Target Excitation Research Articles

Experimental evidence of the target excitation in transfer ionization of He2+ on argon collisions

We have investigated the dependence of electron capture to the continuum (ECC) on the longitudinal momentum of recoil ions in the transfer ionization of 120 and 300 keV He2+ on argon collisions. Compared with theoretical lines predicted by the kinematical relationship between longitudinal momentum of recoil ions and ECC electrons, it is shown that the electron is mainly transferred to the first excited state of He+ ions. The experimental data also show that the electron capture to the ground state of He+ is accompanied by target excitation, where the target excitation results from direct excitation in the transfer ionization processes.

STARLIB: A NEXT-GENERATION REACTION-RATE LIBRARY FOR NUCLEAR ASTROPHYSICS

STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For the first time, this library provides the rate probability density at all temperature grid points for convenient implementation in models of stellar phenomena. The recommended rate and its associated uncertainties are also included. Currently, uncertainties are absent from all other rate libraries, and, although estimates have been attempted in previous evaluations and compilations, these are generally not based on rigorous statistical definitions. A common standard for deriving uncertainties is clearly warranted. STARLIB represents a first step in addressing this deficiency by providing a tabular, up-to-date database that supplies not only the rate and its uncertainty but also its distribution. Because a majority of rates are lognormally distributed, this allows the construction of rate probability densities from the columns of STARLIB. This structure is based on a recently suggested Monte Carlo method to calculate reaction rates, where uncertainties are rigorously defined. In STARLIB, experimental rates are supplemented with: (i) theoretical TALYS rates for reactions for which no experimental input is available, and (ii) laboratory and theoretical weak rates. STARLIB includes all types of reactions of astrophysical interest to Z = 83, such as (p,g), (p,a), (a,n), and corresponding reverse rates. Strong rates account for thermal target excitations. Here, we summarize our Monte Carlo formalism, introduce the library, compare methods of correcting rates for stellar environments, and discuss how to implement our library in Monte Carlo nucleosynthesis studies. We also present a method for accessing STARLIB on the Internet and outline updated Monte Carlo-based rates.

Hard x-ray absorption spectroscopy for pulsed sources

We present a method suitable for the measurement of a high-energy resolved absorption spectrum in the emission mode upon target excitation with an intense monochromatic pulse of x-ray light. The approach is based on a measurement of a single resonant Raman x-ray spectrum at a fixed off-resonant excitation energy by a dispersive-type high-resolution emission spectrometer. Using a simple Lorentzian tail correction function and an energy-scale transformation, we reconstructed the ${L}_{3}$ absorption edge of Xe from the $3d$-$2{p}_{3/2}$ resonant Raman x-ray spectrum recorded at excitation energy 25 eV below the ${L}_{3}$ edge. The energy resolution is well below the ${L}_{3}$ core-hole lifetime broadening and allows for a straightforward extraction of the $[2{p}_{3/2}]n\ensuremath{\ell}$ resonance energies and emission strengths. The reconstructed spectrum is nearly equivalent to the high-energy resolution fluorescence detected absorption spectrum, which is recorded by scanning the photon excitation energy across the absorption edge.

A study of the energy deposition profile of proton beams in materials of hadron therapeutic interest

The energy delivered by a swift proton beam in materials of interest to hadron therapy (liquid water, polymethylmethacrylate or polystyrene) is investigated. An explicit condensed-state description of the target excitation spectrum based on the dielectric formalism is used to calculate the energy-loss rate of the beam in the irradiated materials. This magnitude is the main input in the simulation code SEICS (Simulation of Energetic Ions and Clusters through Solids) used to evaluate the dose as a function of the penetration depth and radial distance from the beam axis.

Two-center convergent-close-coupling calculations of positron scattering on magnesium

The two-center convergent-close-coupling method is extended to the calculation of positron scattering from atoms with two valence electrons above an inert ion core. Calculations of positron scattering on magnesium have been performed over a wide energy range. Elastic scattering, Positronium formation, target excitation, and direct ionization cross sections have been calculated. Results are in reasonably good agreement with the experimental data where available. Current results confirm the existence of a low-energy shape resonance predicted by variational and single-center convergent-close-coupling methods.

