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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on YTl5Cu2(NO2)12 by Materials Project

(Cu)2YTl5(NO2)12 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four copper molecules and one YTl5(NO2)12 framework. In the YTl5(NO2)12 framework, Y3+ is bonded to twelve equivalent O2- atoms to form YO12 cuboctahedra that share edges with six equivalent TlO12 cuboctahedra and faces with four equivalent TlO12 cuboctahedra. All Y–O bond lengths are 2.74 Å. There are two inequivalent Tl1+ sites. In the first Tl1+ site, Tl1+ is bonded to twelve O2- atoms to form distorted TlO12 cuboctahedra that share edges with two equivalent YO12 cuboctahedra, edges with four equivalent TlO12 cuboctahedra, and faces with four equivalent TlO12 cuboctahedra. There are a spread of Tl–O bond distances ranging from 3.20–3.57 Å. In the second Tl1+ site, Tl1+ is bonded to twelve O2- atoms to form TlO12 cuboctahedra that share faces with two equivalent YO12 cuboctahedra and faces with six equivalent TlO12 cuboctahedra. There are six shorter (2.85 Å) and six longer (2.95 Å) Tl–O bond lengths. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.26 Å) and one longer (1.27 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three Tl1+ and one N3+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to one Y3+, two Tl1+, and one N3+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K5CeCu2(NO2)12 by Materials Project

K5Ce(NO2)12(Cu)2 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four copper molecules and one K5Ce(NO2)12 framework. In the K5Ce(NO2)12 framework, there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share faces with two equivalent CeO12 cuboctahedra and faces with six equivalent KO12 cuboctahedra. There are six shorter (2.78 Å) and six longer (2.85 Å) K–O bond lengths. In the second K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share edges with two equivalent CeO12 cuboctahedra, edges with four equivalent KO12 cuboctahedra, and faces with four equivalent KO12 cuboctahedra. There are a spread of K–O bond distances ranging from 3.11–3.31 Å. Ce3+ is bonded to twelve equivalent O2- atoms to form CeO12 cuboctahedra that share edges with six equivalent KO12 cuboctahedra and faces with four equivalent KO12 cuboctahedra. All Ce–O bond lengths are 2.86 Å. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.27 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one N3+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two K1+, one Ce3+, and one N3+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K5YNi2(NO2)12 by Materials Project

K5Y(NO2)12(Ni)2 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four nickel molecules and one K5Y(NO2)12 framework. In the K5Y(NO2)12 framework, there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share faces with two equivalent YO12 cuboctahedra and faces with six equivalent KO12 cuboctahedra. There are six shorter (2.76 Å) and six longer (2.85 Å) K–O bond lengths. In the second K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share edges with two equivalent YO12 cuboctahedra, edges with four equivalent KO12 cuboctahedra, and faces with four equivalent KO12 cuboctahedra. There are a spread of K–O bond distances ranging from 3.08–3.35 Å. Y3+ is bonded to twelve equivalent O2- atoms to form YO12 cuboctahedra that share edges with six equivalent KO12 cuboctahedra and faces with four equivalent KO12 cuboctahedra. All Y–O bond lengths are 2.77 Å. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.28 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one N3+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two K1+, one Y3+, and one N3+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K5CeFe2(NO2)12 by Materials Project

K5Ce(NO2)12(Fe)2 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four iron molecules and one K5Ce(NO2)12 framework. In the K5Ce(NO2)12 framework, there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share faces with two equivalent CeO12 cuboctahedra and faces with six equivalent KO12 cuboctahedra. There are six shorter (2.81 Å) and six longer (2.86 Å) K–O bond lengths. In the second K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share edges with two equivalent CeO12 cuboctahedra, edges with four equivalent KO12 cuboctahedra, and faces with four equivalent KO12 cuboctahedra. There are a spread of K–O bond distances ranging from 3.02–3.24 Å. Ce3+ is bonded to twelve equivalent O2- atoms to form CeO12 cuboctahedra that share edges with six equivalent KO12 cuboctahedra and faces with four equivalent KO12 cuboctahedra. All Ce–O bond lengths are 2.83 Å. N+2.83+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.28 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one N+2.83+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two K1+, one Ce3+, and one N+2.83+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K5CeNi2(NO2)12 by Materials Project

