Cross Section Sigma Research Articles

Spin-resolved photoelectron spectroscopy of HBr in the resonance region of electron autoionization

The spin polarization parameters A and xi , the cross section sigma and the angular distribution parameter beta in outer shell photoemission from HBr were measured in the resonance region of electronic autoionization between the HBr+ X 2 Pi and HBr+ A 2 Sigma + thresholds using photoelectron spectroscopy with circularly polarized synchrotron radiation. In this region the photoelectron spectra display pronounced vibrational excitation of the X 2 Pi final ionic states. The energy dependence of the dynamical parameters is strongly influenced by electronic autoionization and predissociation of vibrational progressions of Rydberg states. The data for sigma and A are combined to obtain cross sections for the outgoing partial waves. A comparison of the experimental results with ab initio MQDT calculations of Lefebvre-Brion(1989) indicates that predissociation is more important than theoretically predicted.

Multiple ionization of copper by electron impact

A pulsed cross beam technique previously developed in this laboratory has been used to study the electron impact ionization of copper. Previous measurements have been very limited in scope and exhibit large discrepancies. Relative cross sections sigma n for the formation of 1 to 5 times ionized copper have been measured with high accuracy within the range 7.8-2100 eV. Individual cross sections have been obtained by normalization to absolute values of sigma 2 obtained by Freund et al (1990) at energies below 200 eV using a fast crossed beam technique. Weak structures in sigma 1 can be attributed to Auger decay processes following the creation of 3s subshell and L shell vacancies but there is a lack of other pronounced structures in sigma n for n>1 where many close-lying subshell vacancies are involved. At 2100 eV cross sections sigma 5 are less than three orders of magnitude smaller than sigma 1.

A quantum mechanically complete study of H++Na(3s) to H(2p)+Na+ charge exchange excitation

The H++Na to H(2p)+Na+ charge exchange reaction has been studied at collision energies between 1 and 5 keV. At 1 and 2 keV impact energy, this study is 'quantum mechanically complete'; three quantities (for example, the magnetic subshell excitation cross sections sigma 0= mod f0 mod 2, sigma 0= mod f1 mod 2 and the phase chi between the corresponding amplitudes f0, f1) are necessary and sufficient to fully characterize this process. The experiment involves angle-resolved coincident detection of scattered H particles and Lyman- alpha photons. Comparison is made to the results of eikonal coupled-channel calculations by Shingal (1993) and by Dubois et al. (1993).

Multiple ionization of iron by electron impact

The authors have used a pulsed crossed beam technique incorporating time-of-flight spectroscopy of the collision products to study the electron impact ionization of ground state Fe atoms. Relative cross sections sigma n for the formation of 1 to 4 times ionized iron have been measured within the energy range 8-1250 eV. Individual cross sections have been obtained by normalization to lower energy values of sigma 2 previously measured by Freund et al. (1990) using a fast crossed beam experiment where analysis was complicated by the presence of metastable atoms in Fe beams. Measured cross sections exhibit evidence of contributions from inner shell electrons. The authors' high energy values of sigma 1 are in excellent agreement with theoretical predictions based on the first Born approximation.

Population of He 41D magnetic sublevels by collisions with protons and multiply charged ions in the intermediate velocity range

The authors have investigated the collisional excitation of He by protons and multiply charged ions at intermediate velocities applying the optical method. The population of the magnetic sublevels of He 41D has been determined using level-crossing techniques for a variety of projectiles with charges ranging from 1+ to 23+ and velocities between 1 au and 7 au. The relative excitation cross sections sigma M/ sigma 41D of the Zeeman substates turn out to be universal functions UM( nu 2/q) of the projectile velocity nu and charge q. Total excitation cross sections of the 31P, 41S and the 41D state are included for a discussion of the scaling relation sigma /q=f( nu 2/q).

Neutron scattering from 12C between 15.6 and 17.3 MeV

The differential cross section sigma ( theta ) for neutron elastic scattering from 12C and for inelastic scattering from the 4.44 MeV state was measured at 15.57, 16.75 and 17.29 MeV. The sigma ( theta ) data, together with published analysing power Ay( theta ) data, were analysed in the framework of the spherical optical model and in the coupled-channels formalism. It was concluded that the present 12C(n,n)12C data and published data at higher energies appear to be well suited for determining properties of valence single-particle excitations in 11C and 13C via an iterative-moment approach or a dispersive optical-model analysis.

