Showing papers by "University of Cagliari published in 2008"
TL;DR: The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva).
Abstract: The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.
2,286 citations
TL;DR: The Large Ion Collider Experiment (ALICE) as discussed by the authors is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model.
Abstract: ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 161626 m3 with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.
1,218 citations
TL;DR: It is indicated that BCL11A variants, by modulating HbF levels, act as an important ameliorating factor of the β-thalassemia phenotype, and it is likely they could helpAmeliorate other hemoglobin disorders.
Abstract: -Thalassemia and sickle cell disease both display a great deal of phenotypic heterogeneity, despite being generally thought of as simple Mendelian diseases. The reasons for this are not well understood, although the level of fetal hemoglobin (HbF) is one well characterized ameliorating factor in both of these conditions. To better understand the genetic basis of this heterogeneity, we carried out genome-wide scans with 362,129 common SNPs on 4,305 Sardinians to look for genetic linkage and association with HbF levels, as well as other red blood cell-related traits. Among major variants affecting HbF levels, SNP rs11886868 in the BCL11A gene was strongly associated with this trait (P< 1035). The C allele frequency was significantly higher in Sardinian individuals with elevated HbF levels, detected by screening for -thalassemia, and patients with attenuated forms of -thalassemia vs. those with thalassemia major. We also show that the same BCL11A variant is strongly associated with HbF levels in a large cohort of sickle cell patients. These results indicate that BCL11A variants, by modulat- ing HbF levels, act as an important ameliorating factor of the -thalassemia phenotype, and it is likely they could help amelio- rate other hemoglobin disorders. We expect our findings will help to characterize the molecular mechanisms of fetal globin regula- tion and could eventually contribute to the development of new therapeutic approaches for -thalassemia and sickle cell anemia. globin gene regulation polymorphism sickle cell anemia
576 citations
Karolinska University Hospital1, University of New South Wales2, National Institutes of Health3, University of California, Los Angeles4, St. Vincent's Health System5, Statens Serum Institut6, Imperial College London7, University of California, San Francisco8, University of York9, BC Cancer Agency10, Mario Negri Institute for Pharmacological Research11, Yale University12, Northwestern University13, University of Cagliari14, Dublin City University15, International Agency for Research on Cancer16, Fred Hutchinson Cancer Research Center17, Wayne State University18, Mayo Clinic19, Brigham and Women's Hospital20, University of Southern California21
TL;DR: In this paper, the authors performed a pooled analysis of self-reported autoimmune conditions and risk of non-Hodgkin lymphoma (NHL) and subtypes, including 29,423 participants in 12 case-control studies.
490 citations
University of Zurich1, University of Notre Dame2, CERN3, Brookhaven National Laboratory4, Durham University5, University of Hamburg6, Max Planck Society7, Autonomous University of Madrid8, University of Victoria9, Technische Universität München10, University of Cagliari11, Instituto Superior Técnico12, University of British Columbia13, International School for Advanced Studies14, University of Valencia15, University of Warsaw16, University of Lyon17, Yale University18, University of Manchester19, University of Pisa20, RWTH Aachen University21, University of Wisconsin-Madison22, University of Sussex23, University of Tokyo24, Argonne National Laboratory25, Helsinki Institute of Physics26, University of Oregon27, University of Zagreb28, KEK29, University of Southampton30, Paul Scherrer Institute31, Petersburg Nuclear Physics Institute32, University of California33, University of Paris34, University of Rome Tor Vergata35, Bulgarian Academy of Sciences36, University of Salento37, Boston University38, University of Würzburg39, International Centre for Theoretical Physics40, University of Murcia41, University of Michigan42, Indian Institute of Science43
TL;DR: In this article, the authors discuss the theoretical, phenomenological and experimental issues related to flavor phenomena in the charged lepton sector and in flavor conserving CP-violating processes.
Abstract: This chapter of the report of the “Flavor in the era of the LHC” Workshop discusses the theoretical, phenomenological and experimental issues related to flavor phenomena in the charged lepton sector and in flavor conserving CP-violating processes. We review the current experimental limits and the main theoretical models for the flavor structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the standard model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.
384 citations
TL;DR: This review analyzes recent literature evaluating 3D virtual screening methods, with focus on molecular docking, and highlights problematic issues and provides guidelines on how to improve the quality of computational studies.
