Cross Section Sigma Research Articles

Effects of excitation and ionization in meteor trains

In a previous paper we examined the problem of the determination of the ionization coefficient beta, and we now extend our analysis to the luminous efficiency tau, relating this to an excitation coefficient zeta, essentially the number of times an atom is excited during the thermalization process. We discuss the scattering processes involved and develop integral equations expressing zeta in terms of the relevant scattering cross-sections. The data available in applying these equations are (i) direct measurements of beta and zeta for iron particles moving through an ionization chamber, with velocities between 18 and 46 km s(-1); (ii) scattering cross-sections sigma (i) and sigma (e), for ionization and excitation on collision between Fe atoms and air molecules, measured under controlled single-atom collision in the laboratory for relative velocities between 60 and 120 km s(-1); (iii) theoretical calculations of the diffusion cross-section sigma (d) of iron atoms in air over the relevant velocity range. Employing an approximation in which the scattering is isotropic in the centre-of-mass frame (the random scattering approximation, RSA), we take the laboratory simulation results fur the ionization and luminosity and invert the equations for beta and zeta to obtain values of beta (0) and zeta (0), the ionization and excitation probabilities at the first collision. We find that for velocities above 42 km s(-1) the value of zeta (0) becomes greater than unity. As zeta (0) is a probability, this must be incorrect, leading to the conclusion that the contribution from subsequent collisions is underestimated. This is possible if small-angle scattering is underestimated in the RSA. To investigate this we have taken the extreme case in which the trajectories of atoms are supposed to undergo no deviation at all on ionization or excitation. This now enables us to derive the ratios sigma (i)/sigma (d) and sigma (c)/sigma (d) and hence, using the theoretical values of the diffusion cross-section, values of sigma (i) and sigma (e). The values so obtained appear unacceptably high and inconsistent with the experimental cross-sections. A possible reason for the failure of the equations is the assumption that ionization permanently removes atoms available for excitation, which will not be true if charge transfer takes place. No cross-sections are available for this process, but we can evaluate zeta in the limiting case when the loss of velocity during transfer may be neglected. To compare our results for the ionization and excitation cross-sections with the experimental ones, we have extrapolated them to 60 km s(-1) and find quite satisfactory agreement. We have accordingly extended the results for the excitation and ionization coefficients to velocities above 60 km s(-1) utilizing the experimental cross-sections for this region. We consider the values of zeta and the corresponding luminous efficiencies to be the best available in the present circumstances.

Measurement of the charm production cross section in γγ collisions at LEP

Open charm production in gamma-gamma collisions is studied with data collected at e+e- centre-of-mass energies from 189 GeV to 202 GeV corresponding to a total integrated luminosity of 410 pb-1. The charm cross section sigma(gamma gamma ---> c c~ X) is measured for the first time as a function of the two-photon centre-of-mass energy in the interval from 5 GeV to 70 GeV and is compared to NLO QCD calculations.

On the relationship between the non-photochemical quenching of the chlorophyll fluorescence and the Photosystem II light harvesting efficiency. A repetitive flash fluorescence induction study.

Plants respond to excess light by a photoprotective reduction of the light harvesting efficiency. The notion that the non-photochemical quenching of chlorophyll fluorescence can be reliably used as an indicator of the photoprotection is put to a test here. The technique of the repetitive flash fluorescence induction is employed to measure in parallel the non-photochemical quenching of the maximum fluorescence and the functional cross-section (sigma(PS II)) which is a product of the photosystem II optical cross-section a(PS II) and of its photochemical yield Phi(PS II) (sigma (PS II) = a(PS II) Phi(PS II)). The quenching is measured for both, the maximum fluorescence found in a single-turnover flash (F(M) (ST)) and in a multiple turnover light pulse (F(M) (MT)). The experiment with the diatom Phaeodactylum tricornutum confirmed that, in line with the prevalent model, the PS II functional cross-section sigma (PS II) is reduced in high light and restored in the dark with kinetics and amplitude that are closely matching the changes of the F(M) (ST) and F(M) (MT) quenching. In contrast, a poor correlation between the light-induced changes in the PS II functional cross-section sigma (PS II) and the quenching of the multiple-turnover F(M) (MT) fluorescence was found in the green alga Scenedesmus quadricauda. The non-photochemical quenching in Scenedesmus quadricauda was further investigated using series of single-turnover flashes given with different frequencies. Several mechanisms that modulate the fluorescence emission in parallel to the Q(A) redox state and to the membrane energization were resolved and classified in relation to the light harvesting capacity of Photosystem II.

