Comparison between bremsstrahlung cross section measurements at the short wavelenght limit and theoretical results

Recent precise measurements of the electron bremsstrahlung cross section d²σ/dk dΩ_k at the short wavelength limit for an incident electron energy of 1.84 MeV important discrepancies between experimental results and earlier theoretical calculations. A new exact theoretical result, for a point Coulomb potential is presented: descrepancies with experimental results remain.     

Coupled electron-phonon system in two dimensions and its implications for inversion layers

The first order electron-phonon vertex in two dimensions is calculated in the regime (q_o/qvf)?1 and (q_o/qvf)?1 where, q_o and q are the phonon frequency and wave vector, and v_f is the electron Fermi velocity. In the former case, the vertex correction is found to be λ. (Ωph/E_f)^{$\text{1}\text{2}$} and in the latter case it is λ, where λ is the dimensionless electron-phonon coupling, Ωph a characteristic phonon energy and E_f the Fermi energy. It is shown that for the relevant range of carrier density and phonon branches in the inversion layer, the term (Ωph/E_f)^{$\text{1}\text{2}$} is of order unity, possibly implying significant higher order effects.     

The cross section of14N(n,p)14C at stellar energies and its role as a neutron poison forS-process nucleosynthesis

The absolute average cross section 〈σ〉 of the¹⁴N(n, p)¹⁴C reaction has been measured using neutron spectra that closely resemble Maxwell-Boltzmann distributions with thermal energies of kT=25.0 and 52.4 keV: 〈σ〉=0.81±0.05 and 0.52±0.06 mb, respectively. The resulting reaction rates are nearly the same at T₉=0.29 and 0.61, and their average, N_A〈συ〉=(1.3±0.1)×105 cm³ s^?1 mol^?1, is about a factor of three smaller than the previously adopted values obtained by extrapolation between thermal and higher-energy data. Thus, the¹⁴N(n, p)¹⁴C reaction plays a correspondingly smaller role as a neutron poison fors-process nucleosynthesis.     

The process γγ → X by parton model and prediction of equivalent-photon method for ee → ee + X

A parton model for the virtual photon process γ → X is examined. It is assumed that a scaling property applies and the total cross section is σ_γγ→X = s^?1φ(ξ), where the function ? is dependent on the scaling variable ξ defined by ξ = s/(q²₁q²₂). The cross section for the process ee → + X is calculated using the equivalent-photon technique.     

Yb3+,Er3+:YAG透明陶瓷的制备和1.5 μm波段光谱性能研究

采用固相法和真空烧结技术制备了5at%Yb³⁺,2at％Er³⁺:YAG透明陶瓷.在1760 ℃真空烧结30 h后, 陶瓷样品具有较高透过率.SEM观察表明制备的透明陶瓷在晶粒和晶界处无气孔、第二相的存在.样品的吸收光谱和荧光光谱的测试结果表明: Yb³⁺在940nm波长处有具有较强的吸收系数.样品在1030nm波长的荧光寿命仅为0.274 ms,以及在1.5μm波段的荧光衰减寿命曲线中,初始的荧光强度呈上升趋势,这些表明了Yb,Er:YA 关键词： Er Yb:YAG透明陶瓷 1.5 μm荧光光谱 Judd-Ofelt理论     

Axial contraction in toroidal pinches with non-circular plasma cross-sections

The axial contraction of linear pinches with racetrack-shaped and elliptic cross sections is studied byδW-analysis near the equilibrium shape. A surface current profile and a fixed plasma cross-section area are assumed. It is shown for the corresponding special perturbation thatδW is given by the variation of the surface energy(δW=δW _s =1/8B _p ² ξ _b ² ), and thatδW is nearly independent of the shape of the plasma (racetrack or ellipse). The axial oscillation frequency is found to beΩ _ax=1/2√CB _p /√ρ_l with a constantC<1. The model correctly predicts theB _p and ρ_l-dependences of the experimental belt-pinch results and yields theoreticalΩ _ax-values a factor of 2 above the experimental ones.     

Excitation of Jπ=2+ resonances in 24Mg by the 23Na(p, 12C)12C reaction

Angular distributions of the ²³Na(p, ¹²C_g.s.)¹²C_g.s. reaction were measured at energies on top of resonance-like structures or close to them in the range E_p=8−18 MeV and were analysed using parametrized S-matrix elements. Partial cross sections proceeding through compound nuclear states of ²⁴Mg with various spin J were derived. In was found that the resulting cross sections for J=2 exhibit correlated structures with the “fissioning resonances” recently observed by the ¹²C(¹²C, λ₀) and ²⁴Mg(α, α') ¹²C_g.s + ¹²C_g.s. reactions.     

