Millisecond pulsar observation system using acousto-opticspectrometer

We have developed a unique system using an acoustooptic spectrometer for precise timing of millisecond pulsars. This system can handle a signal of 50 MHz bandwidth with a frequency resolution of 200 kHz and a time resolution of 13 μs, and it can average 2²⁴ (7 h) pulses without any dead time     

Surface physical and chemical changes of pure iron after molybdenum ion implantation and their effects on the tribological behaviour I: Physical and chemical analysis

Molybdenum ions generated by a metal vapour vacuum arc (MEVVA) ion source were implanted into pure iron at doses of 1 × 10¹⁷ and 3 × 10¹⁷ ions cm⁻² with an extraction voltage of 45 kV. Auger electron spectroscopy (AES) sputtering depth profiles, X-ray photoelectron spectroscopy (XPS) analysis, X-ray diffraction (XRD) analysis, microhardness and the residual stress of the implanted specimen were studied. The results show that molybdenum atoms exist in the implanted layer at a maximum concentration 20 at.%. A new phase (Fe₃C) is formed in the specimens implanted higher doses due to carbon incorporation during sputtering of the natural oxide film from the implanted surface. The Fe₂Mo phase is formed in both dose regimes. Residual compressive stresses of 310 and 560 MPa were measured on the surfaces of the specimens after molybdenum ion implantation at 1 × 10¹⁷ and 3 × 10¹⁷ ions/cm² respectively due to a local expansion of the lattice in the near-surface region. Due to the existence of residual compressive stress and the formation of the new phases, the microhardness of pure iron specimens was increased from 264 to 325 and 333 kgf mm⁻² by molybdenum ion implantation at 1 × 10¹⁷ and 3 × 10¹⁷ ions cm⁻² respectively.     

Implantation effect of diamond-like carbon films by 110 keV nitrogen ions

Diamond-like carbon films, grown on microscope slides by a dual-ion beam sputtering system, were implanted by 110 keV N⁺ under the doses of 1 × 10¹⁵, 1 × 10¹⁶ and 1 × 10¹⁷ions cm⁻² respectively. The implantation induced changes in electrical resistivity of the films and in infrared (IR) transmittance of the specimens were investigated as a function of implantation dose. The structural changes of the films were also studied using IR spectroscopy and Raman spectroscopy. It was observed that, with the increase of implantation dose, the diamond-like carbon films display two different stages in electrical and optical behaviours. The first is the increase of both the film resistivity and the IR transmittance of specimen at the dose of 1 × 10¹⁵ ions cm⁻² which, we consider, is attributed to the implantation-induced increase sp³ C---H bonds. However, when the doses are higher than 1 × 10¹⁵ ions cm⁻², the film resistivity and the IR transmittance of specimen decrea significantly and the decrease rates at dose range of 1×10¹⁶ to 1×10¹⁷ ions cm⁻² are smaller than those between 1×10¹⁵ and 1 × 10¹⁶ ions cm⁻². We conclude that the significant reductions of the two parameters at high doses are caused by the decreases of bond-angle disorder and of sp³ C---H bonds, the increases of sp² C---C bonds dominated the crystallite size and/or number and also the sp² C---H bonds. The smaller decrease rates at a dose range of 1 × 10¹⁶ to 1 × 10¹⁷ ions cm⁻² may be caused by further recombination of some retained hydrogen atoms to carbon atoms.     

