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Results for “below- threshold ionization”

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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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Near threshold electron-impact double ionization of helium

We examine the break-up pattern for the electron-impact double ionization of helium using a time-dependent close-coupling approach. This ionization process is one of the simplest examples of the Coulomb four-body problem of three charged particles moving in the Coulomb field of a bare nucleus. Studies of this four-body process in the near-threshold region allow exploration of the break-up dynamics, which have been stimulated by differing predictions of the most likely geometries for break-up in the related processes of triple photoionization of lithium and electron-impact double ionization of helium. While both processes lead to the same final state of three electrons moving in the field of a bare nucleus, the preferred break-up pattern at low energies for the triple photoionization is predicted as a T-shape (Emmanouilidou et al 2008 Phys. Rev. Lett. 100 063002; Colgan et al 2013 Phys. Rev. Lett. 110 063001) and for the electron-impact double ionization as a 'triangle' pattern (Ren et al 2008 Phys. Rev. Lett. 101 093201). Our calculations at an excess energy of 5 eV show that a triangle break-up pattern is favored, confirming earlier experimental results (Ren et al 2008 Phys. Rev. Lett. 101 093201).

Optics↗
NASA NTRS1974

The energy distribution cross section in threshold electron-atom impact ionization

The flatness of the energy differential cross section in impact ionization is derived analytically in the Wannier theory. However it is shown that the Wannier zone is confined to a region of the order E/5 is less than or equal to epsilon is less than or equal to 4E/5, where E is the available energy and epsilon is the energy of the electrons. By contrasting the known results of photoionization and photodetachment, one can cogently argue that in the complementary region where electrons share their energy very unequally the cross section rises to a value independent of E, and that this region determines the form of the threshold law.

Temkin, A.↗
DOE OSTI2021

The physics potential of a reactor neutrino experiment with Skipper CCDs: Measuring the weak mixing angle

We analyze in detail the physics potential of an experiment like the one recently proposed by the vIOLETA collaboration: a kilogram-scale Skipper CCD detector deployed 12 meters away from a commercial nuclear reactor core. This experiment would be able to detect coherent elastic neutrino nucleus scattering from reactor neutrinos, capitalizing on the exceptionally low ionization energy threshold of Skipper CCDs. To estimate the physics reach, we elect the measurement of the weak mixing angle as a case study. We choose a realistic benchmark experimental setup and perform variations on this benchmark to understand the role of quenching factor and its systematic uncertainties, background rate and spectral shape, total exposure, and reactor antineutrino flux uncertainty. We take full advantage of the reactor flux measurement of the Daya Bay collaboration to perform a data driven analysis which is, up to a certain extent, independent of the theoretical un- certainties on the reactor antineutrino flux. We show that, under reasonable assumptions, this experimental setup may provide a competitive measurement of the weak mixing angle at few MeV scale with neutrino-nucleus scattering.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Laser-generated plasmas in length scales relevant for thin film growth and processing: simulation and experiment

In pulsed laser deposition, thin film growth is mediated by a laser-generated plasma, whose properties are critical for controlling the film microstructure. The advent of 2D materials has renewed the interest in how this ablation plasma can be used to manipulate the growth and processing of atomically thin systems. For such purpose, a quantitative understanding of the density, charge state, and kinetic energy of plasma constituents is needed at the location where they contribute to materials processes. Here, we study laser-induced plasmas over expansion distances of several centimeters from the ablation target, which is the relevant length scale for materials growth and modification. The study is enabled by a fast implementation of a laser ablation/plasma expansion model using an adaptive Cartesian mesh solver. Simulation outcomes for KrF excimer laser ablation of Cu are compared with Langmuir probe and optical emission spectroscopy measurements. Furthermore, simulation predictions for the plasma-shielding threshold, the ionization state of species in the plasma, and the kinetic energy of ions, are in good correspondence with experimental data. For laser fluences of 1–4 J cm -2 , the plume is dominated by Cu 0 , with small concentrations of Cu+ and electrons at the expansion front. Higher laser fluences (e.g. 7 J cm -2 ) lead to a Cu + -rich plasma, with a fully ionized leading edge where Cu 2+ is the dominant species. In both regimes, simulations indicate the presence of a low-density, high-temperature plasma expansion front with a high degree of ionization that may play a significant role in doping, annealing, and kinetically-driven phase transformations in 2D materials.

