Photomultiplier Tube Detector Research Articles

Transient temperature measurements in an ideal gas by laser-induced Rayleigh light scattering

A laser-induced Rayleigh light-scattering (RLS) system was assembled and used to noninvasively measure the transient molecular number density in an ideal gas. This information was used to find the transient gas temperature when operating at known pressure. The laser was a 4 W argon ion operating on all lines at a total power of about 2.5 W. The theoretically predicted photon arrival rate at the photomultiplier tube detector was calculated and compared well with the observed photon rates. These rates were high enough that sampling could be averaged over a 1 s period with theoretical uncertainty due to electronic shot noise below 0.1%, and below 2% for a 0.001 s averaging time. A propagated error analysis showed uncertainty in the transient temperature due to all sources was 2–4 K. The RLS system was used to record transient air temperature at several locations above a flat plate during heating from room temperature to 475 K. Results showed buoyancy-induced fluctuations of about 3 Hz, and instabilities in temperature in addition to the overall temperature rise due to plate heating. Excellent transient temperature records were obtained, substantiating the predicted 2–4 K uncertainty.

Flow methods for the determination of polycyclic aromatic hydrocarbons using low power photomultiplier tube and charge coupled device chemiluminescence detection

Polycyclic aromatic hydrocarbons (PAHs) are produced during combustion processes (both natural and anthropogenic), and as a consequence are distributed widely throughout environmental systems. This paper describes the application of peroxyoxalate chemiluminescence (CL) detection, with a 12 V low power photomultiplier tube (PMT) detector and a two dimensional charge coupled device (CCD) detector, to the determination of PAHs. The PMT system provides comparable sensitivity to high power (1 kV) PMTs (detection limits in the pg range) and complementary information to that derived from fluorescence detection. The CCD system can be used to obtain full CL spectral information (in contrast to the integrated response from PMTs) and it has been shown that multivariate calibration can be used in conjunction with CCD detection to give good predictive errors (<9%) for simple two-component systems, e.g. benzo[ k]fluoranthene and benzo[ a]pyrene in hexane.

The LightCycler: a microvolume multisample fluorimeter with rapid temperature control.

Experimental and commercial microvolume fluorimeters with rapid temperature control are described. Fluorescence optics adopted from flow cytometry were used to interrogate 1-10-microL samples in glass capillaries. Homogeneous temperature control and rapid change of sample temperatures (10 degrees C/s) were obtained by a circulating air vortex. A prototype 2-color, 32-sample version was constructed with a xenon arc for excitation, separate excitation and emission paths, and photomultiplier tubes for detection. The commercial LightCycler, a 3-color, 24-sample instrument, uses a blue light-emitting diode for excitation, paraxial epi-illumination through the capillary tip and photodiodes for detection. Applications include analyte quantification and nucleic acid melting curves with fluorescent dyes, enzyme assays with fluorescent substrates and techniques that use fluorescence resonance energy transfer. Microvolume capability allows analysis of very small or expensive samples. As an example of one application, rapid cycle DNA amplification was continuously monitored by three different fluorescence techniques, Which included using the double-stranded DNA dye SYBR Green I, a dual-labeled 5'-exonuclease hydrolysis probe, and adjacent fluorescein and Cy5z-labeled hybridization probes. Complete amplification and analysis requires only 10-15 min.

