Observation of muonic HF molecule formation with TF μ− SR

Negative muon polarization has been measured in pure gaseous Ne (24 atm) and in Ne+H₂ mixtures (24 atm Ne+1.8 atm H₂, 8 atm H₂ and 16 atm H₂). The experiment was performed at JINR (Dubna) on aSR-spectrometer [1] with 200 G transverse magnetic field at room temperature. In pure Ne no polarization was observed (a ₀=0.09±0.1%) while in Ne+H₂ mixtures clear precession signals were detected at the free-muon frequency with asymmetries a_1.8=0.33±0.13%,a ₈=0.33±0.14% anda ₁₆=0.59±0.09%. The fact that polarization appears in the muonic HF molecule shows that at the moment of the molecule formation (t10^–10 s) the muon is not completely depolarized. The estimate of the reaction constantk=(2.3±1.6)×10^–11cm³ s^–1 agrees with the experimental values obtained by other methods. The result achieved demonstrates that ^– SR-method can be applied for studying fast kinetics processes in the gas phase and in particular for measuring chemical reaction rates of halogen atoms and ions.     

Missing fractions in heavy gases: Xe and CCl4?

The spin polarization of positive muons thermalized in Xe has been measured as a function of pressure up to 4660 Torr (6.1 atm) by the muon spin rotation (MSR) technique. At 4660 Torr, triplet muonium (F=1, M=1) accounts for about 40% of the initial muon polarization and no significant signal from diamagnetic muons has been observed. The unexpectedly slow recovery of the polarization in Xe at high pressures is discussed in conjunction with similar results seen in CCl₄ and CHCl₃ vapors.     

Charge exchange of muons in gases: Experimental implications from rate theory

The effects of the charge exchange process on muon spin dynamics have been investigated using a density operator formalism with special interest placed upon the diamagnetic muon and paramagnetic muonium signals observed after thermalization. In the charge exchange region the dynamics of the spin density operator is assumed to be determined by the muonium hyperfine interaction and by electron capture and loss processes for muons. Analytical expressions are obtained for the amplitudes and phases of the diamagnetic muon and paramagnetic muonium signals as a function of the duration of the charge exchange region,t _c, which is inversely proportional to the number density of the moderating gas. The theoretical signals exhibit three features which have, as yet, to be experimentally observed, namely: (i) that the amplitudes associated with the muonium Larmor frequency and with the hyperfine frequency are not, in general, equal, (ii) that all the amplitudes are, in general, damped oscillatory functions oft _c (temperature/pressure) and (iii) that phase jumps occur when an amplitude decreases to zero and then increases with falling pressure. Fits to the experimental argon data are discussed in light of the above points.     

Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityD _μ of positive muons (μ⁺) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ⁺SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD _μ through the variation of the magnetic field felt by the μ⁺ during their diffusion through the crystals. For μ⁺ near the centres of the flux lines inNb it givesD _μ(4.6 K)=(8±2)·10⁻¹¹m²S⁻¹. The positive temperature coefficient ofD _μ indicates that at liquid-helium temperatures the diffusivity of μ⁺ inNb is mainly due to phonon-assisted tunnelling processes.     

Fabrication of NdBa2Cu3O7−δ single crystals by the top-seeded solution-growth method in 1%, 21%, and 100% oxygen partial pressure atmosphere

Single crystals of NdBa₂Cu₃O_7−δ (Nd123) have been successfully grown by the top-seeded solution-growth (TSSG) method in 1%, 21% and 100% oxygen partial pressure atmosphere ((P(O₂) = 0.01 atm, P(O)₂) = 0.21 atm and P(O₂) = 1.00 atm). Ba---Cu---O solvent with a Ba to Cu ratio of 3:5 was used in a Nd₂O₃ crucible. Nd is supplied by the reaction between the molten solvent and the Nd₂O₃ crucible. Compositions of Nd123 single crystals grown in different oxygen partial pressure atmospheres were analyzed by inductivity coupled plasma atomic emission spectrometry (ICP-AES) and confirmed to be Nd:Ba:Cu = 1.01:1.97:3.00 for P(O₂) = 0.01 atm, Nd:Ba:Cu = 1.07:1.95:3.00 for P(O₂) = 0.21 atm and Nd:Ba:Cu = 1.10:1.90:3.00 for P(O₂) = 1.00 atm, respectively. The Nd123 single crystals grown in different oxygen partial pressure atmospheres were annealed in a pure oxygen gas flow, and the temperature dependence of the DC magnetization for these crystals was measured using a superconducting quantum interference device (SQUID) magnetometer. The Nd123 single crystal grown in P(O₂) = 0.01 atm, and annealed at 340°C for 200 h showed a steep superconductive transition at 96 K. On the other hand, the Nd123 crystal grown in P(O₂) = 0.21, 1.00 atm and, annealed at 340°C for 200 h exhibited a broad transition at 92 K for P(O₂) = 0.21 atm and at 88 K for P(O₂) = 1.00 atm, respectively. Therefore for Nd123 single crystal production with high quality superconductive characteristics, a low oxygen partial pressure atmosphere during crystal growth is found to be effective for minimizing the substitution of Nd ions into Ba sites.     

