189 research outputs found

    Neutron-induced fission cross sections of <math><mmultiscripts><mi>Th</mi><mprescripts/><none/><mn>232</mn></mmultiscripts></math> and <math><mmultiscripts><mi mathvariant="normal">U</mi><mprescripts/><none/><mn>233</mn></mmultiscripts></math> up to 1 GeV using parallel plate avalanche counters at the CERN n_TOF facility

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    International audienceThe neutron-induced fission cross sections of Th232 and U233 were measured relative to U235 in a wide neutron energy range up to 1 GeV (and from fission threshold in the case of Th232, and from 0.7 eV in case of U233), using the white-spectrum neutron source at the CERN Neutron Time-of-Flight (n_TOF) facility. Parallel plate avalanche counters (PPACs) were used, installed at the Experimental Area 1 (EAR1), which is located at 185 m from the neutron spallation target. The anisotropic emission of fission fragments were taken into account in the detection efficiency by using, in the case of U233, previous results available in EXFOR, whereas in the case of Th232 these data were obtained from our measurement, using PPACs and targets tilted 45∘ with respect to the neutron beam direction. Finally, the obtained results are compared with past measurements and major evaluated nuclear data libraries. Calculations using the high-energy reaction models INCL++ and ABLA07 were performed and some of their parameters were modified to reproduce the experimental results. At high energies, where no other neutron data exist, our results are compared with experimental data on proton-induced fission. Moreover, the dependence of the fission cross section at 1 GeV with the fissility parameter of the target nucleus is studied by combining those (p,f) data with our (n,f) data on Th232 and U233 and on other isotopes studied earlier at n_TOF using the same experimental setup

    Neutron-induced fission cross sections of Th 232 and U 233 up to 1 GeV using parallel plate avalanche counters at the CERN n_TOF facility

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    The neutron-induced fission cross sections of 232^{232}Th and 233^{233}U were measured relative to 235^{235}U in a wide neutron energy range up to 1 GeV (and from fission threshold in the case of 232^{232}Th, and from 0.7 eV in case of 233^{233}U), using the white-spectrum neutron source at the CERN Neutron Time-of-Flight (n_TOF) facility. Parallel plate avalanche counters (PPACs) were used, installed at the Experimental Area 1 (EAR1), which is located at 185 m from the neutron spallation target. The anisotropic emission of fission fragments were taken into account in the detection efficiency by using, in the case of 233^{233}U, previous results available in EXFOR, whereas in the case of 232^{232}Th these data were obtained from our measurement, using PPACs and targets tilted 45∘ with respect to the neutron beam direction. Finally, the obtained results are compared with past measurements and major evaluated nuclear data libraries. Calculations using the high-energy reaction models INCL++ and ABLA07 were performed and some of their parameters were modified to reproduce the experimental results. At high energies, where no other neutron data exist, our results are compared with experimental data on proton-induced fission. Moreover, the dependence of the fission cross section at 1 GeV with the fissility parameter of the target nucleus is studied by combining those (p,f) data with our (n,f) data on 232^{232}Th and 233^{233}U and on other isotopes studied earlier at n_TOF using the same experimental setup

    Zr 92 (n,g) and (n,tot) measurements at the GELINA and n_TOF facilities

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    92^{92}Zr (n,y) and (n,tot) measurements at the GELINA and n_TOF facilities