K5Ce(NO2)12(Ni)2 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four nickel molecules and one K5Ce(NO2)12 framework. In the K5Ce(NO2)12 framework, there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share faces with two equivalent CeO12 cuboctahedra and faces with six equivalent KO12 cuboctahedra. There are six shorter (2.79 Å) and six longer (2.86 Å) K–O bond lengths. In the second K1+ site, K1+ is bonded to twelve O2- atoms to form KO12 cuboctahedra that share edges with two equivalent CeO12 cuboctahedra, edges with four equivalent KO12 cuboctahedra, and faces with four equivalent KO12 cuboctahedra. There are a spread of K–O bond distances ranging from 3.10–3.35 Å. Ce3+ is bonded to twelve equivalent O2- atoms to form CeO12 cuboctahedra that share edges with six equivalent KO12 cuboctahedra and faces with four equivalent KO12 cuboctahedra. All Ce–O bond lengths are 2.85 Å. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.27 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one N3+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two K1+, one Ce3+, and one N3+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Cs5CeNi2(NO2)12 by Materials Project

Cs5Ce(NO2)12(Ni)2 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four nickel molecules and one Cs5Ce(NO2)12 framework. In the Cs5Ce(NO2)12 framework, there are two inequivalent Cs1+ sites. In the first Cs1+ site, Cs1+ is bonded to twelve O2- atoms to form CsO12 cuboctahedra that share faces with two equivalent CeO12 cuboctahedra and faces with six equivalent CsO12 cuboctahedra. There are six shorter (3.10 Å) and six longer (3.15 Å) Cs–O bond lengths. In the second Cs1+ site, Cs1+ is bonded to twelve O2- atoms to form CsO12 cuboctahedra that share edges with two equivalent CeO12 cuboctahedra, edges with four equivalent CsO12 cuboctahedra, and faces with four equivalent CsO12 cuboctahedra. There are a spread of Cs–O bond distances ranging from 3.34–3.62 Å. Ce3+ is bonded to twelve equivalent O2- atoms to form CeO12 cuboctahedra that share edges with six equivalent CsO12 cuboctahedra and faces with four equivalent CsO12 cuboctahedra. All Ce–O bond lengths are 2.97 Å. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.28 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a single-bond geometry to three Cs1+ and one N3+ atom. In the second O2- site, O2- is bonded in a single-bond geometry to two Cs1+, one Ce3+, and one N3+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Rb5CeCu2(NO2)12 by Materials Project

Rb5Ce(NO2)12(Cu)2 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four copper molecules and one Rb5Ce(NO2)12 framework. In the Rb5Ce(NO2)12 framework, there are two inequivalent Rb1+ sites. In the first Rb1+ site, Rb1+ is bonded to twelve O2- atoms to form RbO12 cuboctahedra that share faces with two equivalent CeO12 cuboctahedra and faces with six equivalent RbO12 cuboctahedra. There are six shorter (2.90 Å) and six longer (2.99 Å) Rb–O bond lengths. In the second Rb1+ site, Rb1+ is bonded to twelve O2- atoms to form distorted RbO12 cuboctahedra that share edges with two equivalent CeO12 cuboctahedra, edges with four equivalent RbO12 cuboctahedra, and faces with four equivalent RbO12 cuboctahedra. There are a spread of Rb–O bond distances ranging from 3.23–3.56 Å. Ce3+ is bonded to twelve equivalent O2- atoms to form CeO12 cuboctahedra that share edges with six equivalent RbO12 cuboctahedra and faces with four equivalent RbO12 cuboctahedra. All Ce–O bond lengths are 2.82 Å. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.27 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three Rb1+ and one N3+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two Rb1+, one Ce3+, and one N3+ atom.

