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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Dissociative recombination measurements of NH{sup +} using an ion storage ring

We have investigated dissociative recombination (DR) of NH{sup +} with electrons using a merged beams configuration at the TSR heavy-ion storage ring located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We present our measured absolute merged-beams recombination rate coefficient for collision energies from 0 to 12 eV. From these data, we have extracted a cross section, which we have transformed to a plasma rate coefficient for the collisional plasma temperature range from T {sub pl} = 10 to 18,000 K. We show that the NH{sup +} DR rate coefficient data in current astrochemical models are underestimated by up to a factor of approximately nine. Our new data will result in predicted NH{sup +} abundances lower than those calculated by present models. This is in agreement with the sensitivity limits of all observations attempting to detect NH{sup +} in interstellar clouds.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Bootstrapping dielectronic recombination from second-row elements and the orion nebula

Dielectronic recombination (DR) is the dominant recombination process for most heavy elements in photoionized clouds. Accurate DR rates for a species can be predicted when the positions of autoionizing states are known. Unfortunately such data are not available for most third- and higher-row elements. This introduces an uncertainty that is especially acute for photoionized clouds, where the low temperatures mean that DR occurs energetically through very low-lying autoionizing states. This paper discusses S{sup 2+} → S{sup +} DR, the process that is largely responsible for establishing the [S iii]/[S ii] ratio in nebulae. We derive an empirical rate coefficient using a novel method for second-row ions, which do have accurate data. Photoionization models are used to reproduce the [O iii]/[O ii]/[O i]/[Ne iii] intensity ratios in central regions of the Orion Nebula. O and Ne have accurate atomic data and can be used to derive an empirical S{sup 2+} → S{sup +} DR rate coefficient at ∼10{sup 4} K. We present new calculations of the DR rate coefficient for S{sup 2+} → S{sup +} and quantify how uncertainties in the autoionizing level positions affect it. The empirical and theoretical results are combined and we derive a simple fit to the resulting rate coefficient at all temperatures for incorporation into spectral synthesis codes. This method can be used to derive empirical DR rates for other ions, provided that good observations of several stages of ionization of O and Ne are available.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Untangling the recombination line emission from H ii regions with multiple velocity components

H ii regions are the ionized spheres surrounding high-mass stars. They are ideal targets for tracing Galactic structure because they are predominantly found in spiral arms and have high luminosities at infrared and radio wavelengths. In the Green Bank Telescope H ii Region Discovery Survey (GBT HRDS), we found that >30% of first Galactic quadrant H ii regions have multiple hydrogen radio recombination line (RRL) velocities, which makes determining their Galactic locations and physical properties impossible. Here we make additional GBT RRL observations to determine the discrete H ii region velocity for all 117 multiple-velocity sources within 18{sup ∘}<ℓ<65{sup ∘}. The multiple-velocity sources are concentrated in the zone 22{sup ∘}<ℓ<32{sup ∘}, coinciding with the largest regions of massive star formation, which implies that the diffuse emission is caused by leaked ionizing photons. We combine our observations with analyses of the electron temperature, molecular gas, and carbon recombination lines to determine the source velocities for 103 discrete H ii regions (88% of the sample). With the source velocities known, we resolve the kinematic distance ambiguity for 47 regions, and thus determine their heliocentric distances.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

The 21-cm Signal from the cosmological epoch of recombination

The redshifted 21-cm emission by neutral hydrogen offers a unique tool for mapping structure formation in the early universe in three dimensions. Here we provide the first detailed calculation of the 21-cm emission signal during and after the epoch of hydrogen recombination in the redshift range of z ∼ 500–1,100, corresponding to observed wavelengths of 100–230 meters. The 21-cm line deviates from thermal equilibrium with the cosmic microwave background (CMB) due to the excess Lyα radiation from hydrogen and helium recombinations. The resulting 21-cm signal reaches a brightness temperature of a milli-Kelvin, orders of magnitude larger than previously estimated. Its detection by a future lunar or space-based observatory could improve dramatically the statistical constraints on the cosmological initial conditions compared to existing two-dimensional maps of the CMB anisotropies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

