Boxcar Averager Research Articles

Development of a fibre optic luminescence lifetime spectrometer

A spectrometer for measuring luminescence lifetimes has been developed. The design includes a bifurcated fibre optic cable for delivering the light from the source to the sample and from the sample to the detector. It uses a fast photomultiplier tube as the detector, a boxcar averager for data acquisition and a PC clone for data analysis. A nitrogen pumped dye laser is used as the light source and this has a pulse width of 1·2 ns. The spectrometer has been used to measure lifetimes from a minimum of 0·75 ns (rose bengal) to 597 μs (solid uranyl nitrate hexahydrate). A study has been made of the effects of concentration and temperature on the decay of uranyl ions (UO 2 2+ ) in aqueous solution. The decay is found to be biexponential, except at temperature above 60°C, where a three-component decay analysis gives the best results for matching the data to an Arrhenius Plot. The nature of the third emissive state, appearing at high temperatures is unknown and no spectroscopic state assignment has yet been made.

Comment on ‘Magnetic appraisal using simulated annealing’ by S. E. Dosso and D. W. Oldenburg

During the last few years the electromagnetic community has enthusiastically adopted extremal inversion as a tool for constructing models from data. The concept is simple: one minimizes or maximizes some penalty on the model, m, whilst demanding that the model fit the observed data to within some reasonable level of misfit, x+. A Lagrange multiplier formulation is usually employed to realize this concept, so that an unconstrained functional U U=R(m) + p-’x(m), (1) is minimized. Dosso & Oldenburg (1991, this issue) use this approach, as can be seen by inspection of their equation (9). The first term, R(m) is a functional of the model which returns a property which one wishes to penalize; Dosso & Oldenburg choose a boxcar average of the model. The term x(m) measures the misfit obtained by the model. The relative importance of the model penalty and the misfit is

Small-signal recovery and analysis with a novel, phase-locked digital filter

A new technique for recovering small signals from large background disturbances is described. The technique combines the narrow-band sensitivity of synchronous detection via lock-in amplifier with the waveform-recovery capabilities of boxcar averaging. However, unlike a boxcar averager, this technique operates in real time. A digital filter-based realization of this technique phase-locks to a reference signal at frequencies up to 500 Hz. It samples input at a rate of 256 kHz with 12-bit precision, recovers the complete synchronous and asynchronous components of the input signal, and reconstructs them with a precision of 12 bits. This allows for either the extraction of or deletion from the original signal of phase-lockable waveforms. We describe here the design and performance of this instrument and present the results of its use in a signal-recovery problem made difficult by the presence of power-line noise.

Synchronous thermal wave IR video imaging for nondestructive evaluation

We describe a system for real-time processing of infrared video images in synchronism with the time-dependence of the target object's temperature. The system can either be used either with periodic or pulsed heating of the target. With periodic heating, the system operates as if it were a collection of lock-in amplifiers, one for each of the quarter of the million pixels of the image. With pulsed heating, it operates as if it consisted of a similar number of box-car averagers. In both cases, the signal-to-noise ratio and temperature sensitivity of the infrared camera are improved. The technique lends itself to a wide spectrum of NDE applications.

On the Feasibility of Nonthermal Optoacoustic Spectroscopy of Solids

In conventional optoacoustic spectroscopy, an incident laser beam is absorbed by the sample, the radiant energy is converted to heat, and the sample undergoes thermal expansion, generating an acoustic wave which is detected. The incident light is from either a cw modulated or a pulsed laser. In the former case, phase-sensitive detection of the resultant acoustic wave is performed by a lock-in amplifier; while in the latter case, data acquisition is done by some means suited to the pulsed nature of the experiment—for example, with a boxcar averager. In either case, the intensity of the acoustic signal is monitored and yields the desired information. Optoacoustic techniques offer one of the most sensitive diagnostic methods available today.

Behaviour and analytical applications of a modulated power microwave-induced plasma (surfatron)

An a.c. modulated power has been applied to a microwave-induced plasma (MIP) produced by a surfatron structure. A temporal study of the plasma behaviour was carried out with a boxcar averager. It was confirmed that the best signal intensity was obtained at about 50 Torr and it was shown that inter-element effects are concentration dependent in the MIP source. An application of source modulation to sulphur detection in gas chromatography is given, with detection limits lying in the picogram range.

Transient space charge limited current spectroscopy method of measuring the position and concentration of trap levels in semi-insulating materials

A new method is reported to measure the position and concentration of trap levels in semi-insulating materials where the regular deep level transient spectroscopy method is not applicable. In the proposed method, a transient space charge limited current (TSCLC) associated with the trap levels is measured using a capacitance balanced square wave voltage source, a voltage limiter-amplifier and a double box car averager. The TSCLC method is demonstrated on a p-type Y2.01Ca0.99Ge0.91Fe4.09O12 magnetic garnet sample ρ300=107 Ω cm. The trap level is located at 0.36 eV above the valence band edge and the concentration of the trapped holes is found to be 1013–1015 cm−3.

