φ production in In–In collisions at 158 A GeV

The NA60 experiment has measured muon pair production in In–In collisions at 158 AGeV at the CERN SPS. This paper presents a high statistics measurement of φ→μ μ meson production. Differential spectra, yields, mass and width are measured as a function of centrality and compared to previous measurements in other colliding systems at the same energy. The width of the rapidity distribution is found to be constant as a function of centrality, compatible with previous results. The decay muon polar angle distribution is measured in several reference frames. No evidence of polarization is found as a function of transverse momentum and centrality. The analysis of the p _T spectra shows that the φ has a small radial flow, implying a weak coupling to the medium. The T _eff parameter measured in In–In collisions suggests that the high value observed in Pb–Pb in the kaon channel is difficult to reconcile with radial flow alone. The absolute yield is compared to results in Pb–Pb collisions: though significantly smaller than measured by NA50 in the muon channel, it is found to exceed the NA49 and CERES data in the kaon channel at any centrality. The mass and width are found to be compatible with the PDG values at any centrality and at any p _T : no evidence for in-medium modifications is observed.     

Rotational hysteresis loss and anisotropy in yttrium garnet ferrite single crystals

The rotational hysteresis loss in Y₃Fe₅O₁₂ single crystals is found to increase with the field up to 2.5–3 kOe at all temperatures between –183 and 250 ° C, this being followed by a fall, a further peak at 8–10 kOe, and a slow fall, with some residual effect even at 20 kOe. This loss is sensitive to heat treatment; annealing for 2 hr at 275 ° C greatly reduces the loss at –183 ° C and halves the main (second) peak at 20 ° C. These anomalies do not occur in ferrites with the spinel structure. The first anistropy constant is slightly reduced at all temperatures as a result of annealing.     

X-ray diffraction studies of specific features in the atomic structure of Fe-Si alloys in the α area of the phase diagram

The atomic structure of Fe-Si alloys with a silicon concentration of 5–8 at % (α-area of the phase diagram) was studied using X-ray diffraction. The effect of quenching after annealing at a disordering temperature of 850°C on the structural state of the alloys was elucidated. It is shown that the quenched samples are characterized by a short-range ordering; namely, there is a local B2-type order at a concentration of 5–6 at % Si and, in addition, DO₃-phase clusters are formed at 8 at % Si. The atomic structure of B2 clusters and their nearest surroundings is established.     

Localization of nonequilibrium current carriers close to Cr ions in ZnSe:Cr single crystals

The photoluminescence (PL) of Cr-doped ZnSe single crystals is investigated in a temperature interval from 83 up to 297 K and in a wavelengths region from 440 up to 2700 nm. The doping was carried out during a high-temperature annealing of ZnSe crystals in CrSe vapors and in chrome chlorides medium. It is revealed that the doping results in an appearance of both luminescence bands located at 0.54, 0.97, and 2.15 μm and edge luminescence bands located at 454, 457, and 460 nm at 83 K. It is shown that the PL bands located at 457 and 460 nm are caused by the radiative recombination with the participation of holes located on hydrogen-like orbits close to Cr⁺ centers, having a binding energy of 99 meV. The excitons bound with centers responsible for the radiation located at 0.54 μm and having a binding energy of 65-68 meV are considered. The energy of a lattice relaxation at recharge of centers responsible for green radiation is estimated and equals 40-170 meV.     

Dynamics of the anisotropic two-dimensional XY model

In this paper we study the dynamics of the two-dimensional XY model with single-ion anisotropy, and spin S = 1, in the large D phase, and low temperatures, using the bond operator formalism. The in-plane structure factor is a delta function. The out of plane shows a three peak structure, which merges in a single peak at the Brillouin zone boundary. We analyze also spin currents generated by a magnetic field gradient. The spin conductivity is calculated, at finite temperature, using the Kubo formula. The model shows unconventional ballistic spin transport at finite temperature. The computed spin conductivity exhibits a nonzero Drude weight at finite temperature. For ω< 2m, where m is the energy gap, the spin conductivity is described solely by the Drude weight. There is a regular contribution to the spin conductivity for ω> 2m, which persist in the zero temperature limit. The conductivity at the critical point, and for small frequencies, is (gμ_B)²/ħ times a universal scaling function of ħω/k_B T.     