RATE OF FORMATION OF HYDROGEN MOLECULES BY THREE-BODY RECOMBINATION DURING PRIMORDIAL STAR FORMATION

Astrophysical models of primordial star formation require rate constants for three-body recombination as input. The current status of these rates for H{sub 2} due to collisions with H is far from satisfactory, with published rate constants showing orders of magnitude disagreement at the temperatures relevant for H{sub 2} formation in primordial gas. This Letter presents an independent calculation of this recombination rate constant as a function of temperature. An analytic expression is provided for the rate constant which should be more reliable than ones currently being used in astrophysical models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Structural basis for human PRDM9 action at recombination hot spots

The multidomain zinc finger (ZnF) protein PRDM9 (PRD1–BF1–RIZ1 homologous domain-containing 9) is thought to influence the locations of recombination hot spots during meiosis by sequence-specific DNA binding and trimethylation of histone H3 Lys4. The most common variant of human PRDM9, allele A (hPRDM9A), recognizes the consensus sequence 5'-NCCNCCNTNNCCNCN-3'. We cocrystallized ZnF8–12 of hPRDM9A with an oligonucleotide representing a known hot spot sequence and report the structure here. ZnF12 was not visible, but ZnF8–11, like other ZnF arrays, follows the right-handed twist of the DNA, with the α helices occupying the major groove. Each α helix makes hydrogen-bond (H-bond) contacts with up to four adjacent bases, most of which are purines of the complementary DNA strand. The consensus C:G base pairs H-bond with conserved His or Arg residues in ZnF8, ZnF9, and ZnF11, and the consensus T:A base pair H-bonds with an Asn that replaces His in ZnF10. Most of the variable base pairs (N) also engage in H bonds with the protein. These interactions appear to compensate to some extent for changes from the consensus sequence, implying an adaptability of PRDM9 to sequence variations. We investigated the binding of various alleles of hPRDM9 to different hot spot sequences. Allele C was found to bind a C-specific hot spot with higher affinity than allele A bound A-specific hot spots, perhaps explaining why the former is dominant in A/C heterozygotes. Allele L13 displayed higher affinity for several A-specific sequences, allele L9/L24 displayed lower affinity, and allele L20 displayed an altered sequence preference. These differences can be rationalized structurally and might contribute to the variation observed in the locations and activities of meiotic recombination hot spots.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Improved VOC in RbF-Treated Cu(In,Ga)Se2 Solar Cells via Passivation of Recombination Centers

Cu(In,Ga)Se 2 (CIGS) solar cells have benefited in recent years from the addition of heavy alkali elements, such as Rb, which increase the solar cell open-circuit voltage ( V OC ). To investigate the source of this improvement, here, we compare samples with and without Rb to perform a quantitative comparison of electronic defects and minority carrier lifetime. Deep-level transient and optical spectroscopy measurements were performed on two sets of rubidium fluoride (RbF)-treated and untreated CIGS, and three distinct traps were identified regardless of RbF treatment. The RbF treatment was found to reduce the concentration of the H2 trap, which was previously found to act as a recombination center and is located preferentially at CIGS grain boundaries. Time-resolved photoluminescence measurements showed an increase in effective lifetime after RbF and nearly all lifetime improvement resulted from reductions in bulk recombination. The observed V OC improvement is well correlated with increased minority carrier lifetime and acceptor concentration, which led to increases and decreases in electron and hole quasi-Fermi levels, respectively.

Cu(In Ga)Se2 (CIGS)↗
DOE OSTI2016

Temperature- and Intensity-Dependent Photovoltaic Measurements to Identify Dominant Recombination Pathways

In novel photovoltaic absorbers, it is often difficult to assess the root causes of low open-circuit voltages, which may be due to bulk recombination or sub-optimal contacts. In the present work, we discuss the role of temperature- and illumination-dependent device electrical measurements in quantifying and distinguishing these performance losses - in particular, for determining bounds on interface recombination velocities, band alignment, and minority carrier lifetime. We assess the accuracy of this approach by direct comparison to photoelectron spectroscopy. Then, we demonstrate how more computationally intensive model parameter fitting approaches can draw more insights from this broad measurement space. We apply this measurement and modeling approach to high-performance III-V and thin-film chalcogenide devices.

