Cross-section Σa Research Articles

Growth and spectroscopic characteristics of Cr3+:KSc(WO4)2 Crystal

The Cr3+:KSc(WO4)2 crystal was grown by top seed solution method. The absorption and luminescence spectra were investigated. The absorption cross section σa is 1.16×10−19cm−2 at 469nm for the 4A2→4T1 transition and 4.98×10−20cm−2 at 684nm for the 4A2→4T2 transition, respectively. The emission cross section σe of 4T2→4A2 transition is 4.86×10−20cm2 at 916nm with FWHM of 200nm and fluorescence lifetime is 19.83μs. Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ħω and the Huang-Rhys factor S were calculated. Cr3+ ions in KSc(WO4)2 crystal occupy the weak crystal field sites and have stronger coupling to crystal lattice.

A complementary Doppler Broadening formalism and its impact on nuclear reactor simulation

Abstract The Boltzmann Transport equation is the governing formalism upon which simulations of nuclear reactors are performed, in particular when strong absorption or anisotropic scattering are significant. On the left (loss) hand side of the balance equation one finds the absorption and the scattering cross section Σa(E′), Σs(E′) respectively. Those cross sections are energy and temperature dependent i.e. Doppler Broadened. The scattering cross section appears explicitly again on the right (production) hand side of the equation in its differential form ∫∫0 ∞Σ(E → E′); Ω → Ω′)dE dΩ. However, this term is commonly evaluated at 0 K and it does not account for the existing resonances which are the underlying characteristic for Doppler Broadening. Evidently one gets an inherent inconsistency between the integral and differential scattering cross section within the transport solver codes. In this study this missing Doppler Broadened formalism for the differential scattering cross section is introduced in its stochastic and deterministic form. The impact on core criticality is shown to be up to 600 pcm and the change in the nuclides' inventory significant, in particular the 239PU content can be changed by several percents.

Growth and spectroscopic properties of Yb:BSO single crystal

Yb3+:Bi4Si3O12 crystal up to ϕ25×120mm2 has been grown by the modified vertical Bridgman method. The concentration of Yb3+ in as-grown crystal was measured by the inductively coupled plasma atomic emission spectrometer (ICP-AES) and the segregation coefficient was calculated as 3.5. The effect of different melt composition for BSO crystal growth will be discussed. The spectral properties have been investigated. The absorption cross section σa is 2.92×10−20cm2 at 975nm. The emission cross section σe is 0.468×10−20cm2 at 1033nm wavelength. The fluorescence lifetime is 1.01ms at room temperature.

Growth and optical properties of Yb3+ doped Sr3Y2(BO3)4 crystal

A crystal of Yb3+:Sr3Y2(BO3)4 with dimension of ϕ15 × 25 mm has been grown by the Czochralski method. The spectroscopic characterisations have been investigated at room temperature. The crystal exhibits broad absorption at 977 nm with a full width at half maximum of 8 nm. The absorption cross-section σa is 2·07 × 10−20 cm2 at 977 nm. The emission cross-section σe is 0·70 × 10−20 cm2 at 1054 nm. The fluorescence lifetime is 1·18 ms.

Re-Analysis of Recent Neutron Diffusion and Transmission Measurements in Nuclear Graphite

We revisit the determination by Bowman et al. of unusual neutron transport characteristics for a newly fabricated form of graphite [Nucl. Sci. Eng., 159, 182 (2008); Nucl. Sci. Eng., 161, 68 (2009)]. From MCNP modeling and consideration of data from other experiments, we determine revised values for the neutron transport parameters of this graphite. Our reanalysis gives a coherent scattering cross section σcoh ˜ 4 b at 50 meV, a small-angle neutron scattering cross section σsans ˜ 11 to 13 b at 1 meV, and an effective capture cross section σa = 5.8 ± 0.5 mb. Scaled to a graphite reference density of 1.60 g/cm3, we find a diffusion coefficient D̄ = 0.94 ± 0.03 cm and a diffusion length L = 47.7 ± 3.7 cm. Apart from the somewhat larger values of σa and D̄, these are not untypical parameters for graphite. Based on our investigation, the recent experiments and analysis of Bowman et al. do not give evidence for different transport properties for this newly fabricated graphite.

