13,438 research outputs found
Meson-Exchange Currents and the Strangeness Radius of 4he
Meson-exchange current contributions to the strangeness radius of He are
computed in the one-boson exchange approximation. It is found that these
contributions introduce a \lapp10\% correction to the one-body contribution.
They should not, therefore, hamper the extraction of the nucleon strangeness
radius from the parity-violating electron-He asymmetry.Comment: 9 tex pages and 2 figures (not included, available from authors on
request), CEBAF Preprint #TH-93-16 and MIT Preprint CTP#223
Potential uses for bracken (Pteridium aquilinum (L.) Kuhn) in organic agrculture
Bracken is a weed species due to its toxic nature and adverse effects on agriculture and ecology. This poster reviews research into historical uses for fronds and litter harvested as part of organically approved control methods. The use of bracken as an over winter mulch reduced losses of nitrogen and potassium from bare soil and maintained soil temperatures. Bracken litter was found to be a viable biofuel, with a calorific value comparable to wood and low ash and alkali metal contents. The contents of frond ash were investigated, with high concentrations of potassium found giving them a value as organic fertiliser. The addition of frond ash to soil significantly increased clover growth and number of nodules. The addition of frond ash to soil significantly increased the yield of saleable main crop potatoes. These finding have shown that bracken has a value especially within organic agriculture
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U.S. International Trade: Trends and Forecasts
[Excerpt] This report provides an overview of the current status, trends, and forecasts for U.S. import and export flows as well as certain balances. The purpose of this report is to provide current data and brief explanations for the various types of trade flows along with a brief discussion of trends that may require attention or point to the need for policy changes. The use of trade policy as an economic or strategic tool is beyond the scope of this report but can be found in various other CRS reports. Further detail on trade in specific commodities, with particular countries or regions, or for different time periods, can be obtained from the Department of Commerce, U.S. International Trade Commission, or by contacting the authors of this report
Designing stem cell niches for differentiation and self-renewal
Mesenchymal stem cells, characterized by their ability to differentiate into skeletal tissues and self-renew, hold great promise for both regenerative medicine and novel therapeutic discovery. However, their regenerative capacity is retained only when in contact with their specialized microenvironment, termed the stem cell niche. Niches provide structural and functional cues that are both biochemical and biophysical, stem cells integrate this complex array of signals with intrinsic regulatory networks to meet physiological demands. Although, some of these regulatory mechanisms remain poorly understood or difficult to harness with traditional culture systems. Biomaterial strategies are being developed that aim to recapitulate stem cell niches, by engineering microenvironments with physiological-like niche properties that aim to elucidate stem cell-regulatory mechanisms, and to harness their regenerative capacity in vitro. In the future, engineered niches will prove important tools for both regenerative medicine and therapeutic discoveries
Dynamic Power Spectral Analysis of Solar Measurements from Photospheric, Chromospheric, and Coronal Sources
An important aspect in the power spectral analysis of solar variability is the quasistationary and quasiperiodic nature of solar periodicities. In other words, the frequency, phase, and amplitude of solar periodicities vary on time scales ranging from active region lifetimes to solar cycle time scales. Here, researchers employ a dynamic, or running, power spectral density analysis to determine many periodicities and their time-varying nature in the projected area of active sunspot groups (S sub act). The Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor (SMM/ACRIM) total solar irradiance (S), the Nimbus-7 MgII center-to-wing ratio (R (MgII sub c/w)), the Ottawa 10.7 cm flux (F sub 10.7), and the GOES background x ray flux (X sub b) for the maximum, descending, and minimum portions of solar cycle 21 (i.e., 1980 to 1986) are used. The technique dramatically illustrates several previously unrecognized periodicities. For example, a relatively stable period at about 51 days has been found in those indices which are related to emerging magnetic fields. The majority of solar periodicities, particularly around 27, 150 and 300 days, are quasiperiodic because they vary in amplitude and frequency throughout the solar cycle. Finally, it is shown that there are clear differences between the power spectral densities of solar measurements from photospheric, chromospheric, and coronal sources
Assembling an isomer grid: the isomorphous 4-, 3- and 2-fluoro-N'-(4-pyridyl)benzamides
The three title isomers, 4-, (I), 3-, (II), and 2-fluoro-N'-(4-pyridyl)benzamide, (III), all C₁₂H₉FN₂O, crystallize in the P21/c space group (No. 14) with similar unit-cell parameters and are isomorphous and isostructural at the primary hydrogen-bonding level. An intramolecular C-H...O=C interaction is present in all three isomers [C...O = 2.8681 (17)-2.884 (2) Å and C-H...O117-118°], with an additional N-H...F [N...F = 2.7544 (15) Å] interaction in (III). Intermolecular amide-pyridine N-H...N hydrogen bonds link molecules into one-dimensional zigzag chains [graph set C(6)] along the [010] direction as the primary hydrogen bond [N...N = 3.022 (2), 3.049 (2) and 3.0213 (17) Å]. These are augmented in (I) by C-H...π(arene) and cyclic C-F...π(arene) contacts about inversion centres, in (II) by C-F...F-C interactions [C...F = 3.037 (2) Å] and weaker C-H...π(arene)/C-H...F contacts, and in (III) by C-H...π(arene) and C=O...O=C interactions, linking the alternating chains into two-dimensional sheets. Typical amide N-H...O=C hydrogen bonds [as C(4) chains] are not present [N...O = 3.438 (2) Å in (I), 3.562 (2) Å in (II) and 3.7854 (16) Å in (III)]; the C=O group is effectively shielded and only participates in weaker interactions/contacts. This series is unusual as the three isomers are isomorphous (having similar unit-cell parameters, packing and alignment), but they differ in their interactions and contacts at the secondary level
Parity violation in quasielastic electron-nucleus scattering within the relativistic impulse approximation
We study parity violation in quasielastic (QE) electron-nucleus scattering
using the relativistic impulse approximation. Different fully relativistic
approaches have been considered to estimate the effects associated with the
final-state interactions. We have computed the parity-violating quasielastic
(PVQE) asymmetry and have analyzed its sensitivity to the different ingredients
that enter in the description of the reaction mechanism: final-state
interactions, nucleon off-shellness effects, current gauge ambiguities.
Particular attention has been paid to the description of the weak neutral
current form factors. The PVQE asymmetry is proven to be an excellent
observable when the goal is to get precise information on the axial-vector
sector of the weak neutral current. Specifically, from measurements of the
asymmetry at backward scattering angles good knowledge of the radiative
corrections entering in the isovector axial-vector sector can be gained.
Finally, scaling properties shown by the interference nuclear
responses are also analyzed.Comment: 15 pages, 11 figure
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