2,027 research outputs found
Reaction cross-section predictions for nucleon induced reactions
A microscopic calculation of the optical potential for nucleon-nucleus
scattering has been performed by explicitly coupling the elastic channel to all
the particle-hole (p-h) excitation states in the target and to all relevant
pickup channels. These p-h states may be regarded as doorway states through
which the flux flows to more complicated configurations, and to long-lived
compound nucleus resonances. We calculated the reaction cross sections for the
nucleon induced reactions on the targets Ca, Ni, Zr and
Sm using the QRPA description of target excitations, coupling to all
inelastic open channels, and coupling to all transfer channels corresponding to
the formation of a deuteron. The results of such calculations were compared to
predictions of a well-established optical potential and with experimental data,
reaching very good agreement. The inclusion of couplings to pickup channels
were an important contribution to the absorption. For the first time,
calculations of excitations account for all of the observed reaction
cross-sections, at least for incident energies above 10 MeV.Comment: 6 pages, 6 figures. Submitted to INPC 2010 Conference Proceeding
The equations of motion for moist atmospheric air
How phase transitions affect the motion of moist atmospheric air remains
controversial. In the early 2000s two distinct differential equations of motion
were proposed. Besides their contrasting formulations for the acceleration of
condensate, the equations differ concerning the presence/absence of a term
equal to the rate of phase transitions multiplied by the difference in velocity
between condensate and air. This term was interpreted in the literature as the
"reactive motion" associated with condensation. The reasoning behind this
"reactive motion" was that when water vapor condenses and droplets begin to
fall the remaining gas must move upwards to conserve momentum. Here we show
that the two contrasting formulations imply distinct assumptions about how
gaseous air and condensate particles interact. We show that these assumptions
cannot be simultaneously applicable to condensation and evaporation. "Reactive
motion" leading to an upward acceleration of air during condensation does not
exist. The "reactive motion" term can be justified for evaporation only; it
describes the downward acceleration of air. We emphasize the difference between
the equations of motion (i.e., equations constraining velocity) and those
constraining momentum (i.e., equations of motion and continuity combined). We
show that, owing to the imprecise nature of the continuity equations,
consideration of total momentum can be misleading and that this led to the
"reactive motion" controversy. Finally, we provide a revised and generally
applicable equation for the motion of moist air.Comment: 11 pages, two figure
The Effect of Retro-Cueing on an ERP Marker of VSTM Maintenance
Previous research has found that Contralateral Delay Activity (CDA) is correlated with the number of items maintained in Visual Short Term Memory from one visual field (VF) (Vogel & Machizawa, 2004). CDA is usually elicited by a to-be-remembered array after a prospective cue (pro-cue) signalling the relevant side of the visual display, and is interpreted as a putative electrophysiological signature of WM maintenance. Attention can also be directed to the contents of VSTM, after the presentation of a visual array, using a retroactive cue (retro-cue) (Nobre, Griffin, & Rao, 2008). Because retro-cueing directs attention within a memory trace, potentially reducing the load of items to be maintained, we hypothesised that this would significantly attenuate the CDA. Participants were initially presented with a spatial pro-cue which reduced the number of to-be-remembered items to one side. After a delay, a memory array of either four (low load) or eight (high load) items was displayed. A retro-cue then cued participants to one location within the relevant VF, further reducing the load of to-be-remembered items; or provided no information, requiring participants to hold all items in the relevant VF. At the end of the trial, participants performed a same/different judgement on a test stimulus. Retro-cues significantly improved VSTM performance. Unexpectedly, the CDA was found to be abolished by the presentation of both spatially predictive and neutral cues, independently of the VSTM load participants had to maintain
Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics
Phase transitions of atmospheric water play a ubiquitous role in the Earth's
climate system, but their direct impact on atmospheric dynamics has escaped
wide attention. Here we examine and advance a theory as to how condensation
influences atmospheric pressure through the mass removal of water from the gas
phase with a simultaneous account of the latent heat release. Building from the
fundamental physical principles we show that condensation is associated with a
decline in air pressure in the lower atmosphere. This decline occurs up to a
certain height, which ranges from 3 to 4 km for surface temperatures from 10 to
30 deg C. We then estimate the horizontal pressure differences associated with
water vapor condensation and find that these are comparable in magnitude with
the pressure differences driving observed circulation patterns. The water vapor
delivered to the atmosphere via evaporation represents a store of potential
energy available to accelerate air and thus drive winds. Our estimates suggest
that the global mean power at which this potential energy is released by
condensation is around one per cent of the global solar power -- this is
similar to the known stationary dissipative power of general atmospheric
circulation. We conclude that condensation and evaporation merit attention as
major, if previously overlooked, factors in driving atmospheric dynamics
Towards an optical potential for rare-earths through coupled channels
The coupled-channel theory is a natural way of treating nonelastic channels,
in particular those arising from collective excitations, defined by nuclear
deformations. Proper treatment of such excitations is often essential to the
accurate description of reaction experimental data. Previous works have applied
different models to specific nuclei with the purpose of determining
angular-integrated cross sections. In this work, we present an extensive study
of the effects of collective couplings and nuclear deformations on integrated
cross sections as well as on angular distributions in a consistent manner for
neutron-induced reactions on nuclei in the rare-earth region. This specific
subset of the nuclide chart was chosen precisely because of a clear static
deformation pattern. We analyze the convergence of the coupled-channel
calculations regarding the number of states being explicitly coupled. Inspired
by the work done by Dietrich \emph{et al.}, a model for deforming the spherical
Koning-Delaroche optical potential as function of quadrupole and hexadecupole
deformations is also proposed. We demonstrate that the obtained results of
calculations for total, elastic and inelastic cross sections, as well as
elastic and inelastic angular distributions correspond to a remarkably good
agreement with experimental data for scattering energies above around a few
MeV.Comment: 7 pages, 6 figures. Submitted to the proceedings of the XXXVI
Reuni\~ao de Trabalho de F\'{\i}sica Nuclear no Brasil (XXXVI Brazilian
Workshop on Nuclear Physics), held in Maresias, S\~ao Paulo, Brazil in
September 2013, which should be published on AIP Conference Proceeding
Series. arXiv admin note: substantial text overlap with arXiv:1311.1115,
arXiv:1311.042
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