Volume 73, Issue 1, January 2002
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
73(2002); http://dx.doi.org/10.1063/1.1416111View Description Hide Description
A facility for subnanosecond time-resolved (pump-probe) infrared spectroscopy has been developed at the National Synchrotron Light Source of Brookhaven National Laboratory. A mode-locked Ti:sapphire laser produces 2 ps duration, tunable near-infrared pump pulses synchronized to probe pulses from a synchrotron storage ring. The facility is unique on account of the broadband infrared from the synchrotron, which allows the entire spectral range from (0.25 meV) to (2.5 eV) to be probed. A temporal resolution of 100 ps, limited by the infrared synchrotron-pulse duration (full width at half maximum), is achievable. A maximum time delay of 170 ns is available without gating the infrared detector. To illustrate the performance of the facility, a measurement of electron–hole recombination dynamics for an HgCdTesemiconductor film in the far- and mid-infrared range is presented.
73(2002); http://dx.doi.org/10.1063/1.1426232View Description Hide Description
Lock-in imaging using multicycle integration focal plane array (MIFPA) is demonstrated to detect extremely weak image and/or spectroscopic signals. Each pixel of the MIFPA has a correlated multicycle integrator to play the roles of lock-in’s phase-sensitive detector and low-pass filter. In lock-in imaging, the signal current is accumulated while the dark or background current is canceled. As a result, the total integration time is increased by several orders, which is equivalent to the reduction of bandwidth by several orders. All advantages associated with the lock-in amplifier are achieved by imaging with MIFPA. Key issues of the MIFPA technology, including the design and performance of the correlated multicycle integrator, are discussed.
73(2002); http://dx.doi.org/10.1063/1.1424904View Description Hide Description
A two-dimensional detector array has been fabricated from a single 10-mm-diam by 100-μm-thick chemical vapor depositiondiamond disk by applying a metallization grid of with centered bias connections. This diamond has been exposed to high power pulsed laser radiation. It has been shown that this kind of diamond array operates as a radiation hard, ultrafast laser beam profiler and can obtain spatial profiles with 500 ps temporal resolution. Ten spatial profiles were obtained within a single 5 ns duration laser pulse, revealing in detail the temporal and spatial development of the laser beam intensity. No attenuation is necessary for this profiler when making single-shot measurements at intensities up to ∼100 MW/cm2.
73(2002); http://dx.doi.org/10.1063/1.1416114View Description Hide Description
The electrical response of a linear lateral photodetector to three different light projections is examined. First, one section of the photodetector is illuminated and the rest of the detector is covered with a piece of black tape forming a bright/dark edge. Second, the whole photodetector is illuminated and then a wire is placed in front casting a shadow onto the detector. Third, one section of the photodetector is illuminated and the rest of the detector is covered with an optical filter forming a bright/dim edge. The response to each projection is expressed by a set of equations and verified by experiments. The tracking of the position of the bright/dark edge, wire, and bright/dim edge is described.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
73(2002); http://dx.doi.org/10.1063/1.1419216View Description Hide Description
The extraction of high-Z high-charge-state ions up to from a high-energy electron-beam ion trap, the SuperEBIT at Lawrence Livermore National Laboratory, is reported. The SuperEBIT provides a 240 mA electron beam with up to 200 keV of energy. Depending on the operating conditions (pulsed, continuous) and charge state, the number of ions extracted from the SuperEBIT varies between and ions per second under the tested conditions. The ions produced in SuperEBIT are extracted at potentials ranging from 0.5 to 20 keV (continuously variable) to provide highly charged low-emittance ion beams with energies between a few keV and several MeV. The performance of the SuperEBIT as an ion source is described and aspects for future developments and potential applications are discussed.
73(2002); http://dx.doi.org/10.1063/1.1427413View Description Hide Description
A micromechanical sensor for charged-particle flux detection utilizing the extreme force sensitivity of microcantilevers is demonstrated with alpha particles. The latter create a charge buildup on an electrically isolated collection plate, a metallic sphere. The cantilever detector measures the electric field emanating from the charge collection sphere using variations in mechanical parameters. Results are compared with responses due to fixed voltages on the sphere. Parameters investigated include cantilever deflection due to total charge and frequency and damping rate variation due to electrostatic force gradient. The minimum detectable particle fluence using this technique was calculated to be around 1000 particles in air, or about
73(2002); http://dx.doi.org/10.1063/1.1427418View Description Hide Description
A Thomson scattering system has been installed at the Tokyo electron beam ion trap for probing characteristics of the electron beam. A green laser beam was injected antiparallel to the electron beam. The image of the Thomson scattering light from the electron beam has been observed using a charged-coupled device camera. By using a combination of interference filters, the spectral distribution of the Thomson scattering light has been measured. The Doppler shift observed for the scattered light is consistent with the beam energy. The beam radius dependence was investigated as a function of the beam energy, the beam current, and the magnetic field at the trap region. The variation of the measured beam radius against the beam current and the magnetic field were similar to those in Herrmann’s prediction. The beam radius as a function of the beam energy was also similar to Herrmann’s prediction but seemed to become larger at low energy.
