Index of content:
Volume 74, Issue 6, June 2003
- REVIEW ARTICLE
74(2003); http://dx.doi.org/10.1063/1.1570945View Description Hide Description
Quartz fibers were first applied in particle physics experiments for detectors that operated at extremely high radiation levels. The superior radiation hardness of high-purity quartz was the driving consideration at that time. However, it turned out that these fibers have many other interesting and beneficial aspects, which derive from the fact that the Čerenkov effect forms the basis of the signals in quartz-based detectors. These aspects are reviewed in this paper. We also discuss some details of the radiation damage characteristics of quartz.
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
A new spectrometer using multiple gratings with a two-dimensional charge-coupled diode array detector74(2003); http://dx.doi.org/10.1063/1.1573744View Description Hide Description
A new spectrometer with no moving parts uses a two-dimensional Si-based charge-coupled diode (CCD) array detector and an integrated grating consisting of three subgratings. The effective spectral range imaged on the detector is magnified threefold. The digitized spectral image in the 200–1000 nm wavelength range can be measured quickly. The nonlinear relationship between CCD pixel position and wavelength is corrected with multiple polynomial functions in the calibration procedure, which fits the data using a mathematical pattern-analysis method. The instrument can be applied for rapid spectroscopicdata analyses in many types of photoelectronic experiments and routine testing.
- PARTICLE SOURCES, OPTICS and ACCELERATION; PARTICLE DETECTORS
74(2003); http://dx.doi.org/10.1063/1.1574393View Description Hide Description
A time- and angle-resolved photoelectron spectrometer based on an amplified picosecond Nd:glass laser system is described. The kinetic energy of photoemitted electrons, excited by 0.4 ps laser pulses at 264 nm, is measured with a time-of-flightelectron energy analyzer. A theoretical model that takes into account the electron drift inside the μ-metal shielded tube as far as the electron analyzer has been introduced to calculate the energy distribution curves of photoelectrons. As a test of the experimental apparatus we present photoemissionspectra of cesiated silver excited with 4.7 eV photons. Good agreement between the experimental and the calculated distribution curves of photoemitted electrons has been found. Finally, we have briefly discussed the electronic properties of silver in terms of free electron and d-band transitions, together with Auger processes for photon energies near 4 eV.
74(2003); http://dx.doi.org/10.1063/1.1573746View Description Hide Description
We have developed a repetitively pulsed cathodic arc plasma source that can operate at high duty cycle and with long lifetime between downtimes for cathode maintenance. The arc discharge current can be up to 300 A, the arc pulse width can be varied from 0.1 to 4 ms, and the source can operate at a duty cycle of up to 30% for many hours. The cathode can be of a diameter from 20 to 40 mm, and can be easily and quickly changed. A 45° magnetic duct is used for macroparticle filtering, employing a pulsed magnetic field. We have operated the source over a wide parameter range, and we report here on the variation of plasma ion deposition current with arc current, duct magnetic field, and duct bias voltage. A momentary ion deposition current of 0.7 A was obtained with the source running stably for more than 8 h with an arc current of 100 A and duty cycle of 20%.
- NUCLEAR PHYSICS, FUSION and PLASMAS
74(2003); http://dx.doi.org/10.1063/1.1571973View Description Hide Description
Use of high mass atomic neutral beams produced from negative ions as drivers for inertial confinement fusion has been suggested recently. Best candidates for the negative ions would be bromine and iodine with sufficiently high mass and electron affinity. These materials require a heated vapor ion source. Chlorine was selected for initial testing because it has similar electron affinity to those of bromine and iodine, and is available in gaseous form. An experiment was set up by the Plasma and Ion Source Technology Group in Lawrence Berkeley National Laboratory to measure achievable current densities and other beam parameters by using a rf driven multicusp ion source [K. N. Leung, Rev. Sci. Instrum. 65, 1165 (1994); Q. Ji et al., Rev. Sci. Instrum. 73, 822 (2002)]. Current density of 45 mA/cm2 was achieved with 99.5% of the beam as atomic negative chlorine at 2.2 kW of rf power. An electron to negative ion ratio as low as 7 to 1 was observed, while the ratio of positive and negative chlorine ion currents was 1.3. This in addition to the fact that the front plate biasing had almost no effect to the negative chlorine ion and electron currents indicates that a very high percentage of the negative charge in the extraction area of the ion source was in form of ions. A comparison of positive and negative chlorine ion temperatures was conducted with the pepper pot emittance measurement technique and very similar transverse temperature values were obtained for positive and negative chlorine ions.
