76 research outputs found
Dynamics of barrier penetration in thermal medium: exact result for inverted harmonic oscillator
Time evolution of quantum tunneling is studied when the tunneling system is
immersed in thermal medium. We analyze in detail the behavior of the system
after integrating out the environment. Exact result for the inverted harmonic
oscillator of the tunneling potential is derived and the barrier penetration
factor is explicitly worked out as a function of time. Quantum mechanical
formula without environment is modifed both by the potential renormalization
effect and by a dynamical factor which may appreciably differ from the
previously obtained one in the time range of 1/(curvature at the top of
potential barrier).Comment: 30 pages, LATEX file with 11 PS figure
The role of slip ratio in rolling contact fatigue of rail materials under wet conditions
Rolling contact fatigue (RCF) of rail is a significant factor affecting the maintenance and service safety of railway track. While the driving parameters are known, clearer relationships are needed with fatigue life. The objective of this study was to explore the role of slip ratio in the development of RCF cracks and fatigue life of rail materials under a water lubricated condition. The results indicate that slip ratio has a vital and interesting influence in the wear and RCF life of rail materials. With an increase of slip ratio from 0 to 0.3%, fatigue life of rail materials has an obvious decrease. As slip ratio increases to 1%, the life increases. Then, RCF life has a drop as slip ratio changes from 1% to 5% and 10%. This may be a comprehensive effect resulting from various contact characteristics of stick area and slip area in the contact area and a competitive relationship between wear and RCF. The increase of slip ratio significantly increases the growth angle of cracks and transforms the damage mechanism of rail materials from slight surface fatigue to serious fatigue and pitting damage
Childhood leukaemia in Europe after Chernobyl: 5 year follow-up.
The European Childhood Leukaemia - Lymphoma Incidence Study (ECLIS) is designed to address concerns about a possible increase in the risk of cancer in Europe following the nuclear accident in Chernobyle in 1986. This paper reports results of surveillance of childhood leukaemia in cancer registry populations from 1980 up to the end of 1991. There was a slight increase in the incidence of childhood leukaemia in Europe during this period, but the overall geographical pattern of change bears no relation to estimated exposure to radiation resulting from the accident. We conclude that at this stage of follow-up any changes in incidence consequent upon the Chernobyl accident remain undetectable against the usual background rates. Our results are consistent with current estimates of the leukaemogenic risk of radiation exposure, which, outside the immediate vicinity of the accident, was small
Perfil de ácidos graxos e composição química do músculo longissimus dorsi de cordeiros alimentados com dietas contendo polpa cítrica
Dust in Supernovae and Supernova Remnants II: Processing and survival
Observations have recently shown that supernovae are efficient dust factories, as predicted for a long time by theoretical models. The rapid evolution of their stellar progenitors combined with their efficiency in precipitating refractory elements from the gas phase into dust grains make supernovae the major potential suppliers of dust in the early Universe, where more conventional sources like Asymptotic Giant Branch (AGB) stars did not have time to evolve. However, dust yields inferred from observations of young supernovae or derived from models do not reflect the net amount of supernova-condensed dust able to be expelled from the remnants and reach the interstellar medium. The cavity where the dust is formed and initially resides is crossed by the high velocity reverse shock which is generated by the pressure of the circumstellar material shocked by the expanding supernova blast wave. Depending on grain composition and initial size, processing by the reverse shock may lead to substantial dust erosion and even complete destruction. The goal of this review is to present the state of the art about processing and survival of dust inside supernova remnants, in terms of theoretical modelling and comparison to observations
Neutron intensity and energy distributions from protons in the energy range 1.95 MeV to 5.5 MeV incident on thick targets of lithium
Analysis of neutron flux distributions generated by sources of fast neutrons in a range of bituminous coals
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