Index of content:
Volume 70, Issue 1, January 1999
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
70(1999); http://dx.doi.org/10.1063/1.1149479View Description Hide Description
We show that the photoacoustic signalsgenerated at the high reflection mirror of a pulsed laser depend linearly on the output energy. This allows us to measure acoustically the output energy, the tuning range, and the optimum temperature and concentration of a dye laser, with the advantage that there is no interference of the output beam, as is the case with optical measurements.
70(1999); http://dx.doi.org/10.1063/1.1149533View Description Hide Description
Pulse stacking, or synchronous pumping, is a novel approach that offers important advantages in cavity ring-down spectroscopy. Using an ultrashort pulse, high repetition rate laser source we have shown that it is possible to resonantly stack pulses in a high finesse cavity, significantly enhancing the decay wave forms obtained when the laser source is abruptly terminated. We have achieved signal-to-noise ratio improvements of several orders of magnitude compared to single pulse injection systems, demonstrating a sensitivity of at 5.38 μm.
70(1999); http://dx.doi.org/10.1063/1.1149534View Description Hide Description
A new type of optical modulator is reported in which a phase grating is used to diffract a beam of light. Piezoelectric elements modulate the phase shift between adjacent portions of the grating, and hence control the diffraction efficiency. Efficiencies of up to about 17% have been observed, compared to the theoretical maximum of about 40% for diffraction into a single order by a pure square wave phase grating.
70(1999); http://dx.doi.org/10.1063/1.1149535View Description Hide Description
Beam line optics for synchrotron facilities are generally designed from geometric calculations wherein the bending magnet, undulator, or wigglerphotonsource is approximated by a high divergence point source. In this article we will, however, show that in the case of a synchrotron radiationsource with very small divergence (such a source can be encountered at an undulator based third generation facility) the imaging properties resemble those of a laser source. This implies certain criteria for the focusing properties of the optical elements in the beam line. For instance, a beam waist is found close to the back focal plane of the optical element, with its position and size dependent on the divergence. If the divergence is determined predominantly by the diffraction part, this also means that the size and position of the beam waist is photon energy dependent. The study has been carried out by analytic treatment, numerical simulations as well as beam profile measurements carried out at beam line I511 at the MAX II synchrotron in Lund, Sweden.
Interfacing a transient digitizer to a step-scan Fourier transform spectrometer for nanosecond time resolved spectroscopy70(1999); http://dx.doi.org/10.1063/1.1149536View Description Hide Description
A new signal processing and data acquisition system has been developed that allows a Fourier transformspectrometer to be interfaced to external transient digitizers for time-resolved spectroscopy.Time resolution is limited only by the transient digitizer and detection system response time. For the present system it is about 1 ns. The capabilities of this system are demonstrated with visible Fourier transform spectra of both scattered laser light and fluorescence from electronically excited gas.
Cryogenic multiple reflection absorption cell and Fourier transform spectrometer system for the far infrared70(1999); http://dx.doi.org/10.1063/1.1149537View Description Hide Description
A far-infrared system used to measure weak absorption spectra of gases at pressures up to a few atmospheres and at temperatures as low as 20 K is described. The principal component is a multiple reflection mirror cell, cooled either with liquid nitrogen or with the boil-off gas from liquid helium. The cell, of 35 l volume, has an optical beam and a pathlength which can be varied from 4 to 60 m. At the longest pathlength diffraction limits the lowest usable frequency to 20 cm−1. The cell is coupled to a Fourier transformspectrometer and a 1.6 K bolometer. The apparatus has been used in a detailed study of the collision-induced pure translational band of and the translation–rotation bands of and mixtures.
