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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

RADIOACTIVE POSITRON EMITTER PRODUCTION BY ENERGETIC ALPHA PARTICLES IN SOLAR FLARES

Measurements of the 0.511 MeV positron-annihilation line from solar flares are used to explore the flare process in general and ion acceleration in particular. In flares, positrons are produced primarily by the decay of radioactive positron-emitting isotopes resulting from nuclear interactions of flare-accelerated ions with ambient solar material. Kozlovsky et al. provided ion-energy-dependent production cross sections for 67 positron emitters evaluated from their threshold energies (some <1 MeV nucleon{sup –1}) to a GeV nucleon{sup –1}, incorporating them into a computer code for calculating positron-emitter production. Adequate cross-section measurements were available for proton reactions, but not for α-particle reactions where only crude estimates were possible. Here we re-evaluate the α-particle cross sections using new measurements and nuclear reaction codes. In typical large gamma-ray line flares, proton reactions dominate positron production, but α-particle reactions will dominate for steeper accelerated-ion spectra because of their relatively low threshold energies. With the accelerated-{sup 3}He reactions added previously, the code is now reliable for calculating positron production from any distribution of accelerated-ion energies, not just those of typical flares. We have made the code available in the online version of the Journal. We investigate which reactions, projectiles, and ion energies contribute to positron production. We calculate ratios of the annihilation-line fluence to fluences of other gamma-ray lines. Such ratios can be used in interpreting flare data and in determining which nuclear radiation is most sensitive for revealing acceleration of low-energy ions at the Sun.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Semi-empirical scaling for ion–atom single charge exchange cross sections in the intermediate velocity regime

We present a semi-empirical scaling law for non-resonant ion–atom single charge exchange cross sections for collisions with velocities from ${10}^{7}\,{\rm{t}}{\rm{o}}\,{10}^{9}\,\mathrm{cm}\,{{\rm{s}}}^{-1}$ and ions with positive charge $q\lt 8$. Non-resonant cross sections tend to have a velocity peak at collision velocities $v\lesssim 1\ {\rm{a}}{\rm{u}}$ with exponential decay around this peak. We construct a scaling formula for the location of this peak then choose a functional form for the cross section curve and scale it. The velocity at which the cross section peaks, v m , is proportional to the energy defect of the collision, ${\rm{\Delta }}E$, which we predict with the decay approximation. The value of the cross section maximum is proportional to the charge state q, inversely proportional to the target ionization energy I T , and inversely proportional to v m . For the shape of the cross section curve, we use a function that decays exponentially asymptotically at high and low velocities. We scale this function with parameters ${v}_{{\rm{m}}},{I}_{{\rm{T}}},{Z}_{{\rm{T}}},\mathrm{and}\ {Z}_{{\rm{P}}}$, where the ${Z}_{{\rm{T}},{\rm{P}}}$ are the target and projectile atomic numbers. In conclusion, for the more than 100 cross section curves that we use to find the scaling rules, the scaling law predicts cross sections within a little over a factor of 2 on average.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Electron– and positron–molecule scattering: development of the molecular convergent close-coupling method

Starting from first principles, this tutorial describes the development of the adiabatic-nuclei convergent close-coupling (CCC) method and its application to electron and (single-centre) positron scattering from diatomic molecules. In this paper, we give full details of the single-centre expansion CCC method, namely the formulation of the molecular target structure; solving the momentum-space coupled-channel Lippmann-Schwinger equation; deriving adiabatic-nuclei cross sections and calculating $V$-matrix elements. Selected results are presented for electron and positron scattering from molecular hydrogen H$_2$ and electron scattering from the vibrationally excited molecular hydrogen ion H$_2^+$ and its isotopologues (D$_2^+$, T$_2^+$, HD$^+$, HT$^+$ and TD$^+$). Finally, convergence in both the close-coupling (target state) and projectile partial-wave expansions of fixed-nuclei electron- and positron-molecule scattering calculations is demonstrated over a broad energy-range and discussed in detail. In general the CCC results are in good agreement with experiments.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