Quantitation of Collagen Fragments and Gelatin by Deconvolution of Polarimetry Denaturation Curves

A method for quantitating nicked or shortened molecules (fragments) in pepsinizedbovine type I collagen preparations using polarimetry thermal denaturation curves is described.^The shortened molcules denature about 4°C lower than intact collagen molecules. The analog output of a polarimeter was digitized and stored on a microcomputer disk. A BASIC program was written which retrieves the specific rotation data from the disk, smooths the data with a boxcar average, and plots the derivative of the denaturation curve. The derivative curve was deconvoluted by fitting three Gaussian curves to the derivative curve using published algorithms. The area of the Gaussian centered at 37°C was proportional to the amount of collagen fragments. A good correlation between the amount of fragments determined by polarimetry and by a trypsin sensitivity assay was observed. The overall precision of the method was about 10% RSD, and the method was repeatable by multiple analysts. Application of the method to reconstituted fibrillar collagen samples showed that more fragments are generated when pepsin digestion time is lengthened. By fitting a fourth Gaussian component to the derivative curve, the method can also be used to determine relative amounts of denatured collagen (helix partially unwound but a chains not nicked). The detection limit for denatured collagen is about 20%.

Digitally timed boxcar averager for analog integration of portions of a video signal

A new system for measuring video signal intensity over selected portions of a complete video scan has been developed and applied to acquisition of LEED IV data. Unlike other systems available, this system is not microprocessor driven, but relies on appropriately timed boxcar averaging such that interfacing to the computer is relatively simple, yet data acquisition is fast. Signal to noise is theoretically better for this system since the input to the analog to digital (A/D) converter can have a lower time constant.

Multichannel boxcar-averaged measurements of plasma parameters made using a digital storage scope

We present a technique for rapidly acquiring time-resolved, ensemble-averaged Langmuir probe characteristics. Fifty probe characteristics are acquired using a digital storage oscilloscope in the time it would take to acquire a one-probe characteristic using a single-channel boxcar averager. A single Langmuir probe is used, and the probe bias is swept quite slowly, so that the probe is always in equilibrium with the plasma. A method for the automatic extraction of electron temperature, electron density, and the plasma potential from the acquired probe characteristics is described. This technique for acquisition and analysis is applied to the study of plasma decay and the effects of rf excitation in a pulsed, strongly magnetized plasma.

Experimental investigation of the radiative transport of the resonance lines of sodium and lithium

Measurements are reported of the radiative transfer in the 589 nm Na line and the 670.7 nm Li line. The experiments were carried out in an atomic beam as well as in a cylindrical sodium resonance cell. A nitrogen pumped dye laser with 14 ns pulse duration was used for resonance excitation. The decay times were derived from the fluorescence signals recorded using a boxcar averager. The atomic particle density was measured by Doppler-free two- photon absorption. This measurement was carried out immediately after resonance excitation thus yielding the density value for each decay time. Our measurements verify the diffusion model for low optical depths 0.2 ⩽ k o l < 4 and Holstein's transport theory for intermediate and high opacities 4 ⩽ k o l < 50. If Holstein's transport equation is modified for k o l >50 to take into account partial redistribution in frequency, our experimental results again agree with theory.

Detection of cadmium, lead and zinc in aerosols by laser-induced breakdown spectrometry

The repetitive breakdown spark from an Nd: YAG laser was used to generate analytical emission spectra of cadmium, lead and zinc in aerosols. The laser was typically operated at a pulse rate of 10 Hz, with an energy of 100 mJ per pulse and a pulse width of approximately 15 ns. Samples were generated in aerosol form by a nebuliser-heat chamber arrangement. The detection apparatus consisted of a monochromator, photomultiplier tube and boxcar averager. The last component provided amplification, averaging and time resolution. Detection limits for cadmium, lead and zinc in aerosols were 0.019, 0.21 and 0.24 µg g–1, respectively, at a signal to noise ratio of 3. The detection limits for cadmium and zinc were well below the threshold limit value-time weighted average (TLV-TWA) and threshold limit value-short-term exposure limit (TLV-STEL) limits for these two metals. Calibration graphs covering two orders of magnitude were generated. The excitation temperature of the plasma was measured from 20 to 40 µs after plasma initiation using the two-line Boltzman method, and was found to decrease from 6500 K at 20 µs to 5150 K at 40 µs. The effects of chemical form and sodium on the spectroscopic signals were investigated.

Noise properties of analog correlators with exponentially weighted average

Detailed calculations of the root-mean-square value of the output noise of correlators with an exponentially weighted average and an arbitrarily chosen weighting function for three commonly used mathematical models of white noise are derived. A comparative analysis has been made of noise properties of correlators with exponentially weighted average and true average based on two figures of merits: the output signal-to-noise ratio and the signal-to-noise improvement ratio. An analysis of noise properties of a boxcar averager for any gate width is performed. Expressions for the output signal-to-noise ratio and the signal-to-noise improvement ratio of a boxcar averager are derived.