R-matrix electron-impact excitation data for the B-like iso-electronic sequence

We have carried-out parallel intermediate-coupling frame transformation R-matrixcalculations for electron-impact excitation amongst the 204 close-coupling levels of the 2s x 2p y (x + y = 3), 2s 2 {3,4}l ,2 s2p{3,4}l, and 2p 2 3l configurations for all boron-like ions from C + to Kr 31+ . We have also included the configuration interaction due to the 2p 2 4l ,2 s3l3land 2p3s3l configurations. A detailed comparison has been made of the target structure and excitation data for four specific ions (viz., Ne 5+ ,A r 13+ ,F e 21+ and Kr 31+ )t hat span the sequence, so as to assess the accuracy over the entire sequence. Effective collision strengths (Ys) are presented at temperatures ranging from 2 ×10 2 (z +1) 2 Kt o 2×10 6 (z +1) 2 K( wherez is the residual charge of the ions, i.e. Z −5). Detailed comparisons for the (effective) collision strengths (Ys) Ω are made with the results of previous calculations for the four representative ions. Furthermore, we examine the iso-electronic trends of the effective collision strengths as a function of temperature.

Generalized Faddeev equations in the Alt-Grassberger-Sandhas form for deuteron stripping with explicit inclusion of target excitations and Coulomb interaction

Theoretical description of reactions in general, and the theory for $(d,p)$ reactions, in particular, needs to advance into the future. Here deuteron-stripping processes off a target nucleus consisting of $A$ nucleons are treated within the framework of the few-body integral equations theory. The generalized Faddeev equations in the Alt-Grassberger-Sandhas (AGS) form, which take into account the target excitations, with realistic optical potentials provide the most advanced and complete description of the deuteron stripping. The main problem in practical application of such equations is the screening of the Coulomb potential, which works only for light nuclei. In this paper we present a formulation of the Faddeev equations in the AGS form taking into account the target excitations with explicit inclusion of the Coulomb interaction. By projecting the $(A+2)$-body operators onto target states, matrix three-body integral equations are derived, which allow for the incorporation of the excited states of the target nucleons. Using the explicit equations for the partial Coulomb scattering wave functions in the momentum space we present the AGS equations in the Coulomb distorted wave representation without screening procedure. We also use the explicit expression for the off-shell two-body Coulomb scattering $T$ matrix, which is needed to calculate the effective potentials in the AGS equations. The integrals containing the off-shell Coulomb $T$ matrix are regularized to make the obtained equations suitable for calculations. For $NN$ and nucleon-target nuclear interactions we assume the separable potentials what significantly simplifies solution of the AGS equations.

Post collision interactions in fully differential cross sections for four-body charge transfer processes

Recently, experimental fully differential cross sections have been reported for proton–helium collisions. In this work we will examine single capture, transfer with target excitation and double capture for proton–helium collisions. For single capture, the proton captures one electron from helium and leaves the other electron in the ground state. For the transfer-excitation case, the target is excited to arbitrary excited states. In the case of double capture, the proton captures both of the electrons from helium and leaves the collision as an H− ion. We introduce a fully quantum mechanical four-body model that includes the final state post collision interactions between all two-particle pairs exactly. In the initial state, the two-particle interactions are included asymptotically by using an Eikonal wavefunction. The results of this model are in better agreement with experimental data than previous quantum mechanical calculations.

A simulated annealing based study to design optimum pulses for selective target excitation in vibrational levels

In this paper, we seek to design optimum pulses for selective target excitation to specified excited levels. We test our proposed idea for vibrational excitation in systems modelled by the Morse potential, using the time dependent Fourier grid Hamiltonian technique and the simulated annealing method.

Measurements and coupled reaction channels analysis of one- and two-proton transfer reactions for the28Si+90,94Zr systems

Measurements of angular distributions for one- and two-proton stripping reactions for ${}^{28}$Si $+$ ${}^{90,94}$Zr systems were performed at 120 MeV. The experiment was carried out with the ${}^{28}$Si beam at Inter University Accelerator Center, New Delhi. The theoretical calculations were performed using the quantum mechanical coupled reaction channels code fresco. The distorted wave Born approximation calculations reproduced the experimental angular distributions for the one-proton transfer channel for both the systems reasonably well but failed for the two-proton transfer channel. Coupled channels calculations including various intermediate states (involving target and projectile inelastic excitations before and/or after transfer) along with the sequential transfer were able to reproduce the two-proton transfer angular distributions for both the systems reasonably well. It seems that at an energy above the Coulomb barrier, there is significant contribution of the indirect multistep and sequential transfer to the two-proton stripping reaction.

Feshbach resonances in positron scattering from sodium

S-, P- and D-wave Feshbach resonances in positron-sodium scattering have been investigated by using the momentum space coupled-channels optical method. The target continuum and positronium (Ps) formation channels are included via an optical potential. Feshbach resonances below the target excitation and Ps (n = 2) formation thresholds are predicted and the effects of channel-coupling scheme, target continuum and Ps formation channels on the resonance energy and resonance width are discussed. We have also found the Wigner cusp structures at the inelastic channel-opening thresholds in positron-sodium scattering cross sections.