The Structure and Evolution of Weakly Self-interacting Cold Dark Matter Halos.

The evolution of halos consisting of weakly self-interacting dark matter particles is investigated using a new numerical Monte Carlo N-body method. The halos initially contain kinematically cold, dense r-1 power-law cores. For interaction cross sections sigma*=sigmawsi&solm0;mp>/=10-100 cm2 g-1, weak self-interaction leads to the formation of isothermal, constant-density cores within a Hubble time as a result of heat transfer into the cold inner regions. This core structure is in good agreement with the observations of dark matter rotation curves in dwarf galaxies. The isothermal core radii and core densities are a function of the halo scale radii and scale masses which depend on the cosmological model. Adopting the currently popular LambdaCDM model, the predicted core radii and core densities are in good agreement with the observations. For large interaction cross sections, massive dark halos with scale radii rs>/=1.4x104 cm2 g-1 (sigma*)-1 kpc could experience core collapse during their lifetime, leading to cores with singular isothermal density profiles.

Optical Amplification on the 3P0 3F2 Transition in Praseodymium-doped Fluorozirconate Fiber

Small-signal amplification on the 3P0 3F2 transition in a praseodymium-doped fluorozirconate optical fiber at the 632.8-nm He-Ne wavelength, has realized a slope efficiency of 0.15 dB mW and up to 14 dB gain. Spectroscopic and fiber parame ters, including the pump absorption cross section (sigma a 0.14 0.02 10 24 m2), p the signal emission cross section (sigma e 2.1 0.2 10 24 m2), and the 3P state ( ) s 0 lifetime tau 3 49.8 0.3 mus , determined experimentally, independently of the gain, were used to model the performance of the amplifier. The predictions of the rate equation model were in good agreement with the measured gain.

Possible probe of the QCD odderon singularity through the quasidiffractiveηcproduction in γγ collisions

The reactions $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}{\ensuremath{\eta}}_{c}{\ensuremath{\eta}}_{c}$ and $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}{\ensuremath{\eta}}_{c}+X$ are discussed within the three-gluon-exchange model. We give predictions for the differential cross sections and discuss the feasibility of measuring these processes at CERN LEP2 and Cornell TESLA. The total cross sections were estimated to be approximately equal to 40 fb and 120 fb for $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}{\ensuremath{\eta}}_{c}{\ensuremath{\eta}}_{c}$ and $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}{\ensuremath{\eta}}_{c}+X,$ respectively, assuming exchange of elementary gluons that corresponds to the odderon with intercept equal to unity. These values can be enhanced by a factor equal to 2.4 and 2.9 for LEP2 and TESLA energies if the odderon intercept is equal to 1.07. The estimate of cross sections $\ensuremath{\sigma}{(e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}{\ensuremath{\eta}}_{c}{\ensuremath{\eta}}_{c})$ and $\ensuremath{\sigma}{(e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}{\ensuremath{\eta}}_{c}X)$ for antitagged ${e}^{+}$ and ${e}^{\ensuremath{-}}$ is also given.

Chiral beta-particles interact differentially with enantiomers--theory to solve a long-existing debate.