Configuration and Performance of a Mobile 129Xe Polarizer

A stand-alone, self-contained and transportable system for the polarization of ¹²⁹Xe by spin exchange optical pumping with Rb is described. This mobile polarizer may be operated in batch or continuous flow modes with medium amounts of hyperpolarized ¹²⁹Xe for spectroscopic or small animal applications. A key element is an online nuclear magnetic resonance module which facilitates continuous monitoring of polarization generation in the pumping cell as well as the calculation of the absolute ¹²⁹Xe polarization. The performance of the polarizer with respect to the crucial parameters temperature, xenon and nitrogen partial pressures, and the total gas flow is discussed. In batch mode the highest ¹²⁹Xe polarization of P_Xe = 40 % was achieved using 0.1 mbar xenon partial pressure. For a xenon flow of 6.5 and 26 mln/min, P _Xe = 25 % and P _Xe = 13 % were reached, respectively. The mobile polarizer may be a practical and efficient means to make the applicability of hyperpolarized ¹²⁹Xe more widespread.     

Measuring the Michel parameter ξ'

Unlike the majority of Michel parameters which are consistent with the Standard Model V-A interaction, the experimental value of ξ″(=0.65±0.36) [1] is poorly known. Our experiment will measure the longitudinal polarization, P _L , of positrons emitted from the decay of polarized muons. The value of P _L , equal to unity in the Standard Model, will decrease for high energy positrons emitted antiparallel to the muon spin if the combination of Michel parameters ξ″/ξξ′ − 1 deviates from the Standard Model value of zero. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polarization Phenomena in Elastic Electron-Proton Scattering

A unified treatment is given by using the scattering matrix in the Pauli spin spaces and polarization density matrices. Various quantities such as polarization P, asymmetry A, spin correlation parameters C_αβ, double polarization asymmetry A_αβ, polarization transfer parameters D_t, A_t, R_t and depolarization and spin rotation parameters D, A, R are expressed in terms of six coefficients which appear in the scattering matrix. Some explicit expressions as well as numerical results for these quantities are also given in the one photon exchange approximation.     

Flavor dependence of spontaneous pair creation

In superstring models, contributions to the muon longitudinal polarizationP _L in the decay \(K_L \to \bar \mu \mu \) can arise from interactions involving the exoticD andD ^c superfields. It is shown that these interactions also contribute to the decay μ→eγ. After using the experimental limit to constrain this process, we find thatP _L arising from these new interactions is less than 10^?4.     

Generation of laser-polarized xenon using fiber-coupled laser-diode arrays narrowed with integrated volume holographic gratings

Volume holographic gratings (VHGs) can be exploited to narrow the spectral output of high-power laser-diode arrays (LDAs) by nearly an order of magnitude, permitting more efficient generation of laser-polarized noble gases for various applications. A 3-fold improvement in ¹²⁹Xe nuclear spin polarization, P_Xe, (compared to a conventional LDA) was achieved with the VHG-LDA’s center wavelength tuned to a wing of the Rb D₁ line. Additionally, an anomalous dependence of P_Xe on the xenon density within the OP cell is reported—including high P_Xe values (>10%) at high xenon partial pressures (1000 torr).     