A fiber-optic-based multichannel time-correlated single-photon-counting device with subnanosecond time resolution70(1999); http://dx.doi.org/10.1063/1.1149538View Description Hide Description
A fiber-optic-based multichannel time-correlated single-photon-counting device with subnanosecond time resolution was developed. A passively mode-locked Ti:sapphire laser provided the excitation source for 12 separate time-correlated single-photon counting channels in which three single-photon avalanche diodedetectors were used to analyze the fluorescence from the channels. Single-mode optical fibers provided the conduit for the excitation light as well as the fluorescence emission. We determined that the excitation pulses undergo a temporal spread of 50 fs, a spectral spread of 0.2 nm, and remained nearly transform limited. The average instrument response function of each channel was determined to be 181 ps (full width at half maximum). The fluorescence lifetime of a representative near-infrared dye, aluminum tetrasulfonated naphthalocyanine, was determined to be 3.08 ns using this fiber-optic-based multichannel time-correlated single-photon-counting device.
70(1999); http://dx.doi.org/10.1063/1.1149539View Description Hide Description
A small, simple, and effective x-ray energy monitor has been built for use in energy-sensitive experiments where normal transmission monitoring is not an option. The instrument uses x raysscattered elastically from air, thin polymer films, or other featureless scatterers as a secondary source for measuring the transmission versus energy through a calibrated x-ray spectroscopy standard. In general, this type of energy monitor can be made from many combinations of detectors and scatterers, including ultrahigh vacuum compatible components. We show experimental results from a miniature, p-i-n diode-based monitor, which fits unobtrusively anywhere along the optical path of the experiment, and can be made easily from inexpensive and readily available parts.
70(1999); http://dx.doi.org/10.1063/1.1149540View Description Hide Description
We describe a new instrumental technique for the excitation, acquisition, and analysis of fluorescence decays from a variety of substances, in the present case plastic scintillators. The fluorescence is excited by β particles from a radioactive source (100 μCi Sr-90). A random photon from the resulting fluorescence decay provides a trigger pulse to start a time-to-amplitude converter (TAC), while another random photon from the same β-excitation event provides the stop pulse. The optical components and geometry for detecting these two photons, i.e., the two photomultipliers (PMT), the filters, and the pulse counting system, are identical. As a consequence, the measuredfluorescence signal is the autocorrelation function of the fluorescence decay from the sample. A delay line of 50 ns is inserted between the “stop” signal PMT and the TAC so that those “stop” pulses which arrive before “start pulses” also are recorded. Thus the acquired fluorescence signal versus time is symmetric about the delay time and contains twice as many counts as without delay. We call the new technique the “time-autocorrelated two-photon counting technique” (TATPC) in distinction to the conventional “time-correlated single-photon counting technique” (TCSPC). We compared both techniques with the same equipment and scintillators, where in the TCSPC case, a β particle is used for the start of the TAC instead of a random photon in the TATPC technique. We find that under similar experimental circumstances, the signal count rate with TATPC is about 50 times larger than with TCSPC. The new method is well suited for obtaining fluorescence decay times from plastic scintillators, which we use in this article to exemplify the technique. More generally, β-particle excitation in combination with TATPC should prove useful for materials with high energy levels or band gaps, which cannot be excited with pulsed lasers in the visible region. The length of our excitation pulse is less than 20 ps and is negligible compared to the temporal response of about 1 ns of the rest of the apparatus. By employing mathematical deconvolution, we are able to measurefluorescence decays from the subnanosecond range and to longer times.
Comparison of digital correlation techniques in time-resolved fluorometry using a radionuclide-scintillation excitation source70(1999); http://dx.doi.org/10.1063/1.1149541View Description Hide Description
We directly compare two digitally based correlation techniques for measuring the fluorescence-decay kinetics of sample solutions excited by a radionuclide-scintillation light source. Time-correlated single-photon counting and digital cross correlation exhibit markedly different results under identical experimental conditions. However, both digital-processing techniques allow analysis even with the low-light intensities provided by the relatively weak scintillation source. The light source is unique among those used for time-resolved fluorescencemeasurements because it is compact, power free, inexpensive, portable, easy to maintain, and delivers a stochastic excitation sequence. The random nature of the pulse train can introduce severe systematic errors if the lifetime measurements are conducted in the wrong manner. In comparing the two digital correlation techniques, we illustrate the behavior of both systems under various conditions of radionuclide activity and light intensity. We find that over a wide range of conditions, digital cross correlation gives superior accuracy, shorter analysis times, and is generally the better approach. However, we also discuss analytical situations where the use of time-correlated single-photon counting might be preferred.
