Nonresonant Cross Section Research Articles

Nonresonant Compton scattering in an intense pulsed laser field

The influence of an intense pulsed light wave on nonresonant scattering of a photon by an electron is investigated in the approximation when the pulse width is considerably greater than the characteristic time of wave oscillation. The partial differential cross-sections for different numbers of absorption (emission) wave photons are analyzed for a range of external field intensities I ∼ 1017 W cm−2. The general relativistic expression for the nonresonant cross-section of laser-modified Compton scattering summed over all possible partial processes is derived. It is shown that for nonrelativistic energies the pulse character of the external field significantly affects the process.

Influence of an intense pulsed electromagnetic field on nonresonant scattering of a photon by an electron for the nonrelativistic energy

The theory of nonresonant scattering of a photon by an electron in the field of an intense pulsed light wave is developed. The approximation when a pulse width is considerably greater than the characteristic time of wave oscillation is considered. The nonresonant kinematic region is determined. The general relativistic expression for the nonresonant cross-section is derived for the range of the external field intensities if η 0 ≲ 1, η0 2 ≪ 1 (see Eq. (12)) is valid. The obtained differential cross-section of the process has form of a sum over partial differential cross-sections. Each of them corresponds to the process with emission (absorption) of a certain number of wave photons. It is shown, that for nonrelativistic energy the cross-section summed over all possible partial processes differs considerably from the cross-section of Compton scattering if the external field is absent and may exceed the latter over than 200%.

Enhancement of fusion rates due to quantum effects in the particles momentum distribution in nonideal plasma media

This study concerns a situation when measurements of the nonresonant cross-section of nuclear reactions appear highly dependent on the environment in which the particles interact. An appealing example discussed in the paper is the interaction of a deuteron beam with a target of deuterated metal Ta. In these experiments, the reaction cross section for d(d,p)t was shown to be orders of magnitude greater than what the conventional model predicts for the low-energy particles. In this paper we take into account the influence of quantum effects due to the Heisenberg uncertainty principle for particles in a non-ideal medium elastically interacting with the medium particles. In order to calculate the nuclear reaction rate in the non-ideal environment we apply both the Monte Carlo technique and approximate analytical calculation of the Feynman diagram using nonrelativistic kinetic Green's functions in the medium which correspond to the generalized energy and momentum distribution functions of interacting particles. We show a possibility to reduce the 12-fold integral corresponding to this diagram to a fivefold integral. This can significantly speed up the computation and control accuracy. Our calculations show that quantum effects significantly influence reaction rates such as p +7Be, 3He +4He, p +7Li, and 12C +12C. The new reaction rates may be much higher than the classical ones for the interior of the Sun and supernova stars. The possibility to observe the theoretical predictions under laboratory conditions is discussed.

Nonresonant photocreation of electron-positron pair on a nucleus in the field of a pulsed light wave

The nonresonant Coulomb photocreation of electron-positron pair on a nucleus in the field of a pulsed light wave is theoretically investigated. The approximation is examined when pulsewidth is considerably greater than the characteristic time of wave oscillations. The interaction of an electron and a positron with a Coulomb potential of a nucleus is considered in the first order of perturbation theory (the Born approximation). An analytic expression for the nonresonant differential cross section was obtained for the range of moderately strong fields in the case relativistic and nonrelativistic energies. It is shown, that the nonresonant cross section of Coulomb photocreation of nonrelativistic pair in the field of a pulsed light wave is two times larger than the cross section of Coulomb photocreation in the absence of an external field.

Nonresonant Cross-Section, Fano-Shape Parameter and Correlation Coefficient for the Photoionization of 2s2p (1P∘) in Heliumlike

The nonresonant cross-section, Fano-shape parameter q and coefficient parameter ρ2 have been calculated for the photoionization of helium and heliumlike under the n=2 threshold of the residual ions. Dipole matrix elements which correspond to the transitions from 1s2(S1e) to (0,1)22+ P1∘ state and from (0,1)22+ state to continuum state 1sεp (P1∘) were determined by choosing, for the ground state, the six-parameter Hylleraas wave function including electron correlation effects and a completely screened Coulomb wave function for the continuum. Energies and widths have been determined in the diagonalization method. The results are compared with available results obtained by other theoretical approaches.

Photon avalanche upconversion in various Tm3+-doped materials

We demonstrate the good agreement between theory and experimental results concerning the effect of the rare-earth ion concentration, the metastable state lifetime and the nonresonant ground state absorption cross section on the photon avalanche efficiency.