Growth and spectral properties Yb3+/Na+:CaMoO4 crystal

The Yb3+/Na+:CaMoO4 crystal with dimensions of ø20 × 30 mm3 has been grown by Czochralski method from a flux of Na2MoO4. The absorption and emission spectra of Yb3+/Na+:CaMoO4 crystals were investigated. The absorption bands at 934 nm have a FWHM of 59 nm for π polarisation and 65 nm for σ polarisation respectively. The absorption cross-sections σa are 1˙73 × 10–20 cm2 for π polarisation and 1˙44 × 10–20 cm2 for σ polarisation at 934 nm respectively. The emission bands have a FWHM of 68 nm at 1022 nm for π polarisation and 65 nm at 1023 nm for σ polarisation respectively. The emission cross-sections σe are 1˙14 × 10–20 cm2 at 1022 nm for π polarisation and 1˙27 × 10–20 cm2 at 1023 nm for σ polarisation respectively. The Yb3+/Na+:CaMoO4 crystal may be considered as a potential laser gain material.

Total interaction cross sections and effective atomic numbers of some biologically important compounds containing H, C, N and O in the energy range 6.4–136 keV

The total interaction cross sections (σt) of some sugars and amino acids and five elements: lithium, carbon, oxygen, aluminium and calcium have been measured for 6.4 keV, 13.95 keV, 14.4 keV, 17.74 keV, 24.14 keV, 30.8 keV, 35 keV, 59.54 keV, 81 keV, 122 keV and 136 keV photons in a narrow beam good geometry set up, by using high resolution detectors such as a Si-PIN diode detector and a high purity germanium detector. The σt values have been used in a matrix method to evaluate the effective atomic numbers Zeff of the samples from their effective atomic cross sections σa. The effective atomic cross section of a sample σa is the total interaction cross section divided by the total number of atoms of all types in it. Further, a quantity called the effective atomic weight Aeff of a sample was defined as the ratio of the molecular weight A to the total number of atoms of all types in it. The variation of Zeff was systematically studied with respect to Aeff as well as σa at each experimental energy separately and new semi-empirical relations for Zeff have been evolved. It is felt that these relations can be very convenient for determining the effective atomic number of any sample having H, C, N and O atoms in the energy range 6.4 keV–136 keV irrespective of its chemical structure.

Application of Neutron Powder-Diffraction to Mineral Structures

The application of neutrons to study the crystal structures of powdered minerals is of growing popularity among earth scientists. This has just recently been demonstrated at a workshop on Neutrons at the Frontiers of Earth Sciences and Environments (NESE) (Rinaldi and Schober 2006). There are a number of physical reasons to study minerals with neutrons. They are related to the fundamental properties of neutrons regarding their interaction with matter (Parise 2006a, this volume). The major points are the isotope specific scattering lengths and absorption cross-sections of the neutron, being independent of the number of electrons. The scattering power of many elements is of similar magnitude (see Table 1⇓ for some elements of interest for minerals). As a consequence neutrons are well suited to localize light elements (also in the direct neighborhood of heavy elements) and to distinguish between ions or atoms having the same or a very similar number of electrons. A large advantage of neutrons over X-rays is the independence of the scattering power of the scattering angle. Consequently, structural data from neutron experiments are in principle of higher precision. View this table: Table 1. Coherent ( b c) and incoherent ( b i) neutron scattering lengths of selected elements, as well as the corresponding absorption cross sections (σa) (adapted from Sears VF in Wilson 1992, Table 4.4.4.1). The focus of this Chapter is on powder diffraction. There are several answers to the question why to study powders. The material may not be available as a single crystal or cannot be grown to the appropriate size. Since the interaction potential of neutrons with matter is small, single crystals should be in the order of about 1 mm3 (Ross and Hoffman 2006, this volume). Many natural samples are multi phased or structurally and chemically heterogeneous. In order to measure a …