Very high Q microwave spectroscopy on trapped171Yb+ ions: application as a frequency standard

A microwave frequency standard based on buffer-gas cooled ¹⁷¹ Yb⁺ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional frequency instability characterized by an Allan deviation σ_y (τ)=3.7×10^-13τ^-1/2 for τ<3000 s. Microwave Ramsey fringes with a Q factor of 1.5×10¹³ have been observed     

Dielectric properties of AlN films prepared by laser-induced chemical vapour deposition

The dielectric properties and electrical conductivity of AlN films deposited by laser-induced chemical vapour deposition (LCVD) are studied for a range of growth conditions. The static dielectric constant is 8.0 ± 0.2 over the frequency range 10²−10⁷ Hz and breakdown electric fields better than 10⁶ V cm⁻¹ are found for all films grown at temperatures above 130°C. The resistivity of the films grown under optimum conditions (substrate temperature above 170°C, NH₃/TMA flow rate ratio greater than 300 and a deposition pressure of 1–2 Torr) is about 10¹⁴ Ω cm and two conduction mechanisms can be identified. At low fields, F < 5 × 10⁵ V cm⁻¹ and conductivity is ohmic with a temperature dependence showing a thermal activation energy of 50–100 meV, compatible with the presumed shallow donor-like states. At high fields, F > 1 × 10⁶ V cm⁻¹, a Poole-Frenkel (field-induced emission) process dominates, with electrons activated from traps at about 0.7–1.2 eV below the conduction band edge. A trap in this depth region is well-known in AlN. At fields between 4 and 7 × 10⁵ V cm⁻¹ both conduction paths contribute significantly. The degradation of properties under non-ideal growth conditions of low temperature or low precursor V/III ratio is described.     

Precision interferometric frequency measurement of the 674 nm2S1/2-2D5/2 transition in asingle cold Sr+ ion

The narrow linewidth 674 nm ²S_1/2-² D_5/2 transition in a cold Sr⁺ ion confined within an RF Paul trap has been probed using an optically narrowed 674 nm diode laser offset-locked to a second diode laser stabilized to a high-finesse ultra-low-expansion reference cavity. The transition frequency has been measured by means of interferometric comparison with an iodine-stabilized 633 nm He-Ne reference standard. A preliminary value for the ²S_1/2-²D_5/2 transition line center is 444 779 045 (9) MHz, limited by residual micromotion and low magnetic field Zeeman splitting     

Electrical conduction mechanisms in thermally evaporated thin films of 2-(4-biphenylyl) -5-(4-tert-butylphenyl) -1,3,4-oxadiazole

The AC electrical behaviour of sandwich devices fabricated entirely in vacuum using thermally evaporated thin films of 99+% pure 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4 oxadiazole (Bu–PBD) and gold electrodes, is observed over the frequency range 10⁴ to 10⁷ Hz and the temperature range 297 to 398 K. The capacitance and dissipation factor decrease with increasing frequency and at high frequency (>2×10⁶ Hz) the capacitance and dissipation factor are not temperature dependent. The results obtained show the relative importance of the variable range hopping model and band theory in describing the film conductivity.     

Instrumentation for time-resolved measurement of ultrasoundvelocity deviation

An inexpensive digital data acquisition unit is described which has the capability to record ultrasound velocity variations up to 20 m/s in ultrasound velocity at rates up to 1000 readings per second. No calibration procedures are required. The intrinsic velocity resolution of the unit is less than 1 part in 10⁶; however, the short-term frequency instability of the phase-locked loop instrument used as a velocity sensor limits the overall resolution to 1 part in 10 ⁵     

Influence of Si doping level on the Raman and IR reflectivity spectra and optical absorption spectrum of GaN

The optical absorption (hν) and Raman and Infra Red (IR) spectra of Si doped GaN layers deposited on sapphire through buffer layers have been recorded for electron concentrations from 5×10¹⁷ to 5×10¹⁹ cm⁻³. The (hν) values deduced from photothermal deflection spectroscopy (0.5–3.5 eV) and IR absorption (0.15–0.5 eV) vary between 50 and 10⁴ cm⁻¹ showing doping dependant free electron absorption at low energy, doping independant band gap at high energy, and slowly doping dependant defect absorption in the medium energy range. In our micro Raman geometry, maxima appear or can be deduced near the frequency expected for either the A₁(LO⁻) or the A₁(LO⁺) modes split from the A₁(LO) mode by plasmon phonon interaction. There is a large systematic evolution in the expected way for the IR reflectivity.     

