3,461 research outputs found
Tracking the phase-transition energy in disassembly of hot nuclei
In efforts to determine phase transitions in the disintegration of highly
excited heavy nuclei, a popular practice is to parametrise the yields of
isotopes as a function of temperature in the form
, where 's are the measured yields
and and are fitted to the yields. Here would be
interpreted as the phase transition temperature. For finite systems such as
those obtained in nuclear collisions, this parametrisation is only approximate
and hence allows for extraction of in more than one way. In this work we
look in detail at how values of differ, depending on methods of
extraction. It should be mentioned that for finite systems, this approximate
parametrisation works not only at the critical point, but also for first order
phase transitions (at least in some models). Thus the approximate fit is no
guarantee that one is seeing a critical phenomenon. A different but more
conventional search for the nuclear phase transition would look for a maximum
in the specific heat as a function of temperature . In this case is
interpreted as the phase transition temperature. Ideally and would
coincide. We invesigate this possibility, both in theory and from the ISiS
data, performing both canonical () and microcanonical ()
calculations. Although more than one value of can be extracted from the
approximate parmetrisation, the work here points to the best value from among
the choices. Several interesting results, seen in theoretical calculations, are
borne out in experiment.Comment: Revtex, 10 pages including 8 figures and 2 table
Resilient Reducibility in Nuclear Multifragmentation
The resilience to averaging over an initial energy distribution of
reducibility and thermal scaling observed in nuclear multifragmentation is
studied. Poissonian reducibility and the associated thermal scaling of the mean
are shown to be robust. Binomial reducibility and thermal scaling of the
elementary probability are robust under a broad range of conditions. The
experimental data do not show any indication of deviation due to averaging.Comment: 5 pages, 6 figures, submitted to Physical Review
The liquid to vapor phase transition in excited nuclei
For many years it has been speculated that excited nuclei would undergo a
liquid to vapor phase transition. For even longer, it has been known that
clusterization in a vapor carries direct information on the liquid- vapor
equilibrium according to Fisher's droplet model. Now the thermal component of
the 8 GeV/c pion + 197Au multifragmentation data of the ISiS Collaboration is
shown to follow the scaling predicted by Fisher's model, thus providing the
strongest evidence yet of the liquid to vapor phase transition.Comment: four pages, four figures, first two in color (corrected typo in Ref.
[26], corrected error in Fig. 4
Laboratory Characterization and Influence of Mineralogy and Grading on the Performance of Treated and Untreated Granular Materials Used as Surface Pavements in Unpaved Road
This paper presents the results of a laboratory study on the influence of grading and granular mineralogy sources (granitic gneiss, basalt and limestone) on the performance of treated and untreated granular materials used as surface pavements in unpaved roads. Results of bearing capacity, resilient modulus and compressive strength tests are discussed. Natural brine, calcium chloride, cement and a polymer emulsion treated samples were tested and compared to untreated samples. The combinations of product, mineralogy and grading showing great performance are finally pointed out
The Role of Surface Entropy in Statistical Emission of Massive Fragments from Equilibrated Nuclear Systems
Statistical fragment emission from excited nuclear systems is studied within
the framework of a schematic Fermi-gas model combined with Weisskopf's detailed
balance approach. The formalism considers thermal expansion of finite nuclear
systems and pays special attention to the role of the diffuse surface region in
the decay of hot equilibrated systems. It is found that with increasing
excitation energy, effects of surface entropy lead to a systematic and
significant reduction of effective emission barriers for fragments and,
eventually, to the vanishing of these barriers. The formalism provides a
natural explanation for the occurrence of negative nuclear heat capacities
reported in the literature. It also accounts for the observed linearity of
pseudo-Arrhenius plots of the logarithm of the fragment emission probability
{\it versus} the inverse square-root of the excitation energy, but does not
predict true Arrhenius behavior of these emission probabilities
Z-dependent Barriers in Multifragmentation from Poissonian Reducibility and Thermal Scaling
We explore the natural limit of binomial reducibility in nuclear
multifragmentation by constructing excitation functions for intermediate mass
fragments (IMF) of a given element Z. The resulting multiplicity distributions
for each window of transverse energy are Poissonian. Thermal scaling is
observed in the linear Arrhenius plots made from the average multiplicity of
each element. ``Emission barriers'' are extracted from the slopes of the
Arrhenius plots and their possible origin is discussed.Comment: 15 pages including 4 .ps figures. Submitted to Phys. Rev. Letters.
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