78 research outputs found
Quasi one dimensional He inside carbon nanotubes
We report results of diffusion Monte Carlo calculations for both He
absorbed in a narrow single walled carbon nanotube (R = 3.42 \AA) and strictly
one dimensional He. Inside the tube, the binding energy of liquid He is
approximately three times larger than on planar graphite. At low linear
densities, He in a nanotube is an experimental realization of a
one-dimensional quantum fluid. However, when the density increases the
structural and energetic properties of both systems differ. At high density, a
quasi-continuous liquid-solid phase transition is observed in both cases.Comment: 11 pages, 3ps figures, to appear in Phys. Rev. B (RC
Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules
We have studied the effect of local Coulomb interactions on the electronic
structure of the molecular magnet Mn12-acetate within the LDA+U approach. The
account of the on-site repulsion results in a finite energy gap and an integer
value of the molecule's magnetic moment, both quantities being in a good
agreement with the experimental results. The resulting magnetic moments and
charge states of non-equivalent manganese ions agree very well with
experiments. The calculated values of the intramolecular exchange parameters
depend on the molecule's spin configuration, differing by 25-30% between the
ferrimagnetic ground state and the completely ferromagnetic configurations. The
values of the ground-state exchange coupling parameters are in reasonable
agreement with the recent data on the magnetization jumps in megagauss magnetic
fields. Simple estimates show that the obtained exchange parameters can be
applied, at least qualitatively, to the description of the spin excitations in
Mn12-acetate.Comment: RevTeX, LaTeX2e, 4 EPS figure
Do Femtonewton Forces Affect Genetic Function? A Review
Protein-Mediated DNA looping is intricately related to gene expression. Therefore any mechanical constraint that disrupts loop formation can play a significant role in gene regulation. Polymer physics models predict that less than a piconewton of force may be sufficient to prevent the formation of DNA loops. Thus, it appears that tension can act as a molecular switch that controls the much larger forces associated with the processive motion of RNA polymerase. Since RNAP can exert forces over 20 pN before it stalls, a ‘substrate tension switch’ could offer a force advantage of two orders of magnitude. Evidence for such a mechanism is seen in recent in vitro micromanipulation experiments. In this article we provide new perspective on existing theory and experimental data on DNA looping in vitro and in vivo . We elaborate on the connection between tension and a variety of other intracellular mechanical constraints including sequence specific curvature and supercoiling. In the process, we emphasize that the richness and versatility of DNA mechanics opens up a whole new paradigm of gene regulation to explore.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41816/1/10867_2005_Article_9002.pd
Effects of gastric inhibitory polypeptide (GIP) and related analogues on glucagon release at normo- and hyperglycaemia in Wistar rats and isolated islets
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Gamma-ray dosimetry measurements of the Little Boy replica
We present the current status of our gamma-ray dosimetry results for the Little Boy replica. Both Geiger-Mueller and thermoluminescent detectors were used in the measurements. Future work is needed to test assumptions made in data analysis
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Source-jerk analysis using a semi-explicit inverse kinetic technique
A method is proposed for measuring the effective reproduction factor, k, in subcritical systems. The method uses the transient response of a subcritical system to the sudden removal of an extraneous neutron source (i.e., a source jerk). The response is analyzed using an inverse kinetic technique that least-squares fits the exact analytical solution corresponding to a source-jerk transient as derived from the point-reactor model. It has been found that the technique can provide an accurate means of measuring k in systems that are close to critical (i.e., 0.95 < k < 1.0). As a system becomes more subcritical (i.e., k << 1.0) spatial effects can introduce significant biases depending on the source and detector positions. However, methods are available that can correct for these biases and, hence, can allow measuring subcriticality in systems with k as low as 0.5. 12 refs., 3 figs
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Neutron dosimetry of the Little Boy device
Neutron dose rates at several angular locations and at distances out to 0.5 mile have been measured during critical operation of the Little Boy replica. We used modified remmetes and thermoluminescent dosimetry techniques for the measurements. The present status of our analysis is presented including estimates of the neutron-dose-relaxation length in air and the variation of the neutron-to-gamma-ray dose ratio with distance from the replica. These results are preliminary and are subject to detector calibration measurements
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Energy response and dose-rate calibration of a Geiger-Mueller gamma-ray detector
To obtain more precise dose-rate measurements, we have taken a commercial gamma-ray dosimeter that uses a Geiger-Mueller (GM) tube as its detector and, by supplying it with an externally regulated high voltage, counted the gamma-ray-induced pulses with suitable scaling and timing circuits. We have now improved the method of calibration by measuring detector response to 13 different sources, each with an independently verified strength, in the energy range from 60 keV to 2.6 MeV. With the use of computer codes, the resulting response curve can be folded into the spectrum of the source to be measured
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