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1.
In a study of ∑ −π + production by stopped K − in liquid helium, (19 ± 6)% of stopped ∑ − were observed to decay. This corresponds to a “cascade time” of 28 ± 8 psec. 相似文献
2.
Methane activation catalyzed over Ag-exchanged ZSM-5 zeolites was investigated by using the density functional theory (DFT) with a cluster model. Two different pathways were taken into account in this work: the “alkyl” and the “carbenium” pathways. The activation barriers obtained are 34.09 and 66.63 kcal/mol for the “alkyl” and the “carbenium” pathway, respectively. The calculated results show that the activation barrier of the “alkyl” pathway is smaller than that of “carbenium” pathway. Consequently, the “alkyl” pathway is the preferential reaction pathway. A new mechanism of methane conversion in the presence of ethene was proposed. In the catalytic cycle, the initial step of methane activation proceeds with the “alkyl” pathway and the Ag + cation acts as an acceptor of the methyl group, then ethene reacts with the Ag +CH 3− group to form propene. In addition, it is found that the Ag + cations play an important role in the methane activation, compared with the reaction of methane activation over H-ZSM-5. 相似文献
3.
The similarity between the weak interactions of electron and muon is extended to the principle that all e and μ interactions in gauge models are invariant under e ↔ μ exchange. This necessitates the existence of two Higgs doublets φ e and φ μ, and an extended e ↔ μ permutation invariance. After symmetry-breaking, a multiplicatively conserved “permutation parity” π = ± 1 for all particles naturally emerges, with π μ = −π e = 1. The model forbids μ → eγ but predicts e −e − → μ −μ − mediated by π = −1 Higgs bosons at 10 −11 times the rate of typical weak cross sections. 相似文献
4.
Thin films of Bi 2Sr 2CaCu 2O 8 and (Bi, Pb) 2Sr 2Ca 2Cu 3O 10 have been prepared on monocrystalline (100) MgO substrates, using a laser ablation method with post annealing treatment. The influence of substrate temperature and oxygen pressure during deposition were investigated. SEM observations, EDS analysis, electric and magnetic measurements have been used to characterize the films. Superconducting “2212” films, with Tc( R = 0) at 80–83 K and Jc (50 K) up to 5 × 10 5 A/cm 2, have been currently achieved, while Pb-doped “2223” films exhibit Tc as high as 110 K with Jc = 5 × 10 4 A/cm 2 at 77 K. The effect of annealing at low temperature (350°C) in an argon flow has been studied for the 2212 phase, it shows the influence of the oxygen non-stoichiometry, i.e. of the hole carrier density upon Tc's which can be measured up to 89 K (zero resistance). 相似文献
5.
“Rare” annihilation channels for antiprotons stopping on heavy (Ag, Br) nuclei of photoemulsion, have been sought; 4872 stops of antiprotons on photoemulsion nuclei are analysed. Events of formation and decay of the hyperfragment Λ4H, escape of 8He and 8Li nuclei, one-prong stars with the mean range 79.5±5.1 μm of secondary slow “b” particles are found among the annihilation stars at
capture on nuclei (Ag, Br). The lower limits for the production probability of Λ4H and 8He, 8Li nuclei per antiproton stopping in the nuclei (Ag, Br) are WΛ4H2×10−4 and W8He,8Li=(1.3±0.6)×10−3. The branching ratio for the production of one-prong stars with the secondary “b” particles is at least (1.3±0.6)×10−3. Possible mechanisms for a production of these events in annihilation processes are considered. 相似文献
6.
We apply the perturbative chiral quark model to give predictions for the electromagnetic O( p2) low-energy couplings of the ChPT effective Lagrangian that define the electromagnetic mass shifts of nucleons and first-order ( e2) radiative corrections to the π N scattering amplitude. We estimate the leading isospin-breaking correction to the strong energy shift of the π −p atom in the 1 s state, which is relevant for the experiment “Pionic Hydrogen” at PSI. 相似文献
7.