Low-threshold gas lasers pumped by plasma-cathode accelerators

The lasing on the electronic transitions of xenon and neon is studied. It is demonstrated that plasma-cathode accelerators serve as effective sources for the pumping of low-threshold lasers (W < 100 W/cm³). The laser energy in xenon at the wavelength λ = 1.73 μm is 5 J at an efficiency of 2%, and the laser energy in neon at the wavelengths λ = 585.3 nm is 0.5 J at an efficiency of 0.3%. The repetitively pulsed mode of the xenon and neon lasers carried out at a repetition rate of 50 Hz. The radiation energy of a wide-aperture laser with an active volume of 600 l is 100 (0) J for a wavelength of 1.73 (2.03) μm at an efficiency of 2% (1%). Original Text ? Astro, Ltd., 2006.     

Optical loss measurements in femtosecond laser written waveguides in glass

The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 μm. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 μm. In straight waveguides more than 90% of the transmission loss is due to scattering.     

Nucleation of the D1a type ordered phase in the near-surface of A4B alloy: II. Nucleation at surface

Nucleation in the D1_a type A₄B ordering alloy was investigated by means of an atomistic calculation based on a phenomenological nucleation theory and the Bragg-Williams approximation. The nucleation rate at the Ni₄Mo surface is presented as a function of temperature, surface orientation, and type of the plane to which the surface is parallel (fundamental plane or superlattice plane). The radius, degree of order and activation free energy of the critical nucleus are also given as a function of temperature. The value of the nucleation rate at the surface is compared with that in the bulk. The results are as follows: (i) at temperatures near T_c. the nucleation rate is higher at the surfaces than in the bulk and the {200}_FCC surfaces are the highest in nucleation rate; (ii) however, at temperatures near the instability temperature t₀, the nucleation rate is lower at the surface than in the bulk; (iii) the present calculated results give reasonable explanations to our experimental results on the Ni₄ Mo alloy: (a) preferential surface ordering at high temperatures near T_c; (b) surface disordering at intermediate temperatures near the nose temperature.     

Adsorption of carbon monoxide on polycrystalline nickel films

The adsorption states of carbon monoxide on polycrystalline nickel films have been investigated by measuring the thermal desorption, the heat of adsorption, the change in resistivity, and the change in work function in dependence on coverage and temperature. It can be shown that there are two chemisorbed (β₂, β₂) and one weakly bound (γ) species. Desorption peaks appear at 170K, 310–360 K, and 460–490 K. The differential heat of adsorption is $\text{30}\text{kcal}\text{mole}$ at low coverages and approximately $\text{25}\text{kcal}\text{mole}$ between 0.3 and 0.6 monolayers. The resistivity of the nickel film is characteristically changed with increasing coverage, and there is a maximum of resistivity at half a monolayer. At low coverages the increase in the work function is proportional to the amount adsorbed; at a monolayer the total increase is 1.26 eV at 77 K and 1.46 eV at 273 K. The two chemisorbed species differ only in the structures they form in the adsorption phase, β₂ being the species that is stable at low coverages, β₁ being the species that is stable at high coverages. These results are in good agreement with those recently found for CO adsorption on single crystal surfaces.     