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    Background: Stellar nucleosynthesis of elements heavier than iron is driven by neutron capture processes. 92^{92} is positioned at a strategic point along the slow nucleosynthesis path, given its proximity to the neutron magic number N=50 and its position at the matching region between the weak and main slow processes. Purpose: In parallel with recent improved astronomical data, the extraction of accurate Maxwellian averaged cross sections (MACSs) derived from a more complete and accurate set of resonance parameters should allow for a better understanding of the stellar conditions at which nucleosynthesis takes place. Methods: Transmission and capture cross section measurements using enriched 92^{92}Zr metallic samples were performed at the time-of flight facilities GELINA of JRC-Geel (BE) and n_TOF of CERN (CH). The neutron beam passing through the samples was investigated in transmission measurements at GELINA using a Li-glass scintillator. The Îł rays emitted during the neutron capture reactions were detected by C6_{6}D6_{6} detectors at both GELINA and n_TOF. Results: Resonance parameters of individual resonances up to 81 keV were extracted from a combined resonance shape analysis of experimental transmissions and capture yields. For the majority of the resonances the parity was determined from an analysis of the transmission data obtained with different sample thicknesses. Average resonance parameters were calculated. Conclusions: Maxwellian averaged cross sections were extracted from resonances observed up to 81 keV. The MACS for kT=30keV is fully consistent with experimental data reported in the literature. The MACSs for kTâ‰Č15keV are in good agreement with those derived from the ENDF/B-VIII.0 library and recommended in the KADoNiS database. For kT higher than 30 keV differences are observed. A comparison with MACSs obtained with the cross sections recommended in the JEFF-3.3 and JENDL 4.0 libraries shows discrepancies even for kTâ‰Č15keV

    92^{92}Zr(n,Îłn,\gamma) and (nn,tot) measurements at the GELINA and n_TOF facilities

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    International audienceBackground: Stellar nucleosynthesis of elements heavier than iron is driven by neutron capture processes. 92Zris positioned at a strategic point along the slow nucleosynthesis path, given its proximity to the neutron magicnumber N = 50 and its position at the matching region between the weak and main slow processes.Purpose: In parallel with recent improved astronomical data, the extraction of accurate Maxwellian averagedcross sections (MACSs) derived from a more complete and accurate set of resonance parameters should allowfor a better understanding of the stellar conditions at which nucleosynthesis takes place.Methods: Transmission and capture cross section measurements using enriched 92Zr metallic samples wereperformed at the time-of flight facilities GELINA of JRC-Geel (BE) and n_TOF of CERN (CH). The neutronbeam passing through the samples was investigated in transmission measurements at GELINA using a Li-glassscintillator. The γ rays emitted during the neutron capture reactions were detected by C6D6 detectors at bothGELINA and n_TOF.Results: Resonance parameters of individual resonances up to 81 keV were extracted from a combined resonanceshape analysis of experimental transmissions and capture yields. For the majority of the resonances the paritywas determined from an analysis of the transmission data obtained with different sample thicknesses. Averageresonance parameters were calculated.Conclusions: Maxwellian averaged cross sections were extracted from resonances observed up to 81 keV. TheMACS for kT = 30 keV is fully consistent with experimental data reported in the literature. The MACSs forkT  15 keV are in good agreement with those derived from the ENDF/B-VIII.0 library and recommendedin the KADONIS database. For kT higher than 30 keV differences are observed. A comparison with MACSsobtained with the cross sections recommended in the JEFF-3.3 and JENDL-4.0 libraries shows discrepancieseven for kT  15 keV

    Measurement of the α ratio and (n,γ) cross section of U 235 from 0.2 to 200 eV at n_TOF MEASUREMENT of the α RATIO and ... J. BALIBREA-CORREA et al.

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    © 2020 the Author(s). Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.We measured the neutron capture-to-fission cross-section ratio (α ratio) and the capture cross section of U235 between 0.2 and 200 eV at the n_TOF facility at CERN. The simultaneous measurement of neutron-induced capture and fission rates was performed by means of the n_TOF BaF2 Total Absorption Calorimeter (TAC), used for detection of γ rays, in combination with a set of micromegas detectors used as fission tagging detectors. The energy dependence of the capture cross section was obtained with help of the Li6(n,t) standard reaction determining the n_TOF neutron fluence; the well-known integral of the U235(n,f) cross section between 7.8 and 11 eV was then used for its absolute normalization. The α ratio, obtained with slightly higher statistical fluctuations, was determined directly, without need for any reference cross section. To perform the analysis of this measurement we developed a new methodology to correct the experimentally observed effect that the probabilities of detecting a fission reaction in the TAC and the micromegas detectors are not independent. The results of this work have been used in a new evaluation of U235 performed within the scope of the Collaborative International Evaluated Library Organisation (CIELO) Project, and are consistent with the ENDF/B-VIII.0 and JEFF-3.3 capture cross sections below 4 eV and above 100 eV. However, the measured capture cross section is on average 10% larger between 4 and 100 eV.Peer reviewe