This paper studies the effects in low energy nuclear physics of a possible time reversal invariance (TRI) violation in the electromagnetic interaction. It is shown that this effect appears as a two body short range TRI violating transition operator or as a two and three body TRI violating potential. Two cases are studied. Firstly a TRI violating NNγ vertex is considered and found to have very little effect. Secondly the TRI violation is assumed to occur in the NN *γ vertex and it is shown that if the violation is “maximal” the contribution to the imaginary part of the “mixing ratio” δ is Im δ ≈ ¦δ¦ × 10 −3. This should be measurable. 相似文献
8.
Anomalies are found in the near-band-edge luminescence properties of Δ 2-light-hole indirect excitons in Si 1−yC y-based tensilely strained quantum wells (QWs). The experimental spectra exhibit a clear signature of phonon-assisted transitions on the lower energy side of the “no-phonon” transition, which indicates the relevance of “virtual” indirect valleys and in-plane k-dispersion, as opposed to the theoretical prediction that the zone-centered Δ 2 valleys take over the conduction band edge. Intervalley scattering between [0 0 1]-Δ 2 valleys and in-plane Δ 4 valleys is suggested as the underlying mechanism. On the other hand, the experimental evidence was found for “apparently enhanced” quantum-confined Stark red shifts for Si 1−yC y-based QWs. However, quantitative estimates are in conflict with the experimental results and predict a blue shift due to exciton weakening which masks the Stark effect as in the case of Δ 4-heavy hole excitons in Si 1−xGe x-based QWs. 相似文献
9.
We show that the transfer matrix of the AN−1(1) open spin chair with diagonal boundary fields has the symmetry Uq( SU( l)) × Uq( SU( N− l)) × U(1), as well as a “duality” symmetry which maps l ↔ N − l. We exploit these symmetries to compute exact boundary S-matrices in the regime with q real. 相似文献
10.
In very rare circumstances, X-ray photoemission spectra of copper in spinel oxides exhibit a “negative binding energy shift”. The origin of such an anomalous XPS chemical shift was investigated. A metastable Ni 0.48Co 0.24Cu 0.6+xMn 1.68−xO 4 (0 < x < 0.6) spinel was fabricated at 600 °C using a low-temperature solution technique. The binding energy of the 2p 3/2 level of copper (930.8 eV) is found 1.9 eV lower than that of Cu 0 (932.7 eV). XPS and EXAFS studies revealed that the post-thermal annealing between 600 and 800 °C undergoes an irreversible cubic-to-tetragonal phase transformation through oxidation–reduction reaction Cu 1+ + Mn 4+ Cu 2+ + Mn 3+, and only tetrahedral Cu 1+ species in the cubic spinel shows this anomalous chemical shift. The negative shift of the core levels was correlated to an equal shift of the Cu 3d valence band levels. XPS valence bands from the samples annealed at different temperatures were compared to DOS calculations. The DOS computations were performed with FEFF-8.1 code using experimental crystal parameters established by the EXAFS analysis. It was found that the tetrahedral Cu 1+ in the 600 °C annealed sample exhibits localization of the 3d orbitals showing behavior characteristic to zinc. The completely filled and isolated 3d electron shell appears as a false valence band edge in the XPS spectrum. The position of the Cu 3d, and other core levels, is established by oxygen pinning the Cu valence band levels and by the fixed value of the p–d gap characteristic to the tetrahedral copper environment in this spinel. 相似文献
11.
It is pointed out that, contrary to the situation with natural Rn, the growing concentration of man-made Pu in environment — natural water soil plants and food — can provide the serious danger for all kind of animals and especially for mankind. Really, as it was established recently, element Pu is accumulated in human body during all the life and the dangerous concentration estimated to be 10 −12 – 10 −13 grapm Pu per gram of human tissues. Unfortunately up to now there are no simple nonexpensive methods of Pu determination at the level of sensitivity 10 −14 – 10 15 g/g which can provide the determination of Pu even in 0.2 g of human tissue. We warn that now the average concentration of Pu in human body is not less than 10 −14 g/g and much higher for some “hot” regions in Europe and Asia. We propose to discuss the problem of organizing world net exploration of Pu in environment — similarly to Rn exploration program — and personnel Pu monitoring for inhibitions of Pu damaged regions. Our approach to the problem is based on chemical separation of Pu, on determination of Plutonium by high fission cross section with thermal neutrons 239Pu(n,f) reaction and by control of possible admixture of 235U isotope by 238U(γ, f) reaction in Pu preparations. The problem of increasing of sensitivity of Pu analysis up to 10 −14 – 10 −15 g/g is discussed in detail. 相似文献
12.