73(2002); http://dx.doi.org/10.1063/1.1426226View Description Hide Description
For Coulomb excitation experiments using a γ-ray detector array, a position-sensitive particle detector system that has a good angular resolution, capability of managing a high-counting rate, robustness for a radiation damage, compactness (under 110 mm φ) and easiness of handling is necessary. For this purpose we developed a new device utilizing plastic and yttriumaluminumperovskite activated by cerium (formula abbreviated YAP Ce) scintillators and a position-sensitive photomultiplier tube. The detector system could operate at a high-counting rate (about and with a good position resolution of 0.5–1.2 mm, which was enough for Doppler correction of the γ rays. It could sustain the radiation damage caused by Coulomb excitation experiments with heavy ions for more than 50 days. The detector chamber was well fitted in the 110 mm φ diameter of the internal space of a compact γ-ray detector array, GEMINI. This was much easier to handle than the position-sensitive gas counter. In Coulomb excitation experiment utilizing this device, the nucleus has been Coulomb excited using a Pb target. The transitional matrix elements have been determined and the device has turned out to be useful in Coulomb excitation experiments with heavy-ion beams.
73(2002); http://dx.doi.org/10.1063/1.1416116View Description Hide Description
We have developed a time pickup system for monitoring the beam bursts from a cyclotron. The time pickup can measure the phase and the width of the beam bursts by detecting secondary electrons emitted from a foil or a wire target inserted into the beam with time resolutions around 200 ps full width at half maximum in spite of its simple structure. By using the foil and the wire targets appropriately, monitoring the bursts of a variety of beams can be achieved. In addition, by adjusting the position of the target, a high enough detection rate of the beam burst is obtained with a minimum of interference with the beam. The present investigation shows that it is needed to adjust the collection voltage of the time pickup because the optimum collection voltages for high resolution measurement are in the range from −0.5 to −1.0 kV and those for high efficiency measurement are not less than −4.0 kV.
73(2002); http://dx.doi.org/10.1063/1.1419227View Description Hide Description
Development efforts have gone into the construction and performance testing of a device that can be used to provide snap shot images of the beam profile. It is intended to function even at very low ion intensities, such as those expected from a rare-isotope accelerator. Intensity profiles and emittance analysis are among the most critical tools used for optimizing beam transport through accelerators. This article describes the design and performance of a beamimage monitor. The device is sensitive at a wide dynamic range which spans from to With the advent of double-plane slits or a pepper pot plate, this system can be used to scan transverse emittance profiles in both the and phase space planes, simultaneously. Conventional diagnostic devices used for heavy ion accelerators generally require at least intensity to carry out similar diagnostics, which is not practical when considering beams with very low intensities, such as rare isotopebeams. Furthermore, the detection system used here can be used for a wide range of incident ion velocities. Compared with solid-state detectors and scintillators that are inserted directly into the beam, this type of detection system is less susceptible to beam induced damage resulting in longer lifetimes and less maintenance. The test was done using single charge state Kr beams at energies ranging from 3.6 to 18 keV/u. The device’s sensitivity was monitored for intensities below and an emittance scan was recorded and analyzed. The spatial resolution was characterized by comparing the emittance profile with that obtained by a wire scanning device which had better resolution but was sensitive only to intensities above Recently, the device has been used to aid in the transport of ∼6 MeV/u radioactive beams, such as produced by pickup reactions with a gas cell target at the ATLAS accelerator facility.
