Photon Emission Cross Sections Research Articles

Emission of High-Energy Protons and Photons in Heavy-Ion Collisions Treated on the Basis of a Hydrodynamic Approach with a Nonequilibrium Equation of State

A hydrodynamic approach involving a nonequilibrium equation of state is used in describing heavy-ion collisions at medium and intermediate energies. The double differential cross sections for proton and photon emission in collisions of various nuclei are calculated in developing this approach with allowance for nuclear-viscosity effects and amendments introduced by the microcanonical distribution. The results agree everywhere, including the cumulative region of the spectrum, with available experimental data on the emission of high-energy particles. The approach in question permits reproducing high-momentum distributions of protons from the reaction $${}^{12}\textrm{C}+\,^{9}\textrm{Be}\to p+X$$ at the $${}^{12}$$ C ion energies of 300, 600, 950, and 2000 MeV per nucleon and the energy spectra of hard photons from the reaction $${}^{12}\textrm{C}+\,^{9}\textrm{Be}\to\gamma+X$$ at the $${}^{12}$$ C ion energies of 2.0 and 3.2 GeV per nucleon.

Revisiting the Fe xvii Line Emission Problem: Laboratory Measurements of the 3s–2p and 3d–2p Line-formation Channels

Abstract We determined relative X-ray photon emission cross sections in Fe xvii ions that were mono-energetically excited in an electron beam ion trap. Line formation for the 3s (3s−2p) and 3d (3d−2p) transitions of interest proceeds through dielectronic recombination (DR), direct electron-impact excitation (DE), resonant excitation (RE), and radiative cascades. By reducing the electron-energy spread to a sixth of that of previous works and increasing counting statistics by three orders of magnitude, we account for hitherto unresolved contributions from DR and the little-studied RE process to the 3d transitions, and also for cascade population of the 3s line manifold through forbidden states. We found good agreement with state-of-the-art many-body perturbation theory (MBPT) and the distorted-wave (DW) method for the 3s transition, while in the 3d transitions known discrepancies were confirmed. Our results show that DW calculations overestimate the 3d line emission due to DE by ∼20%. Inclusion of electron-electron correlation effects through the MBPT method in the DE cross-section calculations reduces this disagreement by ∼11%. The remaining ∼9% in 3d and ∼11% in 3s/3d discrepancies are consistent with those found in previous laboratory measurements, solar, and astrophysical observations. Meanwhile, spectral models of opacity, temperature, and turbulence velocity should be adjusted to these experimental cross sections to optimize the accuracy of plasma diagnostics based on these bright soft X-ray lines of Fe xvii.

On higher–order corrections to nuclear bremsstrahlung cross sections

A quantitative understanding of bremsstrahlung by electrons in the screened field of atomic nuclei has been developed by several researchers over the last 80 years. However, some ranges of the possible parameters can still not be covered with good accuracy. Dirac partial-wave calculations are the best method developed to date, while analytic calculations are the only viable solution at high impinging electron energies. Recent progresses on both fronts will be discussed. In particular, state of the art partial wave results, including screening, allowed to test the Olsen–Maximon–Wergeland additivity rule discovered by analytic means. Closed expressions for the next-to-leading order corrections to the Furry–Sommerfeld–Maue wave functions are known. One next-to-next-to-leading order term is typically included in this type of approach. The effect of this extra term on the angular dependence of the double differential cross sections for photon emission by 2.5 MeV electrons is compared in detail with partial wave calculations which are exact at all orders.

Radiative association of C(3P) and H+: triplet states

The radiative association of C(${}^3P$) and H${}^+$ is investigated by calculating cross sections for photon emission into bound ro-vibrational states of CH${}^+$ from the vibrational continua of initial triplet d$\,{}^3\Pi$ or b$\,{}^3\Sigma^-$ states. Potential energy curves and transition dipole moments are calculated using multi-reference configuration interaction (MRCI) methods with AV6Z basis sets. The cross sections are evaluated using quantum-mechanical methods and rate coefficients are calculated. The rate coefficients are about 100 times larger than those for radiative association of C${}^+({}^2{P^o})$ and H from the A$\,{}^1\Pi$ state. We also confirm that the formation of CH${}^+$ by radiative association of C${}^+({}^2{P^o})$ and H via the triplet c$\,{}^3\Sigma^+$ state is a minor process.

Solar axion flux from the axion-electron coupling

In non-hadronic axion models, where axions couple to electrons at tree level, the solar axion flux is completely dominated by the ABC reactions (Atomic recombination and deexcitation, Bremsstrahlung and Compton). In this paper the ABC flux is computed from available libraries of monochromatic photon radiative opacities (OP, LEDCOP and OPAS) by exploiting the relations between axion and photon emission cross sections. These results turn to be ∼ 30% larger than previous estimates due to atomic recombination (free-bound electron transitions) and deexcitation (bound-bound), which where not previously taken into account.