Remote Laser-Induced Fluorescence Monitoring of Groundwater Contaminants: Prototype Field Instrument

ABSTRACT Direct, in situ monitoring of many groundwater contaminants is possible using the Remote Laser-Induced Fluorescence (RLIF) technique. We have designed, laboratory-tested, and field-tested a prototype mobile instrument capable of performing such monitoring. It consists of a portable, computer-controlled fluorimeter with a frequency-quadrupled Nd:YAG laser as a UV excitation source, detachable fiber optic sensor, and a detection module employing two photomultiplier tubes and a dual-channel boxcar averager for signal recovery. In this report, we give a complete description of the hardware and software utilized in the instrument, along with a summary of our field testing procedures and experiences, and an assessment of the technique's current and potential capabilities.

Real-Time Acquisition and Preprocessing of Kinetic and Spectroscopic Data in Laser Flash Photolysis

An automatic flash photolysis system for the measurement of absorption decays and transient difference spectra is described. The apparatus performs a digital measurement of the light transmitted by the sample and a few on-line preprocessing operations on the measured data, such as the digital averaging of the signal and the computation of the change in absorbance. A boxcar averager, used as a gated integrator, constitutes the sampling head of the apparatus. A microprocessor-based acquisition and control unit performs a digital conversion of the value sampled by the boxcar, provides an on-line preprocessing of the data, and supervises every operation required by the measurement process. This unit is interfaced to a personal computer which allows a friendly interaction with the operator as well as the on-line display of the data. The electronic instrumentation features a time resolution of a few nanoseconds, with a boxcar averager as input head, and of a few hundreds of picoseconds, with a sampling oscilloscope as input head.

Experiments in software data handling

64. An instrumental analysis experiment that facilitates the study of data-handling software and its methods, including sampling, boxcar averaging, smoothing, ensemble averaging, and digital filtering.

Lifetime Measurements of Metastable Levels of Thallium and Lead in the Air-Acetylene Flame by Laser-Enhanced Ionization Spectrometry

Laser-enhanced ionization spectrometry with two-step excitation has been used to evaluate the collisional lifetime of the metastable P levels of thallium and lead in an air-acetylene flame burning at atmospheric pressure and supported by a three-slot burner head fitted on a conventional nebulizing chamber. A water-cooled molybdenum electrode immersed in the flame was maintained at high negative potential with respect to the burner body. The ionization current resulting after the two-step excitation was amplified and measured with a digital storage oscilloscope and a boxcar averager. The two excimer lasers were triggered externally with two trigger pulses, one being delayed in time with respect to the other one. In this way the second laser photon, tuned at a transition starting from the metatable level under study, could be correspondingly delayed from the first photon tuned at a transition starting from the ground state. The lifetimes measured were found to be 81 ns and 360 ns for T1 and Pb, respectively.

Langmuir probing studies of UV-photoionized CO2 laser mixtures

A Langmuir probe incorporated with a boxcar averager is used to measure the temporally and spatially resolved electron density and electron temperature in a UV-photoionization device for several gases and gas mixtures. The predominant ionization mechanism is determined. The data provides information to optimize the effectiveness of the UV preionization operation in high-pressure lasers.

Optimization of Time Resolved Phosphorimetry

Time resolution, in spite of its enormous potential for minimizing spectral interferences in analytical phosphorimetry, has never achieved the widespread application that one might expect. One reason has been the inefficient trial-and-error optimization of the pulsed source gated detection phosphorimeter for a given analysis. In this paper, this problem is addressed by deriving expressions which relate the observed phosphorescence intensity to the temporal characteristics of the experimental system, and using these expressions to ascertain how the individual adjustable parameters, coupled with the analyte lifetime(s), affect the observed intensity. The predicted effects are verified by comparison with experimental results obtained on several phosphors of widely different lifetimes, using a pulsed laser boxcar averager phosphorimeter.

Laser excited analytical atomic and ionic fluorescence in flames, furnaces and inductively coupled plasmas—I. General considerations

Several important parameters for the analytical use of laser excited fluorescence spectrometry in flames, graphite furnaces and inductively coupled plasmas are discussed in some detail. These parameters include the laser characteristics such as peak power, pulse duration, spectral bandwidth and repetition rate, the choice of the excitation line, the optical arrangement and the detection system, this last one centred on the widespread use of the boxcar averager. It is shown that, if the ultimate sensitivity is the goal to be achieved, then the choice must be the electrothermal atomization. However, even for flames and inductively coupled plasmas, excellent results are possible provided that: (i) the laser system allows complete spectral coverage in the ultraviolet: (ii) saturation of the fluorescence signal can be approached over a large sample volume; and (iii) the gated detection parameters and the laser repetition frequency are optimized with respect to each other so as to reach the maximum signal-to-noise ratio.