High-resolution acoustic measurements in fluids and gases by a path length modulation technique

An acoustic technique has been developed that permits high-resolution velocity measurements to be performed in liquids and gases under circumstances where the acoustic attenuation may become very large. This cavity resonance method has been demonstrated in cavities with lengths as small as 150 µm at acoustic frequencies up to 151 MHz. The acoustic path length is continuously adjusted by a piezoelectric bimorph controlled by feedback from a sensitive acoustic impedance spectrometer. The measurement of velocity then simply reduces to measurement of position of the bimorph, and this can be performed with high accuracy using a capacitance bridge. Absolute measurements of the attenuation of sound can also be performed with this arrangement. It is suggested that this approach will be useful for the study of collective excitations in quantum fluids.     

Zero and first sound velocity and Fermi liquid parameterF 2 s in normal3He

A precise velocity measurement of zero and first sound in normal³He was performed over the pressure range of 0.5 bar and 10 bar at a frequency of 389.1 MHz using a phase detection technique. From these measurements, the Fermi liquid parameter F ₂ ^s was determined as a function of pressure. The new value of F ₂ ^s was positive and less than 0.25 at all pressures below 10 bar. It allows propagation of transverse zero sound mode for all pressures within the current theoretical picture.     

Transverse zero sound and the LandauF 2 parameter in liquid3He

The Boltzmann equation for the quasiparticle distribution in theT 0 limit of a Landau Fermi liquid is solved by Fourier-Laplace transformation, following a method described in a previous paper. An initial sinusoidal transverse velocity distribution is assumed, leading in the hydrodynamic limit to an overdamped solution and in the zero-sound limit to damped transverse standing waves. The velocityc _t of transverse zero sound obeysc _t ² = _s /, where _s is the shear modulus. One can estimatec _t and _s from the Fermi velocity, sincec _t V _F. Using the solutions to calculate the pressure and velocity gradient, one can estimate the shear viscosity by taking the ratio of these quantities, giving = _{s s} , where _s is a relaxation time of the order of the relaxation time for shear stress. Similarly, calculation of the heat flux and temperature gradient gives the thermal conductivity in terms of the Landau parametersF ₁ andF ₂ .A fit of the theoretical expression to experimental values of suggests thatF ₂ <0, which is also a necessary condition for convergence of the solution obtained.     

Longitudinal zero-sound dispersion,thermal conduction,and the LandauF 2 in3He

The Boltzmann equation for the distribution of Landau quasiparticles on the surface of the Fermi sphere is solved by an expansion in Legendre polynomials having symmetry appropriate to longitudinal sound. It is assumed that the collision operator can be replaced by three finite relaxation times and that the quasiparticle interaction includes a nonzero Landau parameterF _s ² . From the solution, the heat flux and temperature gradient in a zero-sound wave can be computed, and comparison with a corresponding phenomenological generalization of Fourier's law yields an expression relating thermal conductivity, ,F _s ² , and the parameter -c ₀ /V _F , wherec ₀ is the longitudinal zero-sound velocity andv _F the Fermi velocity. This expression should hold simultaneously with a second equation expressing the condition that zero sound should propagate undamped atO K, and thus we can solve for andF _s ² . We obtain the valueF _s ² =–2.99 (v _F =56.62 m/sec), which depends hardly at all on the experimental value of , specific heat, and sound absorption used. These estimates agree with earlier ones from transverse zero sound as to sign ofF _s ² , but it appears that complete quantitative consistency may necessitate invoking Landau parameters of third and higher order.     

Thermal expansion,velocity of sound,and compressibility in liquid3He under pressure

We have measured the pressure at constant volumeP _v(T), of liquid³He as a function of temperature from 10 to 600 mK and at starting pressures from 1.6 to 28.1 bar. Simultaneously, we determined the hydrodynamic sound velocityc ₁, at the same pressures and temperatures. From these data we have derived very accurate values of the isobaric expansion coefficient α _p and the adiabatic compressibility κ.     

