649 research outputs found

    Density profiles and collective excitations of a trapped two component Fermi vapour

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    We discuss the ground state and the small-amplitude excitations of a degenerate vapour of fermionic atoms placed in two hyperfine states inside a spherical harmonic trap. An equations-of-motion approach is set up to discuss the hydrodynamic dissipation processes from the interactions between the two components of the fluid beyond mean-field theory and to emphasize analogies with spin dynamics and spin diffusion in a homogeneous Fermi liquid. The conditions for the establishment of a collisional regime via scattering against cold-atom impurities are analyzed. The equilibrium density profiles are then calculated for a two-component vapour of 40K atoms: they are little modified by the interactions for presently relevant values of the system parameters, but spatial separation of the two components will spontaneously arise as the number of atoms in the trap is increased. The eigenmodes of collective oscillation in both the total particle number density and the concentration density are evaluated analytically in the special case of a symmetric two-component vapour in the collisional regime. The dispersion relation of the surface modes for the total particle density reduces in this case to that of a one-component Fermi vapour, whereas the frequencies of all other modes are shifted by the interactions.Comment: 14 pages, 4 figure

    The high-lying 6^6Li levels at excitation energy around 21 MeV

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    The 3^3H+3^3He cluster structure in 6^6Li was investigated by the 3^3H(╬▒\alpha,3^3H 3^3He)n kinematically complete experiment at the incident energy E╬▒E_\alpha = 67.2 MeV. We have observed two resonances at ExÔłŚE_x^* = 21.30 and 21.90 MeV which are consistent with the 3^3He(3^3H, ╬│\gamma)6^6Li analysis in the Ajzenberg-Selove compilation. Our data are compared with the previous experimental data and the RGM and CSRGM calculations.Comment: 12 pages, 6 figures. Accepted for publication in J. Phys. Soc. Jp

    Set up and application of an underwater ╬Ĺ-ray spectrometer for radioactivity measurements

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    The set up and control of an underwater measuring instrument for radioactivity pollution in the marine environment is described. The detection system is based on a NaI scintillator (RADAM III) with modifications for use in the marine environment with on-line measurements. The system is simple, has low power consumption and is stable for long-term monitoring (10 months). Before its deployment, the sensor was calibrated in the laboratory in a tank full of water to reproduce the marine environment. The calibrations were performed, by detecting the 661keV and 1461 keV gamma rays of known activity liquid sources 137 Cs and 40 K, respectively. The measured spectra in the laboratory were compared with spectra from a similar detector as acquired in the field. The analysis of the parallel measurement gave satisfactory agreement for the concentration of the potassium (40 K), as calculated from the salinity in the seawater, thus enabling the system for quantitative measurement of the seawater radioactivity

    Measurement of the 242Pu(n,f) cross section at n_TOF

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    Knowledge of neutron cross sections of various plutonium isotopes and other minor actinides is crucial for the design of advanced nuclear systems. The 242Pu(n,f) cross sections were measured at the CERN n-TOF facility, taking advantage of the wide energy range (from thermal to GeV) and the high instantaneous flux of the neutron beam. In this work, preliminary results are presented along with a theoretical cross section calculation performed with the EMPIRE code. ┬ę Owned by the authors, published by EDP Sciences, 2014

    Neutron-induced fission cross section of 234 U measured at the CERN n_TOF facility

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    The neutron-induced fission cross section of 234U has been measured at the CERN n-TOF facility relative to the standard fission cross section of 235U from 20 keV to 1.4 MeV and of 238U from 1.4 to 200 MeV. A fast ionization chamber (FIC) was used as a fission fragment detector with a detection efficiency of no less than 97%. The high instantaneous flux and the low background characterizing the n-TOF facility resulted in wide-energy-range data (0.02 to 200 MeV), with high energy resolution, high statistics, and systematic uncertainties bellow 3%. Previous investigations around the energy of the fission threshold revealed structures attributed to ╬▓-vibrational levels, which have been confirmed by the present measurements. Theoretical calculations have been performed, employing the talys code with model parameters tuned to fairly reproduce the experimental data

    Measurement of the240Pu(n,f) cross-section at the CERN n-TOF facility: First results from EAR-2

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    The accurate knowledge of neutron cross-sections of a variety of plutonium isotopes and other minor actinides, such as neptunium, americium and curium, is crucial for feasibility and performance studies of advanced nuclear systems (Generation-IV reactors, Accelerator Driven Systems). In this context, the240Pu(n,f) cross-section was measured with the time-of-flight technique at the CERN n-TOF facility at incident neutron energies ranging from thermal to several MeV. The present measurement is the first to have been performed at n-TOF's newly commissioned Experimental Area II (EAR-2), which is located at the end of an 18 m neutron beam-line and features a neutron fluence that is 25-30 times higher with respect to the existing 185 m flight-path (EAR-1), as well as stronger suppression of sample-induced backgrounds, due to the shorter times-of-flight involved. Preliminary results are presented. ┬ę 2015, CERN. All rights reserved.Postprint (published version