2D materials↗
NASA NTRS1974

Formation of HO2/+/ by reaction of metastable O2/+/ ions with H2

The photoionization efficiency curves of H2(+), O2(+), and HO2(+) have been studied in a mixture of hydrogen and oxygen over the wavelength range from 650 to 810 A. The HO2(+) ion appears at 804 A, the threshold for ionization of H2, by the reaction H2(+) + O2 yields HO2(+) + H. The relative photoionization efficiency curves of H2(+) and HO2(+) are the same from 804 to 764 A. Below 764 A production of the 4 Pi u metastable electronic state of O2(+) leads to the formation of HO2(+) by the reaction O2(+)(a 4 Pi u) + H2 yields HO2(+) + H.

Ajello, J. M.↗
NASA NTRS1980

Cosmic ray studies with a gas Cherenkov counter in association with an ionization spectrometer

The results from a balloon-borne gas Cherenkov counter (threshold 16.5 GeV/nucleon) and an ionization spectrometer are presented. The gas Cherenkov counter provides an absolute energy distribution for the response of the calorimeter for 5 or = Z 26 nuclei of cosmic rays. The contribution of scintillation to the gas Cherenkov pulse height was obtained by independently selecting particles below the gas Cherenkov threshold using the ionization spectrometer. Energy spectra were derived by minimizing the chi squared between Monte Carlo simulted data and flight data. Best fit power laws, dN/dE = AE-gamma, were determined for C, N, O, Ne, Mg, and Si. The power laws, all consistent with E (-2.7) are not good fits to the data. A better fit is obtained using the spectrum derived from the spectrometer. The data from the ionization calorimeter and the gas Cherenkov are thus completely self-consistent.

Balasubrahmanyan, V. K.↗
NASA NTRS1980

Cosmic ray studies with a gas Cerenkov counter in association with an ionization spectrometer

The results from a balloon-borne gas Cerenkov counter (threshold 16.5 GeV/nuc) and an ionization spectrometer are presented. The gas Cerenkov counter provides an absolute energy calibration for the response of the calorimeter for the Z range of 5-26 nuclei of cosmic rays. The contribution of scintillation to the gas Cerenkov pulse height has been obtained by independently selecting particles below the gas Cerenkov threshold using the ionization spectrometer. Energy spectra were derived by minimizing the chi-squared between a Monte Carlo simulated data and flight data. Best fit power laws were determined for C, N, O, Ne, Mg, and Si. The power laws, all consistent with E exp-2.7, are not good fits to the data. A better fit is obtained using the spectrum derived from the spectrometer.

Balasubrahmanyan, V. K.↗
DOE OSTI2024

Non-DNA radiosensitive targets that initiate persistent behavioral deficits in rats exposed to space radiation

Predicting future CNS risks for astronauts during deep-space missions will rely substantially on ground-based rodent data with space-relevant ions and behaviors. For rats, the accumulated evidence indicates that less densely ionizing radiation, such as 4 He and 12 C ions, induce behavior deficits at lower doses than densely ionizing ions, such as 48 Ti and 56 Fe. However, this observation conflicts with standard somatic radiobiology, in which densely ionizing ions are generally more effective than less densely ionizing ions, and where the DNA/nucleus is the accepted target for radiation-induced tumorigenesis, cytogenetic aberrations, genetic mutations, and reproductive cell death. To gain deeper insight into the subcellular nature of the radiation targets for behavior risks, we compared the effects of dose, fluence, and linear energy transfer (LET) of 4 He and 56 Fe particles using existing datasets for four distinct behavioral outcomes in rats: elevated plus maze (EPM-anxiety), novel object recognition (NOR-memory), operant responding (OR-response to environmental stimuli), and attentional set-shifting (ATSET-cognitive flexibility). We confirmed that less densely ionizing particles (except protons) showed ~100-fold lower threshold doses than densely ionizing particles for behavioral deficits (0.1–1 cGy for 4 He vs. 15–100 cGy for 56 Fe). However, when analyzed by fluence the behavioral responses converged, indicating that 4 He and 56 Fe were equally effective on a per-track basis. When analyzed by LET, there were ~100-fold differences in the LET for maximum effectiveness for behavioral deficits and DNA endpoints (~1 vs ~100 keV/μm, respectively). These unique features of radiation-induced behavioral deficits (high sensitivity to particles in the 1-keV/μm range, insensitivity to protons in the 0.2 keV/μm range, and isofluence dependence for particles with LET>1 keV/μm) provide evidence in support of a new hypothesis of sub-micron sized radiosensitive targets for behavioral effects consistent with the thickness of plasma membranes and/or small subcellular structures, smaller than a whole synapse. Like our behavior findings, mouse immature oocyte killing which is known to have a plasma membrane target was also better explained by fluence, rather than dose. In contrast, fluence analyses for DNA/nuclear endpoints in somatic cells (e.g., tumor induction, chromosome aberrations) showed opposite results, suggesting that behavior targets are not DNA. Our findings raise questions regarding the identity of subcellular targets and the multi-cellular functional unit for behavior risks, low-dose susceptibility, and generalizability from rat to other species and astronauts.