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na0.4Y0.6F2.2:Er3+: I. Intensities of luminescence spectra and kinetics of luminescence

Na0.4Y0.6F2.2:Er3+ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na0.4Er0.6F2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σa for the band peaked at λ=970.4 nm is 0.15×10−20 cm2. On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ωt were determined by the Judd-Ofelt method: Ω2=1.65×10−20 cm2, Ω4=0.56× 10−20 cm2, and Ω6=1.01×10−20 cm2. These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the 4 G 11/2, 2 G(H)9/2, and 4 F 9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the 4 I 9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the 4 I 11/2 and 4 I 13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm−1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er 3+ crystals are promising candidates for active media of tunable diode-pumped lasers.

Spectral characterizations of Nd3+: KLa(WO4)2 crystal

The absorption and emission of Nd3+: KLa (WO4)2 crystal were investigated. The uniaxial crystal Nd3+: KLa(WO4)2 shows strong polarization dependence. The intensity of the absorption bands in E//c is about four times than that in E//a. The absorption cross section σa is 22.6 × 10−20 cm2 for E//c. The emission cross section at 1.069 μm is 2.09 × 10−19 cm2. The fluorescence lifetime is 157 μs at 300 K. The relationship of Nd3+ concentrations with fluorescence intensity and lifetime was discussed. The results show the ca 3 at % Nd3+ concentration in Nd3+: KLa(WO4)2 crystal is appropriate.

Spectral characterizations of Nd3+:KLa(WO4)2 crystal

The absorption and emission of Nd3+:KLa(WO4)2 crystal were investigated. The uniaxial crystal Nd3+:KLa(WO4)2 shows strong polarization dependence. The intensity of the absorption bands in E//c is about four times than that in E//a. The absorption cross section σa is 22.6 × 10–20 cm2 for E//c. The emission cross section at 1.069 μm is 2.09 × 10–19 cm2. The fluorescence lifetime is 157 μs at 300 K. The relationship of Nd3+ concentrations with fluorescence intensity and lifetime was discussed. The results show the ca 3 at % Nd3+ concentration in Nd3+:KLa(WO4)2 crystal is appropriate.

Spectral studies and radiative characteristics of naphthalocyanine molecules in DMF

Optical absorption and stimulated emission characteristics of free base (H2Pc) and metallated (Eu, VO, Zn) naphthalocyanines in dimethyl formamide (DMF) solvent are reported. Various spectroscopic and radiative parameters such as molar extinction coefficient (ε), electric dipole strength of the absorption band (q), oscillator strength (f), absorption and emission cross sections (σa and σe), excited state decay time (τ), fluorescence bandwidth (Δλ) and optical gain (G) of the system are measured and compared with those of reported solid matrix.

Infrared spectroscopy of chromium-doped cadmium selenide

The maximum optical-absorption cross section of Cr2+ ions was evaluated from near-infrared (NIR) absorption spectroscopy and direct measurements of the chromium concentration in Cr2+:CdSe crystals. The emission lifetime of the excited state, 5E, of Cr2+ was measured as a function of Cr2+ concentration in the 2 × 1017-2 × 1018 ions/cm3 range and as a function of temperature from 77-300 K. Lifetime values were as high as ∼6 µs in the 77-250 K range and decreased to ∼4 µs at 300 K because of nonradiative decays. Assuming that most of the Cr dopant is in the Cr2+ state, an optical-absorption cross section σa of (1.94 ± 0.56) × 10-18 cm2 was calculated. Implications for laser performance are discussed.