Response of fcc metals and L12 and D022 type trialuminides to uniaxial loading along [100] and [001]: ab initio DFT calculations

Ab initio density-functional calculations have been used to investigate the response of the face-centred cubic (fcc) metals Al and Cu, and of the L1₂- and D0₂₂-type trialuminides Al₃(Sc,Ti,V) to uniaxial loading along the [100] and [001] directions. The results obtained under uniaxial strains are compared to the response to biaxial (epitaxial) strains. The ideal tensile and compressive strengths and their limitation by shear instabilities along these deformation paths have been calculated. Although the response of both pure fcc metals could be expected to be very similar, our results show a fundamental difference: whereas for Cu a special invariant state with C ₂₂?=?C ₂₃, leading to a bifurcation from the tetragonal to an orthorhombic deformation path, is reached at a strain of 10%, for Al this state is reached only at a strain of 33% close to the critical strain defining the ideal tensile strength. The reaction of the L1₂-type trialuminides is comparable to the response of Al; no bifurcation to an orthorhombic deformation path is predicted. The response of the D0₂₂-type trialuminides is different from that of the L1₂-type phases because of the difference in the stacking of the atomic planes along the [001] direction. For D0₂₂-type trialuminides, the uniaxial compression along this direction or epitaxial tension in the (001) plane leads to the formation of a stress-free D0₃ structure, in complete analogy to the fcc???bcc transformations observed for the pure metals. Under uniaxial [100] loading the guiding symmetry along the deformation path is orthorhombic and leads to the formation of special structures under both tension and compression parts, which are related to the D0₃ structure in the same way as the parent D0₂₂-lattice is related to the L1₂ structure.     

Muonium formation and the “missing fraction” in vapors

The vapor phase fractional polarizations of positive muons thermalizing as the muonium atom (P _M) and in diamagnetic environments (P _D) has been measured in H₂O, CH₃OH, C₆H₁₄, C₆H₁₂, CCl₄, CHCl₃, CH₂Cl₂ and TMS, in order to compare with the corresponding fractions measured in the condensed phases. There is a marked contrast in every case, with the vapor phase results being largely understandable in terms of a charge exchange/hot atom model. Unlike the situation in the corresponding liquids, there is no permanent lost fraction in the vapor phase in the limit of even moderately high pressures (1 atm); at lower pressures, depolarization is due to hyperfine mixing and is believed to be well understood. For vapor phase CH₃OH, C₆H₁₄, C₆H₁₂, and TMS therelative fractions are found to be pressure dependent, suggesting the importance of termolecular hot atom (or ion) reactions in the slowing down process. For vapor phase H₂O and the chloromethanes, the relative fractions are pressure independent. For CCl₄,P _M=P _D0.5 in the vapor phase vs.P _D=1.0 in the liquid phase; fast thermal reactions of Mu likely contribute significantly to this difference in the liquid phase. For H₂O,P _M 0.9 andP _D0.1 in the vapor phase vs.P _D 0.6 andP _M0.2 in the liquid phase. Water appears to be the one unequivocal case where the basic charge exchange/hot atom model is inappropriate in the condensed phase, suggesting, therefore, that radiation induced spur effects play a major role.     

Condensation of monosilane-argon and monosilane-helium mixtures in free jets

Condensation of gaseous monosilane-argon and monosilane-helium mixtures was investigated in free jets by a Rayleigh scattering laser diagnostic technique. The condensation of a SiH₄-Ar mixture begins to develop at a lower stagnation pressure and at a shorter distance from the nozzle, and proceeds at a higher rate, as compared to condensation in monosilane-helium and pure argon jets. The results of Rayleigh scattering measurements in condensing monosilane-argon jets scale with the parameter P ₀ d ^0.8. An analysis of the results obtained in this study and found in literature suggests that simultaneous monosilane-argon condensation lead-ing to the formation of mixed clusters takes place in the monosilane-argon mixture.     

Rare gas dimers in a nozzle beam

Supersonic expansion from a nozzle produces noble gas clusters under various source conditions. The characteristics of atoms and dimers in a nozzle beam are examined. A study of the dimer collision cross-section suggests the existence of a temperature-dependent limiting oven pressure (P _L ) for the observation of pure dimers. This is further supported by the dependence of the beam intensity on stagnation pressure. The reduced pressure-temperature coordinates for noble gas dimers behave in accordance with the model of corresponding jets. The velocity distribution of atoms and dimers in a beam corresponding to stagnation pressure on either side ofP _L is measured to determine the effect of condensation on the distribution pattern.     