- NUCLEAR PHYSICS, FUSION and PLASMAS
70(1999); http://dx.doi.org/10.1063/1.1149542View Description Hide Description
A “test function” sensitive to the electron distribution function is used to determine the plasma parameters by Langmuir probes. It was successfully applied to determine the parameters of the hot-electron group in a multipolar, magnetically confined Ar plasma, even when the ratio of bulk to hot-electron densities is greater than 50. The method can also be used to measure the positive-ion density from a probe potential slightly below the plasma potential, and to estimate the electron and negative-ion density and temperature in an plasma. A characteristicplasma potential is also introduced and is correlated with the plasma parameters.
70(1999); http://dx.doi.org/10.1063/1.1149575View Description Hide Description
For the investigation of high-Z impurities in the ASDEX Upgrade tokamak a sublimation probe was developed and tested. With this probe it is possible to inject materials, that sublimate at temperatures from about 50 to into the plasma through a controllable valve. For the investigation of the tungsten transport in ASDEX Upgrade the probe was operated with tungsten carbonyl. The flux of tungsten, which is difficult to determine directly because of the uncertain atomic data, can be determined using the fluxes of oxygen and carbon, the atomic data of which are better known. In this article the setup of the probe and first experiments are described. Here the layer of deposited tungsten was investigated and the number of emitted photons per ionization (the ratio) for the 400.8 nm line of W I was estimated.
Inframetrics infrared scanner upgraded to a 12-bit digital thermography system for heat load measurements at nuclear fusion experiments70(1999); http://dx.doi.org/10.1063/1.1149508View Description Hide Description
A commercially available Inframetrics Scanner (Modell 760) has been upgraded to 12-bit resolution in order to derive heat fluxes on tokamak devices (TEXTOR-94). The temperature resolution is high enough to detect—at the same camera settings—the small sawtoothing heat flux modulation and the large disruptive heat fluxes. Eight new boards have been developed and installed at TEXTOR-94 to perform the measurement, transmit the data from the experiment to the control room, and store the data in a PC. To ensure reliable data transmission, the data stream (1.1 MSPS) is transferred via optical fibers from the camera to the PC in the control room. The PC receives timing information from the TEXTOR-94 timing system such that a synchronization of the cameraclock and the experimental clock is guaranteed. The new system includes a time reference to correct a small frequency drift of the original Inframetrics scanner. This feature permits to compare the heat flux with signals from other TEXTOR-94 diagnostics during fast events like disruptions for which the camera can also be operated in the fast line scan mode.
Determination of fluorine atom density in reactive plasmas by vacuum ultraviolet absorption spectroscopy at 95.85 nm70(1999); http://dx.doi.org/10.1063/1.1149543View Description Hide Description
Vacuum ultraviolet absorption spectroscopy was developed for the measurement of absolute fluorine (F) atom density in reactive plasmas. In order to minimize the influence of radiation trapping (self-absorption) in the light source,fluorescence at a wavelength of 95.85 nm from the F atoms in an electron–cyclotron resonance (ECR) plasma, which was operated with a low microwave power (0.1 kW) and a low gas pressure (1 mTorr), was employed as the probe emission. A windowless transmission system for the probe emission was constructed by connecting the ECR light source with the target plasma and the detection system using vacuum tubes having small slits. The connection tubes were differentially evacuated with turbomolecular pumps to prevent neutral particles from passing through between the ECR and target plasmas. The present method was applied to high-density and plasmas produced by helicon-wave discharges. The accuracy of the measurement was examined carefully by evaluating various sources of error. In the present article, we have emphasized the evaluation of the radiation trapping effect in the light sourceplasma.