Abstract: Within the last few years a considerable amount of evaluative studies has been published that investigate the performance of 3D virtual screening approaches. Thereby, in particular assessments of protein-ligand docking are facing remarkable interest in the scientific community. However, comparing virtual screening approaches is a non-trivial task. Several publications, especially in the field of molecular docking, suffer from shortcomings that are likely to affect the significance of the results considerably. These quality issues often arise from poor study design, biasing, by using improper or inexpressive enrichment descriptors, and from errors in interpretation of the data output. In this review we analyze recent literature evaluating 3D virtual screening methods, with focus on molecular docking. We highlight problematic issues and provide guidelines on how to improve the quality of computational studies. Since 3D virtual screening protocols are in general assessed by their ability to discriminate between active and inactive compounds, we summarize the impact of the composition and preparation of test sets on the outcome of evaluations. Moreover, we investigate the significance of both classic enrichment parameters and advanced descriptors for the performance of 3D virtual screening methods. Furthermore, we review the significance and suitability of RMSD as a measure for the accuracy of protein-ligand docking algorithms and of conformational space sub sampling algorithms.
328 citations
TL;DR: The necessary and sufficient role of α4β2- and α6 β2-subunit containing nicotinic receptors, but not α7*-nAChRs, present in cell bodies of the VTA, and their axons, for systemic nicotine reinforcement in drug-naive mice are defined.
Abstract: The identification of the molecular mechanisms involved in nicotine addiction and its cognitive consequences is a worldwide priority for public health. Novel in vivo paradigms were developed to match this aim. Although the β2 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) has been shown to play a crucial role in mediating the reinforcement properties of nicotine, little is known about the contribution of the different α subunit partners of β2 (i.e., α4 and α6), the homo-pentameric α7, and the brain areas other than the ventral tegmental area (VTA) involved in nicotine reinforcement. In this study, nicotine (8.7–52.6 μg free base/kg/inf) self-administration was investigated with drug-naive mice deleted (KO) for the β2, α4, α6 and α7 subunit genes, their wild-type (WT) controls, and KO mice in which the corresponding nAChR subunit was selectively re-expressed using a lentiviral vector (VEC mice). We show that WT mice, β2-VEC mice with the β2 subunit re-expressed exclusively in the VTA, α4-VEC mice with selective α4 re-expression in the VTA, α6-VEC mice with selective α6 re-expression in the VTA, and α7-KO mice promptly self-administer nicotine intravenously, whereas β2-KO, β2-VEC in the substantia nigra, α4-KO and α6-KO mice do not respond to nicotine. We thus define the necessary and sufficient role of α4β2- and α6β2-subunit containing nicotinic receptors (α4β2*- and α6β2*-nAChRs), but not α7*-nAChRs, present in cell bodies of the VTA, and their axons, for systemic nicotine reinforcement in drug-naive mice.
318 citations
TL;DR: In this paper, the authors evaluated the effectiveness of marine protected areas (MPAs) and found that 3 of 15 Italian marine reserves investigated had adequate enforcement, and that patterns of recovery of target fish were related to enforcement.
314 citations
TL;DR: In this paper, a pooled case-control study data was used to provide robust estimates of the risk of non-Hodgkin's lymphoma (NHL) subtypes after hepatitis C virus (HCV) infection.
301 citations
TL;DR: The decomposition analysis for predicted mortality shows that, after allowing for endogeneity, lifestyles contribute strongly to inequality in mortality, reducing the direct role of socio-economic status.
294 citations
TL;DR: The data provide clear evidence that protocols of continuing intensive surveillance and monitoring of compliance with anti-infective regimens may significantly improve the quality of life and long-term survival in patients with CGD.
TL;DR: A review of compositional data of the major explosive eruptions of Vesuvius is presented, comparing compositions (major elements) of whole rock with glass shards from the proximal deposits, hopefully useful for long-distance correlation.
TL;DR: Experimental results on the KDD-Cup 1999 dataset show that the proposed anomaly IDS achieves high attack detection rate and low false alarm rate at the same time.
31 Oct 2008
TL;DR: The paper describes the concept of computational resources outsourcing, referred to computational grids and a real application, and utilises the results by the Cybersar Project managed by the COSMOLAB Consortium (Italy).
Abstract: ldquoCloud Computingrdquo is becoming increasingly relevant, as it will enable companies involved in spreading this technology to open the doors to Web 3.0. In this work the basic features of cloud computing are presented and compared with those of the original technology: Grid Computing. The new categories of services introduced will slowly replace many types of computational resources currently used. In this perspective, grid computing, the basic element for the large scale supply of cloud services, will play a fundamental role in defining how those services will be provided. The paper describes the concept of computational resources outsourcing, referred to computational grids and a real application. This work utilises the results by the Cybersar Project managed by the COSMOLAB Consortium (Italy).
