2,887 research outputs found
The magnetic form factor of the deuteron in chiral effective field theory
We calculate the magnetic form factor of the deuteron up to O(eP^4) in the
chiral EFT expansion of the electromagnetic current operator. The two LECs
which enter the two-body part of the isoscalar NN three-current operator are
fit to experimental data, and the resulting values are of natural size. The
O(eP^4) description of G_M agrees with data for momentum transfers Q^2 < 0.35
GeV^2.Comment: 4 pages, 2 figure
Study of material homogeneity in the long fiber thermoset injection molding process by image texture analysis
To quantify the homogeneity of fiber dispersion in short fiber-reinforced polymer composites, a method for image texture analysis of 3-dimensional X-ray micro computed tomography (µCT) images is presented in this work. The adaption of the method to the specific requirements of the composite material is accomplished using a statistical region merging approach. Subsequently, the method is applied for evaluating the homogeneity of specimens from an intermediate step of the long fiber thermoset injection molding process as well as molded parts. This new injection molding process enables the manufacturing of parts with a flexible combination of short and long glass fibers. By using a newly developed screw element based on the Maddock mixing element design, the material homogeneity of parts molded in the long fiber injection molding process is improved
Field-Scale Soil Property Changes under Switchgrass Managed for Bioenergy
The capacity of perennial grasses to affect change in soil properties is well documented but information on switchgrass (Panicum virgatum L.) managed for bioenergy is limited. An on-farm study (10 fields) in North Dakota, South Dakota, and Nebraska was sampled before switchgrass establishment and after 5 years to determine changes in soil bulk density (SBD), pH, soil phosphorus (P), and equivalent mass soil organic carbon (SOC). Changes in SBD were largely constrained to near-surface depths (0–0.05 m). SBD increased (0–0.05 m) at the Nebraska locations (mean=0.16 Mgm-3), while most South Dakota and North Dakota locations showed declines in SBD (mean=-0.18 Mgm-3; range=-0.42–0.07 Mgm-3). Soil pH change was significant at five of the 10 locations at near surface depths (0–0.05 m), but absolute changes were modest (range=-0.67–0.44 pH units). Available P declined at all sites where it was measured (North Dakota and South Dakota locations). When summed across the surface 0.3 m depth, annual decreases in available P averaged 1.5 kg P ha-1 yr-1 (range=0.5–2.8 kg P ha-1 yr-1). Averaged across locations, equivalent mass SOC increased by 0.5 and 2.4 Mg Cha-1 yr-1 for the 2500 and 10 000 Mg ha-1 soil masses, respectively. Results from this study underscore the contribution of switchgrass to affect soil property changes, though considerable variation in soil properties exists within and across locations
Cellular Automaton Study of Time-Dynamics of Avalanche Breakdown in IMPATT Diodes
Employing a recently developed efficient cellular automaton technique for solving
Boltzmann's transport equation for realistic devices, we present a detailed study of the
carrier dynamics in GaAs avalanche p-i-n (IMPATT) diodes. We find that the impact
ionization in reverse bias p-i-n diodes with ultrathin (less than 50 nm) intrinsic regions is
triggered by Zener tunneling rather than by thermal generation. The impact generation
of hot carriers occurs mainly in the low-field junction regions rather than in the high
field intrinsic zone. The calculations predict significantly more minority carriers on the
n-side than on the p-side
Soil Carbon Storage by Switchgrass Grown for Bioenergy
Life-cycle assessments (LCAs) of switchgrass (Panicum virgatum L.) grown for bioenergy production require data on soil organic carbon (SOC) change and harvested C yields to accurately estimate net greenhouse gas (GHG) emissions. To date, nearly all information on SOC change under switchgrass has been based on modeled assumptions or small plot research, both of which do not take into account spatial variability within or across sites for an agro-ecoregion. To address this need, we measured change in SOC and harvested C yield for switchgrass fields on ten farms in the central and northern Great Plains, USA (930 km latitudinal range). Change in SOC was determined by collecting multiple soil samples in transects across the fields prior to planting switchgrass and again 5 years later after switchgrass had been grown and managed as a bioenergy crop. Harvested aboveground C averaged 2.5± 0.7 Mg C ha−1 over the 5 year study. Across sites, SOC increased significantly at 0–30 cm (P=0.03) and 0–120 cm (P=0.07), with accrual rates of 1.1 and 2.9 Mg C ha−1 year−1 (4.0 and 10.6 Mg CO2 ha−1 year−1), respectively. Change in SOC across sites varied considerably, however, ranging from −0.6 to 4.3 Mg C ha−1 year−1 for the 0–30 cm depth. Such variation in SOC change must be taken into consideration in LCAs. Net GHG emissions from bioenergy crops vary in space and time. Such variation, coupled with an increased reliance on agriculture for energy production, underscores the need for long-term environmental monitoring sites in major agro-ecoregions