Reflectance spectroscopy of interplanetary dust particles

Abstract Reflectance spectra were collected from chondritic interplanetary dust particles (IDPs), a polar micrometeorite, Allende (CV3) meteorite matrix, and mineral standards using a microscope spectrophotometer. Data were acquired over the 380–1100 nm wavelength range in darkfield mode using a halogen light source, particle aperturing diaphrams, and photomultiplier tube (PMT) detectors. Spectra collected from titanium oxide (Ti4O7), magnetite (Fe3O4), and Allende matrix establish that it is possible to measure indigenous reflectivities of micrometer‐sized (&gt;5 μm in diameter) particles over the visible (VIS) wavelength range 450–800 nm. Below 450 nm, small particle effects cause a fall‐off in signal into the ultraviolet (UV). Near‐infrared (IR) spectra collected from olivine and pyroxene standards suggest that the ∼1 μm absorption features of Fe‐bearing silicates in IDPs can be detected using microscope spectrophotometry.Chondritic IDPs are dark objects (&lt;15% reflectivity) over the VIS 450–800 nm range. Large (&gt;1 μm in diameter) embedded and adhering single mineral grains make IDPs significantly brighter, while surficial magnetite formed by frictional heating during atmospheric entry makes them darker. Most chondritic smooth (CS) IDPs, dominated by hydrated layer silicates, exhibit generally flat spectra with slight fall‐off towards 800 nm, which is similar to type CI and CM meteorites and main‐belt C‐type asteroids. Most chondritic porous (CP) IDPs, dominated by anhydrous silicates (pyroxene and olivine), exhibit generally flat spectra with a slight rise towards 800 nm, which is similar to outer P and D asteroids. The most C‐rich CP IDPs rise steeply towards 800 nm with a redness comparable to that of the outer asteroid object Pholus (Binzel, 1992). Chondritic porous IDPs are the first identified class of meteoritic materials exhibiting spectral reflectivities (between 450 and 800 nm) similar to those of P and D asteroids.Although large mineral grains, secondary magnetite, and small particle effects complicate interpretation of IDP reflectance spectra, microscope spectrophotometry appears to offer a rapid, nondestructive technique for probing the mineralogy of IDPs, comparing them with meteorites, investigating their parent body origins, and identifying IDPs that may have been strongly heated during atmospheric entry.

Construction and Evaluation of a Visible Spectrometer Using Digital Micromirror Spatial Light Modulation

Digital micromirror devices (DMDs), also known as spatial light modulators, have been produced in a wide variety of configurations specific to their applications such as joint-transform correlator systems, optical neural networks, and high-definition televisions. The characteristics of DMD technology and flexibility of design lend themselves to a new application in optical spectrometers. Medium-resolution optical spectrometers, with a spectral bandwidth on the order of 1 nm, are widely used in instrumentation designed to record molecular absorption spectra in the ultraviolet and visible regions and are among the most widely used laboratory instruments. Modern UV-visible spectrometers usually are designed to use a multichannel detector, such as a photodiode array (PDA), in conjunction with a compact fixed-resolution spectrograph and can record spectra with reasonable speed, ∼ 30 ms. These spectrographs have no moving parts and are used for on-line detection of chromatographic eluents, for routine analytical determinations, and for industrial applications such as measurements made in process streams. However, diode array detectors are generally more expensive and are less sensitive than photomultiplier tubes (PMTs), particularly in the UV, and require cooling when a long integration time and low dark current are necessary. In addition, the diode array cannot acquire spectra fast enough for most kinetic studies to be made. A medium-resolution spectrometer which incorporates DMD technology and a PMT for detection has the potential of obtaining a spectrum on the order of a few milliseconds with high sensitivity at a lower cost than that for current PDA or charged-coupled device (CCD) spectrometers. Another advantage of the DMD spectrometer is that it possesses the capability of repetitively scanning a small portion of the spectrum without collecting the entire spectrum (random pixel access). The high sensitivity of a DMD spectrometer using a PMT also makes it ideal for fluorescence and phosphorescence detection.

