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At least 739 records · Page 41

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NASA NTRS1994

The Realuminizing of the 7-Meter Diameter Solar Simulator Collimating Mirror

This paper describes the modification of a three-electron-beam (EB) gun system for vacuum depositing a highly reflective aluminum coating on a 7.01-m (23-ft) -diam nickel-plated aluminum collimating mirror. The mirror is part of the JPL 7.62-m space simulator that was recently modernized with a new high vacuum pumping system, solar lamp power supplies, solar optic lens system, and refurbished collimating mirror. The mirror is the primary reflector for the solar simulation system and the aluminized reflective surface is its most critical performance element. The uniformity of thickness and high reflectivity of the coating is visible and near-ultraviolet (UV) light governs the accuracy of the beam for solar testing. The uniformity of the thin-film thickness also controls the durability of the mirror over time.

solar↗

DOE OSTI2016

TREAT reactor LEU fuel-clad chemical interaction empirical modeling analysis

DOE is converting TREAT (Transient Reactor Test) facility from its existing highly enriched uranium (HEU) core to a low-enriched uranium (LEU) core. In order to assess the magnitude of chemical interaction between fuel and cladding materials during physical contact under expected TREAT operational limits, planned transients tests, and reactivity accident scenarios, a combination of experimental testing and thermodynamic modeling was performed to predict the expected chemical interactions among fuel and cladding chemical constituents. Thermodynamic calculations were then validated with empirical data from experiments that emulate TREAT's expected upper operational limits. Pellet samples composed of LEU oxide powder dispersed in a graphite matrix had intimate contact with zirconium-based alloy cladding. The samples were subjected to long-term isothermal heating under high vacuum. Specimen characterization consisted of scanning electron microscopy, x-ray diffraction analysis, and x-ray tomography. ThermoCalc software and Ellingham's diagrams were used for thermodynamic calculations. The X-ray tomography and SEM analysis of the pellets showed a homogeneous distribution of UO{sub 2} particles within the graphite matrix with isolated larger UO{sub 2} agglomerates. The SEM analysis showed the formation of a lamellar structure between the large UO{sub 2} agglomerate and the graphite matrix. This is an indication that diffusion is taking place at a rather moderate temperature and reaction time during the pellet fabrication. Secondary phases, with high Al and Mg content, were detected inside a UO{sub 2} agglomerate using SEM in SE/EBSD mode and chemical analysis with EDS. LEU feedstock impurities, as Al and Mg, seem to promote the chemical reactivity between UO{sub 2} particles and graphite matrix. XRD analysis of a pellet sample showed the presence of UO{sub 2} and graphite, but no other crystalline phases were detected with this technique. Thermodynamic analysis through the use of Ellingham diagrams showed that within TREAT's operational to reactivity accident temperature (278-820 Celsius degrees), UO{sub 2} can be reduced to U metal by Zr due to a more favorable ΔG for the oxidation of Zr.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2017

Peculiarities of the Short-Pulse Dielectric Strength of Vacuum Insulation

Results of a study of the short-pulse dielectric strength of millimeter plane vacuum gaps with electrodes that have been treated with an electron beam are presented. It is shown that the electric field strength of the first breakdown of vacuum gaps with pure metal electrodes is determined to a significant extent by the crystal structure of the metal. The development of the first short-pulse breakdown is accompanied by a very abrupt growth of the electric current. The short duration of the test pulses rules out the influence of all well-known inertial mechanisms of breakdown with characteristic action times greater than 20 ns. Some general assumptions regarding the nature of the factors stimulating the short-pulse breakdown of vacuum gaps are considered.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Dissociative electron recombination and rotational cooling of the deuterated triatomic hydrogen ions H 2 ⁡D + and D 2 ⁢H +

We have measured the dissociative recombination (DR) of the deuterated triatomic hydrogen ions H 2 ⁡D + and D 2 ⁢H + as a function of storage time at the Cryogenic Storage Ring (CSR). Both molecular ions were stored for up to 1000 s inside the cryogenic vacuum of the CSR prior to the electron recombination measurements, allowing them to cool to their lowest rotational states. We implement a comprehensive model for all relevant processes to predict the internal state evolution of the ions during storage inside the CSR, employing calculated radiative transition strengths and state-selective rate coefficients for electron collisions. Our DR rate coefficient measurements with deuterated triatomic hydrogen ions in defined quantum states allow for meaningful comparisons with state-of-the-art theoretical calculations, paving the way for a better understanding of the complex DR process for polyatomic molecular ions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2012

Growth of oriented Au nanostructures: Role of oxide at the interface

We report on the formation of oriented gold nanostructures on Si(100) substrate by annealing procedures in low vacuum ({approx_equal}10{sup -2} mbar) and at high temperature ({approx_equal}975 deg. C). Various thicknesses of gold films have been deposited with SiO{sub x} (using high vacuum thermal evaporation) and without SiO{sub x} (using molecular beam epitaxy) at the interface on Si(100). Electron microscopy measurements were performed to determine the morphology, orientation of the structures and the nature of oxide layer. Interfacial oxide layer, low vacuum and high temperature annealing conditions are found to be necessary to grow oriented gold structures. These gold structures can be transferred by simple scratching method.