Measurements of theΔ(1232)transition form factor and the ratioσn/σpfrom inelastic electron-proton and electron-deuteron scattering

Measurements of inclusive electron-scattering cross sections using hydrogen and deuterium targets in the region of the $\ensuremath{\Delta}(1232)$ resonance are reported. A global fit to these new data and previous data in the resonance region is also reported for the proton. Transition form factors have been extracted from the proton cross sections for this experiment over the four-momentum transfer squared range $1.64l{Q}^{2}l6.75 (\mathrm{GeV}{/c)}^{2}$ and from previous data over the range $2.41l{Q}^{2}l9.82 (\mathrm{GeV}{/c)}^{2}.$ The results confirm previous reports that the $\ensuremath{\Delta}(1232)$ transition form factor decreases more rapidly with ${Q}^{2}$ than expected from perturbative QCD. The ratio of ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}$ in the $\ensuremath{\Delta}(1232)$ resonance region has been extracted from the deuteron data for this experiment in the range $1.64l{Q}^{2}l3.75 (\mathrm{G}\mathrm{e}\mathrm{V}/{c)}^{2}$ and for a previous experiment in the range $2.4l{Q}^{2}l7.9 (\mathrm{GeV}{/c)}^{2}.$ A study has been made of the model dependence of these results. This ratio ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}$ for $\ensuremath{\Delta}(1232)$ production is slightly less than unity, while ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}$ for the nonresonant cross sections is approximately 0.5, which is consistent with deep inelastic scattering results.

Convergent multichannel continuum states by a general configuration interaction expansion in a B-spline basis: application to photodetachment

We report the theoretical calculation of the total and partial cross section and asymmetry parameter for photoionization up to the N = 3 threshold. Multichannel continuum wavefunctions are obtained via a least squares solution of the Schrödinger equation employing a basis set expansion based on a primitive B-spline radial basis and a general configuration interaction (CI) form. This allows the use of a mixed full-CI and close coupling expansion in a completely general approach to improve correlation description in the continuum channels. The present algorithm correctly describes the full range of phenomena encountered with remarkable accuracy both on nonresonant cross sections and resonance structures. An accuracy of better than 0.01 Mb is estimated on both the total and partial cross sections, except for the shape resonance. In addition, Fano resonance parameters compare very well with the best methods available based on Hylleraas-type wavefunctions and complex coordinate rotation.

Photoactivation of isomeric level in 115In

In this paper we have studied the photoactivation of an isomeric level of 115 In(336 keV, T 1 2 ∼ 4.5 h) using a strong 60 Co source and a set of intervening iron absorbers. We have considered the possiblity of the isomeric level activation by nonresonants processes involving the 1173 and 1332 keV 60Co photons. The existence of the nonresonant contribution to the activation of 115In has been confirmed. The nonresonant cross section σ NR T = (5.5 ± 0.8)×10 −32cm −2 is the same order of magnitude as those that have been found in our previous investigations.

Photodetachment of the1s2s2pt4P state ofHe−sfrom threshold to 100 eV

Calculations of cross sections and photoelectron angular distributions from the photodetachment of the 1s2s2p${\mathrm{}}^{4}$ P metastable state of ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ have been performed from threshold to 100 eV using multiconfiguration Hartree-Fock wave functions for discrete states and coupling continuum channels with closed channels to include resonances. Ejection of each electron was considered and ejection plus excitation was included in connection with 1s photoabsorption. The ${1\mathrm{s}2\mathrm{p}}^{2}$ and ${2\mathrm{s}2\mathrm{p}}^{2}$ ${\mathrm{}}^{4}$ P resonances were found in good agreement with recent experiments and calculations; the reason that the former lies above the 2s threshold while the latter lies below the 1s threshold is discussed. The nonresonant 2p cross section is in excellent agreement with experiments and other calculations. The 2s results, in which Cooper minima are predicted, are not in agreement with a similar calculation, particularly for the \ensuremath{\mathrm{B}} parameter. The 1s cross section shows a large maximum just above threshold, much larger than the 1s cross section in neutral He, and at somewhat higher energy the cross section is dominated by 1s detachment plus excitation to the (2s3p\ifmmode\pm\else\textpm\fi{}3s2p)${\mathrm{}}^{3}$ P and 3s3p${\mathrm{}}^{3}$ P autoionizing states of He.

Accurate multichannel continuum states by a general configuration interaction expansion in a B-spline basis: application to He photoionization

We report theoretical calculations of the total and partial cross section and asymmetry parameter for He photoionization up to the N = 3 threshold. Multichannel continuum wavefunctions are obtained via a least squares solution of the Schrödinger equation employing a basis set expansion based on a primitive B-spline radial basis and a general configuration interaction form. This allows use of a mixed full CI and close-coupling expansion in a completely general approach to improve correlation description in the continuum channels. The present algorithm correctly describes the full range of phenomena encountered with full details and a remarkable accuracy both on non-resonant cross sections and resonance structures. An accuracy of better than 0.001 Mb is estimated both on total and partial cross sections, also Fano resonance parameters compare very well with the best methods available based on Hylleraas type wavefunctions and complex coordinate rotation.

