 1.1% using the 3pt method for the highest 100 energy events. In view of the multiple (correlated) scans performed on the data set, these catalog-independent methods do not yield strong evidence of anisotropy in the highest energy cosmic rays. PublicationReconstruction of inclined air showers detected with the pierre Auger Observatory(IOP Publising LTD, 2014) Arqueros Martínez, Fernando; Blanco Ramos, Francisco; García Pinto, Diego; Minaya Flores, Ignacio Andrés; Ortiz Ramis, Montserrat; Rosado Vélez, Jaime; Vázquez Peñas, José RamónWe describe the method devised to reconstruct inclined cosmic-ray air showers with zenith angles greater than 60 degrees detected with the surface array of the Pierre Auger Observatory. The measured signals at the ground level are fitted to muon density distributions predicted with atmospheric cascade models to obtain the relative shower size as an overall normalization parameter. The method is evaluated using simulated showers to test its performance. The energy of the cosmic rays is calibrated using a sub-sample of events reconstructed with both the fluorescence and surface array techniques. The reconstruction method described here provides the basis of complementary analyses including an independent measurement of the energy spectrum of ultra-high energy cosmic rays using very inclined events collected by the Pierre Auger Observatory. PublicationA targeted search for point sources of eev neutrons(IOP Publishing, 2014-06-02) Arqueros Martínez, Fernando; Blanco Ramos, Francisco; García Pinto, Diego; Minaya Flores, Ignacio Andrés; Ortiz Ramis, Montserrat; Rosado Vélez, Jaime; Vázquez Peñas, José RamónA flux of neutrons from an astrophysical source in the Galaxy can be detected in the Pierre Auger Observatory as an excess of cosmic-ray air showers arriving from the direction of the source. To avoid the statistical penalty for making many trials, classes of objects are tested in combinations as nine "target sets," in addition to the search for a neutron flux from the Galactic center or from the Galactic plane. Within a target set, each candidate source is weighted in proportion to its electromagnetic flux, its exposure to the Auger Observatory, and its flux attenuation factor due to neutron decay. These searches do not find evidence for a neutron flux from any class of candidate sources. Tabulated results give the combined p-value for each class, with and without the weights, and also the flux upper limit for the most significant candidate source within each class. These limits on fluxes of neutrons significantly constrain models of EeV proton emission from non-transient discrete sources in the Galaxy. PublicationMeasurement of the proton-air cross section at root s=57 TeV with the Pierre Auger Observatory(American Physical Society, 2012-08-10) Arganda, E.; Arqueros Martínez, Fernando; Blanco Ramos, Francisco; García Pinto, Diego; Ortiz Ramis, Montserrat; Rosado Vélez, Jaime; Vázquez Peñas, José RamónWe report a measurement of the proton-air cross section for particle production at the center-of-mass energy per nucleon of 57 TeV. This is derived from the distribution of the depths of shower maxima observed with the Pierre Auger Observatory: systematic uncertainties are studied in detail. Analyzing the tail of the distribution of the shower maxima, a proton-air cross section of [505 +/- 22(stat)(-36)(+28)(syst)] mb is found. PublicationElectron-impact excitation of the (4d(10)5s) S-2(1/2)-> (4d(9)5s(2)) D-2(3/2) and (4d(10)6s) S-2(1/2) ->(4d(10)6s) 2S(1/ 2) transitions in silver: Experiment and theory(Amer Physical Soc, 2021-08-10) Marinkovic, B. P.; Tosic, S. D.; Sevic, D.; McEachran, R. P.; Blanco Ramos, Francisco; García, G.; Brunger, M. J.We present angle-differential and angle-integrated cross sections for electron-impact excitation of the (4d(10)5s) S-2(1/ 2) -> (4d(9)5s(2)) D-2(3/ 2) and (4d(10)5s) S-2(1/ 2) ->(4d(10)6s) S-2(1/ 2) transitions in atomic silver. Experimental data for four incident electron energies between 10 and 60 eV are compared with predictions from our relativistic distorted wave (RDW) and nonrelativistic atomic optical potential models. Agreement between our measured and calculated data is only fair, although in the case of the RDW it is seen to improve with increasing incident electron energy. However, only for the (4d(10)6s) S-2(1/2) excitation process, agreement of our measured data with earlier relativistic convergent close coupling results from McNamara et al. [J. Phys. B 51, 085203 (2018)] was, with a few exceptions, typically observed to be very good, to within the uncertainties on the data.