73(2002); http://dx.doi.org/10.1063/1.1427302View Description Hide Description
We present experimental evidence that electrons of modest energy are making multiple passes through thin targets placed inside a betatron toroid, thus increasing their bremsstrahlung emission efficiency. Thin Cu, Be, W, and Mylar targets of thicknesses between to radiation lengths were used. The number of passes through the thin radiators were obtained using either the bremsstrahlung photon densities or angular distributions. The number of passes of 33-MeV electrons through the thin radiators was estimated to be 590 for a 1-μm-thick Cu foil, 171 for 5-μm Cu, 30 for 15-μm Cu, 200 for 20-μm Be, 51 for 60-μm Be, 20 for 200-μm Be, 460 for 3-μm-thick Mylar, and 123 for 2-μm W foils. The multiple-pass effect can be useful for increasing the efficiency of novel x-ray sources such as transition and parametric radiators.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Simple, discriminative measurement technique for radon and thoron concentrations with a single scintillation cell73(2002); http://dx.doi.org/10.1063/1.1416121View Description Hide Description
A simple, discriminative measurement technique for radon and thoron concentrations is discussed. In this technique, a single scintillation cell is used for both radon and thoron measurements. It consists of two measurements that use the difference of the half life between the two isotopes. Alpha counting efficiencies for their associated radionuclides were estimated by a Monte Carlo calculation. When evaluating the conversion factor for concentration on two types of scintillation cells, both agreed well with experimental values. Optimum measurement conditions on the timetable are also discussed. This technique can provide two concentrations promptly. Although it is not highly sensitive, it is applicable to performance tests for radon/thoron monitors and simultaneous exhalation rate measurements for both radon and thoron.
Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma73(2002); http://dx.doi.org/10.1063/1.1425777View Description Hide Description
We report on quantitative, spatially resolved density, temperature, and velocity measurements on ground-state atomic hydrogen in an expanding thermal Ar–H plasma using two-photon excitation laser-induced fluorescence(LIF). The method’s diagnostic value for application in this plasma is assessed by identifying and evaluating the possibly disturbing factors on the interpretation of the LIF signal in terms of density, temperature, and velocity. In order to obtain quantitative density numbers, the LIF setup is calibrated for H measurements using two different methods. A commonly applied calibration method, in which the LIF signal from a, by titration, known amount of H generated by a flow-tube reactor is used as a reference, is compared to a rather new calibration method, in which the H density in the plasma jet is derived from a measurement of the two-photonLIF signal generated from krypton at a well-known pressure, using a known Kr to H detection sensitivity ratio. The two methods yield nearly the same result, which validates the new H density calibration. Gauging the new “rare gas method” by the “flow-tube reactor method,” we find a krypton to hydrogen two-photon excitation cross section ratio of 0.56, close to the reported value of 0.62. Since the H density calibration via two-photonLIF of krypton is experimentally far more easy than the one using a flow-tube reactor, it is foreseen that the “rare gas method” will become the method of choice in two-photonLIF experiments. The current two-photonLIF detection limit for H in the Ar–H plasma jet is The accuracy of the density measurements depends on the accuracy of the calibration, which is currently limited to 33%. The reproducibility depends on the signal-to-noise (S/N) ratio in the LIFmeasurements and is orders of magnitude better. The accuracy in the temperature determination also depends on the S/N ratio of the LIF signal and on the ratio between the Doppler-width of the transition and the linewidth of the excitation laser. Due to the small H mass, the current linewidth of the UV laser radiation is never the accuracy limiting factor in the H temperature determination, even not at room temperature. Quantitative velocity numbers are obtained by measuring the Doppler shift in the H two-photon excitation spectrum. Both the radial and axial velocity components are obtained by applying a perpendicular and an antiparallel excitation configuration, respectively. The required laser frequency calibration is accomplished by simultaneously recording the absorptionspectrum with the fundamental frequency component of the laser system. This method, which is well-established in spectroscopic applications, enables us to achieve a relative accuracy in the transition frequency measurement below corresponding to an accuracy in the velocity of approximately 200 m/s. This accuracy is nearly laser linewidth limited.
73(2002); http://dx.doi.org/10.1063/1.1426229View Description Hide Description
The conventional rigid rotation model of thermal equilibrium does not apply to a toroidalnon-neutral plasma trap; the nonuniformity of the magnetic field produces an inhomogeneous flow. For sufficiently large cyclotron frequency; plasma frequency), the flow can be approximated by the drift velocity. The toroidal equilibrium of an electron plasma has been analyzed for a rather complex geometry of magnetic shear configuration.