TL;DR: Interestingly, liposomes prevented the cytotoxicity of resveratrol at high concentrations, even at 100microM, avoiding its immediate and massive intracellular distribution, and increased the ability of res veratrol to stimulate the proliferation of the cells and their ability to survive under stress conditions caused by UV-B light.
TL;DR: In this article, a Vesuvius Event Tree (ET) was created to summarize in a numerical-graphical form, at different levels of detail, all the relative likelihoods relating to the genesis and style of eruption, development and nature of volcanic hazards, and the probabilities of occurrence of different volcanic risks in the next eruption crisis.
TL;DR: The Italian Society of Hematology appointed a panel of experts to produce clinical practice-guidelines for the management of iron overload in thalassemia major and related disorders, and the resulting recommendations advise about which examination to carry out in order to planIron chelation therapy, when to start iron chelation, which iron chelator to choose in regularly transfused patients, how to monitor iron chelated therapy, and when and how to switch standard therapy.
Abstract: New measures of iron accumulation in liver and heart (superconducting quantum inference device and magnetic resonance imaging), and oral iron chelators (deferiprone and deferasirox) are available for managing iron overload in thalassemia major. To assure appropriate use of these new health technologies, the Italian Society of Hematology appointed a panel of experts to produce clinical practice-guidelines for the management of iron overload in thalassemia major and related disorders. The analytical hierarchy process, a technique for multicriteria decision analysis, was applied to relevant key questions in order to identify the alternative strategies, generate explicit criteria for their evaluation, and check how well the alternatives fulfilled the criteria. The result of a comprehensive systematic review of articles released from 1990 to 2007 was used as a source of scientific evidence to compare the decisional options pairwise, and select the final recommendation. Every step in the model was developed from questionnaires and group discussion. The resulting recommendations advise about which examination to carry out in order to plan iron chelation therapy, when to start iron chelation, which iron chelator to choose in regularly transfused patients, how to monitor iron chelation therapy, and when and how to switch standard therapy.
TL;DR: The findings of this study suggest that the observed TMPRSS6 mutation leads to overproduction of hepcidin and, in turn, to defective iron absorption and utilization, and confirm in humans the inhibitory effect of matriptase-2 on hePCidin synthesis already demonstrated in mice.
Abstract: Background Hepcidin plays a key role in body iron metabolism by preventing the release of iron from macrophages and intestinal cells. Defective hepcidin synthesis causes iron loading, while overproduction results in defective reticuloendothelial iron release and iron absorption.
Design and Methods We studied a Sardinian family in which microcytic anemia due to defective iron absorption and utilization is inherited as a recessive character. Five members showed iron deficiency anemia that was not responsive to oral iron and only partially responsive to parenteral iron administration. To investigate the involvement of known genes implicated in iron metabolism we carried out linkage analysis with microsatellite markers mapping close to these genes. Afterwards, a genome-wide search was performed.
Results No linkage was found between the phenotype of the patients and several known human genes involved in iron metabolism ( DMT1, TF, TFRC, ZIRTL, HAMP, HJV ). Genome-wide scanning by microsatellites and single nucleotide polymorphisms showed a multipoint LOD score of 5.6 on chromosome 22q12.3–13.1, where the matriptase-2 (also known as transmembrane protease, serine 6 or TMPRSS6 ) gene is located. Its murine counterpart ( Tmprss6 ) has recently been found to be an essential component of a pathway that detects iron deficiency and suppresses hepcidin production. Sequencing analysis of TMPRSS6 revealed a homozygous causal mutation, predicting a splicing error and a truncated TMPRSS6 protein in affected members. Homozygous subjects had inappropriately elevated levels of serum and urinary hepcidin.
Conclusions The findings of this study suggest that the observed TMPRSS6 mutation leads to overproduction of hepcidin and, in turn, to defective iron absorption and utilization. More generally, they confirm in humans the inhibitory effect of matriptase-2 on hepcidin synthesis already demonstrated in mice.
TL;DR: In this paper, the authors investigate the impact of trade openness on the distribution of firms across marginal cost levels and find that more open industries are characterized by a smaller dispersion of costs across active firms.
Abstract: We use Italian firm-level data to investigate the impact of trade openness on the distribution of firms across marginal cost levels. In so doing, we implement a procedure that allows us to control not only for the standard transmission bias identified in firm-level TFP regressions but also for the omitted price bias due to imperfect competition. We find that more open industries are characterized by a smaller dispersion of costs across active firms. Moreover, in those industries the average cost is also smaller.
TL;DR: It is demonstrated that in this simple system, a very high superconducting critical temperature can be reached via electron-phonon and Coulomb electron-electron interactions.