Ultrasound characterization of normal ocular tissue in thefrequency range from 50 MHz to 100 MHz

The ultrasonic properties of ocular tissues including sclera, cornea, ciliary body and iris have been quantitatively evaluated over the frequency range from 50 MHz to 100 MHz at 37°C. Measurements were made with a wideband 60 MHz PVDF copolymer transducer in conjunction with a C-scan microscopy system developed in the authors' laboratory. Using this system, high resolution overview images were produced to identify homogeneous tissue regions for detailed quantitative analysis. The speed of sound for the four eye tissues ranged from 1542 m/s for iris to 1622 m/s for sclera. At 50 MHz the attenuation coefficient ranged from 1.3 dB/mm for cornea to 4.3 dB/mm for sclera. Scleral tissue also had the highest backscatter coefficient (0.0157 Sr^-1 mm^-1), while iris had the lowest (0.00184 Sr^-1 mm^-1). The measured ultrasonic properties are qualitatively related to their histological structure and imaging characteristics     

Absolute frequency control of a 1560 nm (192 THz) DFB laser lockedto a rubidium absorption line using a second-harmonic-generated signal

We demonstrate second-harmonic generation from a DFB laser at 1560 nm in a type I critically phase-matched KNbO₃ crystal. We obtain 2.2 nW at 780 mm with 11.3 mW at 1560 nm incident on the crystal. The conversion efficiency is 17.2 pW/(mW)². The 780 nm beam is used to interrogate a resonance of the ⁸⁷Rb-D₂ line at 780 nm and lock the laser frequency. To characterize the absolute frequency stability, two 1560 nm DFB lasers are respectively stabilized on a Doppler resonance of the ⁸⁷Rb-D₂ line (780.246 nm) and of the ⁸⁵Rb-D₂ line (780.244 nm). The square root of the Allan variance measured from the beat note is around 1.5×10^-9 for averaging times between 3 and 100 s. To improve the precision of the frequency locking, we realize a setup to observe a saturated absorption profile. We use a 780 nm stabilized laser as a pump and the SHG signal as a probe. A saturated absorption profile is observed over the Doppler envelope. Work is under progress to use this saturated resonance for an improved frequency control     

Effects of Gallium on the Structure and Electrical Properties of 0.65 Bi0.94La0.06) (GaxFe1?x)O3-0.35PbTiO3 Ceramics

Crystalline solutions of 0.65 (Bi_0.94La_0.06) (Ga_xFe_1-x)O₃-0.35PbTiO₃ ceramics (BLGF-PT) for x = 0 and 0.05 have been fabricated by the solid-state reaction method. X-ray diffraction (XRD) was utilized to characterize the crystal structure and examine any possible impurities existing in the ceramics. The effects of Ga substitution on dielectric properties of the samples were studied at frequencies from 10² to 10⁶ Hz over a temperature range from 20 to 620degC. The results indicate that Ga modification can reduce the room temperature dielectric loss. The conduction mechanism of the material was investigated using ac conductivity. It is concluded that electrons originating from Fe²⁺ and oxygen ion vacancies are the main charge carriers, and Ga doping could decrease the electronic conduction effectively. The frequency dependence of ac conductivity was found to follow Jonscher's universal power law.     

