Photoabsorption Data Research Articles

Photoion-yield study of the 3p–3d giant resonance excitation region of isolated Cr, Mn and Fe atoms

Using the charge-separated photoion-yield method combined with monochromatized synchrotron radiation, single (M+) and double (M2+) photoionization spectra in the 3p excitation region were observed for isolated atoms M = Cr, Mn and Fe. The relaxation processes of excited states to double ionization, as well as to single ionization, were considered with the help of existing photoabsorption and photoelectron data and some calculations of highly-excited states of M+ using the atomic code GRASP92. In the 3p→3d giant resonance region before the 3p→n(s, d) Rydberg series starts, the spectral features in the M2+ channel are similar to those in the M+ channel. The ratios M2+/M+ in this region are rather constant accordingly against the photon energy with values 0.045 ± 0.005 for Cr, 0.17 ± 0.03 for Mn and 0.30 ± 0.08 for Fe at the giant resonance. The increase in the M2+/M+ ratio with increasing the atomic number Z can be explained by the increasing contribution of two-step double ionization via highly-excited M+ states. It is expected that, at the 3p→3d giant resonance, the one-step double ionization process occurs within a few per cent of the entire decay.

Large-scale continuum random-phase approximation predictions of dipole strength for astrophysical applications

Large-scale calculations of the E1 strength are performed within the random phase approximation (RPA) based on the relativistic point-coupling mean field approach in order to derive the radiative neutron capture cross sections for all nuclei of astrophysical interest. While the coupling to the single-particle continuum is taken into account in an explicit and self-consistent way, additional corrections like the coupling to complex configurations and the temperature and deformation effects are included in a phenomenological way to account for a complete description of the nuclear dynamical problem. It is shown that the resulting E1-strength function based on the PCF1 force is in close agreement with photoabsorption data as well as the available experimental E1 strength data at low energies. For neutron-rich nuclei, as well as light neutron-deficient nuclei, a low-lying so-called pygmy resonance is found systematically in the 5-10 MeV region. The corresponding strength can reach 10% of the giant dipole strength in the neutron-rich region and about 5% in the neutron-deficient region, and is found to be reduced in the vicinity of the shell closures. Finally, the neutron capture reaction rates of neutron-rich nuclei is found to be about 2-5 times larger than those predicted on the basis of the nonrelativistic RPA calculation and about a factor 50 larger than obtained with traditional Lorentzian-type approaches.

Ozone: Unresolved discrepancies for dipole oscillator strength distributions, dipole sums, and van der Waals coefficients

Dipole oscillator strength distributions (DOSDs) for ozone are constructed from experimental photoabsorption cross-sections combined with constraints provided by the Kuhn-Reiche-Thomas sum rule, the high-energy behavior of the dipole-oscillator-strength density, and molar refractivity data. A lack of photoabsorption data in the intermediate energy region from 24 to 524 eV necessitates the use of a mixture rule in that region. For this purpose, a DOSD for O(2) is constructed first. The dipole properties for O(2) are essentially the same as those obtained in earlier work even though most of the input data is from more recent experiments. A discrepancy is found between the refractivity data and photoabsorption data in the 10-20.6 eV range for ozone. A reliable ozone DOSD of the sort obtained for many other species remains out of reach. However, it is suggested that the true dipole properties of ozone lie between those predicted by two distributions that we present.

Electric dipole polarizabilities of hydrogen and helium isotopes

The electric dipole polarizabilities of $^3$H, $^3$He, and $^4$He are calculated directly using the Schr\"odinger equation with the latest generation of two- and three-nucleon interactions. These polarizabilities are necessary in order to obtain accurate nuclear-polarization corrections for transitions involving S-waves in one- and two-electron atoms. Our results are compared to previous results, and it is shown that direct calculations of the electric polarizability of $^4$He using modern nuclear potentials are smaller than published values calculated using experimental photoabsorption data. The status of this topic is assessed in the context of precise measurements of transitions in one- and two-electron atoms.

Optical constants of magnetron-sputtered boron carbide thin films from photoabsorption data in the range 30 to 770 eV

This work discusses the experimental determination of the optical constants (refractive index) of DC-magnetron-sputtered boron carbide films in the 30-770 eV photon energy range. Transmittance measurements of three boron carbide films with thicknesses of 54.2, 79.0, and 112.5 nm were performed for this purpose. These are believed to be the first published experimental data for the refractive index of boron carbide films in the photon energy range above 160 eV and for the near-edge x-ray absorption fine structure regions around the boron K (188 eV), carbon K (284.2 eV), and oxygen K (543.1 eV) absorption edges. The density, composition, surface chemistry, and morphology of the films were also investigated using Rutherford backscattering, x-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and extreme ultraviolet reflectance measurements.