Optical absorption and emission spectral studies of phthalocyanine molecules in DMF

Optical absorption and emission spectral studies of various phthalocyanine ( Pc ) molecules in dimethyl formamide (DMF) solvent are reported in this paper. Measurements have been done for free base, Eu , Fe , Mn , Sm , Cu , Zn , Nd and La Pc - s . The principal optical transitions viz B and Q are identified and some of the important spectral parameters such as molar extinction coefficient (ε), absorption cross section (σa), dipole strength (q) and oscillator strength (f) are evaluated for the prominent Q-band. The measured radiative parameters viz emission cross section (σe), fluorescence bandwidth (Δλ) and optical gain (G) show wide variation among different systems. The results obtained are compared with other solid matrices such as glass and polymer.

Collisional dynamics of Ca1D + HBr reactions: evidence for transition-state motions

The total cross-sections σA (E) and σB(E) for the exoergic Ca1D + HBr → CaBr A2Π, B2Σ+ + H reactions have been measured as a function of reagent relative translational energy in a crossed atomic and molecular beam experiment. For CaBr A 2Π production, σA(E) proceeds to a maximum after exceeding a threshold energy, and then decreases with further translational excitation of reagents. The energy dependence of σB(E), though similar in overall form, appears to be characterized by a series of maxima in the post-threshold region. These may be attributed to the opening of successive bending vibrational channels through an excited, bent van de Waals transition state [Ca…HBr]*‡ the widths of the resonance features in the excitation function give lifetimes of 435 fs and 610 fs for the two lowest vibrational levels of [Ca…HBr]*‡. An heuristic model of the dynamics of CaBr B2Σ+ formation is developed in terms of Landau-Zener transition probabilities for neutral—ionic curve crossing at two locations in the entrance channel coupled with a dynamic preference for utilization of vibrational energy in the transition state, which enables trajectories to surmount the energy barrier for electron transfer into the ionic exit channel with conservation of electronic orbital angular momentum. It is argued that this mechanism is consistent with theoretical calculations of the entrance channel and transition state potentials, and with information from laser spectroscopy of [Ca-HBr]*.

The absolute cross sections of photoabsorption, photodissociation, and photoionization of the group VIB metal hexacarbonyls at 300–1600 Å

The absolute photoabsorption cross sections σa and photoionization quantum yields η of Cr(CO)6, Mo(CO)6, and W(CO)6 are measured in the wavelength range of ∼300–1600 Å. We have also estimated the photoionization cross sections σi and the neutral photodissociation cross sections σd from σa and η. Main absorption features are tentatively assigned with the help of the MO diagram and the photoelectron spectroscopic data of M(CO)6(M=Cr,Mo,W). Large optical oscillator strength distributions and neutral photodissociation cross sections are obtained at the photon energies far beyond the first ionization potential. Our studies revealed the valence excitation nature of the superexcited states in the extreme ultraviolet region. It is suggested that the neutral photodissociation in this region shares a similarity with that in the UV region below the first ionization potential because they both involve a repulsive ejection of the first CO ligand.

Neutron Moisture Meter Measurement in Waste Packages

A neutronic method based on neutron therinalization by hydrogen nuclei is used to measure the moisture content in packages of radioactive waste embedded in hydraulic binders. The two steps of the measurement are (a) acquisition of the neutron characteristics of the embedded waste considered (or of a chemically similar material) and (b) the measurement itself obtained with a neutron moisture meter. The neutron characteristics required are the adsorption and diffusion cross sections Σa and Σd for thermal neutrons of the dried material. These two parameters are used to calculate the calibration curve (valid only for the material considered) as follows: N = (α. Ds + β) Hv + γ. Ds + δ, which allows the counting N of the neutron moisture meter to be converted into free-water content Hv (α, β, γ, and δ are deduced from Σa and Σd,- Ds is the dry density of the material). The neutron moisture meter (containing a fast neutron source and a thermalized neutron detector) is portable. Measurements are taken at various depths in a core hole made in the package to draw a water profile. The measurements are taken in materials used for waste solidification and in active or inactive packages. The results obtained (free-water content) are in good agreement with those obtained by determining the weight loss at 120° C (the differences between these two measurements are generally ∼ 10% when the free-water content is ∼20 to 25 %). The water profiles allow one to detect the presence of excessive free water.