Neutron irradiation of Nb3Sn and NbTi multifilamentary composites

NbTi and Nb3Sn multifilamentary composites have been irradiated with fast-neutrons at 60 ± 5°C to fluences of 1.2×10²⁰n/cm²(E > 1 MeV). The NbTi samples show only a moderate reduction of Icas a function of neutron fluence in an applied field of 40 kG. Reductions in Icwere observed for fluences greater than 3 × 10¹⁷n/cm²and saturate at 18% for fluences greater than 3-4 × 10¹⁹n/cm². The Nb3Sn composites showed large neutron radiation induced changes in Tc, Icand Hc2. Reductions in Tcwere observed for fluences greater than 7 × 10¹⁷n/cm². No measurable changes in Ic(40 kG) were observed below 10¹⁸n/cm². Between 2 and 3×10¹⁸n/cm², however, there is an apparent threshold where a very rapid reduction in Ic(40 kG) is initiated. At the threshold the decrease in Tcis 13%. Between the threshold and 1.1 × 10¹⁹n/cm², I2(40 kG) has been reduced to 4% of the unirradiated value. These changes in superconducting properties in NbTi and Nb3Sn are analyzed in terms of the radiation induced defects. The impact of the response to irradiation of both materials on their applications in fusion reactor magnets is discussed.     

Dielectric properties of vacuum-evaporated films of tungsten oxide

The dielectric constant of vacuum-evaporated films of tungsten oxide has been measured in the frequency range 300 Hz–10 MHz at various temperatures (25°C–155°C) and with various thickness (300–-5000Å). The frequency response exhibits a dispersion region between 10 and 100 kHz at room temperature (25°C) which shifts to higher frequencies as the temperature rises. The thickness dependence of the dielectric constant shows a rapid rise at low thicknesses but gradually attains a saturation value at thicknesses at which the density of the films is seen to approach its bulk value. Electron diffraction patterns show that these films are stoichiometric, possibly due to the very slow rate of evaporation. The films have a dielectric strength of the order of 5.5 × 10⁶ V cm⁻¹.     

Fabrication and Characterization of Potassium Ion-Selective Electrode Based on Porous Silicon

Porous-silicon (PS)-based potassium ion-selective microelectrode (K⁺ISME) was fabricated by using a microelectronic planar process and an electrochemical anodization etching technique. The apparent sensing area of the K⁺ISME is 100times100 mum ². The response time t_95% is 20 and 31 s when the concentration change is from low to high and vice versa, respectively. The potentials are constant at pH 2-8. The calibration curve for the K ⁺ISME is linear within a wide range of pK=2.0~6.0. Its average slope during six months is 56.5 mV per decade, which is close to the Nernst response. The detection limit was found to be on the order of 5times10^-7 M. The potentiometric selectivity coefficients (K_i,j ^pot) of the K⁺ ISE were 1.8 for NH ₄ ⁺, 3.6 for Li⁺, 4.1 for Na⁺, 4.5 for Mg²⁺, and 4.8 for Ca²⁺, respectively. Good performances of the K⁺ISME are attributed to large specific surface area and excellent adhesion between sensing membrane and the surface of PS     

A novel technique for fabrication of metallic structures on polyimide by selective electroless copper plating using ion implantation

The successful use of palladium ion implantation into polyimide to seed an electroless plated film of copper on the polyimide surface is reported. Polyimide (Hitachi PIX 3400) was implanted with palladium ions to doses of 1.5 × 10¹⁵ − 1.2 × 10¹⁷ ions cm⁻² using a MEVVA ion implanter. The implanted ions acted as sites for nucleation of copper film. A copper film was then deposited on implanted polyimide using a commercial electroless plating solution. The ion energy was kept low enough to facilitate a low critical ‘seed’ threshold dose that was measured to be 3.6× 10¹⁶ Pd ions cm⁻². Test patterns were made using polyimide to study the adaptability of this technique to form thick structures. Plated films were studied with optical microscopy, Rutherford Backscattering Spectrometry (RBS) and Profilometry. The adhesion of films was qualitatively assessed by a ‘scotch tape test’. The film growth (thickness) was observed to be linear with plating time. A higher implantation dose led to greater plating rates. The adhesion was found to improve with increasing dose.     

