210 research outputs found
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
Properties of the Intermediate Mass Fragment Emission Source in the 270 MeV 3-He + 232-Th Reaction
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Emission of Energetic Protons into the Backward Hemisphere
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
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
Calibration of the response function of CsI(Tl) scintillators to intermediate-energy heavy ions
Abstract The response function of 2-cm-thick CsI(Tl) scintillators with photodiode readouts were studied by directly exposing the detectors to beams of heavy ions (2≤Z≤36) with energy up to 25 MeV/u. The dependence of the light output on the energy (E) as well as on the atomic number and the mass of the ion is analyzed and discussed, and a parameterization of the light output as a function of Z and E is proposed
Heavy Residue Formation in 20 MeV/nucleon 197Au + 90Zr collisions
The yields and velocity distributions of heavy residues and fission fragments
from the reaction of 20 MeV/nucleon 197Au + 90Zr have been measured using the
MSU A1200 fragment separator. A bimodal distribution of residues is observed,
with one group, resulting from peripheral collisions, having fragment mass
numbers A=160-200, while the other group, resulting from ``hard'' collisions,
has A=120-160. This latter group of residues can be distinguished from fission
fragments by their lower velocities. A model combining deep-inelastic transfer
and incomplete fusion for the primary interaction stage and a statistical
evaporation code for the deexcitation stage has been used to describe the
properties of the product distributions.Comment: 19 pages, 6 figures, preprint submitted to Nucl. Phys.
Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis
Although the geological record indicates that eukaryotes evolved by 1.9–1.4 Ga, their early evolution is poorly resolved taxonomically and chronologically. The fossil red alga Bangiomorpha pubescens is the only recognized crown-group eukaryote older than ca. 0.8 Ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis. Because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast, B. pubescens is uniquely important for calibrating eukaryotic evolution. However, molecular clock estimates for the divergence between the red and green algae are highly variable, and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1.2 Ga for B. pubescens. As a result, many molecular clock studies reject this fossil ex post facto. Here we present new Re-Os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of B. pubescens in the Bylot Supergroup of Baffin Island and revise its first appearance to 1.047 +0.013/–0.017 Ga. This date is 150 m.y. younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution. Using cross-calibrated molecular clock analyses with the new fossil age, we calculate that photosynthesis within the Eukarya emerged ca. 1.25 Ga. This date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the Proterozoic biosphere
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