14 SOLAR ENERGY↗
DOE OSTI2019

Temperature and Excitation Dependence of Recombination in CIGS Thin Films with High Spatial Resolution

Alkali post-deposition treatments significantly improve the performance of CuInGaSe2 (CIGS) devices, but there is still room for improvement. Here, we investigate the effects of potassium fluoride alkali post-deposition treatment on the defect chemistry and recombination at grain boundaries and grain interiors using temperature- and injection-dependent cathodoluminescence (CL) spectrum imaging. We study CIGS thin films grown on alkali-free sapphire substrates to isolate the effects of alkali treatment from alkali metals that can diffuse from standard soda-lime glass substrates. We find that alkali treatment affects the energy and temperature dependence of the luminescence peaks, as well as the defect activation energies. CL spectrum images reveal that the luminescence transitions at grain boundaries have a distinct power dependence after alkali treatment and substantially different defect chemistry. This work shows that temperatureand injection-dependence CL spectrum images can provide unique insight into the defect chemistry and recombination behavior of CIGS thin films.

14 SOLAR ENERGY↗
DOE OSTI2016

Experimental verification of gain drop due to general ion recombination for a carbon-ion pencil beam

Purpose: Accurate dose measurement in radiotherapy is critically dependent on correction for gain drop, which is the difference of the measured current from the ideal saturation current due to general ion recombination. Although a correction method based on the Boag theory has been employed, the theory assumes that ionized charge density in an ionization chamber (IC) is spatially uniform throughout the irradiation volume. For particle pencil beam scanning, however, the charge density is not uniform, because the fluence distribution of a pencil beam is not uniform. The aim of this study was to verify the effect of the nonuniformity of ionized charge density on the gain drop due to general ion recombination. Methods: The authors measured the saturation curve, namely, the applied voltage versus measured current, using a large plane-parallel IC and 24-channel parallel-plate IC with concentric electrodes. To verify the effect of the nonuniform ionized charge density on the measured saturation curve, the authors calculated the saturation curve using a method which takes into account the nonuniform ionized charge density and compared it with the measured saturation curves. Results: Measurement values of the different saturation curves in the different channels of the concentric electrodes differed and were consistent with the calculated values. The saturation curves measured by the large plane-parallel IC were also consistent with the calculation results, including the estimation error of beam size and of setup misalignment. Although the impact of the nonuniform ionized charge density on the gain drop was clinically negligible with the conventional beam intensity, it was expected that the impact would increase with higher ionized charge density. Conclusions: For pencil beam scanning, the assumption of the conventional Boag theory is not valid. Furthermore, the nonuniform ionized charge density affects the prediction accuracy of gain drop when the ionized charge density is increased by a higher dose rate and/or lower beam size.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

SU-F-T-584: Investigating Correction Methods for Ion Recombination Effects in OCTAVIUS 1000 SRS Measurements

Purpose: PTW’s Octavius 1000 SRS array performs IMRT QA measurements with liquid filled ionization chambers (LICs). Collection efficiencies of LICs have been shown to change during IMRT delivery as a function of LINAC pulse frequency and pulse dose, which affects QA results. In this study, two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. Methods: For the first correction, Matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction, MU/min in daily 1000 SRS calibration was chosen to match average MU/min of the VMAT plan. Usefulness of derived corrections were evaluated using 6MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam equipped with an HD- MLC. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. Results: After complex Matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex Matlab corrections. Conclusion: Ion recombination effects can potentially be clinically significant for OCTAVIUS 1000 SRS measurements, especially for higher pulse dose unflattened beams when using tighter gamma tolerances. Matching daily 1000 SRS calibration MU/min to average planned MU/min is a simple correction that greatly reduces ion recombination effects, improving measurements accuracy and gamma pass rates. This work was supported by PTW.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Kinetics of Atomic Recombination on Silicon Samples in Chlorine Plasma