Abstract: We present a first-principles study of the electron-phonon interaction and the prediction of the superconducting critical temperature in molecular metallic hydrogen. Our study is able to single out the features which drive the system towards superconductivity: mainly, a rich and complex Fermi surface and strongly coupled phonon modes driving the intra- or intermolecular charge transfer. We demonstrate that in this simple system, a very high superconducting critical temperature can be reached via electron-phonon and Coulomb electron-electron interactions.
TL;DR: Analysis of modulation of nicotine effects by the endocannabinoid system on dopamine neurons in the ventral tegmental area indicates for the first time that the anorexic lipids OEA and PEA possess neuromodulatory properties as endogenous ligands of PPAR-α in the brain and provide a potential new target for the treatment of nicotine addiction.
Abstract: Nicotine stimulates the activity of mesolimbic dopamine neurons, which is believed to mediate the rewarding and addictive properties of tobacco use. Accumulating evidence suggests that the endocannabinoid system might play a major role in neuronal mechanisms underlying the rewarding properties of drugs of abuse, including nicotine. Here, we investigated the modulation of nicotine effects by the endocannabinoid system on dopamine neurons in the ventral tegmental area with electrophysiological techniques in vivo and in vitro. We discovered that pharmacological inhibition of fatty acid amide hydrolase (FAAH), the enzyme that catabolizes fatty acid ethanolamides, among which the endocannabinoid anandamide (AEA) is the best known, suppressed nicotine-induced excitation of dopamine cells. Importantly, this effect was mimicked by the administration of the FAAH substrates oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), but not methanandamide, the hydrolysis resistant analog of AEA. OEA and PEA are naturally occurring lipid signaling molecules structurally related to AEA, but devoid of affinity for cannabinoid receptors. They blocked the effects of nicotine by activation of the peroxisome proliferator-activated receptor-α (PPAR-α), a nuclear receptor transcription factor involved in several aspects of lipid metabolism and energy balance. Activation of PPAR-α triggered a nongenomic stimulation of tyrosine kinases, which might lead to phosphorylation and negative regulation of neuronal nicotinic acetylcholine receptors. These data indicate for the first time that the anorexic lipids OEA and PEA possess neuromodulatory properties as endogenous ligands of PPAR-α in the brain and provide a potential new target for the treatment of nicotine addiction.
TL;DR: The progressive increase in the thickness of the dentinal walls and subsequent apical development suggest that appropriate biologic responses can occur with this type of treatment of the necrotic immature permanent tooth with sinus tract.
TL;DR: Evidence of gender differences in drug addiction, with particular emphasis on women's health and implications, is synthesized, and the effects of hormones and estrous/menstrual cycle will be reviewed.
Abstract: Addiction research has historically neglected research on women, and most studies have been conducted on men only, with the concluding results generalized to the female population. The role of sex differences in vulnerability to drug abuse, their repercussions on prevention and treatment strategies all require detailed studies, as does the progression from recreational drug use to dependence. This review synthesizes evidence of gender differences in drug addiction, with particular emphasis on women’s health and implications. We first reviewed behavioral studies showing sex differences in the preference for and self-administration of licit (i.e., alcohol and nicotine) and illicit (i.e., cocaine, amphetamine, heroin and cannabis) substances as revealed by animal models of addiction. Clinical studies demonstrating differences between men and women in craving, drug use, abstinence and relapse will then be examined. For both animal and human studies, the effects of hormones and estrous/menstrual cycle will be ...
TL;DR: Precocious persistent antibodies found in patients with thymoma and/or myasthenia gravis show 98% or higher sensitivity and APS-I specificity and are thus a simpler diagnostic option than detecting AIRE mutations.
Abstract: Context: In autoimmune polyendocrinopathy syndrome type I (APS-I), mutations in the autoimmune regulator gene (AIRE) impair thymic self-tolerance induction in developing T cells. The ensuing autoimmunity particularly targets ectodermal and endocrine tissues, but chronic candidiasis usually comes first. We recently reported apparently APS-I-specific high-titer neutralizing autoantibodies against type I interferons in 100% of Finnish and Norwegian patients, mainly with two prevalent AIRE truncations. Objectives: Because variability in clinical features and age at onset in APS-I frequently results in unusual presentations, we prospectively checked the diagnostic potential of anti-interferon antibodies in additional APS-I panels with other truncations or rare missense mutations and in disease controls with chronic mucocutaneous candidiasis (CMC) but without either common AIRE mutation. Design: The study was designed to detect autoantibodies against interferon-α2 and interferon-ω in antiviral neutralization as...