    STUDIES FOR AN ALTERNATIVE LHC NON-LINEAR COLLIMATION SYSTEM*

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    Abstract A LHC non-linear betatron cleaning collimation system would allow larger gap for the mechanical jaws, reducing as a consequence the collimator-induced impedance, which may limit the LHC beam intensity. In this paper, the performance of the proposed system is analyzed in terms of beam losses distribution around the LHC ring and cleaning efficiency in stable physics condition at 7TeV for Beam1. Moreover, the energy deposition distribution on the machine elements is compared to the present LHC Betatron cleaning collimation system in the Point 7 Insertion Region (IR)

    Simulations and Measurements of Cleaning with 100 MJ Beams in the LHC

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    Abstract The CERN Large Hadron Collider is routinely storing proton beam intensities of more than 100 MJ, which puts extraordinary demands on the control of beam losses to avoid quenches of the superconducting magnets. Therefore, a detailed understanding of the LHC beam cleaning is required. We present tracking and shower simulations of the LHC&apos;s multi-stage collimation system and compare with measured beam losses, which allow us to conclude on the predictive power of the simulations

    Atazanavir/ritonavir with lamivudine as maintenance therapy in virologically suppressed HIV-infected patients: 96 week outcomes of a randomized trial

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    Objectives: To investigate the long-term safety and efficacy of a treatment switch to dual ART with atazanavir/ritonavir+lamivudine versus continuing a standard regimen with atazanavir/ritonavir+2NRTI in virologically suppressed patients. Methods: ATLAS-M is a 96 week open-label, randomized, non-inferiority (margin -12%) trial enrolling HIV-infected adults on atazanavir/ritonavir+2NRTI, with stable HIV-RNA &lt; 50 copies/mL and CD4 counts.200 cells/mm 3 . At baseline, patients were randomized 1:1 to switch to atazanavir/ritonavir+lamivudine or to continue the previous regimen. Here, we report the 96 week efficacy and safety data. The study was registered with ClinicalTrials.gov, number NCT01599364. Results: Overall, 266 subjects were enrolled (133 in each arm). At 96 weeks, in the ITT population, patients free of treatment failure totalled 103 (77.4%) with atazanavir/ritonavir+lamivudine and 87 (65.4%) with triple therapy (difference +12.0%, 95% CI +1.2/+22.8, P=0.030), demonstrating the superiority of dual therapy. Two (1.5%) and 9 (6.8%) virological failures occurred in the dual-therapy arm and the triple-therapy arm, respectively, without development of resistance to any study drug. Clinical adverse events occurred at similar rates in both arms. A higher frequency of grade 3-4 hyperbilirubinemia (66.9% versus 50.4%, P=0.006) and hypertriglyceridaemia (6.8% versus 1.5%, P=0.031) occurred with dual therapy, although this never led to treatment discontinuation. A significant improvement in renal function and lumbar spine bone mineral density occurred in the dual-therapy arm. The evolution of CD4, HIV-DNA levels and neurocognitive performance was similar in both arms. Conclusions: In this randomized study, a treatment switch to atazanavir/ritonavir+lamivudine was superior over the continuation of atazanavir/ritonavir+2NRTI in virologically suppressed patients, with a sustained benefit in terms of improved renal function and bone mineral density

    Measurement and resonance analysis of the 33^{33}S(n,α) 30^{30}Si cross section at the CERN n_TOF facility in the energy region from 10 to 300 keV

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