The crucial organic molecules associated with life are chiral. Beta irradiation on the causal origin for the great preference of biomolecules in life over their corresponding enantiomers has been extensively studied. The left helical beta electrons, with spin and momentum antiparallel, should propagate with different velocities in the two enantiomers. Much effort has been done to investigate possible mechanism of inducing asymmetry, using longitudinally polarized beta rays to decompose asymmetrically racemic mixtures of biomolecules, but the results were inconclusive and contradictory. In this paper we present our viewpoint that the direct inelastic scattering between polarized electrons and chiral molecules is the dominant one in producing the asymmetry of cross sections for both beta+ and beta- irradiation. The inelastic cross section sigma (0 --> n) is dependent on the dipole strength Dn, rotatory strength Rn of the molecule. According to the theoretical study, the asymmetry of cross section F is approximately 10(-6) and it depends on the sign of Rn+(L type). When Rn+ > 0, sigma D > sigma L, it favors L-amino acid surviving. When Rn+ < 0, sigma D < sigma L, it favours D-amino acid surviving. Our results show that helical electrons do distinguish between molecules of opposite chirality, and the disputation and suspicion about the controversial experimental results between Garay and Darge & Thiemann can be explained.

The Use of Pulsed-Neutron Capture Logs for Reservoir Management in the Midway-Sunset Field

Summary Systematic time-lapse pulsed-neutron capture (PNC) logging was conducted during a 2-year steam-foam mechanistic field trial in the steamflooded Monarch reservoir, which is composed of interbedded sandstones, conglomerates, siltstones, and diatomaceous mudstones. Observation wells were drilled with one continuously cored through the reservoir interval and two emptied to facilitate better temperature logging. Uncalibrated PNC log responses in the air-filled wells were normalized to water-filled conditions and were used to generate a series of steam/gas saturation profiles that indicate clearly the dynamics of the fluid and foam in the reservoir during the experiment. Correlations between neutron capture cross sections (sigma), mineralogy, and rock chemistry from core samples were examined for additional use of the PNC logs for steamflood reservoir characterization. Sigma was found to increase with increasing clay/mica content and diatomite content. This finding may result in improved stratigraphic correlations of the more laterally continuous diatomaceous mudstones based on PNC logs. The PNC log data from the field trial, when used in conjunction with core, temperature, and pressure data, were critical in developing a better understanding of foam generation and propagation in the reservoir. Furthermore, they minimized the time and cost required to complete the field trial successfully. They also led to opportunities for vertical expansion of steamflooding in the area.

QCD description of diffractiveρmeson electroproduction

We critically review the QCD predictions for the cross sections sigma_L and sigma_T for diffractive rho meson electroproduction in longitudinally and transversely polarised states in the HERA energy region. We show that both perturbative and non-perturbative approaches which involve convolution with the rho meson wave function predict values of sigma_T which fall-off too quickly with increasing Q^2, in comparison with the data. We present a perturbative QCD model based on the open production of light qqbar pairs and parton-hadron duality, which describes all features of the data for rho electroproduction at high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of sigma_L/sigma_T. We find that precise measurements of the latter can give valuable information on the Q^2 behaviour of the gluon distribution at small x.

Optical Studies on Rare Earth Lasting Materials

Neodymium-doped borate and phosphate glass specimens have been prepared by melt-quenching technique. Their absorption spectra have been recorded in the region 400 -900 nm. From observed absorption spectra,Judd Ofelt (J0), Omega/sub t/, parameters (t = 2, 4 and 6) have been computed. They have been used to calculate the laser parameters, viz., spontaneous emission probability (A), fluorescence branching ratio (Beeta) and stimulated emission cross-section (Sigma). They exhibit the suitability of the present glass specimen for laser action. The effects of composition and concentration of the dopant on these parameters have been discussed. A comparative study of stimulated emission cross-section of different types of doped glasses has also been made.

Disappearance of flow and the in-medium nucleon-nucleon cross section for 64Zn+27Al collisions at intermediate energies

Light charged particles emitted from the reactions Zn-64+Al-27 at beam energies 49, 57, 69 and 79 MeV/u have been measured with a 4 pi multidetector array. The collective transverse momentum in the reaction plane (in-plane flow) is observed to vary strongly with the impact parameter and the emitted fragments. The flow parameter decreases monotonically from 49 to 79 MeV/u and falls to zero at a beam energy about 79 MeV/u in central collisions. This energy of vanishing flow increases with impact parameter and may be the same for different fragments. The disappearance of flow is also studied with the Boltzmann-Uehling-Uhlenbeck equation and it is demonstrated that the flow parameter and E(VF) are very sensitive to the in-medium cross section sigma(NN) and weakly sensitive to the nuclear compressibility in central collisions. Comparisons with experimental data indicate that sigma(NN) is around 35 to 45 mb in central collisions.