Exact Analytical Formulism for Erbium Doped Fiber Lasers

An exact analytical formulism obtained directly by solving a system of rate and propagation equations in the presence of pump excited-state absorption is presented on the basis of four-band transition scheme in radially symmetric and longitudinally uniform monomode erbium doped silica fiber lasers. The exact analytical expressions are determined for overall gain, small-signal gain, pump intensity threshold, lasing threshold condition, and optimal fiber length in terms of alleviative measurable quantities of photon intensities, transition rates, absorption and emission cross sections, and reflectivity of mirrors. Mutatis mutandis, the formulas bear a verisimilitude to analytical expressions appearing in the literature for these parameters.

Analysis of photon emission from 50--350-keV proton impact onH2O

We have measured photon emission cross sections from neutral fragments produced by collisions of 50--350 keV protons with ${\mathrm{H}}_{2}$O molecules. Balmer $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\delta}$ emissions from both the target and projectile were recorded. We also analyzed $A {}^{2}{\ensuremath{\Sigma}}^{+}\ensuremath{-}X {}^{2}\ensuremath{\Pi}$ (0,0) and (1,0) emission from the excited OH fragment produced during target dissociation. Trends in the cross sections revealed two key properties of the collision process: (1) The Bethe theory accurately describes target emission from both H and OH fragments and (2) the ratio of any two Balmer emission cross sections for both the target and projectile can be approximated by simple functions of the respective optical oscillator strengths. Finally, we provide the Bethe fit parameters necessary to calculate the target emission cross sections at all nonrelativistic impact energies.

Excitations from dissociative fragments produced inH++H2Ocollisions

We report on photon emissions in the $200--800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ region resulting from collisions of $200\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ protons with ${\mathrm{H}}_{2}\mathrm{O}$. The most prominent features observed in the spectrum are the Balmer series of hydrogen and two OH molecular bands. Several less intense ${\mathrm{O}}^{+}$ as well as neutral O lines are also observed. The absolute photon emission cross sections of the major lines and bands were measured. The results indicate that a primary dissociation pathway involves the formation of ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ by removing a $1{b}_{2}$ electron. The unstable ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ ion further dissociates into ${\mathrm{H}}^{+}+\mathrm{O}\mathrm{H}$ or $\mathrm{O}{\mathrm{H}}^{+}+\mathrm{H}$. The dominant presence of neutral hydrogen lines and ${\mathrm{O}}^{+}$ lines leads to the conclusion that the subsequent dissociation of $\mathrm{O}{\mathrm{H}}^{+}$ into $\mathrm{H}+{\mathrm{O}}^{+}$ prevails over the other possible dissociation pathway leading to ${\mathrm{H}}^{+}$ and neutral oxygen fragments.

Bremsstrahlung during electron capture to continuum

Theoretical approaches for radiative ionization (RI) in energetic collisions of highly stripped projectiles with target atoms are reviewed and set into context with related processes. The interrelation between RI and electron-nucleus bremsstrahlung is displayed with the help of inverse kinematics. Particular emphasis is laid on the forward-peak region of the electron spectrum resulting from target electrons which are slowed down to approximately zero velocity in the projectile frame of reference. The forward-peak intensity and shape for RI is contrasted to the one obtained from nonradiative capture to continuum. The ridge in the photon spectrum related to forward-peak electrons can unambiguously be identified in measured doubly differential photon emission cross sections resulting from ion–atom collisions at relativistic impact energies.

Cross Sections for Photon Emission Following Electron Capture by C2+, N3+, N4+ and O3+ Ions in Collision with Li(2s) Atoms

We report measurements of photon emission cross sections following electron capture of C2+, N3+, N4+ and O3+ ions (obtained from an ECR ion source) in collision with ground state lithium atoms at keV energies. We examine the sources of error and obtain best estimates of the relative values of the cross sections having accuracies in the range from 15% to 50%. The accuracy of the absolute cross sections may be lower for capture by N3+ and N4+ ions because of the uncertainty in the fraction of metastable ions. We discuss this problem.

Absolute emission cross sections for electron capturereactions of C2+, N3+, N4+ and O3+ ions in collisionswith Li(2s) atoms

Absolute photon emission cross sections following electron capturereactions have been measured for C2+, N3+, N4+ and O3+ions colliding with Li(2s) atoms at keV energies. The results are comparedwith calculations using the extended classical over-the-barrier model byNiehaus. We explore the limits of our experimental method and present adetailed discussion of experimental errors.

Angular distribution of radiation emitted after resonant transfer and excitation

We have calculated the angular distribution of photons emitted after dielectronic recombination of highly charged uranium ions and obtained differential cross sections for photon emission in resonant transfer and excitation (RTE) in collisions of with a graphite target using the impulse approximation. The results are compared with experimental data.