Few-nucleon systems with state-of-the-art chiral nucleon-nucleon forces
We apply improved nucleon-nucleon potentials up to fifth order in chiral
effective field theory, along with a new analysis of the theoretical truncation
errors, to study nucleon-deuteron (Nd) scattering and selected low-energy
observables in 3H, 4He, and 6Li. Calculations beyond second order differ from
experiment well outside the range of quantified uncertainties, providing truly
unambiguous evidence for missing three-nucleon forces within the employed
framework. The sizes of the required three-nucleon force contributions agree
well with expectations based on Weinberg's power counting. We identify the
energy range in elastic Nd scattering best suited to study three-nucleon force
effects and estimate the achievable accuracy of theoretical predictions for
various observables.Comment: 5 pages, 5 figure
Low-energy neutron-deuteron reactions with N3LO chiral forces
We solve three-nucleon Faddeev equations with nucleon-nucleon and
three-nucleon forces derived consistently in the framework of chiral
perturbation theory at next-to-next-to-next-to-leading order in the chiral
expansion. In this first investigation we include only matrix elements of the
three-nucleon force for partial waves with the total two-nucleon
(three-nucleon) angular momenta up to 3 (5/2). Low-energy neutron-deuteron
elastic scattering and deuteron breakup reaction are studied. Emphasis is put
on Ay puzzle in elastic scattering and cross sections in symmetric-space-star
and neutron-neutron quasi-free-scattering breakup configurations, for which
large discrepancies between data and theory have been reported.Comment: 22 pages, 7 figure
Management strategies/practices for increasing storage of organic C and N in soil in cropping systems in the Northern Great Plains of North America
Non-Peer Reviewe
Interaction of Ultrashort-Laser Pulses with Induced Undercritical Plasmas in Fused Silica
Ultrafast light-material interactions near the damage threshold are often studied using postmortem analysis of damaged dielectric materials. Corresponding simulations of ultrashort pulse propagation through the material are frequently used to gain additional insight into the processes leading to such damage. However, comparison between such experimental and numerical results is often qualitative, and pulses near to but not exceeding the damage threshold leave no permanent changes in the material for postmortem analysis. In this article, a series of experiments is presented that measures the near- and far-field properties of a 140-fs laser pulse after propagation through a fused silica sample in which a noncritical electron plasma was generated. Concurrently, results from simulations in which the laser pulse was numerically constructed according to the nearfield beam profile and frequency resolved optical gating (FROG) trace are presented. It is found that to extract a quantitative comparison of such data, cylindrical symmetry of the laser pulse in simulations should be abandoned in favor of a fully 3+1D Cartesian representation. Further comparison of experimental and calculated damage thresholds shows that time-corrective effects predicted by the Drude model play a critical role in the physics of both pulse evolution and plasma formation. The influence of resulting spatiotemporal dependences of the pulse in far-field measurements leads to unretrievable FROG traces. However, it is shown through both simulation and experiment that the use of an appropriate beam aperture will eliminate this effect when measuring the temporal pulse amplitude
Use of Ecological Sites in Managing Wildlife and Livestock: An Example with Prairie Dogs
On the Ground The perception of prairie dogs among Native Americans living on the Standing Rock Sioux Reservation is mixed. Some Native Americans focus on the loss of forage productivity, whereas others are interested in the cultural and ecological aspects of prairie dogs. The use of ecological sites may provide a mechanism for developing a management framework that would consider both livestock and prairie dogs. The three ecological sites we surveyed had large differences in off-colony standing crop, but in 2 of the 3 years we surveyed, there were no differences between standing crop on-colony. This suggests that management of prairie dogs on rangelands should focus on limiting prairie dogs on more productive ecological sites with less productive sites receiving less emphasis
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