Growth responses of plants to gravity
Gravity effects on auxin transport, growth and foliage spread of green plants for efficient radiation capture, using horizontal clinostat experiments
Lyon, C. J.↗
SEARCH · Engineering Papers
Results for “Radiative capture”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Radiative recombination of complex ions
The rates of radiative capture by atomic systems is computed for the first four ionization stages of the abundant elements C, N, O, Ne, Mg, Si, S, and Ar. A simple prescription is given for calculating the rates for systems in higher ionization stages. Results for capture to atomic helium are also given. In the general case, recent calculations of photoionization cross sections have been used to compute capture rates to levels of the ground-state configuration of the recombined species. Captures to other levels of the valence shell and to the higher shells are evaluated using hydrogenic formulae but with a correction factor which takes into account the nonhydrogenic nature of these states. This correction factor is due to the incomplete shielding by the inner electrons and represents essentially an effective charge for the recombining ion; it is determined semiempirically from an appropriate weighting of values derived from the observed level structure for each species. In some cases, for the capture to neutral atomic systems, its application increases the resulting computed recombination rate by a significant amount (about 50%). Computed recombination rates are tabulated for the four ionization stages at an electron temperature of 10,000 K and for the first stage at 100 K. A convenient procedure is outlined for evaluating the rates at other temperatures in the general neighborhood of these values.
Gould, R. J.↗
Solar observations
High energy processes that take place in the Sun's atmosphere and the relationship of these phenomena to the basic problems of solar activity are discussed. Gamma ray emission exhibits characteristics of the conditions in regions where accelerated high energy particles interact. A number of gamma ray production mechanisms are considered. These include: the Compton effect, magnetobremsstrahlung, pi meson production by proton-proton interaction or by proton-antiproton annihilation, fission and neutral of charged particle radiative capture on inelastic scatter.
Source record↗
Accelerator simulation of astrophysical processes
Phenomena that involve accelerated ions in stellar processes that can be simulated with laboratory accelerators are described. Stellar evolutionary phases, such as the CNO cycle, have been partially explored with accelerators, up to the consumption of He by alpha particle radiative capture reactions. Further experimentation is indicated on reactions featuring N-13(p,gamma)O-14, O-15(alpha, gamma)Ne-19, and O-14(alpha,p)F-17. Accelerated beams interacting with thin foils produce reaction products that permit a determination of possible elemental abundances in stellar objects. Additionally, isotopic ratios observed in chondrites can be duplicated with accelerator beam interactions and thus constraints can be set on the conditions producing the meteorites. Data from isotopic fractionation from sputtering, i.e., blasting surface atoms from a material using a low energy ion beam, leads to possible models for processes occurring in supernova explosions. Finally, molecules can be synthesized with accelerators and compared with spectroscopic observations of stellar winds.
Tombrello, T. A.↗
Exploring the limits of crop productivity: beyond the limits of tipburn in lettuce
The productivity of lettuce in a combination of high light, high temperature, and elevated CO2 has not been commonly studied because rapid growth usually causes a calcium deficiency in meristems called tipburn, which greatly reduces quality and marketability. We eliminated tipburn by blowing air directly onto the meristem, which allowed us to increase the photosynthetic photon flux (PPF) to 1000 micromoles m-2 s-1 (57.6 mol m-2 d-1); two to three times higher than normally used for lettuce. Eliminating tipburn doubled edible yield at the highest PPF level. In addition to high PPF, CO2 was elevated to 1200 micromoles m-2 mol-1, which increased the temperature optimum from 25 to 30 degrees C. The higher temperature increased leaf expansion rate, which improved radiation capture and more than doubled yield. Photosynthetic efficiency, measured as canopy quantum yield in a whole-plant gas exchange system, steadily increased up to the highest temperature of 32 degrees C in high CO2. The highest productivity was 19 g m-2 d-1 of dry biomass (380 g d-1 fresh mass) averaged over the 23 days the plants received light. Without the limitation of tipburn, the combination of high PPF, high temperature, and elevated CO2 resulted in a 4-fold increase in growth rate over productivity in conventional environments.