Photon-emission studies of slowC4+-Ne collisions

We present absolute term- and level-selective cross sections for specific single- and double-electron capture processes in slow (16 keV) ${\mathrm{C}}^{4+}$-Ne collisions. The results are deduced through a combined analysis of earlier translational energy-gain data and the present photon-emission cross sections in the region 40--240 nm. We find that double-electron capture is dominated by processes in which two electrons appear to change orbitals in a single interaction of two diabatic quasimolecular potential curves---a type of process which rarely has been isolated before. Furthermore, we observe a strong preference for population of 2s2p over ${2\mathrm{p}}^{2}$ configurations in these one-step two-electron capture reactions. This is a remarkable configuration selectivity, since reaction channels leading to 2s2p and ${2\mathrm{p}}^{2}$ configurations cross with the incident channel at almost the same internuclear distances.

Continuous emission due to radiative ion-atom association and charge exchange in weakly ionized plasmas of H, He, Li and Na

The contribution of radiative charge exchange and radiative association in symmetrical ion-atom collisions to continuous EM radiation from weakly ionized gaseous plasmas has been considered within the semiclassical adiabatic theory. The differential (in lambda ) cross sections for spontaneous photon emission and the general expressions for the spectral coefficients of emission and absorption are given. The hydrogen and helium plasmas that are representative of two different optical types of gaseous medium have been studied in a broad range of T and lambda , 4000<or=T (K)<or=20000 and 200<or= lambda (nm) <or=1000. The domain of T and lambda , where ion-atom collisions contribute significantly to continuous plasma spectra, has been established. The case of weakly ionized alkali metal plasmas of Li and Na has been studied in the same interval of lambda but at lower temperatures, 1500<or=T (K)<or=3500. The relevance of the results to studies of laboratory plasmas is discussed.

Photon spectroscopy of slow He q+ + He ( q = 1,2) collisions

Photon emission following slow ion-atom collisions has been observed for the collision systems He q+ + He( q = 1,2) at the energy 10 keV q −1. In particular relative photon emission cross sections for the 1 s 2 1 S 0−1s n p 1 P 1 series in neutral helium are reported for both charges of the incoming helium ion. The work has been performed at the Uppsala University ECR ion source facility.

Bremsstrahlung produced by colliding particles in an external electric field

We have used the nonrelativistic dipole Born approximation to study bremsstrahlung from two colliding charged particles in a uniform electric field. We have calculated the differential cross sections for photon emission as the system transitions into various states of internal motion. We find the cross sections exhibit spatial anisotropy due to the uniform electric field, and that oscillations can arise that are probably due to interference effects between the colliding particles.

Investigation of single and double electron capture in Ar7+ – He, H2 collisions at 105 keV by photon spectroscopy

A thorough analysis of photon spectra in the 2000-6000 Å wavelength range for the 105 keV Ar7+ - He, H2 collision systems is presented. Numerous lines due to single and double electron capture processes were observed. They have been interpreted with the help of experimental data and ab initio calculations. For the single electron capture process, transitions with Δn = 0 for n = 4 and n = 5 states of Ar VII have been identified. Fine structure splittings of the 3s5p 3P and 3s5d 3D terms have been determined. Strong lines due to the double electron capture process were observed and identified as Rydberg transitions 3s2nl-3s2n'l' or 3s3p(3P, 1P)nl-3s3p(3P, 1P)n'l' in Ar VI. Photon emission cross sections for individual lines are given.

Photon production from collisions of 100-350-keV positive ions with CO, CF4, and CH4.

Various photon emissions from collisions of H{sup +}, H{sub 2}{sup +}, and He{sup +} in the 100--350-keV energy range with targets of CO, CF{sub 4}, and CH{sub 4} were studied. The wavelength range of the investigation ran from 2000 to 7000 A. Photon-emission cross sections were measured for all significant features. The cross-section data were compared to the Bethe-Born theory through the use of Fano plots. The results show that the theory could be applicable in the case of the CO target, but is probably not applicable to the tetrahedral molecular targets as the emissions are from dissociated atoms.

Spectroscopic analysis of visible and near UV light emitted by Ar7+ and Ar6+ ions produced in Ar8+ -He and Ar8+ -H2 collisions at 120 keV

A spectroscopic analysis of light emitted in the 2000-6000 Å wavelength range by Ar7+ and Ar6+ ions produced in Ar8+ -He and Ar8+ -H2 collisions at 120 keV is performed. Well resolved fine structure components of 5s-5p and 5p-5d transitions in Ar VIII following single electron capture are precisely measured. Predominant lines due to double electron capture are observed. In particular, strong lines observed both in Ar8+ -He and Ar8+ -H2 collisions are attributed to Rydberg transitions 3snl-3sn'l' in Ar VII. Attempts at identifications are made for the transitions 3dnl-3dn'l' (n=4,5) with the help of ab initio calculations. Photon emission cross sections for individual lines are determined from the measured data.

Subshell selective electron capture (2-105 keV amu-1) studied by VUV spectroscopy in O6++He collisions

Photon emission cross sections have been measured between 100 and 600 AA in collisions of 2-105 keV amu-1 O6+ ions on helium. The results extend existing data at low energy (0.56-7.13 keV amu-1) to higher energy. The data are compared with the existing atomic orbital calculations for one-electron capture in (quasi-) two-electron systems.