The differential geometry of an imbedded (e.g. string or membrane world sheet) surface in a higher-dimensional background is shown to be conveniently describable (except in the null limit case) in terms of what are designated as its first, second, and third fundamental tensors, which will have the respective symmetry properties η μν = η (μν) as a trivial algebraic identity, Kμνρ = K(μν)ρ as the “generalised Weingarten identity”, which is the (Frobenius type) integrability condition for the imbedding, and Ξ λμνρ = Ξ (λμν)ρ as a “generalised Codazzi equation”, which depends on the background geometry being flat or of constant curvature, needing replacement by a more complicated expression for a generic value of the background curvature Bκλμν. The “generalised Gauss equation” expressing the dependence on this background curvature of the internal curvature tensor Rκλμν of the imbedded surface is converted into terms of the first and second fundamental tensors, and it is thereby demonstrated that the vanishing of the (conformally invariant) “conformation tensor”, i.e. the trace free part Cμνρ of the second fundamental tensor Kμvρ, is a sufficient condition for conformal flatness of the imbedded surface (and thus in particular for the vanishing of its (Weyl type) conformal curvature tensor Cκλμν) provided the background is itself conformally flat. In a trio of which the first two members are the generalised Gauss and Codazzi equations, the “third” member is shown to give an expression in terms of Cμνρ for the (trace free, conformally invariant) “outer curvature” tensor Ω κλμν whose vanishing is the condition for feasibility of the natural generalisation of the Walker frame transportation ansatz. The vanishing of Cμνρ is shown to be sufficient in a conformally flat background for the vanishing also of Ω κλμν. 相似文献
13.
A new type of nuclear spectroscopy to study hadron-nucleus bound states is described. The first successful experiment was to search for deeply bound π − states in heavy nuclei using the 208Pb(d, 3He) reaction at GSI, in which a narrow peak arising from the 2p π − orbital coupled with the neutron-hole states was observed at 135 MeV excitation energy. An improved experiment has just been carried out to separately identify the 1s and 2p π − states. These experiments provide important information on the local potential strength, from which the effective mass of π − is deduced to be 20 MeV. This method will be extended to search for η and ω bound states as well as for K − bound states. The advantage of the bound-state spectroscopy versus invariant mass spectroscopy is emphasized. 相似文献
14.
The Co-sublattice anisotropy in Lu 2Co 17 consists of four competitive contributions from Co atoms at crystallographically different sites in the Th 2Ni 17-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu 2Co 17−xSi x ( x=0−3.4) grown by the Czochralski method. The SRT in Lu 2Co 17 was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at TSR1≈680 K and “easy-cone”–“easy-axis” at TSR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu 2Co 16Si ( TSR1≈75 K and TSR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu 2Co 17−xSi x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy. 相似文献
15.
The reactions π −p → 2π −π +p, π −p → 2π −π +π op and π −p → 2π −2π +n are analysed at 11 and 16 GeV/ c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π −p → 2π −π +p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π −p → (2π −π +)p, and of the proton, π −p → π −(π −π +p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π −p → (2π)(πp) decrease with increasing energy. In the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π−p → π−(π−π+πop); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π−p → π−(π−2π+)n. Another strong maximum corresponds to the pion dissociation π−p → (2π−π+πo)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π−p → (2π−2π+)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π−p → (2π−π+)(πop) and π−p → (2π−π+)(π+n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed. An appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy. 相似文献
16.