Electrofission of238U and232Th

Absolute electrofission cross sections for²³⁸U and²³²Th in the energy regionE _e =7 ?65 MeV and fission fragment angular distributions forE _e =7–30 MeV have been measured. The angular distributions show strong anisotropies for low energies. The relative dipole and quadrupole contributions as a function of excitation energy are discussed in terms of the low lying fission transition states above the fission barriers. The cross sections show significant deviations from the results of some earlier measurements, in particular in the energy region above the giant dipole resonance. From the difficulties of absolute electrofission cross section measurements and the ambiguities in their interpretation it is concluded that by this time the quantitative analysis of electrofission cross sections with respect to the contributions of the giant quadrupole resonances to the fission decay channel should be regarded as rather tentative.     

Optical spectroscopy and laser parameters of Zn2+/Er3+/Yb3+-tridoped LiNbO3 crystal

The Zn/Er/Yb:LiNbO₃ and Er/Yb:LiNbO₃ crystals were grown by the Czochralski technique. The laser characteristics of 1.54 μm emission were predicted based on the Judd–Ofelt theory, and the intensity parameters Ω_t (Ω₂=7.23×10^?20 cm², Ω₄=3.15×10^?20 cm² and Ω₆=1.43×10^?20 cm²) were obtained. The stimulated emission cross sections (σ_em) at 1.54 μm emission in Zn/Er/Yb:LiNbO₃ were calculated based on the McCumber theory and the Füchtbauer–Ladenburg theory. The gain cross section spectrum of Zn/Er/Yb:LiNbO₃ crystal was also investigated. Under 980 nm excitation, a lenghthening lifetime of 1.54 μm emission and an enhancement of green upconversion emission were observed for Zn/Er/Yb:LiNbO₃ crystal. The studies on the power pump dependence and the upconversion mechanism suggested that both green and red upconversion emissions were populated via the three-photon process, and Zn²⁺ ion tridoping increases the probability of cross relaxation process between the two neighboring Er³⁺ ions.     

Relativmessung integrierter Wirkungsquerschnitte für den Kernphotoeffekt: Die Reaktion O16(γ, n)O15

Li₂CO₃ and Na₂C₄H₈O₈ have been irradiated by 34 MeV bremsstrahlung. The yield of the induced O¹⁵ activity has been measured relative to the yield of the C¹²(γ, n)C¹¹ resp. Cu⁶³(γ, n)Cu⁶² reaction by detecting the annihilation quanta. Additional yield measurements with the bremsstrahlung beam attenuated by 18 mm of lead were made to determine the ratio of the integrated cross sections for the reactions O¹⁶(γ, n)O¹⁵ and C¹²(γ, n)C¹¹ resp. Cu⁶³(γ, n)Cu⁶². Using the cross section ofBarber et al. for C¹²(γ, n)C¹¹, the cross section integrated up to 33 MeV for O¹⁶(γ, n)O¹⁵ was obtained to 61±7 MeVmb.     

Muo-production of neutrino pairs and the number of generations

The cross section for production of neutrino pairs by high energy muons in the nuclear Coulomb field is calculated analytically. For right (left)-handedμ ^?(μ ⁺) helicity, the process is only mediated by neutral currents, which opens the possibility to look for the number of generations. Assuming three generations, the calculated cross section turns out to be 1.1×10^?40 cm² for⁵⁶Fe and 6.6×10^?40cm² for²⁰⁸Pb at an incident muon energy of 300 GeV. Some comments about the equivalent photon approximation are made.     

Lattice dynamics of an isotopic defect

Using a spectrum ～Ω ²√Ω _m ² ?Ω ² for the frequenciesΩ of the ideal crystal simple analytic expressions are derived for the response function, phonon scattering cross section and related quantities. The interpretation of resonances and localized vibrations as damped and undamped harmonic oscillators is discussed in detail.     

Sum rules fore + e − annihilation in general electroweak mixing models and extended gauge theories

We derive a lower bound for the integrated cross section for production of weak neutral bosons ine ⁺ e ^? annihilation within general electroweak theories containing an arbitrary number of weak bosons. The low energy parameterC of such theories is expressed in terms of integrals over the weighted cross section fore ⁺ e ^? annihilation at high energies.     

Positron-hydrogen-like-ion scattering

Scattering of positrons by hydrogen-like-ions (He⁺, Li²⁺, Be³⁺ and B⁴⁺) has been studied using two variants of the polarized orbital method. The positronium formation channel has not been included in the calculations,s-wave phase shifts obtained using the two variants differ appreciably from each other. Moreover, the polarized orbitals-wave phase shifts fore ⁺-B⁴⁺ scattering differ from the corresponding variational results qualitatively in the low energy region. The differential cross sections are reported.     