Fully integrated current-mode CMOS gated baseline restorer circuits

Design and performance results for three different fully-integrated gated baseline restorer (BLR) circuits used in a new PET current-mode front-end CMOS ASIC are presented. The BLR for each of the three gated integrator channels is a differential current-in to single ended current-out circuit with a correction bandwidth of 100 kHz set by a 40 pF on-chip capacitor using pole splitting techniques. The BLRs for the constant fraction discriminator (CFD) constant fraction and arming comparators are differential current-in to voltage-out. Circuits with correction bandwidths of 5 MHz and 1 MHz set by on-chip capacitors of 10 pF and 2.5 pF respectively. The BLR circuits are capable of correcting differential input current offsets of /spl plusmn/40 /spl mu/A for the gated integrator circuits, /spl plusmn/100 /spl mu/A for the CFD constant fraction comparator circuit, and /spl plusmn/160 /spl mu/A for the CFD arming comparator circuit. Use of the BLR circuits allows photomultiplier tube (PMT) detector inputs to be ac coupled and all slow (gated integrator) and fast (CFD timing) signal processing channels to be dc coupled. The BLR circuits correct for count-rate dependent baseline shifts due to detector ac coupling and correct for accumulated CMOS dc offsets in the signal processing channels. Gated integrator input offset currents are maintained below 50 nA, keeping the gated integrator output error below 10 mV for an 850 ns integration period. CFD constant fraction comparator input offset is maintained at submillivolt levels, and arming comparator threshold is maintained at a 0-0.48 V level under on-board DAC control.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electrically heated wire-loop, in-torch vaporization (ITV) sample introduction system for ICP-AES with photomultiplier tube detection and ICP-MS

In this first report on an electrically heated, Re wire-loop in-torch vaporization (ITV) sample introduction system for ICP-AES with photomultiplier tube detection and ICP-MS, some preliminary results obtained for Pb, Cd, Zn and Sr are presented. An un-optimized, manually operated ITV system and 10 μl volumes have been used. Estimated absolute detection limits are (in pg) Zn (26), Pb (25), Cd (41) and Sr (0.12) for ITV-ICP-AES and Zn (0.04), Pb (0.12), Cd (0.11) and Sr (0.32) for ITV-ICP-MS.

Photon emission from an alumina insulator in vacuum subjected to DC electrical fields

Radial direct current electric fields were applied to a cylindrical alumina (94% Al2O3) insulator, which was operated under ultra-high vacuum (pressure <10-8 mbar), with current magnitudes limited to pre-breakdown values of less than 50 mu A. A high-resolution monochrome charge-coupled device camera was used to capture images of photon emission from the alumina; the spectrum of the emitted light was quantified using a high-speed, high-resolution scanning monochromator with a photo-multiplier tube detector. Optical images of the observed luminescence are presented, together with wavelength spectra recorded at fixed values of direct current bias voltage. An observed time-dependence of the photon emission intensity is correlated with an apparent time-variation in the direct current-voltage characteristic of the bridged alumina insulator. The features of the recorded spectra are discussed in terms of solid-state photon-emission processes, arising from radiative electron recombination at impurity and structural defect centres within the surface layers of the alumina ceramic.

Simultaneous multielement graphite furnace atomic absorption measurements using a photodiode array detector

A photodiode array detector multichannel analyser system has been coupled to a graphite furnace atomizer and tested for simultaneous multielement atomic absorption analysis. Multielement hollow cathode lamps are used as light sources and spectral lines are dispersed through a spectrograph with three selectable gratings. Multiple transmitted spectra are recorded to simultaneously determine the atomic absorption profiles of the analyte elements during the atomization stage. Atomic absorbance of individual elements is obtained by integrating the respective peak areas of the appropriate time-resolved atomic absorption spectra. The obtained sensitivities for Ni-Co-Fe are within the same order of magnitude as those from conventional single element determinations using photomultiplier tube detection. The system has also been applied for simultaneous multielement flame atomic absorption spectrometry (AAS) measurements and it has been demonstrated that background absorption can be readily corrected for both flame and graphite furnace AAS by a two-line method where non-atomic absorption lines can be chosen from the simultaneously recorded spectra.

Copper vapor laser drilling of copper, iron, and titanium foils in atmospheric pressure air and argon

A copper vapor laser (511 and 578 nm) is used to drill submillimeter diameter holes in 0.025–0.127 mm thick foils of copper, iron, and titanium. Foils are machined in atmospheric pressure air and argon. The laser is repetitively pulsed at 10 kHz with a per pulse energy of 0.5 mJ giving an average power of 5 W at the sample surface for a pulse width of 40 ns. A p-i-n photodiode and a photomultiplier tube detector are connected to a digital-display timing circuit that records the number of incident laser pulses used to drill through the sample. The number of pulses is converted to an average drilling time and can provide an estimate for the average laser energy used to drill the hole. Typical data for all three materials with a per-pulse fluence of 0.7 J/cm2 ranged from 0.1 to 500 s to produce holes of ∼0.3 mm diameter. Drilling times decreased in some cases by an order of magnitude when machining in air. This is attributed to the increased laser absorption of the metal-oxide layer formed in air and was especially noticeable with titanium. A continuous wave thermal model is used to compare experimental data as well as verify the thermal machining mechanism.