Neutral and charged excitations in carbon fullerenes from first-principles many-body theories

We investigate the accuracy of first-principles many-body theories at the nanoscale by comparing the low-energy excitations of the carbon fullerenes C(20), C(24), C(50), C(60), C(70), and C(80) with experiment. Properties are calculated via the GW-Bethe-Salpeter equation and diffusion quantum Monte Carlo methods. We critically compare these theories and assess their accuracy against available photoabsorption and photoelectron spectroscopy data. The first ionization potentials are consistently well reproduced and are similar for all the fullerenes and methods studied. The electron affinities and first triplet excitation energies show substantial method and geometry dependence. These results establish the validity of many-body theories as viable alternative to density-functional theory in describing electronic properties of confined carbon nanostructures. We find a correlation between energy gap and stability of fullerenes. We also find that the electron affinity of fullerenes is very high and size independent, which explains their tendency to form compounds with electron-donor cations.

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

The latest generation of high quality, narrow gap, superconducting tunnel junctions (STJs) exhibits a steady-state and time-dependent behavior which cannot be described satisfactorily by previous treatments of nonequilibrium quasiparticle (qp) dynamics. These effects are particularly evident in experiments using STJs as detectors of photons, over the range from near infrared to x ray. In this paper, we present a detailed theoretical analysis of the spectral and temporal evolution of the nonequilibrium qp and phonon distributions in such STJs excited by single photons, over a wide range of excitation energy, bias voltage, and temperature. By solving the coupled set of kinetic equations describing the interacting excitations, we show that the nonequilibrium qp distribution created by the initial photoabsorption does not decay directly back to the initial undisturbed state in thermal equilibrium. Instead, it undergoes a rapid adiabatic relaxation to a long-lived, excited state, the spectral distribution of which is nonthermal, maintained by a balance between qp creation, recombination, and trapping. The model is able to describe successfully photoabsorption data taken on several different aluminum STJs, using a single set of parameters. Of particular note is the conclusion that the local traps responsible for qp loss are situated specifically in the region of Nb contacts.

The Δ(1232) at RHIC

We investigate properties of the Δ(1232) and nucleon spectral functions at finite temperature and baryon density within a hadronic model. The medium modifications of the Δ consist of a renormalization of its pion–nucleon cloud and resonant πΔ scattering. Underlying coupling constants and form factors are determined by the elastic πN scattering phase shift in the isobar channel, as well as empirical partial decay widths of excited baryon resonances. For cold nuclear matter the model provides reasonable agreement with photoabsorption data on nuclei in the Δ-resonance region. In hot hadronic matter typical for late stages of central Au–Au collisions at RHIC we find the Δ-spectral function to be broadened by ∼65 MeV together with a slight upward mass shift of 5–10 MeV, in qualitative agreement with preliminary data from the STAR collaboration.

Microscopic HFB + QRPA predictions of dipole strength for astrophysics applications

Large-scale QRPA calculations of the E1-strength are performed on top of HFB calculations in order to derive the radiative neutron capture cross sections for the whole nuclear chart. The HFB + QRPA predictions based on various Skyrme forces with different pairing prescriptions and parameterizations are confronted with experimental photoabsorption data for spherical nuclei. Within the approximations of the present model, the effective nucleon–nucleon interaction is probed with the GDR data. The dipole strengths obtained with the present HFB + QRPA model is finally used to estimate the radiative neutron capture cross section for all nuclei of relevance in astrophysics applications. The resulting rates are shown to be close to the one predicted with the previously determined HFBCS + QRPA model, but for neutron-rich nuclei to differ significantly from the ones obtained with the empirical Lorentzian-like formula.

Quasi-free Compton scattering and the polarizabilities of the neutron

Differential cross-sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz photon tagging spectrometer at the Mainz MAMI accelerator together with the Mainz [48]cm ;SPMOslash; × [64]cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. The data cover photon energies ranging from [200]MeV to [400]MeV at θLAB γ = 136.2°. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction p(γ,π+ n). The “free” proton Compton scattering cross-sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross-section for free scattering from quasi-free data. Differential cross-sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron has been determined to be αn - βn = 9.8±3.6(stat)+2.1 -1.1(syst)±2.2(model) in units of [10-4]fm 3. In combination with the polarizability sum αn + βn = 15.2±0.5 deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, αn = 12.5±1.8(stat)+1.1 -0.6(syst)±1.1(model) and βn = 2.7±1.8(stat)+0.6 -1.1(syst)±1.1(model) are obtained. The backward spin polarizability of the neutron was determined to be γ(n) π = (58.6±4.0)×10-4 fm 4.

