400 research outputs found
Calculation of the number of partitions with constraints on the fragment size
This article introduces recursive relations allowing the calculation of the
number of partitions with constraints on the minimum and/or on the maximum
fragment size
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
Statistical Exploration of Fragmentation Phase Space Source Sizes in Nuclear Multifragmentation
The multiplicity distributions for individual fragment Z values in nuclear
multifragmentation are binomial. The extracted maximum value of the
multiplicity is found to depend on Z according to m=Z_0/Z, where Z_0 is the
source size. This is shown to be a strong indication of statistical coverage of
fragmentation phase space. The inferred source sizes coincide with those
extracted from the analysis of fixed multiplicity charge distributions.Comment: 13 pages, 4 revised figures, some revised tex
Statistical Interpretation of Joint Multiplicity Distributions of Neutrons and Charged Particles
Experimental joint multiplicity distributions of neutrons and charged
particles emitted in complex nuclear reactions provide an important test of
theoretical models. The method is applied to test three different theoretical
models of nuclear multi-fragmentation, two of which fail the test. The
measurement of neutrons is decisive in distinguishing between the Berlin and
Copenhagen models of nuclear multi-fragmentation and challenges the
interpretation of pseudo- Arrhenius plots. Statistical-model evaporation
calculations with GEMINI give a good reproduction first and second moments of
the experimental multiplicity correlations.Comment: 12 pages, 3 figures Added GEMINI calculations of multiplicity
correlations Added brief discussion of how neutron emission is treated in
MMM
Scaling Laws and Transient Times in 3He Induced Nuclear Fission
Fission excitation functions of compound nuclei in a mass region where shell
effects are expected to be very strong are shown to scale exactly according to
the transition state prediction once these shell effects are accounted for. The
fact that no deviations from the transition state method have been observed
within the experimentally investigated excitation energy regime allows one to
assign an upper limit for the transient time of 10 zs.Comment: 7 pages, TeX type, psfig, submitted to Phys. Rev. C, also available
at http://csa5.lbl.gov/moretto/ps/he3_paper.p
A statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy
Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV
has been studied with multidetector systems covering nearly 4-pi in solid
angle. The correlations of both the intermediate mass fragment and light
charged particle multiplicities with the transverse energy are explored. A
comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The
experimental trends are compared to statistical model predictions.Comment: 7 pages, submitted to Phys. Rev.
Size Matters: Origin of Binomial Scaling in Nuclear Fragmentation Experiments
The relationship between measured transverse energy, total charge recovered
in the detector, and size of the emitting system is investigated. Using only
very simple assumptions, we are able to reproduce the observed binomial
emission probabilities and their dependences on the transverse energy.Comment: 14 pages, including 4 figure
Statistical Models of Nuclear Fragmentation
A method is presented that allows exact calculations of fragment multiplicity
distributions for a canonical ensemble of non-interacting clusters.
Fragmentation properties are shown to depend on only a few parameters.
Fragments are shown to be copiously produced above the transition temperature.
At this transition temperature, the calculated multiplicity distributions
broaden and become strongly super-Poissonian. This behavior is compared to
predictions from a percolation model. A corresponding microcanonical formalism
is also presented.Comment: 12 pages, 5 figure
Temperature-induced pair correlations in clusters and nuclei
The pair correlations in mesoscopic systems such as -size superconducting
clusters and nuclei are studied at finite temperature for the canonical
ensemble of fermions in model spaces with a fixed particle number: i) a
degenerate spherical shell (strong coupling limit), ii) an equidistantly spaced
deformed shell (weak coupling limit). It is shown that after the destruction of
the pair correlations at T=0 by a strong magnetic field or rapid rotation,
heating can bring them back. This phenomenon is a consequence of the fixed
number of fermions in the canonical ensemble
Multifragmentation of non-spherical nuclei
The shape influence of decaying thermalized source on various characteristics
of multifragmentation as well as its interplay with effects of angular momentum
and collective expansion are first studied and the most pertinent variables are
proposed. The analysis is based on the extension of the statistical
microcanonical multifragmentation model.Comment: 5 pages, 4 figure
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