One-center close-coupling approach to two-center rearrangement collisions

Calculations of ionization and electron-capture cross sections in ion-atom collisions usually require solving the Schrödinger equation governing the collision system by expanding the total scattering wave function in a basis of target- and projectile-centered pseudostates. This approach leads to the two-center close-coupling equations which in some cases may become ill-conditioned due to the non-orthogonality of the underlying combined basis. Here we develop a technique which allows to accurately extract the necessary collision information, including that for the rearrangement channels, from the computationally more convenient one-center close-coupling equations which are built from only target-centered pseudostates. The robustness of the method is demonstrated by considering the proton-hydrogen scattering problem across a wide incident energy range. The developed method is then applied to study proton scattering on multielectron target of lithium. Here, the obtained results for the 2s → 2p excitation and the total electron-capture cross sections are in good agreement with corresponding experimental data. It is concluded that the presented technique could be a simpler alternative when integrated cross sections are required.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Mixed stochastic-deterministic time-dependent density functional theory: application to stopping power of warm dense carbon

Warm dense matter (WDM) describes an intermediate phase, between condensed matter and classical plasmas, found in natural and man-made systems. In a laboratory setting, WDM is often created dynamically. It is typically laser or pulse-power generated and can be difficult to characterize experimentally. Measuring the energy loss of high energy ions, caused by a WDM target, is both a promising diagnostic and of fundamental importance to inertial confinement fusion research. However, electron coupling, degeneracy, and quantum effects limit the accuracy of easily calculable kinetic models for stopping power, while high temperatures make the traditional tools of condensed matter, e.g. time-dependent density functional theory (TD-DFT), often intractable. In this study, we have developed a mixed stochastic-deterministic approach to TD-DFT which provides more efficient computation while maintaining the required precision for model discrimination. Recently, this approach showed significant improvement compared to models when compared to experimental energy loss measurements in WDM carbon. Here, we describe this approach and demonstrate its application to warm dense carbon stopping across a range of projectile velocities. We compare direct stopping-power calculation to approaches based on combining homogeneous electron gas response with bound electrons, with parameters extracted from our TD-DFT calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Omega Electroproduction

The differential cross section for p(e, e'ω)p has been studied at Q{sup 2} ~ 5.5 (GeV/c)2. Here Q{sup 2} represents the four momentum squared of the virtual photon in the excitation of baryonic resonances by an electron projectile. In order to extract the ω-meson differential cross section from the JLAB data, the data was compared to a full Monte Carlo simulation of the detector based on events generated for omega production in a way that the production cross section was varied to achieve a match to the data. The bin selected for this procedure takes into account the measure of robustness of the stripping of the ω peak from the multi-pion background as well as the statistics in the measured data and the Monte Carlo simulation of the signal and background physics. An error estimation technique for the cross section was based on determining the dependence of the extracted cross section parameters on the experimental set-up (including parameters for the spectrometer, target beam geometeries and performance). We compare our results with a Regge-based model for hadronic content in the t-channel exchange of a photon in Q{sup 2} region of overlap. There is an extension of this data into a completely new region, which is the highest yet measured.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Structure and decay correlations of two-neutron systems beyond the dripline

The two-neutron unbound systems of 16 Be, 13 Li, 10 He, and 26 O have been measured using the Modular Neutron Array (MoNA) and 4 Tm Sweeper magnet setup. The correlations of the 3-body decay for the 16 Be and 13 Li were extracted and demonstrated a strong correlated enhancement between the two neutrons. The measurement of the 10 He ground state resonance from a 14 Be(-2p2n) reaction provided insight into previous predictions that wavefunction of the entrance channel, projectile, can influence the observed decay energy spectrum for the unbound system. Lastly, the decay-in-target (DiT) technique was utilized to extract the lifetime of the 26 O ground state. The measured lifetime of 4.5$+1.1\atop{-1.5}$ (stat.)±3(sys.) ps provides the first indication of two-neutron radioactivity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Merging First Principle Structure Studies and Few-Body Reaction Formalism

Calculations for nucleon knockout from a Li-7 beam due to the collision with a proton target at 400 Mev/u are shown based on al) initio Quantum Monte Carlo (QMC) and conventional shell-model nuclear structure approaches to describe the relative motion between the knockout particle and the heavy fragment of the projectile. Structure effects on the total cross section are shown.