- BASIC PHENOMENA
70(1999); http://dx.doi.org/10.1063/1.1149544View Description Hide Description
A magnetometer is described for measurements in pulsed magnetic fields up to 43 T. The compensation system allows removal of pickups produced both by the longitudinal and transverse components of the pulsed field. The compensation level achieves (of the signal from the working coil with an inner diameter 1.4 or 1.75 mm) for the longitudinal and for the transverse components. Sensitivity of the magnetometer is about for the fields up to 10 T and decreases down to at the pulse amplitude of 35 T. A deviation of the compensation is below for the temperature increase from 77 up to 300 K and below after the sample replacement. The sample temperature is controlled by a fast-acting temperature control system in the range from 6 up to 300 K with accuracy of 0.3–0.05 K.
Harmonic detection of multipole moments and absolute calibration in a simple, low-cost vibrating sample magnetometer70(1999); http://dx.doi.org/10.1063/1.1149545View Description Hide Description
The design and fabrication of a low-cost, highly sensitive, fully automated vibrating sample magnetometer for dc magnetization measurements is described. Harmonic detection of magnetic multipole moments is demonstrated experimentally in an inhomogeneously magnetized nickel sample at room temperature and in a gadolinium sample across the magnetic phase transition. A method is presented to nullify the earth’s effective field even without any shielding. More significant, the setup uses only one lock-in amplifier for signal detection, vibration excitation, and temperature measurement and control. First principles calculations give an absolute determination of the instrumental conversion factor that agrees with the experimental calibration factor. The setup is stable from 77 to 300 K, the overall sensitivity is and the reproducibility is ⩽1%.
70(1999); http://dx.doi.org/10.1063/1.1149546View Description Hide Description
A high-frequency transducer for generation of shear acoustic waves in arbitrary substrates has been considered. It exploits thickness-shear resonance in thin piezoelectric films, excited by a lateral electric field through a specific electrode configuration. The resonant equation for the center-of-band frequencies has been derived and possible applications including efficient excitation of surface acoustic waves have been discussed.
A self-contained, portable variable-pressure hydrostatic cell for use in low gauge pressure electromagnetic, ultrasonic, and photoacoustic studies70(1999); http://dx.doi.org/10.1063/1.1149547View Description Hide Description
A simple, manually adjustable hydrostatic cell for electromagnetic, ultrasonic, and photoacoustic studies in absolute pressure ranges from 40 kPa to 5.5 MPa is described. The cell consists of two 3-mm-thick quartz windows enclosing a 1.3-cm-diameter hole bored through a 2.54-cm-long cube block of stainless steel. Four 3-mm-diameter counterbored and taper-threaded holes on the cube walls provide minimally intrusive, chemically inert ports for temperature and acoustical monitoring. Pressure is easily varied within the cell by means of a stainless steel threaded shaft with an O-ring seal at the end, situated inside a matched internally threaded housing. An example of photoacoustic waveforms acquired under conditions of varying hydrostaticpressure is provided.
70(1999); http://dx.doi.org/10.1063/1.1149548View Description Hide Description
In this article a cryogenicmirage setup to perform thermal diffusivitymeasurements of gases in the temperature range 77–500 K is presented. The device was calibrated by measuring the temperature dependence of the thermal diffusivity of pure nitrogen and pure helium that have very different thermal properties. The ability of the device was demonstrated by measuring the thermal diffusivity of gas binary mixtures that fit very well the theoretical values obtained from the kinetic theory.
70(1999); http://dx.doi.org/10.1063/1.1149549View Description Hide Description
This article describes the behavior of Cernox™ zirconium oxynitride film temperature sensors from Lake Shore Cryotronics, Inc. in magnetic fields up to 32 T at temperatures between 2 and 286 K. Results from a number of sensors with different dimensionless temperature sensitivities and from different production batches are analyzed and compared with previous results on carbon-glass sensors that extend to 19 T. In that field range, the Cernox™ sensors appear to be a real alternative to carbon glass. Furthermore, the magnitude of their uncorrected error, is smaller than other sensors at most temperatures in fields less than 20 T, and their temperature correctabilities appear to apply to off-the-shelf sensors with dimensionless temperature sensitivities in the range of −0.74 to −1.9. The sensors show a negligible (<0.05%) orientation dependence of their at 78 K; at 4.2 K, that dependence can be as high as ∼0.7% at 20 T.