Single- and double-electron detachment in collisions of two negative hydrogen ions

Employing the crossed-beams technique in conjunction with a beam-pulsing method, we have measured absolute cross sections sigma tot for the process H-+H- to H0+... for CM energies between 2.5 and 100 keV. Combining the present results with our previously measured cross sections for double-electron detachment ( sigma 00) and triple-electron detachment ( sigma 0+). We obtain cross sections sigma 0- for the single-electron detachment process from the relation sigma 0-= sigma tot- sigma 00- sigma 0+ The experimental data of singleand double-electron detachment are well described by theoretical results based on the non-stationary tunnelling approach. Both cross sections are calculated for CM energies between 0.15 and 300 keV. A new two-electron simultaneous transition mechanism is introduced. The importance of this process for the theoretical assessment of the experimental results is discussed.

Broadening of noise spectra in semiconductors and the 1/f1.5 problem

It has been shown that the low-frequency 1/f1.5 noise in GaAs and SiC, and occasionally in Si, exists in the temperature region where a generation-recombination (GR) noise predominates that is due to a local level. The 1/f1.5 noise can be accounted for by energy level broadening, which inevitably occurs in any real crystal as a result of structural or doping inhomogeneities and defects. GR noise simulations were performed for GaAs and SiC, taking into account the effect of broadening. It has been shown that, to explain the experimental results, consideration must be given to the exponential energy dependence of the capture cross section sigma n= sigma 0 exp(-E1/kT) and the dependence of the energy E1 on the level position E0. The simulated and experimental results are in good agreement.

Induction of HPRT- mutants in Chinese hamster V79 cells after heavy ion exposure.

The induction of resistance to 6-thioguanine by heavy ion exposure was investigated with various accelerated ions (oxygen-uranium) up to linear energy transfer (LET) values of about 15,000 keV/microns. Survival curves are exponential with fluence; mutation induction shows a linear dependence. Cross-sections (sigma i: inactivation, sigma m: mutation) were derived from the respective slopes. Generally, sigma i rises over the whole LET range, but separates into different declining curves for single ions with LET values above 200 keV/microns. Similar behaviour is seen for sigma m. The new SIS facility at GSI, Darmstadt, makes it possible to study the effects of ions with the same LET but very different energies and track structures. Experiments using nickel and oxygen ions (up to 400 MeV/u) showed that inactivation cross-sections do not depend very much on track structure, i.e. similar values are found with different ions at the same LET. This is not the case for mutation induction, where very energetic ions display considerably smaller induction cross-sections, compared with low-energy ions of identical LET. Preliminary analyses using the polymerase chain reaction (PCR) demonstrate that even heavy ions cause "small alterations" (small deletions or base changes). The proportion of the total deletions seems to increase with LET.

Single and double ionization of atomic oxygen by electron impact

A pulsed crossed beam technique incorporating time-of-flight spectroscopy (previously developed in this laboratory) has been used to study the single and double ionization of ground-state oxygen atoms produced by an iridium tube furnace source. Relative cross sections sigma 1 and sigma 2 for single and double ionization have been determined in the respective energy ranges 14.1-2000 eV and 90-2000 eV which are wider than any previous measurements. These values have been normalized by reference to the absolute values of sigma 1 previously measured by Brook et al at energies (50-997 eV) where the possible influence of an unknown admixture of long-lived excited atoms in their experiment should be small. However at energies below about 25 eV our values of al become much smaller than the values of Brook at al. Above 40 eV, the present ratios sigma 2/ sigma 1 are in satisfactory agreement with other measurements which extend to 400 eV. Our values of al have been compared with selected theoretical predictions. Some evidence of structure in the energy dependence of sigma 1 in the region of the cross section maximum is consistent with predicted contributions from inner-shell 2s ionization and from autoionization in addition to outer 2p electron removal.