74(2003); http://dx.doi.org/10.1063/1.1571974View Description Hide Description
We describe a recently designed and constructed system based on a 1 m normal incidence vacuum monochromator with corrected (toroidal) optics that produces a wavelength tuneable and collimated vacuum-ultraviolet (VUV) (λ=30–100 nm) beam. The VUV continuum source is a laser-generated goldplasma. The primary function of the system is the measurement of time resolved “images” or spatial distributions of photoabsorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest (in our case a second synchronised plasma) and recording the “footprint” of the attenuated beam on a charge coupled device. Using this VUV photoabsorption imaging or “shadowgraphy” technique we track and extract column density distributions in expanding plasma plumes. We can also measure the plume front velocity. We have characterized the system, particularly in relation to spectral and spatial resolution and the experimental results meet very well the expectations from ray tracing done at the design phase. We present first photoabsorption images and column density distributions of laser produced Ca plumes from the system.
- MICROSCOPY and IMAGING
74(2003); http://dx.doi.org/10.1063/1.1571952View Description Hide Description
An image-processing procedure that assures correct determination of the magnetic field distribution of magneto-opticalimages is presented. The method remedies image faults resulting from sources that are proportional to the incident light intensity, such as different types of defects in the indicator film and unevenness of light, as well as additive signals from detector bias, external light sources, etc. When properly corrected a better measurement of the local magnetic field can be made, even in the case of heavily damaged films. For superconductors the magnetic field distributions may be used for accurate determination of the current distributions without the spurious current loops associated with defects in the films.
- CONDENSED MATTER; MATERIALS
The deformation-DIA: A new apparatus for high temperature triaxial deformation to pressures up to 15 GPa74(2003); http://dx.doi.org/10.1063/1.1570948View Description Hide Description
A new deformation apparatus has been developed, based on the widely used cubic-anvil apparatus known as the DIA. Two differential rams, introduced in the upper and lower guide blocks, allow independent control of the differential strain and stress field under high confining pressure.Testing experiments with synchrotron x rays have demonstrated that this deformation DIA (D-DIA) is capable of generating up to 30% axial strain on a 1–2 mm long sample under confining pressures up to 15 GPa at simultaneous high temperatures. Various compressional strain rates from to about have been achieved. Extensional experiments have also been carried out successfully. Strains are measured by x-ray imaging of the sample which has a length measurement precision of ∼0.1 μm; pressures are monitored using standard materials with well established equations of state.X-ray transparent anvils made of sintered polycrystalline cubic boron nitride have been successfully tested, with a two-dimensional x-ray charge coupled device detector. Distortions in the diffraction lines due to differential stress can be measured with a precision of about 20 MPa.
74(2003); http://dx.doi.org/10.1063/1.1575926View Description Hide Description
Amorphous metallic alloys lack long-range atomic order and consequently exhibit excellent homogeneity, no microstructure discontinuities, and no sharp x-ray diffraction peaks. Moreover they have higher tensile fracture strength and hardness than those of traditional crystalline metals. These excellent physical properties make bulk metallic glasses good candidates for high pressure gaskets for in situ x-ray/neutron diffraction experiments. We tested the amorphous alloy as a gasket material in three experiments. The behavior of the amorphous alloy under a hydrostatic/nonhydrostatic pressure of over 100 GPa was investigated by in situ synchrotron x-ray diffraction. The pressure gradient of the amorphous metallic gasket in the diamond anvil cell was measured on the ruby fluorescence scale during compression and decompression. The results show that bulk metallic glasses are good high-pressure gasket materials for in situx-ray diffraction experiments.
74(2003); http://dx.doi.org/10.1063/1.1571953View Description Hide Description
Synthetic diamond has been successfully used as a backing-plate in the diamond-anvil cell up to 223 GPa. Use of diamond for the backing-plate has a great advantage for powderx-ray diffraction experiments at ultrahigh pressures, for it makes it possible to measure full Debye–Scherrer rings to a maximum scattering angle of The mechanical strength of the diamond backing-plate should contribute to the extension of the current pressure limit of the diamond-anvil cell. Problems and future improvements of the design are discussed.
