Scintillation efficiency of liquid argon in low energy neutron-argon scattering

Experiments searching for weak interacting massive particles with noble gases such as liquid argon require very low detection thresholds for nuclear recoils. A determination of the scintillation efficiency is crucial to quantify the response of the detector at low energy. We report the results obtained with a small liquid argon cell using a monoenergetic neutron beam produced by a deuterium-deuterium fusion source. The light yield relative to electrons was measured for six argon recoil energies between 11 and 120 keV at zero electric drift field.Comment: 21 pages, 19 figures, 4 table

Study of nuclear recoils in liquid argon with monoenergetic neutrons

For the development of liquid argon dark matter detectors we assembled a setup in the laboratory to scatter neutrons on a small liquid argon target. The neutrons are produced mono-energetically (E_kin=2.45 MeV) by nuclear fusion in a deuterium plasma and are collimated onto a 3" liquid argon cell operating in single-phase mode (zero electric field). Organic liquid scintillators are used to tag scattered neutrons and to provide a time-of-flight measurement. The setup is designed to study light pulse shapes and scintillation yields from nuclear and electronic recoils as well as from {\alpha}-particles at working points relevant to dark matter searches. Liquid argon offers the possibility to scrutinise scintillation yields in noble liquids with respect to the populations of the two fundamental excimer states. Here we present experimental methods and first results from recent data towards such studies.Comment: 9 pages, 8 figures, proceedings of TAUP 2011, to be published in Journal of Physics: Conference Series (JCPS

Evaluation of nitrous oxide emission by soybean inoculated with Bradyrhizobium strains commonly used as inoculants in South America

Aims: The purpose of this work was to analyze the agronomic and environmental performance of soybean plants inoculated with the Bradyrhizobium strains widely used as soybean biofertilizers in South America and to determine if these strains possess any functional or taxonomic trait associated with the NO emission. Methods: Bradyrhizobium japonicum E109 and CPAC 15, B. diazoefficiens USDA 110 and CPAC 7, and B. elkanii SEMIA 5019 and SEMIA 587 were used to inoculate soybean seeds. The field experiment was carried out in a soil without history of soybean cultivation in the Argentinian Humid Pampa. The natural N abundance method was applied to estimate N-fixation, and NO production was evaluated using gas chromatography. Among other physiological parameters, shoot dry weight, shoot N content, and crop yield were estimated after harvest. Results: B. japonicum inoculation produced the greatest increases in soybean growth and crop yield but also led to higher NO emissions compared to all other inoculated treatments. Plants inoculated with B. diazoefficiens released the lowest amount of NO, and their growth and yield were the least affected. Inoculation with B. elkanii resulted in intermediate NO emission fluxes and crop yield compared with B. japonicum and B. diazoefficiens. Conclusions: We found that soybean inoculation with strains of B. japonicum and B. elkanii that lack the nosZ gene led to the highest NO emissions under field conditions, but also to the highest crop yield, while inoculation with strains that carry out complete denitrification, nosZ-containing B. diazoefficiens, showed lower NO emission and lower crop yield.To the Instituto de Investigaciones Agrobiotecnológicas (INIAB); Universidad Nacional de Río Cuarto (UNRC); Consejo Nacional de Investigaciones Científcas y Tecnológicas (CONICET), Fondo Nacional de Ciencia y Tecnología (FONCyT); Instituto Nacional de Tecnología Agropecuaria (INTA). FC is Researcher of CONICET at the UNRC. DT and FD are Postdoc and PhD students at the UNRC granted by CONICET. MOC is a former PhD student at the UNRC granted by CONICET. To Mariano Cicchino from INTA Chascomús, who was in charge of sowing and yield estimation at R8. To Juan Pedro Ezquiaga from INTA Castelar, for their contribution to N2O measurements

The ArDM experiment

The aim of the ArDM project is the development and operation of a one ton double-phase liquid argon detector for direct Dark Matter searches. The detector measures both the scintillation light and the ionization charge from ionizing radiation using two independent readout systems. This paper briefly describes the detector concept and presents preliminary results from the ArDM R&D program, including a 3 l prototype developed to test the charge readout system.Comment: Proceedings of the Epiphany 2010 Conference, to be published in Acta Physica Polonica