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗
NASA NTRS1983

The threshold laws for electron-atom and positron-atom impact ionization

The Coulomb-dipole theory is employed to derive a threshold law for the lowest energy needed for the separation of three particles from one another. The study focuses on an electron impinging on a neutral atom, and the dipole is formed between an inner electron and the nucleus. The analytical dependence of the transition matrix element on energy is reduced to lowest order to obtain the threshold law, with the inner electron providing a shield for the nucleus. Experimental results using the LAMPF accelerator to produce a high energy beam of H- ions, which are then exposed to an optical laser beam to detach the negative H- ion, are discussed. The threshold level is found to be confined to the region defined by the upper bound of the inverse square of the Coulomb-dipole region. Difficulties in exact experimental confirmation of the threshold are considered.

Temkin, A.↗
DOE OSTI2026

Bond Dissociation Energy, Ionization Energy, and Electronic Structure of Thorium Dimer

Diatomic thorium, Th 2 , has been investigated using a laser ablation, supersonic expansion source to produce the molecule and resonant two-photon ionization spectroscopy to measure its bond dissociation energy (BDE) and ionization energy (IE). The molecule has a high density of states in the vicinity of its bond dissociation energy, leading to rapid predissociation as soon as this energy is exceeded. The BDE is identified from this predissociation threshold as D 0 (Th 2 ) = 2.857(7) eV, where the assigned error limit is provided in parentheses in units of the last quoted digit. Similarly, the one-photon ionization threshold has been measured, providing the ionization energy IE(Th 2 ) = 5.042(4) eV. Together with a thermochemical cycle and the atomic ionization energy, these values provide the BDE of the cation, giving D 0 (Th 2 + ) = 4.122(8) eV. Computations show that Th 2 has three nearly degenerate low-lying electronic states (1 3 Σ u + , 1 1 Σ g + , and 1 3 Δ g ) with bonding dominated by 7s and 6d orbitals, indicating predominantly transition-metal-like behavior. The 1 3 Σ u + state exhibits a triple bond, whereas the 1 1 Σ g + and 1 3 Δ g states possess quadruple-bond character and correspondingly shorter bonds. Although 1 3 Σ u + is predicted to be the lowest state without spin–orbit coupling, the large spin–orbit stabilization of the 1 3 Δ g state makes its Ω = 1 g component the ground state. Furthermore, the calculated dissociation energy (2.840 eV) and ionization energy of Th 2 (5.098 eV) are both in excellent agreement with experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1989

Late cataractogenesis caused by particulate radiations and photons in long-lived mammalian species

Radiation cataractogenesis induced by small acute doses of particulate radiations and photons in the New Zealand white rabbit (Oryctolagus cuniculus), the beagle dog (Canis familiaris) and the rhesus monkey (Macaca mulatta) is discussed in the context of the use of animal models to assess the radiation hazards faced by humans during lengthy sojourns in deep space. Attention is paid to (1) the importance of lifespan studies with long-lived species - the above animals have median lifespans in captivity of 5-7, 13-14 and 25 years, respectively; and (2) the magnitudes of possible dose thresholds for cataractogenesis from sparsely ionizing radiations and the modifications of those thresholds by the late degenerative phase of the phenomenon.

Lett, J. T.↗