Kinetic analysis of n-decane–hydrogen blend combustion in premixed and non-premixed supersonic flows

Numerical analysis of ignition and combustion of an n-decane–hydrogen fuel blend in a premixed supersonic flow and in a model scramjet duct is performed using a reduced reaction mechanism built especially to describe the oxidation of blended n-C₁₀H₂₂–H₂ fuel in air at the temperature T₀ > 900–1000 K in the pressure range P₀ = 0.1–13 atm. The developed kinetic mechanism involves the principal reactions responsible for chain mechanism development both for n-decane and for hydrogen oxidation. It has been shown that using blended n-C₁₀H₂₂–H₂ fuel makes it possible to enhance the ignition and combustion both in premixed and in non-premixed supersonic fuel–air flows compared to burning pure hydrogen–air and n-decane–air mixtures. This allows high combustion completeness in the scramjet duct at the distance of ～1 m even at extremely low air temperature T₀ = 1000 K and pressure P₀ = 0.3 atm. This is due to the interaction of kinetics of the formation of highly reactive atoms and radicals, carriers of chain mechanism, in H₂–air and n-C₁₀H₂₂–air mixtures.     

μ + Residual polarization in water,CS2, C6F6 and hydrocarbons under strong decoupling fields

The ⁺ decay asymmetry was measured as a function of longitudinal magnetic field (LF) between zero and 10 kG in spin decoupling experiments on muons in water, c-hexane, CS₂, C₆F₆, C₆H₆ etc. The results were compared both with the diamagnetic polarization,P _D, measured by transverse-field (TF) experiments ⁺SR, and with the asymmetry observed in ⁺ spin resonance experiments under longitudinal field. These results have been interpreted in terms of spin conversion and chemical reactions of muonium (Mu) with paramagnetic species such as free radicals in the muon track or spur. It is argued that spin-conversion reactions are responsible for the so-called missing fraction of ⁺ polarization in various media.     

Relative stability of homochiral and heterochiral dialanine peptides. Effects of perturbation pathways and force-field parameters on free energy calculations

The relative stability of homochiral (D,D or L,L) and heterochiral (D,L or L,D) dipeptides may have been a decisive factor in the evolutionary propagation of a symmetry-breaking event leading to the present-day predominance of L-amino acids in natural proteins. Kinetic resolution in the solid-phase peptide synthesis of blocked dialanine suggests the activation free energy difference of formation of (D,D or L,L)- and (D,L or L,D)-dialanine to be 0.22?kJ?mol^?1 in favour of the formation of the homochiral dipeptide. Computer simulation studies were performed on water-solvated dialanine, applying a thermodynamic integration protocol using the GROMOS force field. Five different pathways and three force-field parameter sets have been used to assess the possibility of a computational prediction of the chiral preference. Inversion of the configuration around either one of the C_α-atoms by changing the improper dihedral angle with concomitant relaxation of the bond angles, leads to an excellent reproduction of the experimental result.     

Narrowing and broadening parameters for H2O lines perturbed by helium,argon and xenon in the 1170–1440 cm−1 spectral range

Collision effects on water vapor at low concentration in a mixture with noble gases (helium, argon and xenon) have been studied by Fourier transform spectroscopy in the pressure range where line narrowing by dynamic confinement (Dicke effect) and collision broadening are observable, i.e. when the Voigt function cannot reproduce the observed profiles. Precise values of the broadening parameter have been obtained for the P and Q branches of the H₂O ν ₂ band taking into account molecular confinement (hard or soft collisions). The broadening parameter value derived from a Voigt profile for H₂O lines perturbed by helium is smaller by about 10% than values determined using the soft or hard collision model. For H₂O lines perturbed by argon or xenon this difference can reach more than 50% for the narrowest lines.     

Nuclear and muon hyperfine techniques in the study of heavy-fermion superconductors

Nuclear magnetic resonance and relaxation, and the related techniques of muon spin rotation and relaxation, have been used to study local spin polarization and quasiparticle excitations in the heavy-fermion superconductors CeCu₂Si₂, pure and thoriated UBe₁₃, and UPt₃. Measurements of nuclear and positive muon Knight shifts, linewidths, and spinlattice relaxation rates give some hints as to the nature of Cooper pairing in these exotic materials.     

Muon transverse polarization in K_{l2_\gamma } decay due to the electromagnetic final-state interaction

For the decay process K ⁺ → μ ⁺ νγ, the effect of muon transverse polarization caused by electromagnetic final-state interaction is analyzed. It is shown that, in the one-loop approximation, the muon transverse polarization varies from ?1.3×10^?3 to zero in the region of the Dalitz plot. The mean value of the muon polarization, 〈P _T〉, in the kinematical region E _γ ≥20 MeV is ?5.44×10^?4.