First results on light readout from the 1-ton ArDM liquid argon detector for dark matter searches

ArDM-1t is the prototype for a next generation WIMP detector measuring both the scintillation light and the ionization charge from nuclear recoils in a 1-ton liquid argon target. The goal is to reach a minimum recoil energy of 30\,keVr to detect recoiling nuclei. In this paper we describe the experimental concept and present results on the light detection system, tested for the first time in ArDM on the surface at CERN. With a preliminary and incomplete set of PMTs, the light yield at zero electric field is found to be between 0.3-0.5 phe/keVee depending on the position within the detector volume, confirming our expectations based on smaller detector setups.Comment: 14 pages, 10 figures, v2 accepted for publication in JINS

Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy beta and nuclear recoils in liquid argon with DEAP-1

The DEAP-1 low-background liquid argon detector was used to measure scintillation pulse shapes of electron and nuclear recoil events and to demonstrate the feasibility of pulse-shape discrimination (PSD) down to an electron-equivalent energy of 20 keV. In the surface dataset using a triple-coincidence tag we found the fraction of beta events that are misidentified as nuclear recoils to be $<1.4\times 10^{-7}$ (90% C.L.) for energies between 43-86 keVee and for a nuclear recoil acceptance of at least 90%, with 4% systematic uncertainty on the absolute energy scale. The discrimination measurement on surface was limited by nuclear recoils induced by cosmic-ray generated neutrons. This was improved by moving the detector to the SNOLAB underground laboratory, where the reduced background rate allowed the same measurement with only a double-coincidence tag. The combined data set contains $1.23\times10^8$ events. One of those, in the underground data set, is in the nuclear-recoil region of interest. Taking into account the expected background of 0.48 events coming from random pileup, the resulting upper limit on the electronic recoil contamination is $<2.7\times10^{-8}$ (90% C.L.) between 44-89 keVee and for a nuclear recoil acceptance of at least 90%, with 6% systematic uncertainty on the absolute energy scale. We developed a general mathematical framework to describe PSD parameter distributions and used it to build an analytical model of the distributions observed in DEAP-1. Using this model, we project a misidentification fraction of approx. $10^{-10}$ for an electron-equivalent energy threshold of 15 keV for a detector with 8 PE/keVee light yield. This reduction enables a search for spin-independent scattering of WIMPs from 1000 kg of liquid argon with a WIMP-nucleon cross-section sensitivity of $10^{-46}$ cm$^2$, assuming negligible contribution from nuclear recoil backgrounds.Comment: Accepted for publication in Astroparticle Physic

Scanning Tunneling Microscopy and Scanning Electron Microscopy Observations of the Early Stage of Silver Deposition on Graphite Single Crystal Electrodes

The early stages of Ag overpotential deposition on highly oriented pyrolytic graphite (HOPG) from Ag+-ion-containing acid solutions have been studied by ex situ scanning tunneling microscopy and scanning electron microscopy imaging complemented with electrochemical, energy dispersive X-ray analysis, and Auger electron spectroscopy data. Nucleation and 3D Ag growth initiate at surface defects. Unstructured 3D Ag nuclei decorating HOPG steps and flat geometric Ag islands are formed around the steps. The island structure is compatible with a local layer-by-layer growth. The entire morphology of the Ag deposit is consistent with a growth mechanism involving Ag atom diffusion from 3D nuclei at step edges toward Ag flat domains. Images with atomic resolution reveal large uncovered HOPG areas and Ag submonolayer domains with the nearest-neighbor distance d = 0.33 ± 0.02 nm, whereas flat Ag islands exhibit d = 0.29 ± 0.02 nm as expected for the nearest-neighbor distance in the Ag lattice. A model for these structures is discussed.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