Selectivity with respect to the orbital polarization in the energy-pooling process with sodium atoms

Cross sections sigma (m1m2)nl for the energy pooling processes in the collision between two excited sodium atoms are calculated for the case when the partners are prepared in the 3p-excited states with the definite orbital momentum projections m1 and m2. As a result of the process one of the colliding atoms comes to the ground state and the other partner is excited to nl state with nl=5s, 4d, 4f. The collision velocity is equal to or higher than thermal. The Landau-Zener-locking model (LZ-locking) is developed: the sudden change of the type of the coupling between the electron orbital momentum and the internuclear axis (the locking approximation) is combined with the multicrossing Landau-Zener model. The validity of the LZ-locking model is analysed. The results show strong cross section dependence on the initial quantum numbers m1 and m2, especially in the 5s channel.

An assessment of veering wind effects on scatterometry from the sea surface

To characterize scatterometer returns from the sea surface near meteorological fronts, we investigated microwave scattering from seas in which long waves are at oblique angles to short waves. We simulate the effects of veering winds on C- and K-u-band scatterometers by using models in which the short waves align with the wind friction velocity u*, but the long waves are at oblique angles to the u* direction. The analysis reveals two main effects due to the rotation of the long wave slope probability density distribution. Azimuthally averaged normalized radar cross-section a(0) decreases as the oblique angle increases. Additionally, two regimes exist. In the small angle regime, azimuthal scans of normalized radar cross-section sigma degrees exhibit features similar to the classic double-maxima pattern for non-veering wind conditions, but the axis of sigma degrees maxima is rotated toward the long-wave axis. In the large angle regime, more than two maxima are apparent in azimuthal scans. Therefore it may be inappropriate to use standard three term Fourier cosine models for some veering wind conditions.

Coherent excitation of H(n=2) induced in H+-Ne and H+-Ar collisions

The collision-induced excitation of atomic hydrogen to the H(n=2) states in H+-Ne and H+-Ar collisions was measured in an energy range of 1 to 25 keV. From a polarization analysis of the emitted Lyman- alpha radiation as a function of an external electric field the partial cross sections sigma lm, for excitation to the H(2s) and the H(2pm) magnetic substates (m=0,+or-1) and the real part of the s0-p0 coherence were extracted. For both collision systems an energy dependent forward-backward asymmetry corresponding to a shift of the centre-of-charge relative to the centre-of-mass (electric dipole moment) was observed. The collision-induced electric dipole moment (Dz) is generally small or negative at incident energies below 10 keV and rises steeply towards higher energies. The present results are in reasonable agreement with close-coupling calculations employing a molecular representation.

Neutron cross sections for 56Fe and 238U, from 0.2 to 22 MeV energy

Neutron total and differential elastic cross sections for 56Fe and 238U nuclei were calculated from different global spherical optical potential (SOP) sets. The total cross sections sigma t for these nuclei for different neutron energies were calculated. The results were compared with those of the experimental data obtained from the EXFOR data file to select the best-fit potential parameters. In addition, the angular distributions of elastically scattered neutrons at different neutron energies were also studied. It was observed that the best overall fit to the experimental values of the total cross sections and angular distributions are obtained from SOP parameter set 2 of Wilmore and Hodgson for 56Fe and set 12 of Bersillon and Cindro for 238U.

On the range of validity of Wannier's threshold law for inner-shell ionization by electron impact: Ne 1s

We have measured the relative cross section sigma rel(1s) of Ne is ionization by electron impact in the range of excess energy E1=5-430 eV via the intensity of KL2.3L2.3 1D2 Auger electrons. The Auger line intensities were obtained by using calculated PCI disturbed lineshapes for the decomposition procedure. For the range of E1=10-50 eV the experimental cross section could be fitted by a power law sigma rel(1s) approximately E11mu mu with mu =1.08(6). This value is smaller than the Wannier exponent mu =1.127. A possible reason for such a deviation is discussed.