36 MATERIALS SCIENCE↗

DOE OSTI2012

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 {mu}A) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 {+-} 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

A 4 x 4 SiPM Sensor Array with Fast Preamplified Output Engineered for the PWO Detectors of the EIC EEEMCAL

The PbWO4 scintillation detectors for the Electron-Ion Collider Electron Endcap Electromagnetic Calorimeter (EEEMCAL) cannot use vacuum photomultipler tubes due to the high magnetic field environment. We are presenting a readout solution with a 4 x 4 SiPM array directly coupled to a preamplification and summing stage. A second amplification stage with variable gain and offset adjustments is also included in the compact electronics module. The integrated power supply feeds the signal processing chain and the adjustable bias control. The parameter settings are saved locally on a EEPROM and are adjustable via the integrated communication interface through USB or RS485. The design was optimized to maximize signal collection energy resolution, for fast timing and wide dynamic range while keeping a small footprint with low power consumption and heat dissipation. The output response is fast enough to meet the 100 ? 200 ns digitization gate requirements for the detector. The electronics assembly requires no active cooling and includes a temperature sensor and a gain compensation of thermal variations which are important features for stable operation in large scale detector structures. A detector prototype was constructed with 20 mm x 20 mm x 200 mm PWO crystals affixed with individual SiPM array readouts and arranged in a 3 x 3 array. This detector was tested at the Thomas Jefferson National Accelerator Facility with 5GeV positrons. We will present the results of these detector characterization measurements.

Philip, O.↗

DOE OSTI2019

Asymptotic theory of ponderomotive dynamics of an electron in the field of a focused relativistically intense electromagnetic envelope

Based on a formal asymptotic solution of Maxwell’s equations for a field propagating in a vacuum and of the relativistic Newton equation for an electron driven by the corresponding Lorentz force, we construct a description of the relativistic ponderomotive dynamics of an electron in the field of an intense focused electromagnetic envelope. A small parameter for the asymptotic expansion is proportional to the ratio of the radiation wavelength to the focal spot radius. Using the obtained averaged model of ponderomotive dynamics, electron beam scattering patterns are plotted with regard to an angle relative to the electromagnetic field propagation axis for laser pulses with a Gaussian transverse intensity distribution, as well as the energy spectra of scattered electrons corresponding to individual ranges of this angle. Scattered particles are absent in some ranges of the polar angle. (interaction of laser radiation with matter. laser plasma)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2012

Recent developments of the ion sources at Tri University Meson Factory/Isotope Separator and ACcelerator Facility

This paper describes the recent progresses concerning the on-line ion source at the Tri University Meson Factory/Isotope Separator and ACcelerator (TRIUMF/ISAC) Radioactive Ion-Beam Facility; description of the new design of the surface-ion-source for improved stability of the beam intensity, description of the transport path to the east target station at ISAC, description of the new brazing techniques that solved recurrent problems with water leaks on the target/ion source assembly in the vacuum system, finally, recent developments concerning the Forced Electron Beam Induced Arc Discharge (FEBIAD) ion source are reported. In particular, a study on the effect of the plasma chamber volume on the ionization efficiency was completed.

43 PARTICLE ACCELERATORS↗

NASA NTRS1976

Neutral-atom electron binding energies from relaxed-orbital relativistic Hartree-Fock-Slater calculations for Z between 2 and 106

Electron binding energies in neutral atoms have been calculated relativistically, with the requirement of complete relaxation. Hartree-Fock-Slater wave functions served as zeroth-order eigenfunctions to compute the expectation of the total Hamiltonian. A first-order correction to the local approximation was thus included. Quantum-electrodynamic corrections were made. For all elements with atomic numbers ranging from 2 to 106, the following quantities are listed: total energies, electron kinetic energies, electron-nucleus potential energies, electron-electron potential energies consisting of electrostatic and Breit interaction (magnetic and retardation) terms, and vacuum polarization energies. Binding energies including relaxation are listed for all electrons in all atoms over the indicated range of atomic numbers. A self-energy correction is included for the 1s, 2s, and 2p(1/2) levels. Results for selected atoms are compared with energies calculated by other methods and with experimental values.

Huang, K.-N.↗

DOE OSTI2014

WE-E-18A-05: Bremsstrahlung of Laser-Plasma Interaction at KeV Temperature: Forward Dose and Attenuation Factors