36 MATERIALS SCIENCE↗
NASA NTRS1995

High rotational CO lines in post-AGB stars and PNe

A significant fraction of a star's initial mass is lost while it is on the Asymptotic Giant Branch (AGB). Mass loss rates range from 10(exp -7) solar mass/yr for early AGB stars to a few 10(exp -4) solar mass/yr for stars at the tip of the AGB. Dust grains condense from the outflow as the gas expands and form a dust shell around the central star. A superwind (approximately 10(exp -4) to 10(exp -3) solar mass/yr) is thought to terminate the AGB phase. In the post-AGB phase, the star evolves to a higher effective temperature, the mass loss decreases (approximately 10(exp -8) solar mass/yr), but the wind velocity increases (approximately 1000 km/s). During this evolution, dust and gas are exposed to an increasingly harsher radiation field and when T(sub eff) reaches about 30,000 K, the nebula is ionized and becomes a planetary nebula (PN). Photons from the central star can create a photodissociation region (PDR) in the expanding superwind. Gas can be heated through the photoelectric effect working on small grains and polycyclic aromatic hydrocarbons (PAH's). This gas can cool via the atomic fine structure lines of O I (63 microns and 145 microns) and C II (158 microns), as well as the rotational lines of CO. In the post-AGB phase, the fast wind from the central star will interact with the material ejected during the AGB phase. The shock caused by this interaction will dissociate and heat the gas. This warm gas will cool through atomic fine structure lines of O I and the rotational lines of (newly formed) CO.

Justtanont, K.↗
DOE OSTI2022

How effective is the Brink–Axel hypothesis for astrophysical weak rates?

We explore the effectiveness of the Brink–Axel hypothesis (BAH) for the computation of stellar electron capture (EC) and β-decay (BD) rates, namely that the transition strength function depends only upon the transition energy and not upon the details of the initial state. For this purpose, we calculated Gamow–Teller (GT) strength distributions for a selection of sd-shell nuclides, using two different microscopic models, namely the proton–neutron quasiparticle random phase approximation and the full configuration-interaction shell model, taking into account the first 100 states of both the initial and final nuclides. The GT transition strengths among these levels evolve with initial state energy. These transition strength functions we folded into weak-interaction mediated rates in stellar matter, specifically EC and BD rates, for a range of densities 10 g cm –3 ≤ ρ ≤ 10 11 g cm –3 and range of temperatures 1 GK ≤ T ≤ 30 GK. When transitions from excited states were approximated using the BAH, augmented by so-called 'back-resonance' transitions, the rates were affected by up to three orders of magnitude or more at high temperatures and densities. Furthermore the BAH is not a reliable approximation for the calculation of stellar rates, especially in high temperature–density environments.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2006

A Lunar Communications and Navigation Satellite Concept for the Robotic Lunar Exploration Program

The Second Robotic Lunar Exploration Program (RLEP) mission has two primary objectives. Broadly stated, they are "To See the Light" and "To Touch the Ice", meaning one mission objective is to survey the lighting conditions on the rim of a candidate crater over the course of a year, and the second objective is to descend into th.! s crater to search for water ice. The Space Communications Program Office at NASA/GSFC is supporting the RLEP-2 Project by developing a communications and navigation architecture for the mission. The primary candidate crater studied is the Shackleton Crater near the South Pole of the Moon. The rim of this crater has approximately 14 contiguous days of visibility to the Earth over the course of a month, while mission elements in the crater have no line of sight to either the Earth or potentially an element on the rim of the crater. One solution studied is to launch a relay spacecraft with RLEP-2 to provide communications and navigation services to the elements on the lunar surface. To support operations concepts requiring long duration contacts between the relay and the elements in the crater, an inclined elliptical orbit originally conceived by Todd Ely a t JPL was chosen for the relay. The orbit was adjusted to lower the maximum range while maintaining its "frozen" nature, meaning once placed in this orbit little delta-V is required for orbit maintenance. The orbital parameters were adjusted such that the relay has visibility into the crater for approximately 8.7 hours of the 12 hour orbital period. The relay satellite design draws heavily upon heritage concepts and hardware to achieve a very low risk implementation. The design concept is a 3-axis spacecraft that is capable of supporting return communication links from multiple surface elements concurrently through a four-element S-Band Multiple Access phased array with two forward links provided through single dedicated elements. The payload is a bent-pipe relay with beamforming performed on the ground (Earth). To mitigate concerns with multipath observed during Mars missions, the preliminary design employs PN-spread signal structures which have been used successfully by Tracking and Data Relay Satellite System (TDRSS) users for simultaneous real-time and recorded data playback. Details of the relay communication trade studies and rationale for the selected design elements are discussed in the paper.