74(2003); http://dx.doi.org/10.1063/1.1574394View Description Hide Description
A modified diamond anvil cell(DAC) is used for structural studies of amorphous materials at high pressures using a monochromatic synchrotron x-ray beam. The DAC modification includes (1) the use of x-ray transparent seats for a large angular opening for x-ray scattering, and (2) the introduction of a boron gasket insert to increase the sample thickness and to minimize the gasket–hole deformation. A procedure for absorption correction and background subtraction in DAC experiments is described, together with an optimization process for obtaining accurate data of the structure factor and the corresponding pair distribution function. Data for amorphousiron at 67 GPa are presented for demonstration. It is shown that quantitative structural data can be determined for amorphous materials at very high pressures using the DAC. The apparatus should be also useful for structural studies of liquids at high pressures.
Ion beam charging of silicon nanoparticles in helium background gas: Design of the ion beam aerosol charger74(2003); http://dx.doi.org/10.1063/1.1575922View Description Hide Description
An ion beamaerosol charger that ionizes aerosolnanoparticles of less than 10 nm diameter using an ion beam was designed for use in the electrostatic manipulation of gas-suspended nanoparticles.Pulsed laserablation of a solid target in a high purity helium gas under pressure of 2–10 Torr (266–1330 Pa) was employed to fabricate nanometer-sized silicon particles. The ion beam, which was generated by cold cathode Penning ionization of He atoms, was accelerated with an energy of 0–5 keV, penetrated a skimmer located within the differential pumping system, and then entered the aerosolionization chamber. The siliconnanoparticles were both positively and negatively charged by the direct impact of the ion beam or the secondary electrons generated from the surrounding He gas. The change in the concentration of ions and charged aerosols was measured by ion probes. It was found that the concentration of charged particles was drastically increased to 2–50 times that at baseline.
74(2003); http://dx.doi.org/10.1063/1.1573748View Description Hide Description
We report on the design and performance of a combined heating/cooling stage for the thermalization of clusters in a gas phase time-of-flightmass spectrometer. With this setup the cluster temperature can sensitively be adjusted within the range from 100 up to 800 K and higher. The unique combination of a heating stage with a subsequent cooling stage allows us to perform thermodynamic investigations on clusters at very high temperatures without quality losses in the spectra due to delayed fragmentation in the drift tube of the mass spectrometer. The performance of the setup is demonstrated by the example of clusters.
74(2003); http://dx.doi.org/10.1063/1.1575921View Description Hide Description
A data acquisition system is reported with a number of improvements over similar systems currently used in Rayleigh scattering of Mössbauer radiation (RSMR) experiments. It is based on a data acquisition board including a Mössbauer function generator with adjustable frequency reference signal delivery, an internal single channel analyzer, and an internal 32 kB random access memory for data storage. The system performances were tested in RSMR measurements on pyrolytic graphite C (002) single crystal by using an iron enriched in rhodium matrix absorber. We want to demonstrate that the reported data acquisition system is suitable for the experimental separation of the elastic and inelastic components of the scattered radiation. The system can be used also in the ordinary Mössbauer measurements as a compact and rapid data acquisition system. In this case it presents the advantage of including also the function generator.
74(2003); http://dx.doi.org/10.1063/1.1574395View Description Hide Description
This article describes a high-stability quartz-crystal microbalance (QCM) and the methodology for measuring the change in mass during thin-film growth in deposition and sputter processes. Much lower noise and higher-frequency stability have been achieved than with conventional QCMs. A stability of ±0.1 Hz at 6 MHz has been obtained over 4 h, with a rms stability of 0.03 Hz. The adsorption of one atomic monolayer of oxygen produces a frequency shift of about 5 Hz, so this stability enables the QCM to be used to determine the stoichiometry of submonolayer oxide films, as well as for high-accuracy measurements of adsorbate sticking probability and ion-milling rate.