Elastic and inelastic electron scattering by mercury

Relative differential electron impact cross sections sigma ( theta ) for elastic and inelastic scattering in mercury have been measured. The sigma ( theta ) are normalized to the optical oscillator strength for the 61S0-61P1 transition and put on an absolute scale. Results are presented at 15, 25, 35, 40, 50, 60 and 100 eV for elastic scattering and the 61P1 excitation. In addition the 63P1, 71S0 and 71P1 excitation sigma ( theta ) are presented at 60 eV. The angular ranges are from 2' for inelastic and from 10' for elastic scattering, up to 150'. Results are compared with the other experimental and theoretical data. Comparison with the relativistic calculations is given by Srivastava et al. in the following paper (ibid., vol.26, no.5, p.1025-30 (1993)).

Lethality of heavy ion-induced DNA double-strand breaks in mammalian cells.

Cell killing and the production of DNA double-strand breaks (dsbs) were measured in parallel for V79 cells and a human tumour cell line (Caski) following exposure to radiations of differing linear energy transfer (LET) including accelerated heavy ions. Dsb induction was assessed by neutral filter elution (pH 7.4) and elution from agarose plugs in pulsed-field electrophoresis. The elution data were consistent with linear lesion induction and allowed to derive dsb yields or production cross-sections (sigma dsb), respectively. Both cell lines as well as the two elution approaches gave comparable results. sigma dsb was found to rise up to the heaviest ions used (LET = 11,500 keV/microns) but relative biological effectiveness was always smaller than unity. Slopes of the exponential survival curves (final slope with 60Co-radiation) were normalized to the respective dsb yields and allowed to calculate the relative lethality per induced dsb. This parameter increased with LET up to about 300 keV/microns followed by a steep decline for the very heavy ions. The extent of dsb rejoining was concomitantly reduced with ionization density.

Parameter evaluation in automated digital deep-level transient spectroscopy (DLTS)

A method of directly evaluating the activation energy Delta E, capture cross section sigma , and density N/sub T/, of deep-level traps from the pulsed reverse bias capacitance transient is described. The main advantages of this technique are that it requires only a single temperature scan, and it can resolve nonexponential transients due to closely-spaced energy levels. The test samples used for this paper consisted of Schottky diodes fabricated on nonirradiated and 1-MeV electron-irradiated n-type VPE (vapor-phase epitaxy) GaAs wafers. The well known EL2 trap was identified with Delta E of 0.81 eV, and sigma /sub n/ of 1.0*10/sup -13/ cm/sup 2/ for the nonirradiated sample. These values were found to be in good agreement with published data using established, conventional DLTS techniques. For the irradiated samples a nonexponential capacitance transient was found in the EL2 range of temperatures. The discussed technique was able to resolve two closely spaced deep levels lying at E/sub c/-0.81 eV and E/sub c/-0.84 eV, and with capture cross sections of 1.5*10/sup -13/ cm/sup 2/ and 2.5*10/sup -12/ cm/sup 2/, respectively.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ionization of the cobalt K shell by low-energy protons, deuterons and helium-3 ions

The characteristic X-ray spectra of cobalt (Z=27) have been obtained as a function of energy in a range from 0.7 to 2.0 MeV amu-1 for incident protons, deuterons and 3He ions. K X-ray production cross sections sigma x were measured by a comparison with the simultaneously measured Rutherford scattering cross section. Uncertainties inherent in this method are minimized to approximately 8%. For protons, the measurement was extended at energies up to 3.0 MeV. The results are compared with the predictions of the perturbed-stationary-state with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR) ionization theory and first Born approximation. The values obtained for the ratios K beta /K alpha and sigma x are tabulated. Comparisons of the authors' data with those from other measurements in earlier works are discussed.

Extensions of spacetimes with Killing horizons

The authors consider spacetimes possessing a one-parameter group of isometries with a Killing horizon, N, i.e. an isometry-invariant null hypersurface to which the Killing field is normal. They assume further that the Killing orbits on N are diffeomorphic to R, and that N admits a smooth cross section Sigma , such that each orbit intersects Sigma precisely once. If the surface gravity, kappa , on a generator gamma of N is non-vanishing, then gamma will be null geodesically incomplete. It is proved that any such incomplete generator gamma must terminate in a parallelly propagated curvature singularity whenever the surface gravity has a non-vanishing gradient on gamma . If, however, kappa is constant throughout the horizon, the authors prove that one can extend a neighbourhood of N so that N is a proper subset of a regular bifurcate Killing horizon in the extended spacetime. Since constancy of kappa on N is implied by Einstein's equations and the dominant energy condition, these results indicate that the only physically relevant Killing horizons are bifurcate Killing horizons and horizons with kappa =0. They also prove that for a static or stationary axisymmetric spacetime with a bifurcate Killing horizon, the natural static or stationary axisymmetric hypersurfaces smoothly intersect the bifurcation surface.