Model-independent analysis of the differential cross section for the reaction n+d→π0+t at medium energies

Experimental values of the differential cross section for the reactionn+d→π ⁰+t at large angles (θ?50°) in the neutron energy range 460–560 MeV and the known values of the coupling constants \(G_{nn\pi ^O }^2 \) andG _tdn ² are used to predictdσ/dΩ at smaller angles. The method is based on the use of general analytic properties of the differential cross section in the cosgq-plane.     

On the extraction of elastic neutron electromagnetic form factors from highQ 2 D(e,e′) scattering

The problem of extraction of the neutron magnetic form factor from highQ ² D(e, e′) data is considered. We calculate the inelastic contribution to the cross section using two models of the EMC effect and find that the extracted value ofG _Mn(Q ²) is not sensitive to the deuteron structure forQ ²≦6 GeV². At the same time accurate data atQ ²>10 GeV² and x≧ l would enable to establish presence of the EMC type effect for the deuteron.     

Analytical solution of the double-logarithmic integral equation with QCD running coupling

The analytical solution of the double-logarithmic integral equation with QCD running coupling describing small-x behaviour of the non-singlet structure function ? _NS(x,Q ²) has been found for any cut-off parameter μ. Analytical properties of the solution and a position of the right-most singularity in the complex ρ-plane which determines the asymptotics of ? _NS(x,Q ²) at small x have been studied. The asymptotical formula ? _NS(x,Q ²) = C ₁ x ^-λ1{ln^κ1(Q ²/Λ²) —ln^κ1 (μ ²/Λ²) + κ ₁ ln^κ1-1(Q ²/Λ²)[ψ(1) - ψ(λ₁)]} valid if x ? 1 and ln(Q ²/Λ²) ? 1 has been obtained where C ₁, λ₁ are constants, κ ₁ = g/λ₁, λ₁ < g = 8/(33 - 2gh _f), gh _f is a number of active flavours and ψ(ξ) denotes the digamma function.     

Composition dependent structural and optical properties of Sm-doped sodium borate and sodium fluoroborate glasses

Sodium borate and fluoroborate glasses doped with trivalent samarium (Sm³⁺) were prepared and their detailed spectroscopic analysis was carried out. The FTIR spectra reveal that, the glasses contain BO₃, BO₄, non-bridging oxygen and strong OH⁻ bonds. From the optical absorption spectra, Judd-Ofelt intensity parameters (Ω_λ, λ=2, 4 and 6) have been evaluated and are in turn used to predict radiative properties such as radiative transition probability (A), stimulated emission cross section () and branching ratios (β_R) for the excited levels of Sm³⁺ ions in sodium borate and sodium fluoroborate glasses. The dependence of the spectral characteristics of Sm³⁺ ions due to compositional changes have been examined and reported. The value is found to decrease with the decrease in the sodium content in the glass. The decay from the ⁴G_5/2 level is found to be non-exponential indicating a cross-relaxation among the Sm³⁺ ions.     

Positron-lithium scattering with the inclusion of positronium formation

Detailed studies have been made of elastic scattering and positronium formation in low energy collisions of positrons with lithium atoms for the two partial wavesl=0,1. For this system, as for all alkali atoms, the positronium formation channel is open even at zero positron energy. A two-channel version of the Kohn variational method is used with trial functions containing many variational parameters, and reasonably well converged results are obtained. The s-wave positronium formation cross section is infinite at zero positron energy but it then falls rapidly to become several orders of magnitude smaller than the elastic scattering cross section which has a maximum value of approximately 100 ₀ ² at a positron energy of 0.5 eV. For p-wave scattering the positronium formation cross section rises to a value of approximately 10 ₀ ² at an energy of 0.1 eV, with the elastic scattering cross section rising to a maximum of approximately 60 ₀ ² just below the first excitation threshold at 1.84 eV.     

On pair-absorption in intrinsic vapours

The bound-state pair-absorption bands Cs(6² S _1/2)+Cs(6² S _1/2)+h v→Cs(5² D _5/2,3/2) +Cs(6² P _1/2) and the K — K continuum-state pair-absorptions in the wavelength region 2,350≦λ≦2,850 Å have been investigated experimentally. In the case of the bound-state pair-absorption bands a theoretical approach for the absorption cross section at the band centre is given which is in good agreement with the experimental observation. Differences between our and the theoretical formulas given by the Stanford group are discussed.