Crespo, R.↗
DOE OSTI2018

7 Li( d , p ) 8 Li transfer reaction in the NCSM/RGM approach

Recently, we applied an ab initio method, the no-core shell model combined with the resonating group method, to the transfer reactions with light p-shell nuclei as targets and deuteron as the projectile. In particular, we studied the elastic scattering of deuterium on 7Li and the 7Li(d,p)8Li transfer reaction starting from a realistic two-nucleon interaction. In this contribution, we review of our main results on the 7Li(d,p)8Li transfer reaction, and we extend the study of the relevant reaction channels, by showing the dominant resonant phase shifts of the scattering matrix. We assess also the impact of the polarization effects of the deuteron below the breakup on the positive-parity resonant states in the reaction. For this purpose, we perform an analysis of the convergence trend of the phase and eigenphase shifts, with respect to the number of deuteron pseudostates included in the model space

Physics↗
DOE OSTI2010

WATER/ICY SUPER-EARTHS: GIANT IMPACTS AND MAXIMUM WATER CONTENT

Water-rich super-Earth exoplanets are expected to be common. We explore the effect of late giant impacts on the final bulk abundance of water in such planets. We present the results from smoothed particle hydrodynamics simulations of impacts between differentiated water(ice)-rock planets with masses between 0.5 and 5 M{sub +} and projectile to target mass ratios from 1:1 to 1:4. We find that giant impacts between bodies of similar composition never decrease the bulk density of the target planet. If the commonly assumed maximum water fraction of 75 wt% for bodies forming beyond the snow line is correct, giant impacts between similar composition bodies cannot serve as a mechanism for increasing the water fraction. Target planets either accrete materials in the same proportion, leaving the water fraction unchanged, or lose material from the water mantle, decreasing the water fraction. The criteria for catastrophic disruption of water-rock planets are similar to those found in previous work on super-Earths of terrestrial composition. Changes in bulk composition for giant impacts onto differentiated bodies of any composition (water rock or rock iron) are described by the same equations. These general laws can be incorporated into future N-body calculations of planet formation to track changes in composition from giant impacts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MASS MEASUREMENT OF {sup 45}Cr AND ITS IMPACT ON THE Ca-Sc CYCLE IN X-RAY BURSTS

Masses of neutron-deficient {sup 58}Ni projectile fragments have been measured at the HIRFL-CSR facility in Lanzhou, China employing the isochronous mass spectrometry technique. Masses of a series of short-lived T{sub z} = -3/2 nuclides including the {sup 45}Cr nucleus have been measured with a relative uncertainty of about 10{sup -6}-10{sup -7}. The new {sup 45}Cr mass turned out to be essential for modeling the astrophysical rp-process. In particular, we find that the formation of the predicted Ca-Sc cycle in X-ray bursts can be excluded.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

3 He +𝛼 resonances in 7 Be

Resonances in 7 Be which decay into the 3 He+α exit channel have been measured with improved precision using preexisting data sets. The energy and width of the J π =7/2 - state have been extracted from an invariant-mass study of projectile-breakup products originating from interactions of an E/A=10.7-MeV 10 C beam on Be and C targets. The excitation energy of this state (from the pole of the S-matrix) is determined to be E*=4.545(6)~MeV, a factor of 8 improvement in precision as compared to the ENSDF value. This improvement is enabled by fine tuning the detector calibrations using calibration resonances in 6 Li, 7 Li, 6 Be, 9 B, and 12 C whose decay energies are known to high precision. The J π =7/2- resonance in 7 Be can now itself be used as a calibration resonance in invariant-mass experiments. This utility is particularly helpful for 3 He energy calibrations of CsI(Tl) detectors which are often used in detector arrays employed for measurements with fast beams. This utility is demonstrated with a data set associated with E/A=70 MeV 7 Be beams which are inelastically excited to the 7/2 - and 5/2 - 1 resonances. Again using the pole of the S-matrix as the definition of the resonance parameters, the fitted excitation energy of the 5/2 - 1 resonance is 6.376(17) MeV, approximately 300 keV lower than the ENSDF value. Finally, its width of 565(4) keV is roughly half of the ENSDF value.

energy levels↗
DOE OSTI2010

Absolute total and partial cross sections for ionization of nucleobases by proton impact in the Bragg peak velocity range

We present experimental results for proton ionization of nucleobases (adenine, cytosine, thymine, and uracil) based on an event-by-event analysis of the different ions produced combined with an absolute target density determination. We are able to disentangle in detail the various proton ionization channels from mass-analyzed product ion signals in coincidence with the charge-analyzed projectile. In addition we are able to determine a complete set of cross sections for the ionization of these molecular targets by 20-150 keV protons including the total and partial cross sections and the direct-ionization and electron-capture cross sections.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Single ionization of CH{sub 4} by bare ions: Fully differential cross sections