Variation with temperature of the electron attachment to SO2F2

The total electron attachment rate constant ka(〈ε〉,T) for SO2F2 has been measured, in a buffer gas of N2, as a function of the mean electron energy 〈ε〉 (0.046–0.911 eV) and temperature T (300–700 K) using an electron swarm technique. From the measured ka(〈ε〉,T), the total electron attachment cross sections σa(ε,T) were determined. At 300 K the σa(ε,T) exhibits a maximum at ∼0.22 eV which is due to dissociative electron attachment and an increase below ∼0.1 eV which is due to the formation of parent negative ions SO2F−2 at near zero energy. At T=400 K, σa(ε,T) has only one main peak at ∼0.13 eV which is due only to dissociative electron attachment reflecting the depletion of the parent anions and the prevalence of the fragment negative ions as T increases. The main peak of σa(ε,T) shifts to lower electron energies with increasing T so that at 700 K the peak is located at ∼0.03 eV. The value, σda(εmax), of the total electron attachment cross section at the peak energy εmax increases by a factor of ∼32 as T increases from 300 to 700 K. The analysis of these results—and similar earlier work—leads to the conclusion that the increase in σda(ε,T) for the dissociative electron attachment processes in molecules, with increasing T results mainly from an increase with T of the internal energy (principally vibrational) of the molecule.

Variation of the electron attachment to n-C4F1 with temperature

The total electron attachment rate constant ka(〈ε〉) for n-C4F10 has been measured using an electron swarm technique in the mean electron energy, 〈ε〉, range 0.76 to 4.81 eV and over the temperature, T, range 300 to 750 K. The measured ka(〈ε〉,T) and the total electron attachment cross section σa(ε,T) determined from these by unfolding are reported. The ka(〈ε〉) first decrease slowly with T between 300 and 400 K, then decrease precipitously between 400 and ∼500 K, and subsequently increase for T>500 K. The overall variation of ka with T depends on 〈ε〉. From the measured dependence of ka(〈ε〉) on the total gas number density and T we determined the contribution of dissociative and nondissociative attachment to ka as a function of 〈ε〉; the contribution of nondissociative attachment decreases with T and the contribution of dissociative attachment increases with T. The ratio Rd/t of the dissociative to the total electron attachment is small at low 〈ε〉 and T, but it increases with both 〈ε〉 and T and reaches unity for T>500 K and 〈ε〉>1.7 eV. These results are discussed in relation to similar earlier work on C2F6 and C3F8.

Electron attachment to F2

The rate constant ka for electron attachment to F2 has been measured at ambient (298 K) temperature T in the buffer gas Ar over the mean electron energy 〈ε〉 range 0.40–2.95 eV; similar measurements were made at T=233, 298, and 373 K in the buffer gas N2 over the 〈ε〉 range 0.04–0.75 eV. The ka(〈ε〉) function attains a maximum value of 1.7×10−8 cm3 s−1 (T=298 K) at ∼0.04 eV and decreases monotonically with increasing 〈ε〉 above thermal energy; the magnitude of ka increases only slightly with increasing T. The ka(〈ε〉) data at 298 K were unfolded and the resultant cross section σa(ε) shows a main maximum at ∼0.0 eV, a shoulder at ∼0.5 eV, and a weak maximum at ∼1.1 eV. Possible electron attachment processes are discussed to account for the energy dependence of σa(ε). Also discussed, in the light of the present measurements, are published experimental and theoretical results on ka(〈ε〉) and σa(ε).