Energy pooling reactions between two Na* (3p) atoms specified according to magnetic substate combinations

In a beam experiment the authors have studied the energy pooling reaction between two laser excited Na* (3p) atoms, where in the final state one of the sodium atoms is in the 5s or 4d state and the other in the ground state. They have determined all eight independent partial cross sections and coherence terms, specified according to the various magnetic substate combinations of the colliding Na* (3p) atoms, as a function of the collision velocity. This is the maximum information that can be extracted from the experiment given the symmetry of the detection scheme. By applying a weak magnetic field, to orient the Na* (3p) atoms, the authors were able to distinguish between the two partial cross sections sigma 11 and sigma 1-1, where the subscripts 11 and 1-1 refer to the initial magnetic substate combinations of the two collision partners. By using an electro-optical modulator the authors have furthermore improved the signal-to-noise ratio to an extent where it was possible to measure all coherence terms.

The RBE-LET Relationship for Rodent Intestinal Crypt Cell Survival, Testes Weight Loss, and Multicellular Spheroid Cell Survival after Heavy-Ion Irradiation

This report presents data for survival of mouse intestinal crypt cells, mouse testes weight loss as an indicator of survival of spermatogonial stem cells, and survival of rat 9L spheroid cells after irradiation in the plateau region of unmodified particle beams ranging in mass from 4He to 139La. The LET values range from 1.6 to 953 keV/microns. These studies examine the RBE-LET relationship for two normal tissues and for an in vitro tissue model, multicellular spheroids. When the RBE values are plotted as a function of LET, the resulting curve is characterized by a region in which RBE increases with LET, a peak RBE at an LET value of 100 keV/microns, and a region of decreasing RBE at LETs greater than 100 keV/microns. Inactivation cross sections (sigma) for these three biological systems have been calculated from the exponential terminal slope of the dose-response relationship for each ion. For this determination the dose is expressed as particle fluence and the parameter sigma indicates effect per particle. A plot of sigma versus LET shows that the curve for testes weight loss is shifted to the left, indicating greater radiosensitivity at lower LETs than for crypt cell and spheroid cell survival. The curves for cross section versus LET for all three model systems show similar characteristics with a relatively linear portion below 100 keV/microns and a region of lessened slope in the LET range above 100 keV/microns for testes and spheroids. The data indicate that the effectiveness per particle increases as a function of LET and, to a limited extent, Z, at LET values greater than 100 keV/microns. Previously published results for spread Bragg peaks are also summarized, and they suggest that RBE is dependent on both the LET and the Z of the particle.

Ocean imaging with two-antenna radars

Synthetic-aperture radar (SAR) measurements are affected both by the radar backscattering cross section ( sigma /sup 0/) and by the radial velocity (V/sub r/) of the surface. An analysis of the capabilities of two-antenna SARs using a method for forming radial-velocity images to provide a general framework for evaluating the performance of different possible measurements is presented. Several key results are derived. One can measure sigma /sup 0/ and V/sub r/ separately, given a properly designed SAR. The error in V/sub r/ depends on the thermal noise in the receivers, the spacing of the two antennas, the coherence time of the surface ( tau /sub c/) and the spatial resolution of the measurement. There is an optimal separation of the antennas proportional to v tau /sub c/, where v is the speed of the aircraft. In cases where radial velocities are unimportant, two-antenna SARs can be used to image azimuth-traveling waves with wavenumbers larger than the usual azimuth cutoff. The authors show how phase errors affect the accuracy of the measurement of V/sub r/. In this development, they also provide a simple explanation for velocity bunching, a subject that has caused much controversy. >

L-shell X-ray opacity of many-electron atoms

The study presents a formulation for obtaining the L-shell photoelectric cross section (sigma sub L) for many-electron atomic systems. A scaling law is suggested for the evaluation of the contribution to sigma sub L from 2s and 2p ejection from a general system in any ionization state. The formula is expressed in terms of the effective nuclear charge (Z sub 2s and Z sub 2p) seen by the corresponding bound electrons, with Z sub 2s and Z sub 2p, obtained from improved screening constants derived in a recent investigation. A comparison is made of Hartree-Fock and hydrogenic expressions for sigma sub L with experimental results for atomic neon at X-ray photon energies, providing the basis for the scaling law and an approximate energy dependence for the cross sections. The special case of sigma sub L for atomic Fe is considered in detail. This L edge has structure because of the different threshold for 2s and 2p photoejection; the latter has fine structure.

