Étoile à neutrons

Résumé

thumb|300px|RX J1856.5-3754, une étoile à neutrons isolée proche du Système solaire, dont l'émission de surface est vue par le télescope spatial Hubble. Une étoile à neutrons est un astre principalement composé de neutrons maintenus ensemble par les forces de gravitation. De tels objets sont le résidu compact issu de l'effondrement gravitationnel du cœur de certaines étoiles massives lorsque celles-ci ont épuisé leur combustible nucléaire. Une étoile à neutrons peut présenter différents aspects. Si elle tourne rapidement sur elle-même et qu'elle possède un puissant champ magnétique, elle projette alors le long de son axe magnétique un mince pinceau traversant ou pas de radiations, et un observateur placé approximativement dans la direction de cet axe observera une émission pulsée par un effet de phare, appelée pour cette raison pulsar. Une étoile à neutrons située dans un système binaire peut arracher de la matière à son étoile compagnon et donner lieu à une émission pulsée ou contin

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https://fr.wikipedia.org/wiki/%C3%89toile_%C3%A0_neutrons

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Shape staggering of midshell mercury isotopes from in-source laser spectroscopy compared with density-functional-theory and Monte Carlo shell-model calculations

Neutron-deficient Hg177-185 isotopes were studied using in-source laser resonance-ionization spectroscopy at the CERN-ISOLDE radioactive ion-beam facility in an experiment combining different detection methods tailored to the studied isotopes. These include either alpha-decay tagging or multireflection time-of-flight gating for isotope identification. The endpoint of the odd-even nuclear shape staggering in mercury was observed directly by measuring for the first time the isotope shifts and hyperfine structures of Hg177-185. Changes in the meansquare charge radii for all mentioned isotopes, magnetic dipole, and electric quadrupole moments of the odd-A isotopes and arguments in favor of I = 7/2 spin assignment for Hg-177,Hg-179 were deduced. Experimental results are compared with density functional theory (DFT) and Monte Carlo shell model (MCSM) calculations. DFT calculations using Skyrme parametrizations predict a jump in the charge radius around the neutron N = 104 midshell, with an odd-even staggering pattern related to the coexistence of nearly degenerate oblate and prolate minima This near-degeneracy is highly sensitive to many aspects of the effective interaction, a fact that renders perfect agreement with experiments out of reach for current functionals. Despite this inherent difficulty, the SLy5s1 and a modified UNEDF1(SO) parametrization predict a qualitatively correct staggering that is off by two neutron numbers. MCSM calculations of states with the experimental spins and parities show good agreement for both electromagnetic moments and the observed charge radii. A clear mechanism for the origin of shape staggering within this context is identified: a substantial change in occupancy of the proton pi h(9/2) and neutron nu i(13/2) orbitals.

AMER PHYSICAL SOC2019

Chargement

CERN is presently designing a new chain of accelerators to replace the present Proton Synchrotron (PS) complex: a 160 MeV room-temperature H- linac (Linac4) to replace the present 50 MeV proton linac injector, a 3.5 GeV Superconducting Proton Linac (SPL) to replace the 1.4 GeV PS booster (PSB) and a 50 GeV synchrotron (named PS2) to replace the 26 GeV PS. Linac4 has been funded and the civil engineering work started in October 2008, whilst the SPL is in an advanced stage of design. Beyond injecting into the future 50 GeV PS, the ultimate goal of the SPL is to generate a 4 MW beam for the production of intense neutrino beams. The radiation protection design is driven by the latter requirement. This thesis summarizes the radiation protection studies conducted for Linac4. FLUKA Monte Carlo simulations, complemented by analytical estimates, were performed 1) to evaluate the propagation of neutrons through the waveguide, ventilation and cable ducts placed along the accelerator, 2) to estimate the radiological impact of the accelerator in its low energy section, where the access area is located, and 3) to calculate the induced radioactivity in the air and in the components of the accelerator. The latter study is particularly important for maintenance interventions and final disposal of radioactive waste. Two possible layouts for the CCDTL section of the machine were considered in order to evaluate the feasibility, from the radiological standpoint, of replacing electromagnetic quadrupoles with permanent magnet quadrupoles with high content of cobalt. The present work provides complete information on beam loss assumptions, accelerator structure and duct and maze design, in order to make the present results of sufficiently general interest and provide guidelines for similar studies for intermediate energy proton accelerators. In addition to the Linac4-specific studies, this thesis also discusses FLUKA simulations performed to test the capability of the code, in a proton therapy application, in predicting induced radioactivity from intermediate energy protons. The thesis also reviews the analytical models mostly used for the calculation of neutron streaming through penetration traversing shielding barriers, discusses the FLUKA simulations performed to test the reliability of these models and, on the basis of the simulations, derives a universal expression that can be used to estimate the neutron transmission through a straight duct in direct view of the source, model missing so far in the literature.

EPFL2009

Chargement

Characterisation of the activation in Crocus

The aim of this study is to characterize the activation of the crocus cavity. For this purpose, the concrete plug of the irradiation channel crossing the cavity shielding was used to measure the activation by gamma spectrometry. In a first time measurements were made after a long cooling time to determine the residual activation. Then, the plug was irradiated at 40Watts for 2 hours to provide a sufficient irradiation to the concrete. At the same time, measurements were led thanks to the dosimetry by thermoluminescence in the cavity. TLDs measurements gave reaction rates in Li6 at strategical positions detailed on this report. TLDs positions were defined in a Cartesian system centred on the centre core. 2D plans based on previous plan were designed to improve the calculation accuracy of the neutrons flux simulations. The analysis shown a very low activity of the concrete plug, below the low level waste thresholds. However results provided by TLDs and the Nagra’s calculations allowed to highlight a greater neutrons fluence in the upper part of the cavity. So, it might be relevant to pursue the gamma ray spectrometry measurements with concrete plugs from other cavity areas where the neutron flux should be higher.

2018

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EPFL2009