Mn-Ga二元系的X射线研究

本文从44个缓冷和淬炼Mn-Ga合金摄取了德拜·谢乐照相,并配合了在富Ga部分的差热分析,初步画出了这个系统的相图。这个系统除纯Ga外共有十个相。Mn在Ga中的固溶度是几乎无可觉察的。α相是Ga在α-Mn中的原固溶体,在室温的固溶度为1.95at％Ga。β相在室温的均匀范围为8.6—19.2at％Ga,这是β-Mn结构,因此可看作是β-Mn的固溶体,由于Ga原子无规地替代了部分Mn原子而这个结构得在室温稳定存在。γ相可分成γ₁,γ₂,γ₃,三部分,γ₁是面心立方结构,γ₂是面心四方结构,γ₃是有序的面心四方结构,与Cu-Au系中的CuAuⅠ同型。在室温下稳定的是γ₃,均匀范围为37—45at％Ga,而在高温稳定的却总是γ₁。从γ₁变到γ₂,再从γ₂变到γ₃的变化是二级相变。有序度随Ga含量的递增而递增,随温度的递升而递降。整个γ相可看作是γ-Mn的固溶体,γ-Mn本身是不可能用淬炼的办法在室温获得的。δ相只存在于高温,可看作是δ-Mn的固溶体。由于Ga原子替代了部分Mn原子,因而δ一Mn结构产生了畸变而有序化。ε相是有序的六角密堆积结构,每个晶胞含8个原子,它是在约820℃从γ相同成份地转变而成的,在室温的均匀范围估计为27一30at％Ga。η相在室温约50—60at％Ga处有一宽广的均匀范围。从520到600℃,它经历一多型性变化,转变为λ相。λ相的相区随温度的递升而向富Mn的一边偏移。η和λ结构都很复杂。在富Ga的一边,存在着三个居间相χ,φ和ω,它们是由包析或包晶反应所形成的。ω相的化合式很可能相当于Mn₂Ga₉或MnGa₅,而φ相则与NiHg₄同型,在Mn_2.3Ga_7.7左右有一狭隘的均匀范围。在室温稳定存在的七个居间相中,β,ε,γ₃,X和φ是铁磁性的。铁磁性最强的是Ga含量较富的γ₃和φ相。我们测量了其中若干合金的饱和磁化强度与居里温度。 关键词：     

Autoignition of condensed hydrocarbon fuels in non-premixed flows at elevated pressures

Experimental and computational investigation is carried out to elucidate the fundamental mechanism of autoignition of n-heptane, n-decane, and n-dodecane in non-premixed flows at elevated pressures up to 6 bar. The counterflow configuration is employed. In this configuration, an axisymmetric flow of a gaseous oxidizer stream is directed over the surface of an evaporating pool of liquid fuel. The oxidizer stream is a mixture of oxygen and nitrogen. The experiments are conducted at a fixed value of mass fraction of oxygen and at a fixed low value of strain rate. The temperature of the oxidizer stream at autoignition, T_ig, is measured as a function of pressure, p. Computations are carried out using skeletal mechanisms constructed from a detailed mechanism and critical conditions of autoignition are predicted. The experimental data and predictions show that, for all fuels tested, T_ig decreases with increasing p. At a fixed value of p, T_ig for n-dodecane is the lowest, followed by n-decane and n-heptane. This indicates that n-dodecane is the most easily ignited, followed by n-decane and n-heptane. This is in agreement with previous experimental and computational studies at 1 atm, where a similar order of reactivities for these fuels was observed at low strain rates. Flame structures at conditions before and at conditions immediately after autoignition are calculated. A noteworthy finding is that low temperature chemistry is found to play a dominant role in promoting autoignition. The influence of low temperature chemistry is found to increase with increasing pressure.     

Der Einfluß metastabiler Atome auf den statischen Durchschlag in Argon

A gas discharge in argon is initiated by an ultraviolet light pulse which releases 10⁵ electrons at the cathode within 20 nsec. The time of built-up for the static breakdown is found to be in the order of msec. The oscillogram of the current near breakdown shows a fast component consisting of several successive avalanches caused by secondary electrons liberated by ions at the cathode and a slow component which is due to the electrons liberated by metastable atoms at the cathode. The second Townsend coefficient for argon ions and a nickel cathode is determined to beγ ₊= 2.5·10^?3. The amount of electrons liberated by metastables at static breakdown is 25% of the total number of secondary electrons. The mean life-time of the metastables at 1,1 Torr and an electrode separation of 10 mm is found to be 2.2 msec, which is mainly due to de-exciting collisions at the electrodes.     