    Investigation of the 240Pu(n, f ) reaction at the n_TOF/EAR2 facility in the 9 meVÔÇô6 MeV range

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    Background: Nuclear waste management is considered amongst the major challenges in the field of nuclear energy. A possible means of addressing this issue is waste transmutation in advanced nuclear systems, whose operation requires a fast neutron spectrum. In this regard, the accurate knowledge of neutron-induced reaction cross sections of several (minor) actinide isotopes is essential for design optimization and improvement of safety margins of such systems. One such case is 240 Pu , due to its accumulation in spent nuclear fuel of thermal reactors and its usage in fast reactor fuel. The measurement of the 240 Pu ( n , f ) cross section was previously attempted at the CERN n_TOF facility EAR1 measuring station using the time-of-flight technique. Due to the low amount of available material and the given flux at EAR1, the measurement had to last several months to achieve a sufficient statistical accuracy. This long duration led to detector deterioration due to the prolonged exposure to the high ╬▒ activity of the fission foils, therefore the measurement could not be successfully completed. Purpose: It is aimed to determine whether it is feasible to study neutron-induced fission at n_TOF/EAR2 and provide data on the 240 Pu ( n , f ) reaction in energy regions requested for applications. Methods: The study of the 240 Pu ( n , f ) reaction was made at a new experimental area (EAR2) with a shorter flight path which delivered on average 30 times higher flux at fast neutron energies. This enabled the measurement to be performed much faster, thus limiting the exposure of the detectors to the intrinsic activity of the fission foils. The experimental setup was based on microbulk Micromegas detectors and the time-of-flight data were analyzed with an optimized pulse-shape analysis algorithm. Special attention was dedicated to the estimation of the non-negligible counting loss corrections with the development of a new methodology, and other corrections were estimated via Monte Carlo simulations of the experimental setup. Results: This new measurement of the 240 Pu ( n , f ) cross section yielded data from 9 meV up to 6 MeV incident neutron energy and fission resonance kernels were extracted up to 10 keV . Conclusions: Neutron-induced fission of high activity samples can be successfully studied at the n_TOF/EAR2 facility at CERN covering a wide range of neutron energies, from thermal to a few MeV.Croatian Science Foundation 857

    Isomeric cross sections of fast-neutron induced reactions on 197^{197}Au

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    Recent accurate data obtained for the isomeric cross section of the 197^{197}Au(n,2n)(n,2n) reaction provide a valuable opportunity to consider the question of the effective moment of inertia of the nucleus within a local consistent model analysis of all available reaction data for the 197^{197}Au target nucleus. Thus, a definite proof of a moment of inertia equal to that of the rigid--body has been obtained for 196^{196}Au nucleus while an inference of the half rigid--body value is suggested for the 194^{194}Ir nucleus. The usefulness of further measurements at incident energies up to Ôëą\ge40 MeV has also been proved.Comment: 5 pages, 5 figure

    Measurement of the neutron capture cross section of the s-only isotope 204Pb from 1 eV to 440 keV

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    The neutron capture cross section of 204Pb has been measured at the CERN n_TOF installation with high resolution in the energy range from 1 eV to 440 keV. An R-matrix analysis of the resolved resonance region, between 1 eV and 100 keV, was carried out using the SAMMY code. In the interval between 100 keV and 440 keV we report the average capture cross section. The background in the entire neutron energy range could be reliably determined from the measurement of a 208Pb sample. Other systematic effects in this measurement could be investigated and precisely corrected by means of detailed Monte Carlo simulations. We obtain a Maxwellian average capture cross section for 204Pb at kT=30 keV of 79(3) mb, in agreement with previous experiments. However our cross section at kT=5 keV is about 35% larger than the values reported so far. The implications of the new cross section for the s-process abundance contributions in the Pb/Bi region are discussed.Comment: 8 pages, 3 figures, article submitted to Phys. Rev.

    Ni-62(n,gamma) and Ni-63(n,gamma) cross sections measured at the n_TOF facility at CERN

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    The cross section of the Ni-62(n,gamma) reaction was measured with the time-of-flight technique at the neutron time-of-flight facility n_TOF at CERN. Capture kernels of 42 resonances were analyzed up to 200 keV neutron energy and Maxwellian averaged cross sections (MACS) from kT = 5-100 keV were calculated. With a total uncertainty of 4.5%, the stellar cross section is in excellent agreement with the the KADoNiS compilation at kT = 30 keV, while being systematically lower up to a factor of 1.6 at higher stellar temperatures. The cross section of the Ni-63(n,gamma) reaction was measured for the first time at n_TOF. We determined unresolved cross sections from 10 to 270 keV with a systematic uncertainty of 17%. These results provide fundamental constraints on s-process production of heavier species, especially the production of Cu in massive stars, which serve as the dominant source of Cu in the solar system.Peer reviewedFinal Accepted Versio
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