TL;DR: In this article, an experimental study of the transport properties of a low-bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (Eg-opt ∼ 1.35 eV) is presented.
Abstract: An experimental study of the transport properties of a low-bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (Eg-opt ∼ 1.35 eV) is presented. Using a organic thin film transistor geometry, we demonstrate a relatively high in-plane hole mobility, up to 1.5 · × 10−2 cm2 V−1 s−1 and quantify the electron mobility at 3 × · 10−5 cm2 V−1 s−1 on a SiO2 dielectric. In addition, singular contact behavior results in bipolar quasi-Ohmic injection both from low and high workfunction metals like LiF/Al and Au. X-ray investigations revealed a degree of interchain π-stacking that is probably embedded in a disordered matrix. Disorder also manifests itself in a strong positive field dependence of the hole mobility from the electric field. In blends made with the electron acceptor methanofullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM), the transistor characteristics suggest a relatively unfavorable intermixing of the two components for the application to photovoltaic devices. We attribute this to a too fine dispersion of [C60]-PCBM in the polymer matrix, that is also confirmed by the quenching of the photoluminescence signal measured in PCPDTBT [C60]-PCBM films with various composition. We show that a higher degree of phase separation can be induced during the film formation by using 1,8-octanedithiol (ODT), which leads to a more efficient electron percolation in the [C60]-PCBM. In addition, the experimental results, in combination with those of solar cells seem to support the correlation between the blend morphology and charge recombination. We tentatively propose that the drift length, and similarly the electrical fill factor, can be limited by the recombination of holes with electrons trapped on isolated [C60]-PCBM clusters. Ionized and isolated [C60]-PCBM molecules can modify the local electric field in the solar cell by build-up of a space-charge. The results also suggest that further improvements of the fill factor may also be limited by a strong electrical-field dependence of the hole transport.
TL;DR: A general view of the conceptual elaborations on disablement in the 20th century is provided to discuss the role of these different contributions in developing the current concepts of disablement and introduces major hints for further considerations on how to create virtuous processes of enablement.
Abstract: Purpose. The aim of this work is to provide a general view of the conceptual elaborations on disablement in the 20th century and to discuss the role of these different contributions in developing the current concepts of disablement.Method. A review of the literature on conceptual models of disablement in the past century has been performed.Results. The 20th century has witnessed important theoretical considerations on health, diseases and their consequences. These considerations have generated various conceptual models, some of which share the same focus and point of arrival, the so-called ‘Disablement Process’. Among the models that were developed, two stand out, which were drafted and disseminated under the aegis of the World Health Organization, namely the International Classification of Impairments, Disabilities and Handicaps (ICIDH) and the International Classification of Functioning, Disability and Health (ICF), but these are just one part of the conceptual elaboration in the field. Further conceptu...
TL;DR: This work reviews and compares centrality measures based on spectral properties of graph matrices, and shall focus on PageRank, eigenvector centrality (EV), and the hub and authority scores of the HITS algorithm.
Abstract: Complex networks are characterized by heterogeneous distributions of the degree of nodes, which produce a large diversification of the roles of the nodes within the network. Several centrality measures have been introduced to rank nodes based on their topological importance within a graph. Here we review and compare centrality measures based on spectral properties of graph matrices. We shall focus on PageRank (PR), eigenvector centrality (EV), and the hub and authority scores of the HITS algorithm. We derive simple relations between the measures and the (in)degree of the nodes, in some limits. We also compare the rankings obtained with different centrality measures.
TL;DR: In this article, it was shown that ultracompact objects with large rotation have a strong ergoregion instability when rapidly spinning, which can be detected by next-generation LIGO or LISA.
Abstract: Most of the properties of black holes can be mimicked by horizonless compact objects such as gravastars and boson stars. We show that these ultracompact objects develop a strong ergoregion instability when rapidly spinning. Instability time scales can be of the order of 0.1 seconds to 1 week for objects with mass $M=1\ensuremath{-}{10}^{6}{M}_{\ensuremath{\bigodot}}$ and angular momentum $Jg0.4{M}^{2}$. This provides a strong indication that ultracompact objects with large rotation are black holes. Explosive events due to ergoregion instability have a well-defined gravitational-wave signature. These events could be detected by next-generation gravitational-wave detectors such as Advanced LIGO or LISA.
TL;DR: In this article, the authors review the present understanding of azimuthal and single spin asymmetries for particle production in unpolarized and polarized hadronic collisions at high energy and moderately large transverse momentum.
TL;DR: In this article, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress.
Abstract: Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure.