45 research outputs found
The Generalized Liquid Drop Model Alpha-Decay Formula: Predictability Analysis and Super-Heavy Element Alpha Half-Lives
The predictive accuracy of the generalized liquid drop model (GLDM) formula
for alpha decay half-lives has been investigated in a detailed manner and a
variant of the formula with improved coefficients is proposed. The method
employs the experimental alpha half-lives of the well-known alpha standards
(REFERENCE) to obtain the coefficients of the analytical formula using the
experimental Qalpha values (the DSR-E formula), as well as the finite range
droplet model (FRDM) derived Qalpha values (the FRDMFRDM formula). The
predictive accuracy of these formulae were checked against the experimental
alpha half-lives of an independent set of nuclei (TEST) that span approximately
the same Z,A region as the standards and possess reliable alpha spectroscopic
data, and were found to yield good results for the DSR-E formula but not for
the FRDM-FRDM formula. The two formulae were used to obtain the alpha
half-lives of super-heavy (SHE) and heavy nuclides where the relative accuracy
was found to markedly improve for the FRDM-FRDM, which corroborates the
appropriateness of the FRDM masses and the GLDM prescription for high Z,A
nuclides. Further improvement resulted, especially for the FRDM-FRDM formula,
after a simple linear optimization over the calculated and experimental
half-lives of TEST was used to re-calculate the half-lives of the SHE and heavy
nuclides. The advantage of this optimization was that it required no
recalculation of the coefficients of the basic DSR-E or FRDM-FRDM formulae. The
halflives for 324 medium-mass to super-heavy alpha decaying nuclides,
calculated using these formulae and the comparison with experimental
half-lives, are presented.Comment: 61 pages, 6 figures, PDF file, to appear in Atomic Data and Nuclear
Data Table
The analysis of predictability of alpha-decay half-life formulae and the alpha partial half-lives of some exotic nuclei
The predictabilities of the three alpha-decay half-life formulae, the Royer
GLDM, the Viola-Seaborg and the Sobiczewski-Parkhomenko formulae, have been
evaluated by developing a method based on the ansatz of standard experimental
benchmarking. The coefficients of each formula were re-derived using the
reliable data of the alpha-standards nuclei. The modified formulae that
resulted were used to evaluate the accuracies of the formulae towards the
prediction of half-lives of a set of nuclides with well-studied alpha-
spectroscopic data as well as a set of exotic alpha-emitters. Further, a simple
linear optimization of the modified formulae allowed adjustments for the
insufficient statistics of the primary data set without changing the modified
formulae. While the three modified formulae showed equivalent results for all
the medium heavy nuclei except the odd-odd, the modified GLDM showed relatively
the best figures of merit for the odd-odd and superheavy nuclides.Comment: 16 pages, 4 tables, 2 figure
The Gac-Rsm and SadB Signal Transduction Pathways Converge on AlgU to Downregulate Motility in Pseudomonas fluorescens
Flagella mediated motility in Pseudomonas fluorescens F113 is tightly regulated. We have previously shown that motility is repressed by the GacA/GacS system and by SadB through downregulation of the fleQ gene, encoding the master regulator of the synthesis of flagellar components, including the flagellin FliC. Here we show that both regulatory pathways converge in the regulation of transcription and possibly translation of the algU gene, which encodes a sigma factor. AlgU is required for multiple functions, including the expression of the amrZ gene which encodes a transcriptional repressor of fleQ. Gac regulation of algU occurs during exponential growth and is exerted through the RNA binding proteins RsmA and RsmE but not RsmI. RNA immunoprecipitation assays have shown that the RsmA protein binds to a polycistronic mRNA encoding algU, mucA, mucB and mucD, resulting in lower levels of algU. We propose a model for repression of the synthesis of the flagellar apparatus linking extracellular and intracellular signalling with the levels of AlgU and a new physiological role for the Gac system in the downregulation of flagella biosynthesis during exponential growth
Using the conservative nature of fresh leaf surface density to measure foliar area
For a herbaceous species, the inverse of the fresh
leaf surface density, the Hughes constant, is nearly conserved. We
apply the Hughes constant to develop an absolute method of leafarea
measurement that requires no regression fits, prior calibrations
or oven-drying. The Hughes constant was determined in situ
using a known geometry and weights of a sub-set obtained from the
fresh leaves whose areas are desired. Subsequently, the leaf-areas
(at any desired stratification level), were derived by utilizing the
Hughes constant and the masses of the fresh leaves. The proof of
concept was established for leaf-discs of the plants Mandevilla
splendens and Spathiphyllum wallisii. The conservativeness of the
Hughes constant over individual leaf-zones and different leaftypes
from the leaves of each species was quantitatively validated.
Using the globally averaged Hughes constant for each species, the
leaf-area of these and additional co-species plants, were obtained.
The leaf-area-measurement-by-mass was cross-checked with
standard digital image analysis. There were no statistically significant
differences between the leaf-area-measurement-by-mass and
the digital image analysis measured leaf-areas and the linear
correlation between the two methods was very good. Leaf-areameasurement-
by-mass was found to be rapid and simple with
accuracies comparable to the digital image analysis method. The
greatly reduced cost of leaf-area-measurement-by-mass could be
beneficial for small agri-businesses in developing countries
The origin of atmospheric oxygen on Earth: The innovation of oxygenic photosynthesis
The evolution of O(2)-producing cyanobacteria that use water as terminal reductant transformed Earth's atmosphere to one suitable for the evolution of aerobic metabolism and complex life. The innovation of water oxidation freed photosynthesis to invade new environments and visibly changed the face of the Earth. We offer a new hypothesis for how this process evolved, which identifies two critical roles for carbon dioxide in the Archean period. First, we present a thermodynamic analysis showing that bicarbonate (formed by dissolution of CO(2)) is a more efficient alternative substrate than water for O(2) production by oxygenic phototrophs. This analysis clarifies the origin of the long debated âbicarbonate effectâ on photosynthetic O(2) production. We propose that bicarbonate was the thermodynamically preferred reductant before water in the evolution of oxygenic photosynthesis. Second, we have examined the speciation of manganese(II) and bicarbonate in water, and find that they form Mn-bicarbonate clusters as the major species under conditions that model the chemistry of the Archean sea. These clusters have been found to be highly efficient precursors for the assembly of the tetramanganese-oxide core of the water-oxidizing enzyme during biogenesis. We show that these clusters can be oxidized at electrochemical potentials that are accessible to anoxygenic phototrophs and thus the most likely building blocks for assembly of the first O(2) evolving photoreaction center, most likely originating from green nonsulfur bacteria before the evolution of cyanobacteria