Cross Sections For Transitions Research Articles

Spectroscopy ofB13via theC13(t,He3)reaction at115AMeV

Gamow-Teller and dipole transitions to final states in $^{13}\mathrm{B}$ were studied via the $^{13}\mathrm{C}(t,^{3}\mathrm{He})$ reaction at ${E}_{t}=115A$ MeV. In addition to the strong Gamow-Teller transition to the $^{13}\mathrm{B}$ ground state, a weaker Gamow-Teller transition to a state at 3.6 MeV was found. This state was assigned a spin-parity of $3/{2}^{\ensuremath{-}}$ by comparison with shell-model calculations using the WBP and WBT interactions which were modified to allow for mixing between $n\ensuremath{\hbar}\ensuremath{\omega}$ and $(n+2)\ensuremath{\hbar}\ensuremath{\omega}$ configurations. This assignment agrees with a recent result from a lifetime measurement of excited states in $^{13}\mathrm{B}$. The shell-model calculations also explained the relatively large spectroscopic strength measured for a low-lying $1/{2}^{+}$ state at 4.83 MeV in $^{13}\mathrm{B}$. The cross sections for dipole transitions up to ${E}_{x}$$(^{13}\mathrm{B})=20$ MeV excited via the $^{13}\mathrm{C}(t,^{3}\mathrm{He})$ reaction were also compared with the shell-model calculations. The theoretical cross sections exceeded the data by a factor of about 1.8, which might indicate that the dipole excitations are ``quenched''. Uncertainties in the reaction calculations complicate that interpretation.

Hyperfine transitions in ultracold hydrogen-antihydrogen collisions

We consider the hyperfine transitions in ultracold collisions of hydrogen (H) and antihydrogen $(\overline{\text{H}})$ atoms. The cross sections for transitions between various spin states are calculated. We show that hyperfine transitions in $\overline{\text{H}}\ensuremath{-}\text{H}$ collisions are basically driven by the strong force between proton and antiproton.

Spin exchange in hydrogen–antihydrogen collisions

We consider spin exchange in ultracold collisions of hydrogen (H) and antihydrogen atoms. The cross sections for transitions between various hyperfine states are calculated. We show that the process of spin exchange in –H collisions is basically driven by the strong force between a proton and an antiproton.

Dynamics of He2+ + He+(1s) and He2+ + He+(2s) excitation and electron-capture processes with screened Coulomb interaction

Collision dynamics of He2+ + He+(1s) and He2+ + He+(2s) excitation and electron capture in hot, dense plasmas is studied by the two-centre atomic orbital close-coupling method in the energy range 2–100 keV/u. The screening of the Coulomb interaction due to the plasma effects introduces significant changes in electronic bound state wavefunctions and energies (including lifting of Coulomb degeneracy), coupling matrix elements, and the number of open reaction channels, thus affecting the entire collision dynamics of the system. With increasing interaction screening, the total electron-capture cross sections for the considered collision systems increase with respect to those of the unscreened case in the lower part of the considered energy range, whereas in its high-energy part they show the opposite behaviour. The excitation cross sections generally decrease with increasing screening but show an oscillatory behaviour almost in the entire energy range considered. The cross section for the He2+ + He+(2s) → He2+ + He+(2p) excitation is several orders of magnitude larger than the cross sections for all other transitions and is highly sensitive to the strength of the interaction screening. This strongly affects the intensity and other spectral properties of the associated He+ Ly-α radiation.

Preparation and spectroscopic characterization of Nd3+:Y2O3 nanocrystals suspended in polymethyl methacrylate

We describe a method to fabricate polymethyl methacrylate (PMMA), a polymeric host, in which nanocrystals of Nd3+:Y2O3 are suspended. The spectroscopic properties of this material are analyzed using the standard Judd–Ofelt technique. The phenomenological Judd–Ofelt intensity parameters are used to calculate the radiative decay rates and the branching ratios of the F43/2→I4J (J=9/2, 11/2, 13/2, and 15/2) intermanifold transitions. The room temperature fluorescence lifetime has been measured for the most intense F43/2→I411/2 emission transition. Emission cross sections for the intense intermanifold transitions and peak emission cross sections for the intense inter-Stark transitions are also reported. Assignments to individual Stark levels of the I4J manifolds have been made and compared with the calculated splittings reported earlier. Finally, the spectroscopic properties of the Nd3+:Y2O3 nanocrystals suspended in PMMA are compared with those of Nd3+ doped in various host materials. Detailed optical analysis led to favorable values of fluorescence lifetime and emission cross section for the F43/2→I411/2 transition, which suggest that the Nd3+:Y2O3 nanocrystals embedded in PMMA would have potential for various photonic applications including laser systems and optical communication devices.