Gramling, Jeffrey J.↗
NASA NTRS2007

Carbon Nanotubes for Space Photovoltaic Applications

Carbon nanotubes (CNTs) can be envisioned as an individual graphene sheet rolled into a seamless cylinder (single-walled, SWNT), or concentric sheets as in the case of a multi-walled carbon nanotube (MWNT) (1). The role-up vector will determine the hexagonal arrangement and "chirality" of the graphene sheet, which will establish the nanotube to be metallic or semiconducting. The optoelectronic properties will depend directly on this chiral angle and the diameter of the SWNT, with semiconductor types exhibiting a band gap energy (2). Characteristic of MWNTs are the concentric graphene layers spaced 0.34 nm apart, with diameters from 10-200 nm and lengths up to hundreds of microns (2). In the case of SWNTs, the diameters range from 0.4 - 2 nm and lengths have been reported up to 1.5 cm (3). SWNTs have the distinguishable property of "bundling" together due to van der Waal's attractions to form "ropes." A comparison of these different structural types is shown in Figure 1. The use of SWNTS in space photovoltaic (PV) applications is attractive for a variety of reasons. Carbon nanotubes as a class of materials exhibit unprecedented optical, electrical, mechanical properties, with the added benefit of being nanoscale in size which fosters ideal interaction in nanomaterial-based devices like polymeric solar cells. The optical bandgap of semiconducting SWNTs can be varied from approx. 0.4 - 1.5 eV, with this property being inversely proportional to the nanotube diameter. Recent work at GE Global Research has shown where a single nanotube device can behave as an "ideal" pn diode (5). The SWNT was bridged over a SiO2 channel between Mo contacts and exhibited an ideality factor of 1, based on a fit of the current-voltage data using the diode equation. The measured PV efficiency under a 0.8 eV monochromatic illumination showed a power conversion efficiency of 0.2 %. However, the projected efficiency of these junctions is estimated to be > 5 %, especially when one considers the enhanced absorption (from nanotubes whose bandgap is tailored to illumination) and electromagnetic coupling in a network of nanotubes.