Numerical analysis on auto-ignition of a high pressure hydrogen jet spouting from a tube

In this study, a direct numerical simulation based on compressible flow dynamics has been applied to the autoignition and extinction of a high-pressure hydrogen jet spouting from a tube. The diameter of the tube is 4.8 mm. The length of the tube is 71 mm. At the inlet, pressure is set at 3.6, 5.3 and 21.1 MPa, and temperature is set at 300 K for all cases. To explore the autoignition of hydrogen jet, two-dimensional axisymmetric Navier–Stokes equations with a detailed chemical kinetics and rigorous transport properties have been employed. The hydrogen jet through the tube is choked. The numerical results show that the high-pressure hydrogen jet produces a semi-spherical shock wave in the ambient air at the early time of jetting. The shock wave heats up the air to a high temperature and causes the autoignition of the hydrogen and air mixture in the tube as well as at the tube exit.     

Fe-Ga二元系平衡图

本文主要从X射线的研究,并配合差热分析,测定了1000℃以下的Fe-Ga二元系平衡图。这一系统在室温存在着三个居间相ε,χ和Ψ。ε相是有序面心立方结构,相当于理想化合式Fe₃Ga,相区范围很窄。在550℃上下,这个相转变为另一相ξ,这是在高温从Ga在Fe中的固溶体直接分出来的,这个高温相的结构尚未测定。χ相的均匀范围较大,在室温从55at.％Ga延伸到60at.％Ga,结构似乎非常复杂,可能的化合式是Fe₄Ga₅,Fe₃Ga₄或Fe₂Ga₃,这是在960℃上下由包晶反应形成的。Ψ相由另一包晶反应形成,反应温度为820℃,Ψ相属四方晶系,在20℃,α=6.2628?,c=6.5559?,空间群为D_4h¹⁴-P4₂/mnm,晶包内含四个化合式量FeGa₃。没有观察到Fe在Ga中有任何固溶度。从FeGa₃到Ga,存在着一条共晶等温线,共晶点非常接近纯Ga,在约590℃,存在着一条共析等温线,共析点在50at.％Ga左右,在这里Ga在Fe中的固溶体同时分解成ξ和χ。这个系统中最值得注意的是Ga在Fe中的原生固溶体。在室温的固溶度是15.2at.％,它随温度而逐渐递增,在700℃以上,则突增到约50at.％,在625℃以下的结构是体心立方体,在相图中用α代表。但在625℃以上,这个相又转变为两个不同的结构,并且各占据着一定的相域,在相图中各用α₁和α₂代表。α₁和α₂的结构尚未经测定,最可能是从α结构导生的、由基本单胞依照一定规律堆垜起来而产生的原子的重新排列或空位缺陷。     

Sequential beamforming for synthetic aperture imaging

Synthetic aperture sequential beamforming (SASB) is a novel technique which allows to implement synthetic aperture beamforming on a system with a restricted complexity, and without storing RF-data. The objective is to improve lateral resolution and obtain a more depth independent resolution compared to conventional ultrasound imaging. SASB is a two-stage procedure using two separate beamformers. The initial step is to construct and store a set of B-mode image lines using a single focal point in both transmit and receive. The focal points are considered virtual sources and virtual receivers making up a virtual array. The second stage applies the focused image lines from the first stage as input data, and take advantage of the virtual array in the delay and sum beamforming. The size of the virtual array is dynamically expanded and the image is dynamically focused in both transmit and receive and a range independent lateral resolution is obtained. The SASB method has been investigated using simulations in Field II and by off-line processing of data acquired with a commercial scanner. The lateral resolution increases with a decreasing F#. Grating lobes appear if F# ⩽ 2 for a linear array with λ-pitch. The performance of SASB with the virtual source at 20 mm and F# = 1.5 is compared with conventional dynamic receive focusing (DRF). The axial resolution is the same for the two methods. For the lateral resolution there is improvement in FWHM of at least a factor of 2 and the improvement at −40 dB is at least a factor of 3. With SASB the resolution is almost constant throughout the range. For DRF the FWHM increases almost linearly with range and the resolution at −40 dB is fluctuating with range. The theoretical potential improvement in SNR of SASB over DRF has been estimated. An improvement is attained at the entire range, and at a depth of 80 mm the improvement is 8 dB.     