NASA Discipline Life Support Systems↗
Measurement of Insulation Compaction in the Cryogenic Fuel Tanks at Kennedy Space Center by Fast/Thermal Neutron Techniques
The liquid hydrogen and oxygen cryogenic storage tanks at John F. Kennedy Space Center (KSC) use expanded perlite as thermal insulation. Th ere is evidence that some of the perlite has compacted over time, com promising the thermal performance and possibly also structural integr ity of the tanks. Therefore an Non-destructive Testing (NDT) method for measuring the perlite density or void fraction is urgently needed. Methods based on neutrons are good candidates because they can readil y penetrate through the 1.75 cm outer steel shell and through the ent ire 120 cm thickness of the perlite zone. Neutrons interact with the nuclei of materials to produce characteristic gamma rays which are the n detected. The gamma ray signal strength is proportional to the atom ic number density. Consequently, if the perlite is compacted then the count rates in the individual peaks in the gamma ray spectrum will i ncrease. Perlite is a feldspathic volcanic rock made up of the major elements Si, AI, Na, K and 0 along with some water. With commercially available portable neutron generators it is possible to produce simul taneously fluxes of neutrons in two energy ranges: fast (14 MeV) and thermal (25 meV). Fast neutrons produce gamma rays by inelastic scatt ering which is sensitive to Fe and O. Thermal neutrons produce gamma rays by radiative capture in prompt gamma neutron activation (PGNA) and this is sensitive to Si, AI, Na, Kand H. Thus the two energy ranges produce complementary information. The R&D program has three phases: numerical simulations of neutron and gamma ray transport with MCNP s oftware, evaluation of the system in the laboratory on test articles and finally mapping of the perlite density in the cryogenic tanks at KSC. The preliminary MCNP calculations have shown that the fast/therma l neutron NDT method is capable of distinguishing between expanded an d compacted perlite with excellent statistics.
Livingston, R. A.↗
Capturing and Modeling Radiation Hardness Assurance throughout the Project Lifecycle
This presentation describes how assurance arguments for the radiation reliability of a system can be modeled in Goal Structuring Notation over the life-cycle of a project. A practical implementation of the NASA-STD-8729.1 and Model-Based Mission Assurance.
Model-Based Mission Assurance↗
Angular distribution of 𝛾 rays from a neutron-induced 𝑝-wave resonance of 132 Xe
A neutron-energy dependent angular distribution was measured for individual 𝛾 rays from the 3.2 eV 𝑝-wave resonance of 131 Xe+𝑛, that shows enhanced parity violation owing to a mixing between 𝑠- and 𝑝-wave amplitudes. The 𝛾-ray transitions from the 𝑝-wave resonance were identified, and the angular distribution with respect to the neutron momentum was evaluated as a function of the neutron energy for 7132 keV 𝛾 rays, which correspond to a transition to the 1807 keV excited state of 132 Xe. The angular distribution is considered to originate from the interference between 𝑠- and 𝑝-wave amplitudes, and will provide a basis for a quantitative understanding of the enhancement mechanism of the fundamental parity violation in compound nuclei.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Neutron capture measurements at UMass lowell research reactor
A new facility was designed around the thermal column beam port of the 1 MW Research Reactor at the University of Massachusetts Lowell. Thermal neutrons are collimated to a 1-inch diameter beam and incident on samples to induce the radiative neutron capture. New measurements of capture gamma rays are planned for Mn, Cu, Ni, Cr, and Gd samples in the next few years. The experiments will be carried out in close collaboration with the ENSDF (Evaluated Nuclear Structure Data File) evaluation group at Brookhaven National Laboratory. The gamma rays are measured using an array of high-resolution HPGe detectors. Some of the HPGe detectors use active Compton shields that were designed from scintillation detectors and improve significantly the signal-to-background in the measured gamma-ray spectra with HPGe. The experimental spectra will be validated by the GEANT4 simulations of the array and theoretical models of the emission of gamma rays from the compound nucleus. In conclusion, the new experimental results on the capture gamma ray intensities will be incorporated in future ENSDF evaluations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Orbital electron capture by the nucleus
The theory of nuclear electron capture is reviewed in the light of current understanding of weak interactions. Experimental methods and results regarding capture probabilities, capture ratios, and EC/Beta(+) ratios are summarized. Radiative electron capture is discussed, including both theory and experiment. Atomic wave function overlap and electron exchange effects are covered, as are atomic transitions that accompany nuclear electron capture. Tables are provided to assist the reader in determining quantities of interest for specific cases.
Bambynek, W.↗
Muon to Positron Conversion
Lepton-flavor violation (LFV) has been discovered in the neutrino sector by neutrino oscillation experiments. The minimal extension of the Standard Model (SM) to include neutrino masses allows LFV in the charged sector (CLFV) at the loop level, but at rates that are too small to be experimentally observed. Lepton-number violation (LNV) is explicitly forbidden even in the minimally extended SM, so the observation of an LNV process would be unambiguous evidence of physics beyond the SM. The search for the LNV and CLFV process μ−+N(A,Z)→e++N′(A,Z−2) (referred to as μ−→e+) complements 0νββ decay searches, and is sensitive to potential flavor effects in the neutrino mass-generation mechanism. A theoretical motivation for μ−→e+ is presented along with a review of the status of past μ−→e+ experiments and future prospects. Special attention is paid to an uncertain and potentially dominant background for these searches, namely, radiative muon capture (RMC). The RMC high energy photon spectrum is theoretically understudied and existing measurements insufficiently constrain this portion of the spectrum, leading to potentially significant impacts on current and future μ−→e+ work.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Bremsstrahlung constraints on proton-boron 11 inertial fusion
Proton-boron 11 (pB11) fusion is relatively safe and clean, but difficult to use for net power production, since bremsstrahlung radiation tends to radiate away power more quickly than it can be generated by fusion power, particularly once poisoning by alpha particles is taken into account. While in magnetic confinement fusion, this problem can be addressed by deconfining the alphas, in inertial confinement fusion (ICF), the alphas that heat the plasma linger for the duration of the reaction. Thus, it becomes essential to trap the bremsstrahlung radiation in the hotspot. Through burn simulations incorporating bremsstrahlung emission and reabsorption, we infer the necessary conditions to capture enough radiation to produce scientific breakeven in a pB11 ICF plasma. We find that breakeven requires a stagnation areal density roughly two orders of magnitude higher than the current state-of-the-art, at pressures three orders of magnitude higher.