Comparative study of quantal and semiclassical treatments of charge transfer betweenO+and He

A comparative study for the electron capture process ${\text{O}}^{+}\text{ }({^{4}S}^{0},{^{2}D}^{0},{^{2}P}^{0})+\text{He}\ensuremath{\rightarrow}\text{O}(^{3}P)+{\text{He}}^{+}$ is reported. The cross sections are calculated using fully quantal and semiclassical molecular-orbital close-coupling (MOCC) approaches in the adiabatic representation. Detailed comparison of transition probabilities and cross sections is made from both MOCC approaches and displays close agreement above $\ensuremath{\sim}125\text{ }\text{eV}/\text{u}$. The remarkable discrepancies between the earlier semiclassical and quantal MOCC approaches may be attributed to the insufficient step-size resolution in their semiclassical calculation [M. Kimura et al., Phys. Rev. A 50, 4854 (1994)]. Our results have also been compared with experiment and found to be in good agreement.

PRESSURE-INDUCED ELECTRONIC PHASE TRANSITIONS AND SUPERCONDUCTIVITY IN TITANIUM

The electronic band structure, density of states, structural phase transition, superconducting transition and Fermi surface cross section of titanium ( Ti ) under normal and high pressures are reported. The high pressure band structure exhibits significant deviations from the normal pressure band structure due to s → d transition. On the basis of band structure and total energy results obtained using tight-binding linear muffin-tin orbital method (TB LMTO), we predict a phase transformation sequence of α( hcp ) → ω (hexagonal) → γ (distorted hcp) → β (bcc) in titanium under pressure. From our analysis, we predict a δ (distorted bcc) phase which is not stable at any high pressures. At ambient pressure, the superconducting transition occurs at 0.354 K. When the pressure is increased, it is predicted that, Tc increases at a rate of 3.123 K/Mbar in hcp–Ti . On further increase of pressure, Tc begins to decrease at a rate of 1.464 K/Mbar. The highest value of Tc(P) estimated is 5.043 K for hcp–Ti , 4.538 K for ω– Ti and 4.85 K for bcc – Ti . From this, it is inferred that the maximum value of Tc(P) is rather insensitive to the crystal structure of Ti . The nonlinearities in Tc(P) is explained by considering the destruction and creation of new parts of Fermi surface at high pressure. At normal pressure, the hardness of Ti is in the following order: ω- Ti > hcp - Ti > bcc- Ti > γ- Ti .

Rotational Quenching Rate Coefficients for H2in Collisions with H2from 2 to 10,000 K

Rate coefficients for rotational transitions in H2 induced by H2 impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H2)2 potential energy surface were performed. The potential energy surface used here has been demonstrated to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels of -->J ≤ 8 were computed for collision energies ranging between 10−4 and 2.5 eV, and the corresponding rate coefficients were calculated for the temperature range -->2 ≤ T≤ 10,000 K. In general, agreement with earlier calculations, which were limited to 100-6000 K, is good, although discrepancies are found at the lowest and highest temperatures. Low-density-limit cooling functions due to para- and ortho-H2 collisions are obtained from the collisional rate coefficients. Implications of the new results for nonthermal H2 rotational distributions in molecular regions are also investigated.

Some peculiarities of the energy level, photoelectron and photoexcitation spectra of the atoms with a half-filled shell

The specific features of the atoms with a half-filled shell following from the existence of two classes of its wavefunctions as well as determined by its 2ℓ+2S ground term are considered. The explicit expressions for the photoionization and photoexcitation cross sections for transitions involving such a shell are derived. They are used for a consideration of the spectral regularities of some atoms with p3, d5 and f7 shells.

MgO-codoping effects on the spectroscopic properties of Er3+-doped LiNbO3

Optical absorption, spectrally resolved polarized upconversion fluorescence, and time-resolved luminescence spectra are used to investigate the Er3+ spectroscopic properties in a series of congruent LiNbO3 crystals codoped with Er3+ (1 mol %) and MgO (X mol %, X=0, 2, 4, 6, and 8). The absorption spectra indicate that the transition cross section of Er3+ ions decreases with increasing MgO concentration; it is explained as the improvement of the Er3+-site symmetries. A new Er3+ energy site with low energy, which appears in the long wavelength side of the polarized fluorescence spectra for the samples heavily codoped with MgO, is demonstrated in the time-resolved luminescence spectra. An important finding is that mild codoping with MgO clearly does not facilitate the formation of clustered sites, whereas heavy codoping clearly does facilitate it in Er3+-doped LiNbO3 crystals. These results are important to those who consider the material as a 1.5 μm laser or fiber optical cable.