High-precision measurements of the compressibility and the electrical resistivity of bulk <Emphasis Type="Italic">g</Emphasis>-As<Subscript>2</Subscript>Te<Subscript>3</Subscript> glasses at a hydrostatic pressure up to 8.5 GPa

High-precision studies of the volume and the electrical resistivity of g-As₂Te₃ glasses at a high hydrostatic pressure up to 8.5 GPa at room temperature are performed. The glasses exhibit elastic behavior in compression only at a pressure up to 1 GPa, and a diffuse structural transformation and inelastic density relaxation (logarithmic in time) begin at higher pressures. When the pressure increases further, the relaxation rate passes through a sharp maximum at 2.5 GPa, which is accompanied by softening the relaxing bulk modulus, and then decreases, being noticeable up to the maximum pressure. When pressure is relieved, an unusual inflection point is observed in the baric dependence of the bulk modulus near 4 GPa. The polyamorphic transformation is only partly reversible and the residual densification after pressure release is 2%. In compression, the electrical resistivity of the g-As₂Te₃ glasses decreases exponentially with increasing pressure (at a pressure up to 2 GPa); then, it decreases faster by almost three orders of magnitude in the pressure range 2–3.5 GPa. At a pressure of 5 GPa, the electrical resistivity reaches 10^–3 Ω cm, which is characteristic of a metallic state; this resistivity continues to decrease with increasing pressure and reaches 1.7 × 10^–4 Ω cm at 8.1 GPa. The reverse metal–semiconductor transition occurs at a pressure of 3 GPa when pressure is relieved. When the pressure is decreased to atmospheric pressure, the electrical resistivity of the glasses is below the initial pressure by two–three orders of magnitude. Under normal conditions, both the volume and the electrical resistivity relax to quasi-equilibrium values in several months. Comparative structural and Raman spectroscopy investigations demonstrate that the glasses subjected to high pressure have the maximum chemical order. The glasses with a higher order have a lower electrical resistivity. The polyamorphism in the As₂Te₃ glasses is caused by both structural changes and chemical ordering. The g-As₂Te₃ compound is the first example of glasses, where the reversible metallization under pressure has been studied under hydrostatic conditions.     

Absorption phenomena in multicomponent phosphate glasses with Mn

The absorption and ESR spectra of multicomponent alumino-phosphate glasses doped with manganese (0·2–20 mol %) has been measured and compared with the absorption spectra of manganous and manganic ions in model solutions. The unirradiated glasses show in ultraviolet region increasing absorption below 200 nm with the shoulder at 235 nm and the absorption bands due to manganous oxygen complexes in octahedral symmetry. They are similar to the absorption bands of Mn²⁺ in concentrated H₃PO₄ acid. In irradiated glasses the bands at 200, 235, 275 (only if Mn is present) and a broad band at 540 nm appear. After the annealing at 450 C all radiation-induced bands disappear and the bands at 235 nm is more pronounced. The Mn³⁺ in H₃PO₄ solution exhibit the absorption spectrum with a weak band at 530 nm and a very intense band at 270 nm. It is therefore proposed that both the 540 and 275 nm bands in irradiated glasses can be assigned to octahedral oxygen complexes of Mn³⁺, i.e. to hole centres. The band at 200 nm which is practically independent of the modifiers (Be, Mg, Ca, Sr and Ba) is, therefore, associated with electron centres (electrons trapped in non-bridging oxygen vacancies). It is suggested that the band at 235 nm in irradiated and annealed glasses is associated with irreversible structural changes.The authors wish to express their appreciation to H.Dvoáková for preparing the solutions and E.Linhartová for careful measurements.     