The recombination kinetics of chlorine atoms on the wall of a plasmachemical reactor and on silicon samples in the positive column of a glow discharge in Cl{sub 2} has been studied experimentally. The rate constants and probabilities of the heterogeneous recombination of chlorine atoms on the plasma limiting surfaces, as well as of the chemical interaction of chlorine atoms with silicon, are calculated. The temperature and time dependences of the probabilities of the chemical interaction of chlorine atoms with silicon are analyzed, and optimal conditions for conducting pulse relaxation experiments are determined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of copper on the recombination activity of extended defects in silicon

The effect of copper atoms introduced by high-temperature diffusion on the recombination properties of dislocations and dislocation trails in p-type single-crystal silicon is studied by the electron-beam-induced current technique. It is shown that, in contrast to dislocations, dislocation trails exhibit an increase in recombination activity after the introduction of copper. Bright contrast appearance in the vicinity of dislocation trails is detected after the diffusion of copper and quenching of the samples. The contrast depends on the defect density in these trails.

36 MATERIALS SCIENCE↗
DOE OSTI2016

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si{sub 0.8}5Ge{sub 0.15} layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Spin-dependent tunneling recombination in heterostructures with a magnetic layer

We propose a mechanism for the generation of spin polarization in semiconductor heterostructures with a quantum well and a magnetic impurity layer spatially separated from it. The spin polarization of carriers in a quantum well originates from spin-dependent tunneling recombination at impurity states in the magnetic layer, which is accompanied by a fast linear increase in the degree of circular polarization of photoluminescence from the quantum well. Two situations are theoretically considered. In the first case, resonant tunneling to the spin-split sublevels of the impurity center occurs and spin polarization is caused by different populations of resonance levels in the quantum well for opposite spin projections. In the second, nonresonant case, the spin-split impurity level lies above the occupied states of electrons in the quantum well and plays the role of an intermediate state in the two-stage coherent spin-dependent recombination of an electron from the quantum well and a hole in the impurity layer. The developed theory allows us to explain both qualitatively and quantitatively the kinetics of photoexcited electrons in experiments with photoluminescence with time resolution in Mn-doped InGaAs heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On Recombination Processes in CdS–PbS Films

The transverse and longitudinal photoconductivity, photoluminescence, and cathodoluminescence of sublimated (CdS){sub 0.9}–(PbS){sub 0.1} films at room temperature and upon cooling are studied. The role of inclusions of the narrow-gap phase in the processes is shown. The films are excited over the entire active surface and pointwise (within one crystallite). The surface recombination rate and the lifetime of majority charge carriers at different generation rates and characters of excitation are estimated. A comparative table of recombination parameters of CdS and CdS–PbS films is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Nickel and Copper Introduced at Room Temperature on the Recombination Properties of Extended Defects in Silicon

The change in the recombination properties of individual dislocations and dislocation trails in silicon due to the diffusion of nickel and copper during chemical-mechanical polishing at room temperature is studied by the electron-beam- and light-beam-induced current techniques. It is found that the introduction of nickel results in an increase in the recombination activity of both dislocations and dislocation trails. The introduction of copper does not induce any substantial change in the contrast of extended defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Hyperfine Interaction and Shockley–Read–Hall Recombination in Semiconductors

Experimental and theoretical studies on optical orientation and spin-dependent recombination in a semiconductor in a magnetic field under the normal incidence of circularly polarized radiation onto the sample surface are reviewed. The experiments were carried out on GaAs{sub 1 –x}N{sub x} solid solutions, in which Ga{sup 2+} interstitial displacement defects play the role of deep paramagnetic centers responsible for spin-dependent recombination. It is established that, in the investigated materials, the hyperfine interaction of a localized electron with one nucleus of the paramagnetic center remains strong even at room temperature. The theory is compared with an experiment conducted in the steady-state excitation mode and under two-pulse pump-probe conditions. An analytical formula for spin beats in a magnetic field is derived.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