Propagation of solitary waves in a mixture of liquid3He and superfluid4He

The effect of an admixture of liquid³He on the propagation of solitary waves in superfluid⁴He is investigated. In the local density approximation, the mean field equations are reduced to the two-fluid equations describing the dynamics of the mixture of two liquids. The nonlinear perturbation approach is used to obtain a set of equations describing the density and velocity fluctuations of the homogeneous mixture at rest. The self-consistent solution of these equations is shown to be the solitary waves. Using a pseudopotential method, we perform numerical calculations to study the changes in the general properties of the waves caused by the³He impurity.     

Fluctuation contribution to the velocity and damping of sound in liquid 3He above the superfluid transition temperature

We have developed a theory, valid for arbitrary frequencies, to calculate the change in the velocity and damping of sound due to fluctuations of the order parameter just above the superfluid transition temperature in liquid ³He. While the theory is approximate, it gives the exact high-frequency limit and is off the exact low-frequency limit by less than 0.5%. We show that careful measurements on the frequency dependence of the fluctuation contribution should allow one to obtain information on the nature of quasiparticle interactions in the normal liquid.     

Sharp gap edge and determination of the fermi velocity in Y1Ba2Cu3O7−δ by point contact spectroscopy

We report point contact spectroscopy measurements on Y₁Ba₂Cu₃O_7?δ thin films and textured bulk ceramics. The I(V) curves measured along the copper-oxygenab plane direction are characteristic of the Andreev reflection phenomenon, with a sharp break at the gap edge. This allows a rather precise determination of the (in-plane) gap,Δ = 20 meV. Subgap structure suggests the existence of a smaller gap for some orientations in theab plane, of the order of 12 meV. The ratio of the zero bias conductance to the normal state one, in theab direction measurements, enabled us to set a lower bound to the Fermi velocity in Y₁Ba₂Cu₃O_7?δ in this direction:υ _F>-7 × 10⁷ cm/sec. The value ofυ _F has a crucial importance in the determination of the theoretical explanation for superconductivity in the high-temperature superconductors. For out-of-plane measurements, only a weak zero bias anomaly is observed at energies up to 5meV.     

Behavior of a vibrating wire near the liquid3He-vacuum interface and in a saturated3He-4He mixture

In this paper we describe the effect of the vicinity of a liquid³He-vacuum interface on the behavior of a vibrating wire viscometer. It was found that in the fluid near the liquid interface the quality factor is lower than in the bulk liquid. We further report on the observation of a doubling of the resonance peak of the wire in a saturated³He-⁴He mixture. The frequencies and amplitudes of the two peaks strongly depend on the distance between the phase boundary and the vibrating wire; the temperature and the velocity of the phase boundary have no significant influence on the peak frequencies. The observed peak doubling is attributed to the coupling of the vibrating wire with a standing second-sound wave in the dilute phase where the volume of the dilute phase can be regarded as a resonating cavity for second sound.     

Measurements of the spin diffusion coefficient of normal3He in bulk liquid and a restricted geometry

In this paper we present reliable measurements of the pressure variation of the spin diffusion coefficient of normal liquid³He in the limiting low-temperature Fermi fluid regime. Deviations from the bulk liquid behavior are observed for a sample confined to a 5-µm glass capillary array in which the mean free path of the³He quasiparticles is of the order of the capillary diameter. These deviations are believed to indicate that the spin momentum specular reflection coefficient is less than unity but greater than zero in this system.     

Attenuation and velocity of first sound in a saturated 3He-4He mixture at very low temperatures and high magnetic field

We present experimental results on the attenuation and velocity of first sound in a saturated (X = 0.064) ³He-⁴He mixture. The measurements were performed at very low temperature (3.5 <T< 20 mK) with relatively low frequencies (around 1 MHz) at saturated vapor pressure, using a CW technique. Though the results are found to be in qualitative agreement with the theory for acoustic transmission in degenerate mixtures, they are not quantitatively consistent with theory. A phenomenological explanation is offered for this discrepancy. We compare our data with recent measurements at higher temperatures and frequency. We have also performed the measurements in a magnetic field of 9 T. Within our experimental accuracy we cannot detect any systematic difference from the measurements in zero field. This is expected from theory.     