Nuclear track radiography was applied to identify aerosol “hot” particles which contain elements of nuclear fuel and fallout after Chernobyl NPP accident. For the determination of the content of transuranium elements in radioactive aerosols the measurement of the -activity of “hot” particles by SSNTD was used in this work, as well as radiography of fission fragments formed as a result of the reactions (n,f) and (γ,f) in the irradiation of aerosol filters by thermal neutrons and high energy gamma quanta. The technique allowed the sizes and alpha-activity of “hot” particles to be determined without extracting them from the filter, as well as the determination of the uranium content and its enrichment by 235U, 239Pu and 241Pu isotopes. Sensitivity of determination of alpha activity by fission method is 5×10 −6 Bq per particle. The software for the system of image analysis was created. It ensured the identification of track clusters on an optical image of the SSNTD surface obtained through a video camera and the determination of size and activity of “hot” particles. 相似文献
17.
We analyze the spatial and the luminosity properties of the Perseus-Pisces redshift survey. We find that the two point correlation function (CF) Γ( r) is a power law up to the sample effective depth ( 30 h −1 Mpc), showing the fractal nature of the galaxy distribution in this catalog. The fractal dimension turns out to be D 2. We also consider the CF ξ( r) and in particular the behavior of the “ correlation length” r0 (ξ( r0)1) as function of the sample size. In this respect we find, unambiguously, that the luminosity segregation effect is not supported by any experimental evidence. In addition we have studied the galaxian number-density ( n( r)) and number-counts ( N( m)) in the VL subsamples finding a good agreement with the properties of a fractal distribution. In particular our conclusion is that the n( r) relation permits to extend the analysis of the fractal nature up to a deeper depth than that reached by the CF analysis, and, we find evidence for fractal properties up to the limiting depth of 130 h −1 Mpc. We clarify the role of the small-scale fluctuations in the determination of the galaxy counts. Even in this case the results are in agreement with the previous ones. Finally we have considered the correlations between galaxy positions and luminosities by means of the multifractal analysis. We find clear evidence for self-similar behavior of the whole luminosity-space distribution. These results confirm and extend those of Coleman and Pietronero (1992). 相似文献
18.
Six rotational bands in the odd-odd nucleus 174Ta have been populated with the 160Gd( 19F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π1/2 − [541] ν1/2 − [521] band and semi-decoupled π1/2 − [541] ν7/2 − [633] band are observed, in addition to the high-K couplings of the π9/2 − [514] ν7/2 + [633], π9/2 − [514] ν5/2 − [512], π7/2 + [404] ν7/2 + [633], and π5/2 + [402] ν5/2 − [512] configurations. The signature splitting of the π1/2 − [541] ν7/2 + [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i13/2 neutrons in the π1/2 − [541] bands of odd-Z nuclei. 相似文献
19.
New experimental evidence is given to the approach suggested by the authors to non-radiative transitions in rare earth and transition metal ions as radiationless energy transfer from ion to ligand and solvent molecule vibrations. Using non-radiative transitions in Yb 3+ as an example the participation of combinations of vibrations in the electronic energy dissipation was investigated. Dy 3+ in solutions was used to show the role of anharmonicity in non-radiative transitions. It was proved that the semi-empirical calculations of non-radiative transition rates by Förster's formula gave also good results for Mn 2+ ions luminescence which is an example of “strong” vibronic interaction. The suggested approach permits to explain the anomalous low probability of the 5D 1− 5D 0 non-radiative transition in the Eu 3+ ion. 相似文献
20.
Polarized micro-Raman spectra of different compounds belonging to the Sr 14Cu 24O 41-type structure were studied. In the spectra with parallel polarization along the plane crystal axes of the insulating samples Sr 9R 5Cu 24O 41 (R = La, Y) a broad peak near 3000 cm −1, similar to the well-known two-magnon peak in the layered cuprates, was observed. In addition to the Raman lines characteristic for this structure, we observed only in the spectra with parallel to the chains numerous lines between 100 and 1200 cm −1, probably originating from Raman-forbidden infrared-active only LO phonons and their combinations. In the spectra of the conducting compounds Sr 14−xCa xCu 24O 41 ( x = 0,7) these features were very weak or disappeared. We interpret these results as Raman evidence for hole doping of the Cu 2O 3 “spin-ladder” planes and for a redistribution of holes between chains and planes through Ca substitution in the case of the rare-earth-free samples. 相似文献
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