Critical conditions at liftoff limit of a laminar n-butane-air jet diffusion flame

The stability mechanism of laminar coflow jet diffusion flames in normal gravity has been studied computationally and experimentally. N-butane, the heaviest alkane in a gaseous state at ambient temperature and pressure, is used as the fuel since the reaction mechanism is similar to that of higher (liquid) hydrocarbons. The critical mean n-butane jet and coflowing air velocities at flame stability limits are measured using a small fuel tube burner (0.8 mm inner diameter). The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (58 species and 540 reactions), molecular diffusive transport, and a radiation model, reveals a flame structure. A fuel-lean peak reactivity spot (i.e., reaction kernel), possessing the hybrid nature of diffusion-premixed flame structure at a constant temperature of ≈1560 K, is formed at the flame base and controls the flame stability. In a near-quiescent environment, the flame base resides below the fuel tube exit plane and thereby premixing is limited. As the coflowing air velocity is increased incrementally under a fixed fuel jet velocity, the flame base moves slightly above (≈1 mm) the burner exit and vigorous premixed combustion becomes prevailing. The local heat-release rate at the reaction kernel nearly doubles due to the increased convective oxygen flux (i.e., a blowing effect). The local Damköhler number, newly defined as a ratio of the square root of the local heat-release rate and the local velocity, decreases gradually first and drops abruptly at a critical threshold value and the flame base lifts off from the burner rim. The calculated coflow air velocity at liftoff is ≈0.38 m/s at the fuel jet velocity of 2 m/s, which is consistent with an extrapolated measured value of 0.41 m/s. This work has determined the critical Damköhler number at the stability limit quantitatively, for the first time, for laminar jet diffusion flames.     

Enhancement of cavity selectivity in relativistic gyrotrons operated at axisymmetric modes

Mode selection is a key problem from the viewpoint of maintaining the single-mode generation regime in high-power gyrotrons operated at axisymmetric modes. In this paper, we propose several electrodynamic methods of mode selection, which allow separating the electrically strong axisymmetric higher-order mode from spurious modes. The possibility of suppressing the nearest spurious modes efficiently, by using either wide slits in the cavity, or azimuthal corrugations on the walls of the tapered cavity, is shown. A method of mode selection at cyclotron frequency harmonics is proposed. The results of studying two types of gyrotron cavities experimentally at a low power level confirmed their high selective properties. The cavities were calculated aiming at using them as the basis for creation of a gyrotron operated at the TE0.3 mode and producing a power of 5–10 MW in 1 μs long pulses at a wavelength of 1 cm. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 837–849, October 2008.     

The transport properties in Nd0.75Sr1.25CoO4 film: Evidence for polaronic transport

The experiments of electrical resistivity and thermopower on Nd_0.75Sr_1.25CoO₄ film in the temperature range 90 K<T<310 K were carried out. The great difference in the activation energies estimated from thermopower and resistivity, a characteristic of small polarons, is observed, providing strong evidence for polaron-dominated transport mechanism in this material. Furthermore, the activation energy at intermediate-temperature region is larger than that at low-temperature region in resistivity, but it is not observed in thermopower, indicating that the energy for the creation of the carriers is slightly lower at low-temperature region than that at intermediate-temperature region. At the same time, the abrupt drop in the thermopower and the abnormal peak in the differential curve of resistivity indicate that a phase transition between a paramagnetic state and a ferromagnetic state occurs at temperature about 218 K. The positive thermopower in the whole temperature range measured suggests that the carriers are holes in this system.