8,011 research outputs found
Direct Experimental Evidence for Differing Reactivity Alterations of Minerals following Irradiation: The Case of Calcite and Quartz
Concrete, a mixture formed by mixing cement, water, and fine and coarse
mineral aggregates is used in the construction of nuclear power plants (NPPs),
e.g., to construct the reactor cavity concrete that encases the reactor
pressure vessel, etc. In such environments, concrete may be exposed to
radiation (e.g., neutrons) emanating from the reactor core. Until recently,
concrete has been assumed relatively immune to radiation exposure. Direct
evidence acquired on Ar-ion irradiated calcite and quartz indicates, on the
contrary, that, such minerals, which constitute aggregates in concrete, may be
significantly altered by irradiation. Specifically, while quartz undergoes
disordering of its atomic structure resulting in a near complete lack of
periodicity, i.e., similar to glassy silica, calcite only experiences random
rotations, and distortions of its carbonate groups. As a result, irradiated
quartz shows a reduction in density of around 15%, and an increase in chemical
reactivity, described by its dissolution rate, similar to a glassy silica;
i.e., an increase of around 3 orders of magnitude. Calcite however, shows
little change in dissolution rates - although its density noted to reduce by
around 9%. These differences are correlated with the nature of bonds in these
minerals, i.e., being dominantly ionic or covalent, and the rigidity of the
mineral's atomic network that is characterized by the number of topological
constraints (n) that are imposed on the atoms in the network. The outcomes
are discussed within the context of the durability of concrete structural
elements formed with calcitic/quartzitic aggregates in nuclear power plants
Towards a Unified Quark-Hadron Matter Equation of State for Applications in Astrophysics and Heavy-Ion Collisions
We outline an approach to a unified equation of state for quark-hadron matter
on the basis of a derivable approach to the generalized Beth-Uhlenbeck
equation of state for a cluster decomposition of thermodynamic quantities like
the density. To this end we summarize the cluster virial expansion for nuclear
matter and demonstrate the equivalence of the Green's function approach and the
derivable formulation. For an example, the formation and dissociation of
deuterons in nuclear matter is discussed. We formulate the cluster
derivable approach to quark-hadron matter which allows to take into
account the specifics of chiral symmetry restoration and deconfinement in
triggering the Mott-dissociation of hadrons. This approach unifies the
description of a strongly coupled quark-gluon plasma with that of a
medium-modified hadron resonance gas description which are contained as
limiting cases. The developed formalism shall replace the common two-phase
approach to the description of the deconfinement and chiral phase transition
that requires a phase transition construction between separately developed
equations of state for hadronic and quark matter phases. Applications to the
phenomenology of heavy-ion collisions and astrophysics are outlined.Comment: 35 pages, 3 figures, Special Issue "Compact Stars in the QCD Phase
Diagram
Charmonium Suppression and Regeneration from SPS to RHIC
The production of charmonia is investigated for heavy-ion collisions from SPS
to RHIC energies. Our approach incorporates two sources of yield: (i)
a direct contribution arising from early (hard) parton-parton collisions,
subject to subsequent nuclear absorption, quark-gluon plasma and hadronic
dissociation, and (ii) statistical production at the hadronization transition
by coalescence of and quarks. Within an expanding thermal
fireball framework, the model reproduces centrality dependencies
observed at the SPS in Pb-Pb and S-U collisions reasonably well. The study of
the ratio at SPS points at the importance of the hadronic phase
for interactions, possibly related to effects of chiral symmetry
restoration. Predictions are given for the centrality dependence of the
ratio at full RHIC energy. We also calculate the
excitation function of this ratio. The latter exhibits a characteristic minimum
structure signalling the transition from the standard suppression
scenario prevailing at SPS to dominantly thermal regeneration at collider
energies.Comment: 17 pages, 15 figure
Composition and thermodynamics of nuclear matter with light clusters
We investigate nuclear matter at finite temperature and density, including
the formation of light clusters up to the alpha particle The novel feature of
this work is to include the formation of clusters as well as their dissolution
due to medium effects in a systematic way using two many-body theories: a
microscopic quantum statistical (QS) approach and a generalized relativistic
mean field (RMF) model. Nucleons and clusters are modified by medium effects.
Both approaches reproduce the limiting cases of nuclear statistical equilibrium
(NSE) at low densities and cluster-free nuclear matter at high densities. The
treatment of the cluster dissociation is based on the Mott effect due to Pauli
blocking, implemented in slightly different ways in the QS and the generalized
RMF approaches. We compare the numerical results of these models for cluster
abundances and thermodynamics in the region of medium excitation energies with
temperatures T <= 20 MeV and baryon number densities from zero to a few times
saturation density. The effect of cluster formation on the liquid-gas phase
transition and on the density dependence of the symmetry energy is studied.
Comparison is made with other theoretical approaches, in particular those,
which are commonly used in astrophysical calculations. The results are relevant
for heavy-ion collisions and astrophysical applications.Comment: 32 pages, 15 figures, minor corrections, accepted for publication in
Physical Review
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