Four-body charge transfer processes in proton–helium collisions

Recent advancements in experimental techniques now allow for the study of fully differential cross sections (FDCS) for four-body collisions. The simplest four-body problem is a charged particle collision with a helium atom, in which both atomic electrons change state. This type of collision can result in many different outcomes, such as double excitation, excitation ionization, double ionization, transfer excitation, transfer ionization and double charge transfer. In this paper, we compare absolute experimental proton–helium FDCS for transfer excitation with the fully quantum mechanical 4BTTE (four-body transfer with target excitation) model. This model was previously used to study TTE for proton energies between 25 and 75 keV and reasonable agreement was found with the experimental data for large scattering angles, but not small angles. Since this is a first-order model, which contains contributions from all higher order terms, one would expect improved agreement with increasing energy and the purpose of this work was to look at higher energies. We found that the agreement with the magnitude of the experimental data became worse with increasing energy while the agreement with the shape of the data was reasonably good. Consequently, we conclude that the model contains the physical effects that determine the shape but not the magnitude of the cross section.

Toward a microscopic reaction description based on energy-density-functional structure models

A microscopic calculation of reaction cross sections for nucleon-nucleus scattering has been performed by explicitly coupling the elastic channel to all particle-hole excitations in the target and one-nucleon pickup channels. The particle-hole states may be regarded as doorway states through which the flux flows to more complicated configurations, and subsequently to long-lived compound nucleus resonances. Target excitations for ${}^{40,48}$Ca, ${}^{58}$Ni, ${}^{90}$Zr, and ${}^{144}$Sm were described in a random-phase framework using a Skyrme functional. Reaction cross sections obtained agree very well with experimental data and predictions of a state-of-the-art fitted optical potential. Couplings between inelastic states were found to be negligible, while the pickup channels contribute significantly. The effect of resonances from higher-order channels was assessed. Elastic angular distributions were also calculated within the same method, achieving good agreement with experimental data. Observed absorptions are completely accounted for by explicit channel coupling, for incident energies between 10 and 70 MeV, with consistent angular distribution results.

My Life with LIF: A Personal Account of Developing Laser-Induced Fluorescence

Laser-induced fluorescence (LIF) is a spectroscopic technique that involves the excitation of a molecular target by a beam of laser radiation followed by the detection of the subsequent emission of radiation from the target. LIF detection has several advantages over absorption spectroscopy. First, LIF has excellent detection sensitivity because a signal is observed against a dark background. Second, the emitted radiation can be collected at various angles with respect to the incoming laser beam, making it possible to obtain two- and three-dimensional images because the fluorescence is emitted in all directions. Third, by dispersing the fluorescence, it is also possible to learn about the transitions from the state excited to the various lower levels of the target species. Finally, because of the delay between the excitation and detection events, it is also possible to learn about what processes the excited target undergoes in the intervening time.

A panorama of CDCC calculations for deuteron induced reactions: from elastic cross sections to transfer and inelastic ones

In this contribution, an overview of the calculations that can be performed within the Continuum Discretized Coupled Channels (CDCC) approach to analyze deuteron induced reactions is given. First, we briefly remind the CDCC formalism. In the second part, we present an extension of the CDCC formalism which accounts for the target excitations allowing us to determine (d,d') cross sections off deformed nuclei. After the derivation of the coupled equations, we compare some calculated inelastic cross sections with experimental data. Then it is shown that the CDCC formalism can also be a useful tool to determine (d,p) or (d,n) cross sections. We illustrate this point for nuclei with N≈32.

Reaction cross-section predictions for nucleon induced reactions

A microscopic calculation of the optical potential for nucleon-nucleus scattering has been performed by explicitly coupling the elastic channel to all the particle-hole (p-h) excitation states in the target and to all relevant pickup channels. These p-h states may be regarded as doorway states through which the flux flows to more complicated configurations, and to long-lived compound nucleus resonances. We calculated the reaction cross sections for the nucleon induced reactions on the targets 40,48Ca, 58Ni, 90Zr and 144Sm using the QRPA description of target excitations, coupling to all inelastic open channels, and coupling to all transfer channels corresponding to the formation of a deuteron. The results of such calculations were compared to predictions of a well-established optical potential and with experimental data, reaching very good agreement. The inclusion of couplings to pickup channels was an important contribution to the absorption. For the first time, calculations of excitations account for all of the observed reaction cross-sections, at least for incident energies above 10 MeV.