The Non-Resonant Magnetic X-ray Scattering Cross Section of MnF2. 1. Medium X-ray Energies from 5 to 12keV

To demonstrate the possibilities of non-resonant magnetic X-ray diffraction, data taken on the antiferromagnetic model system MnF2 in a medium X-ray energy range from 5 to 12 keV are presented. An experimental set-up for the measurement of magnetic X-ray diffraction is introduced that employs a new design for a highly flexible polarization analyzer for synchrotron X-ray diffraction based on 90° Bragg reflection from a single-crystal. The analyzer has been successfully tested on beamline W1 at HASYLAB by measuring the polarization of the primary photon beam and of the 400 charge peak of an MnF2 single-crystal. Subsequently, the analyzer was used to explore the polarization dependence of the non-resonant magnetic X-ray scattering cross section of this sample, complemented by measurements of the directional dependencies without polarization analysis. In the latter configuration, count rates as high as 3800 photons s−1 and a peak-to-background ratio of 160:1 were obtained. A method to determine the direction of the magnetic moments with respect to the crystalline axes is presented. The results give experimental support for the theoretical form of the non-resonant magnetic X-ray scattering cross section and at the same time provide a feasibility test for a general magnetic structure determination.

The Non-Resonant Magnetic X-ray Scattering Cross Section of MnF2. 2. High-Energy X-ray Diffraction at 80keV

Results of high-energy non-resonant magnetic X-ray diffraction experiments performed on the model system MnF2 at a photon energy of 80 keV are presented. A surprisingly high peak intensity of the magnetic 300 reflection of 13000 photons s−1 in the three-crystal mode and 19000 photons s−1 in the two-crystal mode, with a peak-to-background ratio of 230:1 and 10:1, respectively, has been achieved. At 80 keV, the penetration depth is 7 mm. When the path length of the beam through the crystal is varied, the effect of volume enhancement of the intensity diffracted by magnetic reflections is demonstrated. The Q dependence of the magnetic and the charge Bragg reflections has been measured and agrees well with theory. The measurement of the temperature dependence of the sublattice magnetization allows a very accurate determination of the critical exponent β = 0.333 (3) and the Néel temperature TN = 67.713 (2) K. Finally, the multiple charge scattering is discussed, which is very pronounced for the magnetic reflections of MnF2.

On the electromagnetic production of the proposed πNN resonance d′

Abstract We examine the electromagnetic decay rates as well as the photo- and electroproduction cross sections of the proposed πNN resonance d ′, for the existence of which evidence has been claimed in the pionic double charge exchange (DCX) on nuclei as well as in the 2π-production in pp -collisions. Based on the d ′ parameters deduced from DCX, we estimate the d ′-production on the deuteron in the reactions γd → NNπ and ed → e ′ NNπ . Comparing to the non-resonant cross sections at the relevant energies and momentum transfers, we discuss the feasibility of a search for a d ′ signal both in γd → npπ 0 and in ed → e ′ NNπ .

Wavelength dependence of the nonresonant photoionization cross section of a two-electron atom near the ionization threshold

We present the theoretical near-threshold photoionization cross sections of He atoms from bound excited states, using a B-spline-based configuration-interaction method. Our study has shown that in the absence of a strongly energy-dependent feature such as a doubly excited resonance, the photoionization cross sections in the immediate vicinity of the ionization threshold depend linearly on the wavelengths of the incident light. Qualitatively, following the quantum-defect theory, this linear dependence can be linked directly to the expected nearly energy-independent scattering phase shift corresponding to the wave function of the ionized electron at near-zero photoelectron energy.

Direct capture in the 3(+) resonance of 2H( alpha, gamma )6Li.

The cross section for the capture reaction[sup 2]H([alpha],[gamma])[sup 6]Li was measured in the energy range of about [ital E][sub [alpha],lab][approx]2 MeV corresponding to the 3[sup +] resonance in [sup 6]Li at [ital E][sub [ital x]]=2186 keV. Calculations in a direct-capture model using double-folded [alpha]-[ital d] potentials reproduce strength and width of this resonance as well as the nonresonant capture cross section at lower and higher energies.