Remote sensing of the Earth's surface with an airborne polarized laser

A new remote sensing instrument, the Airborne Laser Polarization Sensor (ALPS), is making the first multispectral radiometric and polarization measurements of the Earth's surface using a polarized laser light source. Results from data flights taken over boreal forests in Maine at 1060 and 532 nm, using a Nd:YAG laser source, showing depolarization signatures for three broadleaf and five coniferous tree species, are discussed. Measurements made over nonforest ground cover had a large dynamic range in depolarization values at both wavelengths. The ALPS system use twelve photomultiplier tube detectors configurable to measure desired parameters such as the total backscatter and the polarization state, including the azimuthal angle and ellipticity, at different ultraviolet to near-infrared wavelengths simultaneously. Measurements of the azimuth and ellipticity of the backscatter polarization were variable and no specific conclusions have yet been drawn.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Resolution of fluorescence signals from cells labeled with fluorochromes having different lifetimes by phase-sensitive flow cytometry.

A flow cytometric method has been developed that uses phase-sensitive detection to separate signals from simultaneous fluorescence emissions in cells labeled with fluorochromes having different fluorescence decay lifetimes. By CHO cells were stained with propidium iodide (PI) and fluorescein isothiocyanate (FITC). These dyes bind to DNA and protein and the fluorescence lifetimes of the bound dyes are 15.0 and 3.6 ns, respectively. Cells were analyzed as they passed through a modulated (sinusoidal) laser excitation beam. Fluorescence was measured using only a long-pass filter to block scattered laser excitation light and a single photomultiplier tube detector. The fluorescence detector output signals were processed by dual-channel phase-sensitive detection electronics and the phase-resolved PI and FITC signals were displayed as frequency distribution histograms and bivariate plots. By shifting the phase of one detector channel reference signal by pi/2 + phi 1 degrees and the phase of the other detector channel reference signal by - pi/2 + phi 2 degrees, where phi 1 and phi 2 are the phase shifts associated with the PI and FITC lifetimes, the PI and FITC signals were separately resolved at their respective phase-sensitive detector outputs. This technology is also applicable to suppressing background interferences caused by cellular autofluorescence, unbound/free dye, nonspecific dye binding, and Raman and Rayleigh scattering.

Acridinium ester chemiluminescence: pH dependent hydrolysis of reagents and flow injection analysis of hydrogen peroxide and glutamate

An automated analysis system is described for the measurement of hydrogen peroxide based on a chemiluminescence reaction with phenyl 10-methylacridinium-9-carboxylate (PMAC). A reversed FIA experimental arrangement is used to establish the operating conditions for the measurement of submicromolar levels of hydrogen peroxide. The carrier stream consists of hydrogen peroxide standards prepared in a pH 9.0, boric acid buffer and the flow rate for this carrier/sample stream is 4 ml/min. Twenty microliters of a 10 mM PMAC solution, prepared in a pH 3 phosphate buffer, are injected into the carrier/sample stream. Hydrogen peroxide mixes with the PMAC reagent in an incubation coil that is constructed by wrapping 107 cm of polyethylene tubing around a 1 cm o.d. plastic rod. The chemiluminescence reaction is then initiated by adding base just before the sample passes in front of a photomultiplier tube (PMT) detector. The calculated limit of detection (S/N = 3) for hydrogen peroxide is 0.25 μM. In addition, the pH dependent hydrolysis of the PMAC reagent is characterized by an HPLC method which has been specifically developed for the separation and detection of the hydrolysis products of PMAC. Results indicate that a pH of 3.0 is required for long term stability of the PMAC reagent. Finally, this system has been successfully extended to the measurement of glutamate by coupling a bioreactor column of glutamate oxidase with the hydrogen peroxide detection scheme. A detection limit (S/N = 3) of 0.5 μM has been established for glutamate with a throughput of 200 samples per hour.