Mass effects in hard exclusive photoproduction of [formula omitted] mesons

Large-scale QRPA calculations of the E1-strength are performed on top of HFB calculations in order to derive the radiative neutron capture cross sections for the whole nuclear chart. The HFB + QRPA predictions based on various Skyrme forces with different pairing prescriptions and parameterizations are confronted with experimental photoabsorption data for spherical nuclei. Within the approximations of the present model, the effective nucleon–nucleon interaction is probed with the GDR data. The dipole strengths obtained with the present HFB + QRPA model is finally used to estimate the radiative neutron capture cross section for all nuclei of relevance in astrophysics applications. The resulting rates are shown to be close to the one predicted with the previously determined HFBCS + QRPA model, but for neutron-rich nuclei to differ significantly from the ones obtained with the empirical Lorentzian-like formula.

Large-scale QRPA calculation of E1-strength and its impact on the neutron capture cross section

Large-scale QRPA calculations of the E1-strength are performed as a first attempt to microscopically derive the radiative neutron capture cross sections for the whole nuclear chart. A folding procedure is applied to the QRPA strength distribution to take the damping of the collective motion into account. It is shown that the resulting E1-strength function based on the SLy4 Skyrme force is in close agreement with photoabsorption data as well as the available experimental E1 strength at low energies. The increase of the E1-strength at low energies for neutron-rich nuclei is qualitatively analyzed and shown to affect the corresponding radiative neutron capture cross section significantly. A complete set of E1-strength function is made available for practical applications in a table format for all 8⩽ Z⩽110 nuclei lying between the proton and the neutron drip lines.

Photofission of heavy nuclei from 0.2 to 3.8 GeV

Total photofission cross sections for ${}^{237}\mathrm{Np},$ ${}^{233}\mathrm{U},$ ${}^{235}\mathrm{U},$ ${}^{238}\mathrm{U},$ ${}^{232}\mathrm{Th},$ and ${}^{\mathrm{nat}}\mathrm{Pb}$ have been measured simultaneously, using tagged photons in the energy range ${E}_{\ensuremath{\gamma}}=0.17--3.84 \mathrm{GeV}$ in Hall B at Jefferson Lab. The fission fragments were detected using parallel-plate avalanche detectors. Our results show that, of these nuclei, ${}^{237}\mathrm{Np}$ has the largest photofission cross section per nucleon, and that the photofission cross section for ${}^{238}\mathrm{U}$ relative to that for ${}^{237}\mathrm{Np}$ is about 80% over the entire energy range. The relative photofission cross sections per nucleon increase with the fissility parameter ${Z}^{2}/A,$ approaching an apparent asymptote at the ${Z}^{2}/A$ value for ${}^{237}\mathrm{Np},$ and they do not depend strongly on the incident photon energy over this entire energy range. These results, together with a comparison of the ${}^{237}\mathrm{Np}$ photofission cross section per nucleon with total photoabsorption data, indicate that the photofission probability for ${}^{237}\mathrm{Np}$ is almost unity. If we make this assumption, we observe a significant shadowing of the interior nucleons starting below 1.5 GeV, so that the photoabsorption cross section decreases by 20% near 4 GeV.

Neutron polarizabilities investigated by quasifree Compton scattering from the deuteron.

Measuring Compton scattered photons and recoil neutrons in coincidence, quasifree Compton scattering by the neutron has been investigated at MAMI (Mainz) at theta(lab)(gamma) = 136 degrees in an energy range from 200 to 400 MeV. From the data a polarizability difference of alpha(n)-beta(n) = 9.8+/-3.6(stat)+2.1-1.1(syst)+/-2.2(model) in units of 10(-4) fm(3) has been determined. In combination with the polarizability sum alpha(n)+beta(n) = 15.2+/-0.5 deduced from photoabsorption data, the first precise results for the neutron electric and magnetic polarizabilities, alpha(n) = 12.5+/-1.8(stat)+1.1-0.6(syst)+/-1.1(model) and beta(n) = 2.7-/+1.8(stat)+0.6-1.1(syst)-/+1.1(model), are obtained.