Influence of Nd:Zn codoping in near-stoichiometric lithium niobate

Near-stoichiometric lithium niobate (SLN) crystals doped with up to 1.6 mol % Zn and codoped with various Nd concentrations in the melt (0.2, 0.5, 0.9, and 1.5 mol %) (Nd:Zn:SLN) are grown from 58.6 mol % Li(2)O using conventional Czochralski technique. Crystals are pulled at the rate of 0.35 mmh with seed rotation at 9 rpm. Concentrations of Zn and Nd in the crystal are varied by adding appropriate amounts of ZnO and Nd(2)O(3) to the starting composition. Unit cell parameters of the grown crystals are calculated by Rietveld refinement method using FULLPROFF software. Domain structure studies are carried out by chemical etching followed by microscopic examination. Dielectric studies reveal the existence of piezoelectric resonance at high frequencies. Enhancement in dielectric constant and tan delta in Nd doped samples has been attributed to the space charge polarization. Nd doped samples exhibit reduction in the relative permittivity after oxygen annealing. Transmission spectra of Nd:Zn:SLN crystals in the UV region exhibit blueshift in the cutoff wavelength. In Mid Infrared (MIR) region crystals doped with 1.6 mol % Zn have shift in the OH absorption peak from 2873 to 2833 nm. Judd-Ofelt analysis carried out on the absorption spectra of codoped crystal yields the lifetime of 104 mus for the metastable state (4)F(32). The branching ratio for the electronic transition from (4)F(32) to (4)I(112) is high compared to that for (4)F(32) to (4)I(132), indicating a higher emission cross section for the former transition. Laser damage threshold evaluated using 532 nm, 5 ns pulsed neodymium doped yttrium aluminum garnet laser, shows an increase by two orders of magnitude for crystals doped with 1.6 mol % Zn. Photorefractive damage threshold for these crystals shows an enhancement of four orders of magnitude due to increase in the photoconductivity.

Thermal neutron capture cross sections of the palladium isotopes

Precise thermal neutron capture \ensuremath{\gamma}-ray cross sections ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ were measured for all elements with $Z=1\text{\ensuremath{-}}83,90$, and 92, except for He and Pm, at the Budapest Reactor. These data were evaluated with additional information from the literature to generate the Evaluated Gamma-ray Activation File (EGAF). Isotopic radiative neutron cross sections can be deduced from the total transition cross section feeding the ground state, ${\ensuremath{\sigma}}_{0}=\ensuremath{\Sigma}{\ensuremath{\sigma}}_{\ensuremath{\gamma}}(\mathrm{GS})$ if the decay scheme is complete. The EGAF file contains partial \ensuremath{\gamma}-ray cross sections for all stable palladium isotopes. None of these decay schemes are complete, although in each case transitions de-exciting low-lying levels are known. We have performed Monte Carlo simulations of the palladium thermal neutron capture decay schemes using the computer code DICEBOX. The simulated populations of low low-lying levels are normalized to the measured ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ values from EGAF and the total radiative neutron cross section ${\ensuremath{\sigma}}_{0}$ is obtained. The ${\ensuremath{\sigma}}_{0}$ values derived for the palladium isotopes agree well with previous measurements and were in several cases more precise. Complementary use of \ensuremath{\gamma}-ray cross-section data and Monte Carlo calculations has proven effective in determining both the palladium total radiative cross sections and new nuclear structure information.

Production and decay of the 73Ge*(1/2−) metastable state in a low-background germanium detector

The 73Ge*(1/2−) metastable state decays with a very characteristic signature which allows it to be tagged event-by-event. Studies were performed using data taken with a high-purity germanium detector in a low-background laboratory near a nuclear power reactor core where the -flux was . The measured average and equilibrium production rates of 73Ge*(1/2−) were (8.7 ± 0.4) and (6.7 ± 0.3) kg−1 day−1, respectively. The production channels were studied and identified. By studying the difference in the production of 73Ge*(1/2−) between the reactor ON and OFF spectra, limiting sensitivities in the range of for the cross sections of neutrino-induced nuclear transitions were derived. The dominant backgrounds are due to β− decays of cosmic-ray-induced 73Ga. The prospects of enhancing the sensitivities at underground locations are discussed.

Intensity analysis and crystal-field modeling of Ho3+ in KPb2Cl5 host

Optical absorption and emission intensities are investigated for Ho3+ doped into single-crystal KPb2Cl5. Room temperature absorption spectra of Ho3+(4f10), representing transitions from I58 to excited multiplet manifolds observed between 400 and 2100 nm, have been analyzed using the Judd–Ofelt (JO) model to obtain the phenomenological intensity parameters Ω2, Ω4, and Ω6. The JO intensity parameters are used to calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios of the Ho3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds L2S+1J of Ho3+. The room temperature fluorescence spectrum in the near infrared region is reported between 850 and 1550 nm. The emission cross section for the intermanifold transition, F55→I57 (0.98 μm) has been determined. The 8 K absorption spectrum was examined as well. The detailed structure observed in the absorption bands I57, S52, F54, and F53, was analyzed in terms of the crystal-field splitting of each manifold using a charge-compensation model first developed for Er3+ doped into KPb2Cl5. The optical and spectroscopic characteristics of Ho3+:KPb2Cl5 demonstrate that this material has potential as a laser host system at various wavelengths.