Low-energy total and differential cross sections for the electron-impact excitation of Si2+ and Ar6+

The total and angular differential excitation cross sections for the transitions 3${\mathit{s}}^{2}$ $^{1}$S\ensuremath{\rightarrow}3s3p $^{3}$P, 3${\mathit{s}}^{2}$ $^{1}$S\ensuremath{\rightarrow}3s3p $^{1}$P, and 3s3p $^{3}$P\ensuremath{\rightarrow}3s3p $^{1}$P in the Mg-like ions ${\mathrm{Si}}^{2+}$ and ${\mathrm{Ar}}^{6+}$ are determined in the threshold energy region using the close-coupling R-matrix method. The total cross sections for all three transitions in both ions exhibit strong resonance structures near threshold. We focus our analysis on the angular distribution of the scattered electrons. The differential cross sections are found to undergo rapid and strong variations when the incident energy is tuned through the autoionizing resonance transitions. In ${\mathrm{Ar}}^{6+}$, we also examine the angular distribution of the scattered electrons associated with the nonresonant background cross sections for the two transitions from the ground state. For the dipole-allowed transition, the electrons are preferentially backscattered near the excitation threshold; however, the fraction of electrons scattered in the forward direction increases dramatically with energy. The corresponding behavior for the spin-forbidden transition is quite different. We discuss the implications of these results to possible excitation measurements in which the scattered electrons are collected.

Differential cross sections for electronic excitation of molecular hydrogen using the R-matrix method

Differential cross sections are calculated for electron-H2 collisions for scattering energies up to 20 eV. Elastic scattering and excitation from the ground, X 1 Sigma g+, to the lowest six excited electronic states, b3 Sigma u+, a3 Sigma g+, c3 Pi u, B1 Sigma u+, E, F 1 Sigma g+ and C 1 Pi u, is explicitly considered for all total symmetries up to and including 2 Phi g for a single fixed H2 geometry. The target states are represented using a full configuration interaction treatment within a basis of Slater-type orbitals optimized to give accurate vertical excitation energies. Results are presented for both resonant and non-resonant differential cross sections. Comparison is made with the available experimental and theoretical data. Excellent agreement is obtained for elastic differential cross sections; agreement for the inelastic differential cross sections is only moderate. Possible improvements to these calculations are discussed.

Comment on "Resonant and nonresonant contributions to the photoactivation of 111Cd"

The most recent investigation of the photoactivation of isomeric nuclei reported by Kr\ifmmode \check{c}\else \v{c}\fi{}mar et al. continues to contain a strong contribution from nonresonant channels. In this Comment we report that integrated cross sections for resonant photoactivation of isomeric levels in $^{87}\mathrm{Sr}$, $^{111}\mathrm{Cd}$, and $^{113,115}\mathrm{In}$ have been calculated from recent nuclear structure data and compared to those experimental results which assert that nonresonant contributions are important. The latter have shown systematically smaller values and the amount of missing resonant strength can be correlated to the magnitude of nonresonant cross section found in these investigations. Monte Carlo simulations of realistic experimental geometries display important components in the photon fields, resulting from environmental Compton scattering, which have been omitted in previous analyses of experimental data. The strength and shape of this contribution as a function of the experimentally important parameters suggest that the data can be explained entirely on the basis of a resonant excitation mechanism without any need to introduce a nonresonant contribution.

Polarization and resonance studies of x-ray magnetic scattering in holmium.

We report the results of an x-ray-scattering study of the polarization and resonance properties of the magnetic cross section of holmium. Off resonance, we have measured the degree of linear polarization for several different chemical and magnetic reflections, and shown that the results are consistent with a nonresonant description of the x-ray-scattering cross section. When the incident x-ray energy is tuned near the ${\mathit{L}}_{\mathrm{III}}$ absorption edge, large resonant enhancements of the magnetic scattering, and resonant integer harmonics, are observed. Detailed measurements of the dependence of the integrated magnetic intensity on the incident x-ray energy, on the x-ray momentum transfer, and on the sample temperature are described for incident x-ray energies in a range of 250 eV below and above the ${\mathit{L}}_{\mathrm{III}}$ absorption edge. In addition, we have characterized the energy dependence of the linearly \ensuremath{\sigma}- and \ensuremath{\pi}-polarized components of the magnetic scattering in this range, and fitted the resulting line shapes to a simple model of the resonant cross section, including electric dipole and quadrupole transitions among atomic orbitals. The most striking feature of the line shapes is their asymmetry, which results from the interference of the resonant and nonresonant cross sections. Smaller resonant enhancements of the magnetic scattering are reported for incident x-ray energies tuned near the ${\mathit{L}}_{\mathrm{II}}$ and ${\mathit{L}}_{\mathrm{I}}$ absorption edges.