Temperature dependencies of excited states lifetimes and relaxation rates of 3–5 phonon (4–6 μm) transitions in the YAG, LuAG and YLF crystals doped with trivalent holmium, thulium, and erbium

The temperature dependencies of the nanosecond multiphonon relaxation (MR) rates of the ³F₃ state of Tm³⁺ in the YLF crystal and of the ⁵F₅ state of Ho³⁺ ion in the YAG and LuAG crystals and of the microsecond MR rates of the ⁴F_9/2 (²H_9/2) state of Er³⁺ ions in YLF were measured in the wide temperature range using direct laser excitation and selective fluorescence kinetics decay registration. For YLF the observed relations are explained by 4-phonon process in the frame of a single-frequency model with hω_eff=450±30 cm⁻¹ for the ³F₃ state of Tm³⁺ and by 5-phonon process with hω_eff=445 cm⁻¹ for the ⁴F_9/2 (²H_9/2) state of Er³⁺. For YAG and LuAG crystals these dependencies are explained by the 3-phonon process with hω_eff=630 cm⁻¹. The decrease of the relaxation rate with the temperature in the range from 13 to 80 K was observed for the ⁴F_9/2 (²H_9/2) state of Er³⁺ in the YLF crystal. It is explained by the redistribution of excited electronic states population of erbium ions over the higher lying Stark levels with different MR probabilities. A good fit of experimental temperature dependence (including the dropping part of the experimental curve) was obtained for single-frequency model (hω_eff=450 cm⁻¹) with W₀₁=8.0×10⁴ s⁻¹ and W₀₂=4.7×10⁴ s⁻¹ accounting Boltzmann distribution of population over two excited Stark levels of the excited state of erbium ions. Employment of this model improves the fit between the experiment and the theory for the ⁵F₅ state of Ho³⁺ ion in YAG as well. Strong influence of the parameters of the non-linear theory of MR, i.e. the reduced matrix elements U^(k) of electronic transitions and the phonon factor of crystal matrix η on the spontaneous MR rates was observed experimentally. The smaller these parameters the slower the spontaneous MR W₀. This fact can be used for searching new active crystal laser media for the mid-IR generation.     

Hg+ trapped ion standard with the superconducting cavitymaser oscillator

The frequency stability of an atomic standard based on ¹⁹⁹ Hg⁺ ions confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition has been measured to be 17 mHz wide, representing a quality factor greater than 2×10¹². A 160-mHz line is used to steer the output of a 5-MHz crystal oscillator to obtain a stability of 2×10^-15 for 24000-s averaging times. In a separate measurement, a 37-mHz line is used to steer the output of the superconducting cavity maser oscillator to reach 1×10^-15 stability at 10000 s     

Measurement of radio channels using an elastic convolver and spreadspectrum modulation. II. Results

A spread spectrum measurement system using a surface acoustic wave convolver has been used to measure radio wave propagation in steel works at 1.75 GHz with an echo delay resolution of ≈20 ns. Due to the high metal content of the factory halls, path loss is found to be small and its exponent to range between 1.1 and 2.3 only. We observed a delay spread between 82 and 548 ns depending on the size of the steel mill, its construction and machinery. Characterizing the radio channel by a stochastic delay line model the echo amplitude probability distribution is found to fit a Rician or log-normal distribution rather than a Rayleigh distribution. The fit of the amplitude distributions is determined by a χ² hypothesis test. From the channel impulse response the coherence bandwidth is deduced to range between 2.4 and 27.4 MHz     

A fast 2-bit digitizer for radio astronomy

The design and performance details for a 2-bit digitizer operating at a 250-MHz clock rate are presented. The digitizer is part of a new correlator system for a three-element millimeter-wave interferometer. The performance of the digitizer circuit is analyzed in terms of threshold errors, indecision, sampling aperture width, and timing errors. For an input bandwidth of 125 MHz, digitizer distortion actually improves the sensitivity of the interferometer by about 0.3% but limits the dynamic range of the instrument to about 2×10³