Alpha particles↗
Direct Computation of Sound Radiation by Jet Flow Using Large-scale Equations
Jet noise is directly predicted using large-scale equations. The computational domain is extended in order to directly capture the radiated field. As in conventional large-eddy-simulations, the effect of the unresolved scales on the resolved ones is accounted for. Special attention is given to boundary treatment to avoid spurious modes that can render the computed fluctuations totally unacceptable. Results are presented for a supersonic jet at Mach number 2.1.
Mankbadi, R. R.↗
Neutron capture-induced nuclear recoils as background for CE$v$NS measurements at reactors
Nuclear reactors represent a promising neutrino source for CEvNS (coherent-elastic neutrino-nucleus scattering) searches. However, reactor sites also come with high ambient neutron flux. Neutron capture-induced nuclear recoils can create a spectrum that strongly overlaps the CEvNS signal for recoils ≲100 eV for nuclear reactor measurements in silicon or germanium detectors. This background can be particularly critical for low-power research reactors providing a moderate neutrino flux. Here, in this work we quantify the impact of this background and show that, for a measurement 10 m from a 1 MW reactor, the effective thermal neutron flux should be kept below ~7 × 10 –4 n/cm 2 s so that the CEvNS events can be measured at least at a 5σ level with germanium detectors in 100 kg yr exposure time. This flux corresponds to 60% of the sea-level flux but needs to be achieved in a nominally high-flux (reactor) environment. Improved detector resolution can help the measurements, but the thermal flux is the key parameter for the sensitivity of the experiment. For silicon detectors, the constraint is even stronger and thermal neutron fluxes must be near an order of magnitude lower. This constraint highlights the need of an effective thermal neutron mitigation strategy for future low threshold CEvNS searches. In particular, the neutron capture-induced background can be efficiently reduced by active veto systems tagging the deexcitation gamma following the capture.
79 ASTRONOMY AND ASTROPHYSICS↗
High-resolution Laboratory Measurements of K-shell X-ray Line Polarization and Excitation Cross Sections in Helium-like S XV Ions
We report measurements of electron-impact excitation cross sections for the strong K-shell n = 2 -> 1 transitions in S XV, using the LLNL EBIT-I electron beam ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The cross sections are determined by direct normalization to the well-known cross sections of radiative electron capture, measured simultaneously. Using contemporaneous polarization measurements with the two crystal spectrometers, whose dispersion planes are oriented parallel and perpendicular to the electron beam direction, the polarization of the direct excitation line emission is determined, and in turn the isotropic total cross sections are extracted. We further experimentally investigate various line-formation mechanisms, finding that radiative cascades and collisional inner-shell ionization dominate the degree of linear polarization and total line emission cross sections of the forbidden line, z.
atomic data↗
Neutron capture-induced silicon nuclear recoils for dark matter and CEνNS
Not Available
79 ASTRONOMY AND ASTROPHYSICS↗
Measurement of the neutron-induced capture-to-fission cross section ratio in U 233 at LANSCE
Not Available
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Radiation Tolerance of Nanoporous Gadolinium Titanate
Defect sinks play a crucial role in reducing radiation-induced damage accumulation, but their effectiveness varies. This study directly compares the efficacy of grain boundaries and pores (free surfaces) in gadolinium titanate (Gd 2 Ti 2 O 7 ) using in situ ion irradiation transmission electron microscopy (TEM). A unique sample configuration is developed, where each of the three distinct regions, each containing only one primary sink of interest, is in the same TEM lamella for simultaneous irradiation. Regular assessments of crystallinity in each region via nanobeam electron diffraction show that, at both room temperature and 600°C, free surface (pore) defect sinks are more effective than grain boundaries in capturing radiation-induced defects and delaying the onset of amorphization. These findings have significant implications for further microstructural engineering and the design of radiation-resistant microstructures.
36 MATERIALS SCIENCE↗