Magnetization measurements of high-mobility two-dimensional electron gases

De Haas-van Alphen (dHvA) oscillations are observed for Landau levels (LLs) with filling factors between 4 and 52, at temperatures in the range 50 mK to 1 K, in experiments on high mobility GaAs/(Al, Ga)As heterojunctions. The oscillations become sawtooth-shaped at low filling factors, and theoretical fits to the data, assuming the two-dimensional electron gas to be a non-interacting Fermi system, show the shape of LLs to be close to a ?-function. The small residual width (~0.4 meV or less) fits equally well to either a Gausian or a Lorentzian density of states model. In almost all cases, a constant background density of states has to be included to obtain a satisfactory fit. weak odd-filling-factor dHvA peaks are detected at high fields, from which a g-factor enhancement of 15 can be inferred. Comparison of the scattering time derived from the fits before and after illumination, with the momentum relaxation time derived from transport, reveals a counterintuitive behavior in the bulk-modulation-doped sample

Beta-delayed-neutron studies of 135,136^{135,136}Sb and 140^{140}I performed with trapped ions

Beta-delayed-neutron ($\beta$n) spectroscopy was performed using the Beta-decay Paul Trap and an array of radiation detectors. The $\beta$n branching ratios and energy spectra for $^{135,136}$Sb and $^{140}$I were obtained by measuring the time of flight of recoil ions emerging from the trapped ion cloud. These nuclei are located at the edge of an isotopic region identified as having $\beta$n branching ratios that impact the r-process abundance pattern around the A~130 peak. For $^{135,136}$Sb and $^{140}$I, $\beta$n branching ratios of 14.6(11)%, 17.6(28)%, and 7.6(28)% were determined, respectively. The $\beta$n energy spectra obtained for $^{135}$Sb and $^{140}$I are compared with results from direct neutron measurements, and the $\beta$n energy spectrum for $^{136}$Sb has been measured for the first time

Inventory verification measurements using neutron multiplicity counting

This paper describes a series of neutron multiplicity measurements of large plutonium samples at the Los Alamos Plutonium Facility. The measurements were corrected for bias caused by neutron energy spectrum shifts and nonuniform multiplication, and are compared with calorimetry/isotopics. The results show that multiplicity counting can increase measurement throughput and yield good verification results for some inventory categories. The authors provide recommendations on the future application of the technique to inventory verification

SparsePak: A Formatted Fiber Field Unit for The WIYN Telescope Bench Spectrograph. I. Design, Construction, and Calibration

We describe the design and construction of a formatted fiber field-unit, SparsePak, and characterize its optical and astrometric performance. This array is optimized for spectroscopy of low-surface brightness, extended sources in the visible and near-infrared. SparsePak contains 82, 4.7" fibers subtending an area of 72"x71" in the telescope focal plane, and feeds the WIYN Bench spectrograph. Together, these instruments are capable of achieving spectral resolutions of lambda/dlambda ~ 20000 and an area--solid-angle product of ~140 arcsec^2 m^2 per fiber. Laboratory measurements of SparsePak lead to several important conclusions on the design of fiber termination and cable curvature to minimize focal ratio degradation. SparsePak itself has throughput >80% redwards of 5200 A, and 90-92% in the red. Fed at f/6.3, the cable delivers an output 90% encircled energy at nearly f/5.2. This has implications for performance gains if the WIYN Bench Spectrograph had a faster collimator. Our approach to integral-field spectroscopy yields an instrument which is simple and inexpensive to build, yet yields the highest area--solid-angle product per spectrum of any system in existence. An Appendix details the fabrication process in sufficient detail for others to repeat. SparsePak was funded by the National Science Foundation and the University of Wisconsin-Madison Graduate School, and is now publicly available on the WIYN Telescope through the National Optical Astronomical Observatories.Comment: accepted for publication in PASP; 17 pages text, 16 figures (embedded

Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region

Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline silicates at temperatures lower than those required for spectra showing high abundance of other crystalline silicates. A few spectra show 10 micron complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 micron complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. Crystalline silicate abundances correlate positively with other such abundances, suggesting that crystalline silicates are processed directly from amorphous silicates and that neither forsterite, enstatite, nor silica are intermediate steps when producing either of the other two. Disks with more dust settling typically have greater crystalline abundances. Large-grain abundance is somewhat correlated with greater settling of disks. The lack of strong correlation is interpreted to mean that settling of large grains is sensitive to individual disk properties. Lower-mass stars have higher abundances of large grains in their inner regions.Comment: 84 pages, 27 figures, submitted to the Astrophysical Journal on 7 November, 200

Silica in Protoplanetary Disks

Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope show prominent narrow emission features indicating silica (crystalline silicon dioxide). Silica is not a major constituent of the interstellar medium; therefore, any silica present in the circumstellar protoplanetary disks of TTS must be largely the result of processing of primitive dust material in the disks surrouding these stars. We model the silica emission features in our spectra using the opacities of various polymorphs of silica and their amorphous versions computed from earth-based laboratory measurements. This modeling indicates that the two polymorphs of silica, tridymite and cristobalite, which form at successively higher temperatures and low pressures, are the dominant forms of silica in the TTS of our sample. These high temperature, low pressure polymorphs of silica present in protoplanetary disks are consistent with a grain composed mostly of tridymite named Ada found in the cometary dust samples collected from the STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks may arise from incongruent melting of enstatite or from incongruent melting of amorphous pyroxene, the latter being analogous to the former. The high temperatures of 1200K-1300K and rapid cooling required to crystallize tridymite or cristobalite set constraints on the mechanisms that could have formed the silica in these protoplanetary disks, suggestive of processing of these grains during the transient heating events hypothesized to create chondrules.Comment: 47 pages, 9 figures, to appear in the 1 January, 2009 issue of the Astrophysical Journa

INEL test plan for evaluating waste assay systems

A test bed is being established at the Idaho National Engineering Laboratory (INEL) Radioactive Waste Management Complex (RWMC). These tests are currently focused on mobile or portable radioassay systems. Prior to disposal of TRU waste at the Waste Isolation Pilot Plant (WIPP), radioassay measurements must meet the quality assurance objectives of the TRU Waste Characterization Quality Assurance Program Plan. This test plan provides technology holders with the opportunity to assess radioassay system performance through a three-tiered test program that consists of: (a) evaluations using non-interfering matrices, (b) surrogate drums with contents that resemble the attributes of INEL-specific waste forms, and (c) real waste tests. Qualified sources containing a known mixture and range of radionuclides will be used for the non-interfering and surrogate waste tests. The results of these tests will provide technology holders with information concerning radioassay system performance and provide the INEL with data useful for making decisions concerning alternative or improved radioassay systems that could support disposal of waste at WIPP

Nitrogen cycling and budget in crop rotations as influenced by preceding crops and N fertilization

Non-Peer Reviewe

Can agricultural cultivation methods influence the healthfulness of crops for foods

The aim of the current study was to investigate if there are any health effects of long-term consumption of organically grown crops using a rat model. Crops were retrieved over two years from along-term field trial at three different locations in Denmark, using three different cultivation systems(OA, organic based on livestock manure; OB, organic based on green manure; and C, conventional with mineral fertilizers and pesticides)with two field replicates. The cultivation system had an impact on the nutritional quality, affecting γ-tocopherol, some amino acids, and fatty acid composition. Additionally, the nutritional quality was affected by harvest year and location. However, harvest year and location rather than cultivation system affected the measured health biomarkers. In conclusion, the differences in dietary treatments composed of ingredients from different cultivation systems did not lead to significant differences in the measured health biomarkers, except for a significant difference in plasma IgGl evels