Development and validation of a technique for the determination of 226Ra and 228Ra by liquid scintillation in liquid samples

Radium isotopes are dispersed in the environment according to their physicochemical characteristics. Considering their long half-lives and radiological effects, (226)Ra and (228)Ra are very important issues in radiological protection. In Brazil, radium isotopes represent an exposure problem both in nuclear fuel cycle installations and in high natural radiation background areas. The experimental part of this work includes the development of a technique for the determination of (226)Ra and (228)Ra by liquid scintillation with potential application in biological samples. Radium was concentrated and then separated from other constituents of the sample by co-precipitation/precipitation with Ba(Ra)SO(4). The precipitate was filtered and weighted to calculate the chemical yield. The filter containing the precipitate of Ba(Ra)SO(4) was transferred to a scintillation vial. Two methods were used to prepare the sources. The first one consisted of the addition of water (8 ml), Instagel XF (8 ml) and UltimaGold (4 ml) in the vial containing the filter and the precipitate, forming a gel suspension. In the second method, the precipitate was dissolved with 0.2 M ethylene-diamine-tetra-acetic-acid solution (9 ml) and 11 ml of scintillation solution (Optiphase Hisafe 3) was added to the vial, forming an aqueous and an organic phase. The solutions obtained were counted in a low background scintillation spectrometry system (Quantulus) suitable for the detection and identification of both alpha and beta particles for the determination of (226)Ra and (228)Ra. The activity values of (226)Ra and (228)Ra calculated by the two methods are in good agreement with the reference values, indicating that both methods are suitable for the determination of (226)Ra and (228)Ra.     

Analysis of optical switching in a Yb3+-doped fiber Bragg grating by using self-phase modulation and cross-phase modulation

The optical performances of the all-optical switching based on Yb3+-doped fiber Bragg grating (FBG) are investigated under the case of self-phase modulation (SPM) and cross-phase modulation (XPM). For the SPM case, the optical bistability of FBG under different parameters is investigated. It shows that the width of the hysteresis loop and threshold switching power are strongly dependent on the fiber grating length, fiber grating detuning, and coupling constant. For the XPM case, the expressions of the threshold switching power in the different detuning range are given. The influence of parameters about different detuning and coupling constant to the threshold switching power and extinction ratio are also studied. In comparison with the SPM case, the switching power of FBG under XPM case can be reduced to less than 20 mW by optimizing the parameters of FBG.     

A novel technique using DNA denaturation to detect multiply induced single-strand breaks in a hydrated plasmid DNA molecule by X-ray and 4He2+ ion irradiation

To detect multiple single-strand breaks (SSBs) produced in plasmid DNA molecules by direct energy deposition from radiation tracks, we have developed a novel technique using DNA denaturation by which irradiated DNA is analysed as single-strand DNA (SS-DNA). The multiple SSBs that arise in both strands of DNA, but do not induce a double-strand break, are quantified as loss of SS-DNA using agarose gel electrophoresis. We have applied this method to X-ray and (4)He(2+) ion-irradiated samples of fully hydrated pUC18 plasmid DNA. The fractions of both SS-DNA and closed circular DNA (CC-DNA) exponentially decrease with the increasing dose of X rays and (4)He(2+) ions. The efficiency of the loss of SS-DNA was half that of CC-DNA for both types of irradiation, indicating that one of two strands in DNA is not broken when one SSB is produced in CC-DNA by irradiation. Contrary to our initial expectation, these results indicate that SSBs are not multiply induced even by high linear energy transfer radiation distributed in both strands.     

The formation of ilmenite FeTiO3 powders by a novel liquid mix and H2/H2O reduction process

FeTiO₃ powder was prepared via a liquid mix and H₂/H₂O reduction process. First, an intermediate compound was synthesized by a liquid mix process. This precursor compound to FeTiO₃ was characterized by thermal analysis (TG/DTA) and Fourier Transform Infrared spectroscopy (FT-IR). Second, the precursor was annealed in the air, H₂/H₂O(g) mixing gas stream, respectively. The obtained powder was characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis spectroscopy (EDAX) and scanning electron microscope (SEM). It was showed that pure and homogenous FeTiO₃ was successfully obtained below 600 °C.     

Transient photodarkening induced by a nanosecond pulsed laser in amorphous and liquid As2Se3: fractal characteristics in relaxation dynamics

Journal of Materials Science: Materials in Electronics - Both amorphous and liquid As2Se3 exhibit fast decrease of optical transmittance under a nanosecond pulsed laser excitation. This...