Giant resonance spectroscopy of 40Ca with the (e,e′ x) reaction (II): Multipole decomposition of 4 π-integrated spectra and angular correlations

The present article is the second out of three on a study of the 40Ca(e,e′ x) reaction discussing the multipole decomposition of the measured cross sections and the analysis of angular correlations. The decomposition of the strongly overlapping E0, E1 and E2 giant resonance strengths using the (e,e′ x; x=p, α) reaction in 40Ca is discussed for excitation energies between 10 and about 21 MeV. Two extraction methods are presented based on the variation of the form factors for the different multipoles. The resulting B(E1) strength distribution is in good agreement with ( γ, x) photoabsorption data. The summed B(E2) and B(E0) strength is highly fragmented and spread out over the energy region investigated. Microscopic continuum RPA calculations including the coupling of the basic particle–hole states to the low-lying surface vibrations are capable of reproducing the strength distributions quite accurately. Exhaustion of the energy-weighted sum rules (EWSR) for the various decay channels is presented. A complete decomposition of E0, E1 and E2 contributions in 40Ca is possible for (e,e′ α) angular correlations populating the 36Ar ground state. Contrary to expectations, the form factors of isoscalar E0 and E2 strengths in the 40Ca(e,e′ α 0) reaction exhibit increasing differences towards smaller momentum transfers. Angular correlations for proton decay into low-lying states of 39K are compared to a self-consistent continuum RPA calculation which allows a systematic description of the strong variations observed as a function of 40Ca excitation energy and momentum transfer. The success implies that direct knock-out models of the 40Ca(e,e′p) reaction are too simple. Furthermore, the shapes of the angular correlations seem to be determined largely by the final-state interaction, in particular by charge exchange reactions in the nuclear medium.

Giant Resonance in 4d Photoionization of Xe-Like Iso-Electronic Sequence

Making use of linear response method and density functional theory, we have studied the role of dynamical electron correlation in photo-absorption processes. By varying nuclear charge, we have made systematic studies of so-called 4d giant resonances appearing in Xe-like iso-electronic sequences. By changing nucleus from Xe to Cs+ and Ba++ the 4d-ionization threshold moved to the higher energy side but the dynamic correlation effect does not change so much. Particular attention was paid to the analysis of Ba++ photo-absorption data. We could identify the observed excitation peaks and find the role of dynamic correlation in the photo-absorption processes

The photoabsorption spectrum of laser-generated Li+in the 60-190 eV photon energy range

The relative photoabsorption cross-section of the Li+atomic ion was measured using the dual laser plasmaphotoabsorption technique. The measurements were carried out inthe photon energy range from 62 eV (1s2p discrete resonance) upto 190 eV, i.e. about two-and-a-half times the value of thefirst ionization potential (I1 = 75.64 eV). In thecontinuum, the relative photoabsorption data (with an estimateduncertainty of ±14%) are normalized to the theoreticalphotoionization cross-section, given in analytical form byVerner et al (Verner D A, Ferland G J, Korista K T andYakovlev D G 1996 Astrophys. J. 465 487).Comparisons between calculated photoionization cross-sectionsand with relevant experimental data are presented.

Ionic and electronic structure of sodium clusters up toN=59

We determined the ionic and electronic structures of sodium clusters with even electron numbers and 2 to 59 atoms in axially averaged and three-dimensional density functional calculations. A local, phenomenological pseudopotential that reproduces important bulk and atomic properties and facilitates structure calculations has been developed. Photoabsorption spectra have been calculated for ${\mathrm{Na}}_{2},$ ${\mathrm{Na}}_{8},$ and ${\mathrm{Na}}_{9}^{+}$ to ${\mathrm{Na}}_{59}^{+}.$ The consistent inclusion of ionic structure considerably improves agreement with experiment. An icosahedral growth pattern is observed for ${\mathrm{Na}}_{19}^{+}$ to ${\mathrm{Na}}_{59}^{+}.$ This finding is supported by photoabsorption data.

Evaluation of Photonuclear Data of Mo, Zn, S and CI for Medical and Accelerator Applications

As part of IAEA CRP on “Compilation and evaluation of photonuclear data for applications”, we evaluated photonuclear data of Mo, Zn, S and CI isotopes for medical and accelerator applications. Available experimental data were collected and their discrepancies were analyzed to select or reconstruct the representative data set. The photoabsorption cross sections were then evaluated by applying the Giant Dipole Resonance (GDR) model for the energies below about 30 MeV, and the quasi-deuteron model for energies below 140 MeV. The resulting representative photoabsorption data were given as input for the theoretical calculations for the emission process of light nuclei including neutron, proton, deuteron, triton, He-3, alpha particles and gamma rays by use of the Hauser-Feschbach and the preequilibrium model.

Photoabsorption on nuclei in the energy range 0.5–2.6 GeV

The energy and nuclear mass dependences of the total hadronic cross section in the energy range 0.5--2.6 GeV have been measured at Bonn using the SAPHIR tagged photon beam. The measurement, performed on C, Al, Cu, Sn, and Pb, provides the first photoabsorption data in the region 1.2--1.7 GeV. The results show a significant reduction of the photoabsorption strength on the bound nucleon compared to the free-nucleon case in the whole energy region. Above 1.2 GeV this reduction decreases with the average nuclear density.