A theoretical study of fully differential cross sections for the single ionization of CH{sub 4} by collisions with H{sup +}, He{sup 2+}, and C{sup 6+} ions at energies in the order of MeV/amu is presented. We work in terms of the Born-3DW model, which considers a non-Coulomb central potential for the interaction of the active electron with the molecular core. Results obtained with the Born-3DW model are compared to those obtained with the Born-C3 model, which assumes this potential as purely Coulombic. The anisotropic potential of the CH{sub 4} molecule is smoothed through an angular integration, and results are averaged over all the possible orientations of the target molecule. Results for the lesser bound molecular orbitals (1T and 2A{sub 1}) are presented and discussed for different projectile momentum transfers for the coplanar geometry.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Theoretical fully differential cross sections for double-charge-transfer collisions

We present a four-body model for double charge transfer, called the four-body double-capture model. This model explicitly treats all four particles in the collision, and we apply it here to fully differential cross sections (FDCSs) for proton+helium collisions. The effects of initial- and final-state electron correlations are studied, as well as the role of the projectile-nucleus interaction. We also present results for proton+helium single capture, as well as single-capture:double-capture ratios of FDCSs.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Target effects on the linear polarization of photons emitted in radiative electron capture by heavy ions

Radiative electron capture by highly-charged, heavy ions is re-investigated within the framework of the first-order perturbation theory and impulse approximation. In this study, we focus especially on the polarization properties of the emitted radiation and on the effects which arise from the binding of the target electrons. In order to explore such effects, detailed calculations have been carried out for relativistic collisions of bare uranium U{sup 92+} projectiles with neutral low- and medium-Z atoms. From these calculations, it is shown that the linear polarization of K-REC (L-REC) photons [photons originating from radiative electron capture into the K shell (L shell)] can be very sensitive to the momentum distribution of bound electrons if the capture occurs selective for the different shells of the target. The most pronounced effect can be observed for collision studies as performed with well-defined impact parameters or upon suppression of the capture from the outer target shells.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Electron-capture cross sections for Li{sup +} colliding on H and H{sub 2} in the low-keV energy region

Absolute total and differential cross sections have been measured for neutral Li formation in single collisions of Li{sup +} on H{sub 2} from 1 to 5 keV. Total cross sections were obtained by direct integration of measured differential cross sections and are in good agreement with earlier cross-sectional measurements. A comparison of the differential cross sections plotted as a function of the reduced variables {theta}sin{theta}d{sigma}/d{Omega} vs E{theta} is also presented with a comparison of the maximum of the curves of the present data. Theoretically, we report state to state and summed charge transfer at collision energies ranging from 0.1 to 25 keV/amu. Cross sections were calculated using a nonadiabatic time-dependent direct approach for study of ion-atom and ion-molecule interaction processes. We verify that the main electron-capture channel is in the n=2 state of Li with a large probability for electron capture for impact parameters below 2 a.u. for collisions on both H and H{sub 2} in the so-called rotational region of the collision. In the case of atomic hydrogen, the electron-capture probability into the 2s and 2p states shows an oscillating behavior that smooths out when summed. For the molecular case, we find that for projectile energies larger than 1 keV, ionization effects become important. Our results show good agreement when compared to other available experimental and theoretical data.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

C{sup 6+}-impact ionization of helium in the perpendicular plane: Ionization to the ground state, excitation-ionization, and relativistic effects

In search of the discrepancy between theory and experiment in perpendicular plane geometry for C{sup 6+} ionization of He at 100 MeV/amu, we have used the first and second Born approximations to examine whether ionization to an excited He{sup +} state could be significant, whether relativistic effects could be important, and whether there is substantive sensitivity to the He wave functions used in the calculations. We fail to find any explanation of the discrepancy. Of the three possibilities, only relativistic effects turn out to be significant but then only in changing the overall normalization of the cross section, not in changing its shape, which is a prerequisite to getting agreement with experiment. The second Born calculations are in excellent accord with previous impact parameter coupled pseudostate results [Phys. Rev. A 81, 042704 (2010)] and confirm, yet again, that elastic scattering of the projectile by the target nucleus cannot explain the discrepancy. The calculations are extended to the lower impact energy of 2 MeV/amu. Here, in perpendicular plane geometry, ionization to excited He{sup +} states becomes significant and we find an interesting ''oscillatory'' structure in both the ground- and excited-state cross sections. Comparison is made with some relative experimental data and, although the agreement is poor, possibly because of the need to include experimental resolutions, there are nuances in the data that mirror the structures in the calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