73(2002); http://dx.doi.org/10.1063/1.1427415View Description Hide Description
The working characteristic of a new hollow cathode, thermionic-arc dc dischargeionization device was investigated experimentally. Under typical operating conditions the device produces a steady, field-free plasma plume by ionizing a flowing gas column. Experiments were performed with argon, nitrogen, and helium, the typical discharge pressures being 0.2–0.6 Torr. The discharge exploits the inherent stability of the point to plane geometry and the ionization efficiency of the hollow cathode. Electrically, the discharge is fed from a constant–voltage power supply and maintained below the interelectrode gap breakdown threshold. Thus the dischargecurrent is given by Ohm’s law and is a function of the applied field and local plasma parameters. This modus operandi increases the ionization efficiency by maintaining a local thermal nonequilibrium in the discharge. The degree of nonequilibrium maintained in the plume downstream the cathode was higher in nitrogen than in argon and is attributed to a lower collision frequency in the nitrogen plume. At an input power level of 0.9 kW plasmas with electron densities of in nitrogen and in argon were recorded in the plume outside the high-field cathode region. Helium discharge displayed the highest nonequilibrium level but at the same time showed the largest degree of instability and the lowest electron density levels The experiments indicate that the stability and the ionization efficiency of the discharge are enhanced by a supersonic flow field. Flow surveys have shown that the more stable argon and nitrogen plumes were slightly supersonic in the range of pressure investigated while helium flow remained subsonic throughout. The important consequence to the supersonic flow is the development of a normal shock in front of the cathode filament. Analysis of the data indicates that the normal shock is an effective stabilizing boundary.
- BASIC PHENOMENA
73(2002); http://dx.doi.org/10.1063/1.1427416View Description Hide Description
Practical techniques for producing both Hilbert and wavelet transform representations of peak frequency are given. Peak frequency-time plots of simple model signals are produced using both transforms and are presented and compared. This study shows that the peak frequency-time plots produced using the wavelet transform are superior to those produced using the Hilbert transform since the wavelet representation does not show large false frequency indications when the model time series undergoes a large abrupt frequency change. Further, when multiple dominant frequencies are present in the model time series, the wavelet based peak frequency-time plot more clearly identifies the higher of the dominant frequencies while the Hilbert based peak frequency-time plot is affected by the lower frequency and leads to less clear results.
- MICROSCOPY and IMAGING
73(2002); http://dx.doi.org/10.1063/1.1425776View Description Hide Description
We present a novel construction of a scanning tunneling microscope(STM) for investigations of fluid/solid interfaces and, in particular, for in situelectrochemicalmeasurements at elevated temperatures. A special feature of this instrument is a vacuum tight connection of the electrochemical cell with the STMscanner via a flexible metal bellow. This enables measurements with highly reactive and volatile fluids at high temperatures. Details of the mechanical and electronic parts of this setup are described. Test measurements on the electrodeposition of metals from molten saltelectrolytes have been performed. The Ag deposition has been studied in an acidic room temperaturemolten salt composed of 1-butyl-3-methyl-imidazoliumchloride and up to 355 K. As a second example the Al deposition from molten has been tested up to 500 K. First results of these experiments are briefly presented.
- CONDENSED MATTER; MATERIALS
An experimental method to investigate the structure and kinetics of patterned surfaces using laser light diffraction73(2002); http://dx.doi.org/10.1063/1.1425774View Description Hide Description
We describe a novel experimental method using the diffraction of a He–Ne laser beam to study surfacespatterned with structures on mesoscopic to macroscopic length scales. The technique provides high spatial and temporal resolution; it is not limited to periodic, artificial structures, but is also well suited to study the development of self-organizedsurface relief. Measurements can be performed under in situ conditions in a diffraction mode or an imaging mode, providing (1) qualitative and quantitative information on the surface structures, (2) information on time-dependent surface changes with a resolution of 10 μs or better, (3) observation of incubation processes (including determination of incubation time) in first-order, displacive phase transformations, and (4) observation of the surface in real space, in particular, the pattern evolution as a function of temperature or other parameters. As an example we show results of the application of our method to a single crystal undergoing a martensitic transformation.
73(2002); http://dx.doi.org/10.1063/1.1419223View Description Hide Description
A method for measuring the surface tension of free standing smectic films is described which is considerably more precise than the methods used up to now. Data on the precision and the reproducibility of the device are presented. The temperature dependence of the surface tension for the smectic liquid crystal is discussed.
High pressure hydrogen loading cell for photoconductivity measurements down to the milliKelvin regime73(2002); http://dx.doi.org/10.1063/1.1425775View Description Hide Description
A gas loading cell has been developed to load rare earth thin film samples with hydrogen at pressures up to 200 bars at room temperature. A miniature valve closes the gas inlet, after which the cell is suspended from the cold tail of a flow cryostat into the bore of a 16 T superconducting magnet. An ultraviolet stroboscope outside the cryostat illuminates the sample by way of an optical fiber to a window in the cell. Electrical feedthroughs permit photoconductivity and magnetotransport measurements over three decades in temperature. Extension to other materials, different gas atmospheres, and helium dilution refrigeratortemperatures is straightforward.