A combined molecular dynamics and Monte Carlo simulation of the spatial distribution of energy deposition by proton beams in liquid water

We have evaluated the spatial distribution of energy deposition by proton beams in liquid water using the simulation code SEICS (Simulation of Energetic Ions and Clusters through Solids), which combines molecular dynamics and Monte Carlo techniques and includes the main interaction phenomena between the projectile and the target constituents: (i) the electronic stopping force due to energy loss to target electronic excitations, including fluctuations due to the energy-loss straggling, (ii) the elastic scattering with the target nuclei, with their corresponding energy loss and (iii) the dynamical changes in projectile charge state due to electronic capture and loss processes. An important feature of SEICS is the accurate account of the excitation spectrum of liquid water, based on a consistent solid-state description of its energy-loss-function over the whole energy and momentum space. We analyse how the above-mentioned interactions affect the depth distribution of the energy delivered in liquid water by proton beams with incident energies of the order of several MeV. Our simulations show that the position of the Bragg peak is determined mainly by the stopping power, whereas its width can be attributed to the energy-loss straggling. Multiple elastic scattering processes contribute slightly only at the distal part of the Bragg peak. The charge state of the projectiles only changes when approaching the end of their trajectories, i.e. near the Bragg peak. We have also simulated the proton-beam energy distribution at several depths in the liquid water target, and found that it is determined mainly by the fluctuation in the energy loss of the projectile, evaluated through the energy-loss straggling. We conclude that a proper description of the target excitation spectrum as well as the inclusion of the energy-loss straggling is essential in the calculation of the proton beam depth–dose distribution.

Measurement of differential cross-sections and widths of resonances in 32S(p,p′γ) 32S reaction in the 3.0–4.0 MeV region

The paper reports the widths and differential cross-sections of resonances at 3.089, 3.379 and 3.717 MeV in the 32S(p,p′γ) 32S nuclear reaction. The cross-sections are computed at 0° and 90° angles (relative to the beam direction) from thick target excitation curves constructed by measuring 2230 keV γ-rays, characteristic of the reaction. The differential cross-sections of resonances are about 18, 64 and 70 mb/sr respectively at 0° angle and decrease by about half around an angle of 90°. The first resonance, the sharpest among the three, exhibits a width of about 400 eV while those at 3.379 and 3.717 MeV are in 1.0–1.5 keV range. The widths of the resonances are extracted from the respective thick target excitation curves by an interquartile separation method and also by simulating their leading edges. A study of thick target yields in the 3.0–4.0 MeV proton energy region for several sulphide forming elements shows the absence of any significant interference. These resonances, as a result, can be effectively utilised for sensitive and high resolution depth profile measurements of sulphur in films and materials surfaces.

Selective excitation of two‐dimensional arbitrarily shaped voxels with parallel excitation in spectroscopy

Parallel excitation is being studied intensively for applications in MR imaging and in particular for selecting arbitrary shapes as regions of interest. In this work, parallel excitation was applied to arbitrarily shaped voxel selection in spectroscopy and investigated for different excitation k-space trajectories (radial, rectilinear, and spiral) and acceleration factors. Each trajectory was segmented into multiple excitations to increase the overall bandwidth during target selection. Acceleration by parallel excitation was used to decrease the number of segments. Evaluation of spatial and spectral localization of the target of interest was performed in simulation and phantom experiments, and was compared with the point resolved spectroscopy (PRESS) experiment with standard voxels. The selective excitation experiments demonstrated excellent spatial localization and a broad frequency response, although PRESS was superior in direct comparisons with respect to signal-to-noise ratio (SNR) and outer volume suppression. Extensive SNR variation was observed dependent on trajectory (8%-90%), with the preferred radial case producing approximately 40%-60% SNR of the PRESS case. Accelerated trajectories at R = 4 provided comparable artifact signal and target excitation accuracy compared with their nonaccelerated counterparts; however, further acceleration (R = 8) resulted in increased artifact (33% increase at R = 8).

High-resolutionKMMradiative Auger x-ray emission spectra of calcium induced by synchrotron radiation

The KMM radiative Auger (RA) x-ray spectra of solid Ca were induced by monochromatic synchrotron radiation and measured with a high-resolution von Hamos bent crystal spectrometer. Two excitation energies were employed, one in the near K threshold region and the second well above the K absorption edge. The KMM RA spectral structure and relative intensity with respect to the diagram K{beta}{sub 1,3} (K-M{sub 3,2}) line are found to be independent of the excitation energy. The overall RA structure resembles the density of unoccupied s, p, and d states. Due to solid-state effects, however, spectral features resulting from the major discrete shake-up transitions could not be resolved. For the total KMM RA to K{beta}{sub 1,3} yield ratio, a value of 0.053(3) is obtained. The latter is compared to theoretical predictions and available experimental data obtained by various types of target excitation.