SCORE underestimates cardiovascular risk (CVR) of HIV+ patients

The new European Guidelines of Dislipidemia Management of the European Societies of Cardiology and Arteriosclerosis consider HIV+ as patients at high risk of developing cardiovascular events and deaths. The objective of the study was to evaluate cardiovascular events and deaths in a series of HIV+ patients. Observational, cross-sectional study, including a cohort of HIV+ and HIV&#x2212; patients from 2008. CVR was calculated using the SCORE-CVR chart. Variation on lipid profile and incidence of cardiovascular events, cardiovascular death or death related to any cause were recorded. Data was analyzed using SPSS version 20.0 for MAC. 154 HIV+ and 155 HIV&#x2212; patients were included. Mean age: 44.8&#x00B1;9.5 vs 55.2&#x00B1;14.3 y and 69.5% vs 49% males respectively (p&#60;0.01). Mean time since HIV+ diagnosis was 11&#x00B1;6.2 y. Mean BMI and systolic blood pressure were lower in HIV+ (25.1&#x00B1;6.7 kg/m2 vs 28.7&#x00B1;5.1 kg/m2, (p&#60;0.01) and 119.6&#x00B1;19.4 vs 124.7&#x00B1;14.7 mmHg, (p=0.044; respectively)). A lower proportion of hypertense, diabetic and obese patients was observed in HIV+ (25.5% vs 6.5%; 20.6% vs 3.9% and 36.8% vs 12.3%) but a larger proportion of smokers (68.8% vs 29.7%) was observed (p&#60;0.01 in all cases). Mean cholesterol and LDLc were lower in HIV+ (191.2&#x00B1;41.4 vs 218.5&#x00B1;44.6 mg/dl and 109.5&#x00B1;33.9 vs 134.6&#x00B1;37.7 mg/dl; p&#60;0.01; respectively) but with a lower mean HDLc and higher TG (50.3&#x00B1;19 mg/dl vs 55.2&#x00B1;14.9 mg/dl; p=0.013 and 156.7&#x00B1;85.7 vs 135.8&#x00B1;66.2 mg/dl; p=0.017; respectively). There was no significant difference in mean CVR-SCORE (3.5&#x00B1;3.6% vs 4.4&#x00B1;3.8%; p=0.091). With this SCORE, 5.2&#x00B1;5.3 and 6.7&#x00B1;5.8 cardiovascular events or deaths should be expected in HIV+ and HIV&#x2212; respectively at 10 y. Four years later cholesterol, LDLc, HDLc, TG in HIV+ and HIV&#x2212; patients did not vary compared with those obtained 4 y before. 5 events and 1 death were seen at 4 y follow-up in HIV+, and in HIV&#x2212; patients. The incidence of events in HIV+ patients is similar to the expected according to their SCORE at 10 y. We could suppose that once the 10 y follow-up is reached, this incidence would be higher. On the other side, in HIV&#x2212; at 4 y just 3 events ocurred, far from the 6.7 events expected. There were no significant differences between lipid profiles in any of the cohorts. Lipid profile with low HDLc and high TG is persistent in HIV+ patients at 4 y follow-up. Understimation of CVR in HIV+ patients by SCORE charts could be present as soon as 4 y after the first assesment. This supports the stratification of HIV+ patients as high-risk patients in new guidelines

DM: a ton-scale LAr detector for direct Dark Matter searches

The Argon Dark Matter (ArDM-1t) experiment is a ton-scale liquid argon (LAr) double-phase time projection chamber designed for direct Dark Matter searches. Such a device allows to explore the low energy frontier in LAr with a charge imaging detector. The ionization charge is extracted from the liquid into the gas phase and there amplified by the use of a Large Electron Multiplier in order to reduce the detection threshold. Direct detection of the ionization charge with fine spatial granularity, combined with a measurement of the amplitude and time evolution of the associated primary scintillation light, provide powerful tools for the identification of WIMP interactions against the background due to electrons, photons and possibly neutrons if scattering more than once. A one ton LAr detector is presently installed on surface at CERN to fully test all functionalities and it will be soon moved to an underground location. We will emphasize here the lessons learned from such a device for the design of a large LAr TPC for neutrino oscillation, proton decay and astrophysical neutrinos searches