3He on-induced K X-ray emission cross section of elements between Mn and Zn

The K X-ray production cross sections sigma x have been measured for Mn, Fe, Ni, Cu and Zn induced by 3He ions in the energy range 1.5 to 6.0 MeV. The measurements are compared with the predictions of both the first Born and ECPSSR ionization theories using single-hole fluorescence yields. The ECPSSR theory is shown to be in much closer agreement with the data. The obtained values of the Kbeta /Kalpha intensity ratio and sigma x are tabulated. Comparisons of the authors' data with other published results are discussed.

Multiple ionization of magnesium by electron impact

A pulsed crossed beam technique incorporating time-of-flight spectroscopy of the collision products, which was developed previously in this laboratory, has been successfully adapted to allow studies of multiple ionization of metallic atoms. Cross sections sigma n for the formation of n=1-4 times ionized magnesium have been determined over the wide energy range 8-5300 eV by normalization to cross sections for single ionization of Mg recently measured by Freund et al using a fast crossed beam technique. Cross sections sigma 2 are found to closely approach the authors' values of sigma 1 at our highest impact energies. Comparison is made with previous experiments based on different methods and, in the case of single ionization, results are compared with theoretical predictions.

A crossed beam study of the multiple ionization of argon by electron impact

A pulsed crossed beam technique incorporating time-of-flight spectroscopy previously developed in this laboratory has now been applied to studies of the multiple ionization of argon by electron impact. Cross sections sigma n for the production of n=1-5 times ionized argon have been determined for impact energies in the range from near threshold to 5300 eV. The measurements cover a wider energy range and extend to higher charge states n than most previous experiments. The results also provide valuable checks on the results of previous measurements which exhibit unexplained discrepancies. In the case of single ionization, the authors' values of sigma 1 are compared with available threshold predictions.

Hoechst 33258 photosensitization of 5-iododeoxyuridine-substituted DNA to 365 nm light: dependence of dehalogenation on the dye-to-nucleotide ratio.

DNA containing 5-iododeoxyuridine substituted at levels ranging from 1 to 24% was irradiated at 365 nm in the presence of various amounts of the Hoechst dye 33258. The subsequent loss of iodine was measured by following the loss of radioactive iodine (125I) from filter immobilized DNA subjected to repeated washings. A plot of the photochemical cross section (sigma B) for this process as a function of r, the molar ratio of dye added per nucleotide present, shows a maximum at r = 0.04 to 0.06, followed by a decrease with increasing dye-to-phosphate ratios to give a minimum at r = 0.5. Increasing the amounts of dye further results in sigma B reaching a second maximum at r greater than or equal to 1, after which it levels off. An attempt was made to interpret these results in terms of the binding models currently employed for analyzing binding of Hoechst dye to DNA. Chain breaks as measured in alkali were also measured and found to be enhanced by the dye; the ratio of breaks per iodine loss was 0.9.

Quantitative ray methods for scattering of sound by spherical shells

Abstract : The application of ray methods to the scattering of high-frequency plane waves from evacuated elastic spherical shells is investigated. The investigation of ray methods for spherical shells is a precursor to the application of such methods to shells having more complicated shapes. Ray models are developed to synthesize the form function f(Theta, ka) where k is the wavenumber of the incident wave and Theta is the scattering angle. The forward scattering amplitude, f(Theta = 0), is related to the extinction cross section, sigma sub e by the optical theorem. If the absorption by the scatterer is negligible, the sigma sub e is equal to the total scattering cross section sigma sub t. A ray synthesis partition f(Theta = 0) into a component for ordinary forward diffraction about the shell, f sub FD, and contributions from surface guided elastic waves. For high-frequency scattering, the relevant surface guided elastic waves are leaky Lamb waves. A similar ray synthesis of the backscattering amplitude f(Theta = pi) contains a specular reflection component, f sub sp(Theta = pi), and leaky Lamb wave contributions. A generalization of the geometrical theory of diffraction is employed to synthesize f sub l(Theta = 0, ka) and f sub l (Theta = pi, ka) for the lth leaky Lamb wave contribution. The syntheses for forward and backwards scattering correctly describe the leaky Lamb wave contributions and are expressible in a Fabry-Perot resonator form. While the ray description of backscattering ordinarily accurately reproduces exact computations and experiments with tone burst, certain anomalies are discussed.