Efstathiadis, Harry↗
NASA NTRS1996

Silicon Carbide High-Temperature Power Rectifiers Fabricated and Characterized

The High Temperature Integrated Electronics and Sensors (HTIES) team at the NASA Lewis Research Center is developing silicon carbide (SiC) for use in harsh conditions where silicon, the semiconductor used in nearly all of today's electronics, cannot function. Silicon carbide's demonstrated ability to function under extreme high-temperature, high power, and/or high-radiation conditions will enable significant improvements to a far ranging variety of applications and systems. These improvements range from improved high-voltage switching for energy savings in public electric power distribution and electric vehicles, to more powerful microwave electronics for radar and cellular communications, to sensors and controls for cleaner-burning, more fuel-efficient jet aircraft and automobile engines. In the case of jet engines, uncooled operation of 300 to 600 C SiC power actuator electronics mounted in key high-temperature areas would greatly enhance system performance and reliability. Because silicon cannot function at these elevated temperatures, the semiconductor device circuit components must be made of SiC. Lewis' HTIES group recently fabricated and characterized high-temperature SiC rectifier diodes whose record-breaking characteristics represent significant progress toward the realization of advanced high-temperature actuator control circuits. The first figure illustrates the 600 C probe-testing of a Lewis SiC pn-junction rectifier diode sitting on top of a glowing red-hot heating element. The second figure shows the current-versus voltage rectifying characteristics recorded at 600 C. At this high temperature, the diodes were able to "turn-on" to conduct 4 A of current when forward biased, and yet block the flow of current ($quot;turn-off") when reverse biases as high as 150 V were applied. This device represents a new record for semiconductor device operation, in that no previous semiconductor electronic device has ever simultaneously demonstrated 600 C functionality, and 4-A turn-on and 150-V rectification. The high operating current was achieved despite severe device size limitations imposed by present-day SiC wafer defect densities. Further substantial increases in device performance can be expected when SiC wafer defect densities decrease as SiC wafer production technology matures.

Source record↗
DOE OSTI2020

Materials Data on CeTl5Cu2(NO2)12 by Materials Project

(Cu)2CeTl5(NO2)12 crystallizes in the cubic Pn-3 space group. The structure is three-dimensional and consists of four copper molecules and one CeTl5(NO2)12 framework. In the CeTl5(NO2)12 framework, Ce3+ is bonded to twelve equivalent O2- atoms to form CeO12 cuboctahedra that share edges with six equivalent TlO12 cuboctahedra and faces with four equivalent TlO12 cuboctahedra. All Ce–O bond lengths are 2.83 Å. There are two inequivalent Tl1+ sites. In the first Tl1+ site, Tl1+ is bonded to twelve O2- atoms to form TlO12 cuboctahedra that share faces with two equivalent CeO12 cuboctahedra and faces with six equivalent TlO12 cuboctahedra. There are six shorter (2.81 Å) and six longer (2.89 Å) Tl–O bond lengths. In the second Tl1+ site, Tl1+ is bonded to twelve O2- atoms to form TlO12 cuboctahedra that share edges with two equivalent CeO12 cuboctahedra, edges with four equivalent TlO12 cuboctahedra, and faces with four equivalent TlO12 cuboctahedra. There are a spread of Tl–O bond distances ranging from 3.15–3.41 Å. N3+ is bonded in a bent 120 degrees geometry to two O2- atoms. There is one shorter (1.25 Å) and one longer (1.27 Å) N–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three Tl1+ and one N3+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to one Ce3+, two Tl1+, and one N3+ atom.

36 MATERIALS SCIENCE↗
NASA NTRS2003

X-ray QPOs from the Ultra-luminous X-ray Source in M82: Evidence Against Beaming

We report the discovery with the European Photon Imaging Camera (EPIC) CCD cameras onboard XMM-Newton of a 54 mHz quasiperiodic oscillation (QPO) in the greater than 2 keV X-ray flux from the ultra-luminous X-ray source (ULX) X41.4+60 in the starburst galaxy M82. This is the first detection of a QPO in the X-ray flux from an extra-Galactic ULX, and confirms that the source is a compact object. The QPO is detected in the combined PN and MOS data at the approx. 6sigma level, and separately at lower significances in both the PN and MOS instruments. It had a centroid frequency of 54.3 +/- 0.9 mHz, a coherence Q is identical with nu(sub 0)/Delta nu(sub fwhm) is approx. 5, and an amplitude (rms) in the 2 - 10 keV band of 8.5%. Below about 0.2 Hz the power spectrum can be described by a power-law with index approx. 1, and integrated amplitude (rms) of 13.5%. The X-ray spectrum requires a curving continuum, with a disk-blackbody (diskbb) at T = 3.1 keV providing an acceptable, but not unique, fit. A broad Fe line centered at 6.55 keV is required in all fits, but the equivalent width (EW) of the line is sensitive to the choice of continuum model. There is no evidence of a reflection component. The implied bolometric luminosity is approx. 4 - 5 x 10(exp 40) ergs/s. Data from several archival Rossi X-ray Timing Explorer (RXTE) pointings at M82 also show evidence for QPOs in the 50 - 100 mHz frequency range. Several Galactic black hole candidates (BHCs), including GRS 1915+105, GRO J1655-40, and XTE 1550-564, show QPOs in the same frequency range as the 50 - 100 mHz QPOs in X41.4+60, which at first glance suggests a possible connection with such objects. However, strong, narrow QPOs provide solid evidence for disk emission, and thus present enormous theoretical difficulties for models which rely on either geometrically or relativistically beamed emission to account for the high X-ray luminosities. We discuss the implications of our findings for models of the ULX sources.

