52 research outputs found
Liquid-Gas Coexistence and Critical Behavior in Boxed Pseudo-Fermi Matter
A schematic model is presented that allows one to study the behavior of
interacting pseudo-Fermi matter locked in a thermostatic box. As a function of
the box volume and temperature, the matter is seen to show all of the familiar
charactersitics of a Van der Waals gas, which include the coexistence of two
phases under certain circumstances and the presence of a critical point
Probing the Concept of Statistical Independence of Intermediate-Mass Fragment Production in Heavy-Ion Collisions
It is found that the total IMF-transverse-energy (E_t) spectra in multi-IMF
events are well represented by synthetic spectra obtained by folding of the
single-IMF spectrum. Using the experimental IMF multiplicity distribution, the
observed trends in the IMF multiplicity distribution for fixed values of E_t
are reproduced. The synthetic distributions show binomial reducibility and
Arrhenius-like scaling, similar to that reported in the literature. Similar
results are obtained when the above folding-type synthesis is replaced with one
based on mixing events with different IMF multiplicities. For statistically
independent IMF emission, the observed binomial reducibility and Arrhenius-type
scaling are merely reflections of the shape of the single-IMF transverse-energy
spectrum. Hence, a valid interpretation of IMF distributions in terms of a
particular production scenario has to explain independently the observed shape
of the single-IMF E_t spectrum.Comment: 13 pages with 8 figur
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 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
A Simple Method for Rise-Time Discrimination of Slow Pulses from Charge-Sensitive Preamplifiers
Performance of a simple method of particle identification via pulse rise time
discrimination is demonstrated for slow pulses from charge-sensitive
preamplifiers with rise times ranging from 10 ns to 500 ns. The method is based
on a comparison of the amplitudes of two pulses, derived from each raw
preamplifier pulse with two amplifiers with largely differing shaping times,
using a fast peak-sensing ADC. For the injected charges corresponding to energy
deposits in silicon detectors of a few tens of MeV, a rise time resolution of
the order of 1 ns can be achieved. The identification method is applicable in
particle experiments involving large-area silicon detectors, but is easily
adaptable to other detectors with a response corresponding to significantly
different pulse rise times for different particle species.Comment: 10 pages, 7 figure
Liquid-Drop Model and Quantum Resistance Against Noncompact Nuclear Geometries
The importance of quantum effects for exotic nuclear shapes is demonstrated.
Based on the example of a sheet of nuclear matter of infinite lateral
dimensions but finite thickness, it is shown that the quantization of states in
momentum space, resulting from the confinement of the nucleonic motion in the
conjugate geometrical space, generates a strong resistance against such a
confinement and generates restoring forces driving the system towards compact
geometries. In the liquid-drop model, these quantum effects are implicitly
included in the surface energy term, via a choice of interaction parameters, an
approximation that has been found valid for compact shapes, but has not yet
been scrutinized for exotic shapes.Comment: 9 pages with 3 figure
Angular momentum sharing in dissipative collisions
Light charged particles emitted by the projectile-like fragment were measured
in the direct and reverse collision of Nb and Sn at 25 AMeV. The
experimental multiplicities of Hydrogen and Helium particles as a function of
the primary mass of the emitting fragment show evidence for a correlation with
net mass transfer. The ratio of Hydrogen and Helium multiplicities points to a
dependence of the angular momentum sharing on the net mass transfer.Comment: 8 pages, 2 figure
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