Absorption intensities and emission cross section of Er3+ in Sc2O3 transparent ceramics

Spectroscopic investigation of Er3+ doped Sc2O3 transparent ceramics was performed in order to assess its potential as a laser material for different emission wavelengths. The Judd–Ofelt intensity parameters for the f-f transitions of the C2-symmetry center of Er3+ in Sc2O3 transparent ceramics, inferred from the room temperature absolute intensities of the absorption bands in the 370–1600nm range, are Ω2=3.57×10−20cm2, Ω4=1.61×10−20cm2, and Ω6=0.79×10−20cm2. These parameters are comparable to those reported for Er3+ in other sesquioxide single crystals. The Judd–Ofelt intensity parameters are used to determine the emission probabilities, radiative lifetimes, and branching ratios of the different Er3+ transitions. The emission cross sections of the potential laser transitions at ∼1.5μm were determined using the reciprocity method, and the potential laser gain versus wavelength for the I13∕24→I15∕24 was analyzed.

Crystal growth, spectroscopic characterization, and eye-safe laser operation of erbium- and ytterbium-codoped KLu(WO_4)_2

Erbium (Er)- and Ytterbium (Yb)-codoped monoclinic KLu(WO4)2 single crystals were grown by top seeded solution growth-slow cooling method for several different doping concentrations. Growth parameters have been optimized to obtain macrodefect-free single crystals. Er energy levels involved in the 4I13/2-->4I15/2 were determined by 6 K polarized optical absorption. The maximum emission cross section for this electronic transition has been evaluated, being 2.85x10(-20) cm2 for E||Nm at 1535 nm. Laser oscillation in the 1.5 microm range was obtained by pumping the Yb ion at 980 nm and sensitizing Er. The maximum output power achieved was 152 mW, with 1.2% slope efficiency.

Extraction of Weak Transition Strengths via the (He3,t) Reaction at 420 MeV

Differential cross sections for transitions of known weak strength were measured with the (3He, t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn, and 208Pb. Using these data, it is shown that the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in the distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

Study of inner-shell excitation processes from N(1s) orbitals in N2O molecules by electron impact

A combination of the iterative Schwinger variational method with the distorted-wave approximation is applied to study excitations of a core-level electron in a triatomic molecule by electron impact. More specifically, we report calculated differential and integral cross sections for the X1Σ+ → 1,3Π(2σ → 3π) and X1Σ+ → 1,3Π(3σ → 3π) transitions in N2O in the 415–900 eV incident energy range. The RI(1:3) ratios, obtained via dividing the distorted-wave integral cross sections for transitions leading to the singlet core-excited states by those leading to triplet states, are also reported. The generalized oscillator-strength profiles for the singlet transitions have also been calculated at the incident energy of 3400 eV. The comparison of these quantities with the available theoretical and experimental data reported in the literature is encouraging.

Low frequency impedance of tapered transitions with arbitrary cross sections

We study the impedance of a tapered transition at small frequencies for an arbitrary shape of the transition cross section. Our approach does not require a symmetry axis in the system (unlike round geometry). We show that the calculation of the impedance reduces to finding a few auxiliary potential functions that satisfy two-dimensional Poisson equations with Dirichlet boundary conditions. In simple cases such solutions can be obtained analytically; for more complicated geometries they can easily be found numerically. We apply our method to axisymmetric geometry and reproduce results known from the literature. We then calculate the impedance of a taper with rectangular cross section in which the vertical dimension of the cross section is a slowly changing function of the longitudinal coordinate. Finally, we find a transverse kick experienced by a beam passing near a conducting wall with a variable distance from the beam to the wall.

Electroluminescent properties of dibenzoxazolyl biphenyl molecules

We have measured the energy loss spectra of 4,4′-bis[(E)-1-(1,3-benzoxazol-2-yl)-2-ethenyl]-2-n-hexyloxy biphenyl for interaction with electrons with energies 17.5 eV and 50 eV. We used time-dependent density functional theory to calculate spectra of the singlet transitions, which match the experimental data well. We have shown that the cross section for the long-wavelength transitions is greater than the cross section for the short-wave transitions, which is attractive for efficient excitation by low-energy electrons. Electroluminescence was achieved for the studied compound. The threshold voltage was 3.5 V. Introducing an additional layer of copper phthalocyanine increases the brightness of the luminescence several-fold.