Strohmayer, Tod E.↗
NASA NTRS2007

Evidence for a Broad Relativistic Iron Line from the Neutron Star LMXB Ser X-1

We report on an analysis of XMM-Newton data from the neutron star low mass X-ray binary (LMXB) Serpens X-1 (Ser X-1). Spectral analysis of EPIC PN data indicates that the previously known broad iron Ka emission line in this source has a significantly skewed structure with a moderately extended red wing. The asymmetric shape of the line is well described with the laor and diskline models in XSPEC, which strongly supports an inner accretion disk origin of the line. To our knowledge this is the first strong evidence for a relativistic line in a neutron star LMXB. This finding suggests that the broad lines seen in other neutron star LMXBs likely originate from the inner disk as well. Detailed study of such lines opens up a new way to probe neutron star parameters and their strong gravitational fields. The laor model describes the line from Ser X-1 somewhat better than diskline, and suggests that the inner accretion disk radius is less than 6GM/c(exp 2). This is consistent with the weak magnetic fields of LMXBs, and may point towards a high compactness and rapid spin of the neutron star. Finally, the inferred source inclination angle in the approximate range 50-60 deg is consistent with the lack of dipping from Ser X-1.

Bhattacharyya, Sudip↗
NASA NTRS2012

An MSK Radar Waveform

The minimum-shift-keying (MSK) radar waveform is formed by periodically extending a waveform that separately modulates the in-phase and quadrature- phase components of the carrier with offset pulse-shaped pseudo noise (PN) sequences. To generate this waveform, a pair of periodic PN sequences is each passed through a pulse-shaping filter with a half sinusoid impulse response. These shaped PN waveforms are then offset by half a chip time and are separately modulated on the in-phase and quadrature phase components of an RF carrier. This new radar waveform allows an increase in radar resolution without the need for additional spectrum. In addition, it provides self-interference suppression and configurable peak sidelobes. Compared strictly on the basis of the expressions for delay resolution, main-lobe bandwidth, effective Doppler bandwidth, and peak ambiguity sidelobe, it appears that bi-phase coded (BPC) outperforms the new MSK waveform. However, a radar waveform must meet certain constraints imposed by the transmission and reception of the modulation, as well as criteria dictated by the observation. In particular, the phase discontinuity of the BPC waveform presents a significant impediment to the achievement of finer resolutions in radar measurements a limitation that is overcome by using the continuous phase MSK waveform. The phase continuity, and the lower fractional out-of-band power of MSK, increases the allowable bandwidth compared with BPC, resulting in a factor of two increase in the range resolution of the radar. The MSK waveform also has been demonstrated to have an ambiguity sidelobe structure very similar to BPC, where the sidelobe levels can be decreased by increasing the length of the m-sequence used in its generation. This ability to set the peak sidelobe level is advantageous as it allows the system to be configured to a variety of targets, including those with a larger dynamic range. Other conventionally used waveforms that possess an even greater spectral efficiency than the MSK waveform, such as linear frequency modulation (LFM) and Costas frequency hopping, have a fixed peak sidelobe level that is therefore not configurable, and can be exceeded by high contrast targets. Furthermore, in the case of a multistatic experiment observing a target in motion, self-interference from the transmitter to the receiver is mitigated by the MSK waveform. Waveforms that have delay Doppler coupling, such as LFM, provide no such protection.