Design of a triple resonance magic angle sample spinning probe for high field solid state nuclear magnetic resonance74(2003); http://dx.doi.org/10.1063/1.1571951View Description Hide Description
Standard design and construction practices used in building nuclear magnetic resonance(NMR) probes for the study of solid state samples become difficult if not entirely impractical to implement as the resonance frequency approaches the self resonance frequency of commercial capacitors. We describe an approach that utilizes short variable transmission line segments as tunable reactances. Such an approach effectively controls stray reactances and provides a higher Q alternative to ceramic chip capacitors. The particular probe described is built to accommodate a 2.5 mm magic angle spinning rotor system, and is triply tuned to and frequencies for use at 18.8 T (200, 80, and 800 MHz, respectively). Isolation of the three radio frequency (rf) channels is achieved using both a rejection trap and a transmission line notch filter. The compact geometry of this design allows three channels with high power handling capability to fit in a medium bore (63 mm) magnet. Extended time variable temperature operation is integral to the mechanical design, enabling the temperature control necessary for investigation of biological macromolecules. Accurate measurement of the air temperature near the sample rotor is achieved using a fiber optic thermometer, which does not interfere with the rf electronics. We also demonstrate that acceptable line shapes are only readily achieved using zero magnetic susceptibility wire in construction of the sample coil. Computer simulation of the circuit aided in the physical design of the probe. Representative data illustrating the efficiency, rf homogeneity, and signal to noise factor of the probe are presented.
74(2003); http://dx.doi.org/10.1063/1.1571971View Description Hide Description
We demonstrate how room temperature scanning Hall probe microscopy (SHPM) can be applied to noninvasively probe the (meta-)stable magnetic domain states of individual mesoscopic Co bars as a function of applied field. These data are correlated with magneto-optical measurements of the hysteresis loop as well as higher spatial resolutionmagnetic force microscopy imaging. In particular, we show how a combination of visualization techniques is essential for fully understanding the mechanism of magnetization reversal in such microstructures. Micromagnetic calculations of the magnetic states and their corresponding stray fields are correlated with the observations. These experiments prove the usefulness of a noninvasive and quantitative technique such as room temperature SHPM in an applied magnetic field to study magnetic domain stability and switching field distributions.
74(2003); http://dx.doi.org/10.1063/1.1570947View Description Hide Description
We describe an apparatus to measure the diffusion of dilute fluorophores in molecularly thin liquid films within a surface forces apparatus (SFA). The design is a significant modification of the traditional SFA in that it allows one to combine nanorheology with the single-molecule sensitive technique of fluorescencecorrelation spectroscopy. The primary enabling idea was to place a miniaturized SFA onto the stage of an optical microscope equipped with a long working distance objective and illuminated by a femtosecond laser. A secondary enabling idea was that the silver coating on the backside of mica, normally used in the traditional SFA design for interferometric measurements of the film thickness, was replaced by multilayer dielectriccoatings that allowed simultaneous interferometry and fluorescencemeasurements in different regions of the optical spectrum. To illustrate the utility of this instrument, we contrast the translational diffusion of rhodamine dye molecules (in the solvent, 1,2-propane diol), in the unconfined bulk state and confined between mica sheets to the thickness 2.5 nm. The diffusion coefficient is found to decrease by 2 orders of magnitude under confinement.
High-temperature and high-pressure cell for kinetic measurements of supercritical fluids reactions with the use of ultraviolet-visible spectroscopy74(2003); http://dx.doi.org/10.1063/1.1573747View Description Hide Description
A high-temperature high-pressure ultraviolet-visible (UV-Vis) cell is described. The cell has been designed specifically for use with the UVspectrophotometer Hewlett–Packard 8453 but it could work up to 780 K at 100 MPa with any other UV-Vis spectrophotometer, as well as for near infrared (NIR) experiments. Three features of the cell make it convenient for experiments with supercritical fluids: the possibility to choose an optimal path length, the presence of three interchangeable ports into the cell, and a movable thermocouple in the working zone of the cell. The cell has been used to study a range of chemical reactions in water under near-critical and supercritical conditions, as well as for measuring the kinetics of such reactions. Some examples illustrating the performance of the cell are given.
74(2003); http://dx.doi.org/10.1063/1.1574397View Description Hide Description
We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of through the length of the 2 cm tube allows for high yield of radicals (approximately and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical phenyl radical and methylperoxyl radical IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, we have derived the residence time of the gas pulse in the nozzle to be around 30 μs.