Purpose: To obtain an analytical empirical formula for the photon dose source term in forward direction from bremsstrahlung generated from laser-plasma accelerated electron beams in aluminum solid targets, with electron-plasma temperatures in the 10–100 keV energy range, and to calculate transmission factors for iron, aluminum, methacrylate, lead and concrete and air, materials most commonly found in vacuum chamber labs. Methods: Bremsstrahlung fluence is calculated from the convolution of thin-target bremsstrahlung spectrum for monoenergetic electrons and the relativistic Maxwell-Juettner energy distribution for the electron-plasma. Unattenuatted dose in tissue is calculated by integrating the photon spectrum with the mass-energy absorption coefficient. For the attenuated dose, energy dependent absorption coefficient, build-up factors and finite shielding correction factors were also taken into account. For the source term we use a modified formula from Hayashi et al., and we fitted the proportionality constant from experiments with the aid of the previously calculated transmission factors. Results: The forward dose has a quadratic dependence on electron-plasma temperature: 1 joule of effective laser energy transferred to the electrons at 1 m in vacuum yields 0,72 Sv per MeV squared of electron-plasma temperature. Air strongly filters the softer part of the photon spectrum and reduce the dose to one tenth in the first centimeter. Exponential higher energy tail of maxwellian spectrum contributes mainly to the transmitted dose. Conclusion: A simple formula for forward photon dose from keV range temperature plasma is obtained, similar to those found in kilovoltage x-rays but with higher dose per dissipated electron energy, due to thin target and absence of filtration.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2014

WE-E-18A-06: To Remove Or Not to Remove: Comfort Pads From Beneath Neonates for Radiography

07 ISOTOPE AND RADIATION SOURCES↗

NASA NTRS1987

Stigmatically focusing partial pressure analyzer with dual chamber ion source

The partial pressure analyzer developed has a high absolute sensitivity and high resolution. Interaction effects, as they occur in conventional partial pressure analyzers between the heating filament for the emission of electrons and the residual gas, are suppressed by removing the heating filament from the measurement area. The electron beam is generated in an auxiliary vacuum compartment, and passes subsequently through an aperture into the ionization compartment. The probability is small, therefore, that the presence of substances produced at the heating filament by chemical reactions and thermal decompositions will have disturbing effects. The high sensitivity and resolution of the instrument are obtained with the aid of an electron path configuration of unconventional characteristics.

Rasskopf, Klaus F.↗

DOE OSTI2019

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe 2 and WS 2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.

36 MATERIALS SCIENCE↗

NASA NTRS1995

Redesigned Electron-Beam Furnace Boosts Productivity

Redesigned electron-beam furnace features carousel of greater capacity so more experiments conducted per loading, and time spent on reloading and vacuum pump-down reduced. Common mounting plate for electron source and carousel simplifies installation and reduces vibration.

Williams, Gary A.↗

NASA NTRS1999

Super Photon Counters

The perfect photon detector would measure the arrival time, the energy, the polarization, and the position of every arriving quantum, but that is easier said than done. Two groups have now succeeded in doing time-resolved spectroscopy on the Crab Nebula pulsar, measuring everything but the polarization, with reports from Romani et al. at Stanford and from Perryman et al. at ESTEC. Both groups use superconducting detectors to gain the necessary speed and sensitivity. The photon can heat the electrons in a superconductor biased in the middle of its resistive transition, or break bound superconducting electron-hole pairs, which can then be collected. Three years ago, Peacock et al. reported that they had detected single optical photons with a superconducting tunnel junction (STJ), and Paresce wrote a News and Views article. A tunnel junction uses two pieces of conductive material, separated by a tiny gap of insulating material or even vacuum. If the gap is thin enough, electrons can tunnel across anyway, and if the conductors are superconductors, the junction displays very useful quantum mechanical properties and electrical nonlinearities. Amplifiers, detectors, oscillators, and computer circuits can all be made from them. Their special advantage is that they operate at very low temperatures, dissipate very little power, operate very fast, and are very small.

Mather, John↗

DOE OSTI2015

Secondary electron imaging of monolayer materials inside a transmission electron microscope

A scanning transmission electron microscope equipped with a backscattered and secondary electron detector is shown capable to image graphene and hexagonal boron nitride monolayers. Secondary electron contrasts of the two lightest monolayer materials are clearly distinguished from the vacuum level. A signal difference between these two materials is attributed to electronic structure differences, which will influence the escape probabilities of the secondary electrons. Our results show that the secondary electron signal can be used to distinguish between the electronic structures of materials with atomic layer sensitivity, enhancing its applicability as a complementary signal in the analytical microscope.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Synchrotron radiation-based Mössbauer spectra of {sup 174}Yb measured with internal conversion electrons

A detection system for synchrotron-radiation (SR)-based Mössbauer spectroscopy was developed to enhance the nuclear resonant scattering counting rate and thus increase the available nuclides. In the system, a windowless avalanche photodiode (APD) detector was combined with a vacuum cryostat to detect the internal conversion (IC) electrons and fluorescent X-rays accompanied by nuclear de-excitation. As a feasibility study, the SR-based Mössbauer spectrum using the 76.5 keV level of {sup 174}Yb was observed without {sup 174}Yb enrichment of the samples. The counting rate was five times higher than that of our previous system, and the spectrum was obtained within 10 h. This result shows that nuclear resonance events can be more efficiently detected by counting IC electrons for nuclides with high IC coefficients. Furthermore, the windowless detection system enables us to place the sample closer to the APD elements and is advantageous for nuclear resonant inelastic scattering measurements. Therefore, this detection system can not only increase the number of nuclides accessible in SR-based Mössbauer spectroscopy but also allows the nuclear resonant inelastic scattering measurements of small single crystals or enzymes with dilute probe nuclides that are difficult to measure with the previous detection system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