Quirk, Kevin J.↗
DOE OSTI2024

Probing the Isospin Composition of Short-Range Correlated Pairs at Jefferson Lab Hall B

Nucleons in short-range correlated (SRC) pairs, due to their close proximity and high relative momentum, can provide insight into the short-range part of the strong nuclear interaction. In particular, the prevalence of np pairs is due to the dominance of a tensor term for correlated nucleons with momenta of approximately 400?600 MeV/c. This dissertation comprises two studies advancing the community?s understanding of the isospin composition of SRC pairs. First, I performed a study of proton and neutron knockout from initially low-momentum and high-momentum states in 3He. Previous work has shown that protons are disproportionately represented in high-momentum states in neutron-rich nuclei. I demonstrate that spectral functions for the proton-rich nucleus 3He predict, in agreement with data, that neutrons are disproportionately represented in high-momentum states, but that 3He does not display the same strong prevalence of np pairs that is observed in larger nuclei. Second, Generalized Contact Formalism (GCF), a well-supported theory for predicting SRC behavior, predicts the transition from an isospin-dependent, tensor-dominant interaction at intermediate distances to a scalar-dominant, isospin-independent interaction at very short distances. This dissertation uses data from the CLAS12 Nuclear Targets Experiments in Hall B at Jefferson Lab to measure the relative abundances of pp and pn pairs for increasing relative momentum and decreasing separation. I provide an independent confirmation of the previously-observed increase in pp pairs at increasing momentum of the struck nucleon. I also contribute to the application of the new CLAS12 Central Neutron Detector by precisely measuring the neutron detection efficiency and developing a machine learning model for rejecting charged particle background.

Seroka, Erin↗
NASA NTRS2009

Zinc Alloys for the Fabrication of Semiconductor Devices

ZnBeO and ZnCdSeO alloys have been disclosed as materials for the improvement in performance, function, and capability of semiconductor devices. The alloys can be used alone or in combination to form active photonic layers that can emit over a range of wavelength values. Materials with both larger and smaller band gaps would allow for the fabrication of semiconductor heterostructures that have increased function in the ultraviolet (UV) region of the spectrum. ZnO is a wide band-gap material possessing good radiation-resistance properties. It is desirable to modify the energy band gap of ZnO to smaller values than that for ZnO and to larger values than that for ZnO for use in semiconductor devices. A material with band gap energy larger than that of ZnO would allow for the emission at shorter wavelengths for LED (light emitting diode) and LD (laser diode) devices, while a material with band gap energy smaller than that of ZnO would allow for emission at longer wavelengths for LED and LD devices. The amount of Be in the ZnBeO alloy system can be varied to increase the energy bandgap of ZnO to values larger than that of ZnO. The amount of Cd and Se in the ZnCdSeO alloy system can be varied to decrease the energy band gap of ZnO to values smaller than that of ZnO. Each alloy formed can be undoped or can be p-type doped using selected dopant elements, or can be n-type doped using selected dopant elements. The layers and structures formed with both the ZnBeO and ZnCdSeO semiconductor alloys - including undoped, p-type-doped, and n-type-doped types - can be used for fabricating photonic and electronic semiconductor devices for use in photonic and electronic applications. These devices can be used in LEDs, LDs, FETs (field effect transistors), PN junctions, PIN junctions, Schottky barrier diodes, UV detectors and transmitters, and transistors and transparent transistors. They also can be used in applications for lightemitting display, backlighting for displays, UV and visible transmitters and detectors, high-frequency radar, biomedical imaging, chemical compound identification, molecular identification and structure, gas sensors, imaging systems, and for the fundamental studies of atoms, molecules, gases, vapors, and solids.

Ryu, Yungryel↗