Comparison of Interferogram Noises in the Ultraviolet and Visible Regions

The sources of noise in ultraviolet (UV), visible, and near-infrared (NIR) interferograms acquired with silicon and germanium diodes as well as photomultiplier tube (PMT) detectors have been investigated. Similar conditions were used with each detector so that noises could be compared with minimal bias. Interferograms were produced from tungsten and deuterium lamps as well as an inductively coupled plasma (ICP). Data were collected from two instruments; a Mattson Sirius 100 UV/Vis spectrometer, and a Chelsea Instruments FT 500. The signal was the average of 4 to 100 interferograms, while the standard deviation of the data was the noise. In addition to the average and the standard deviation, which are the first two moments of the frequency distribution, the third moment (known as skew) was employed to analyze the symmetry of noise distributions.

Near-IR retinal laser Doppler velocimetry and flowmetry: new delivery and detection techniques

In laser Doppler velocimetry and flowmetry of the human ocular fundus, near infrared diode lasers, coupled with avalanche photodiode detectors, offer much improved signal-to-noise ratio compared to He-Ne laser delivery and photomultiplier tube detection. The increased signal-to-noise ratio is achieved through increased retinal irradiance and better quantum efficiency of the avalanche photodiode in the near infrared. Furthermore, the small size of the diode lasers allows delivery of two individually steerable beams for simultaneous measurements in different parts of the fundus. Applications are given as illustrations of the capabilities of the system: retinal laser Doppler velocimetry in humans without need for pupil dilatation and optic nerve head laser Doppler flowmetry in the cat using flicker as a stimulus to induce blood flow changes.

First photoelectron timing error evaluation of a new scintillation detector model

A general timing system model for a scintillation detector developed by Hero et al. (1990) was experimentally evaluated. The detector consists of a scintillator and a photodetector such as a photomultiplier tube (PMT) or an avalanche photodiode. The model uses a Poisson point process to characterize the light output from the scintillator. This timing model was used to simulate a bismuth germanate (BGO) scintillator with a Burle 8575 PMT using first photoelectron timing detection. Evaluation of the model consisted of comparing the root-mean-square error from the simulations with the error from the actual detector system. It is found that the general model compares well with the actual error results for the BGO/8575 PMT detector. >

Measurement of Incident Intensity of UV Radiation Delivered by Optical Fibers

Fiber optics are being used more and more frequently to deliver light to a sample for remote spectroscopic analysis, or to induce photochemistry or photoablation on an otherwise inaccessible sample, e.g., living tissue. For accurate quantitative results, the photon flux at the sample must be known. This paper presents two techniques for simple and accurate measurement of the intensity of ultraviolet light in the range 260 to 369 nm delivered to a distant sample by a fused-silica optical fiber. One technique uses a home-built evanescent wave coupler near the sample to couple excitation light into a monitoring fiber; the other uses a twin-fiber system, with equal amounts of source radiation launched into an excitation and a monitoring fiber. In each method, two Rhodamine B quantum-counter photomultiplier tube detectors are used simultaneously to assess the effectiveness with which the incident intensity is monitored. A critical evaluation of the results is presented. The twin fiber method is, theoretically, very simple; in practice, however, it is difficult to obtain a good wavelength-independent match between the two outputs. The evanescent wave coupling is also wavelength-dependent, but the observed results are in agreement with the predictions of simple evanescent field theory.

Flame Atomic Absorption Spectroscopy Using a Single-Mode Laser Diode as the Line Source

The laser diode recently has been applied to atomic spectroscopy in laser-enhanced ionization spectroscopy, laser-excited atomic fluorescence spectroscopy, resonance ionization mass spectrometry, and atomic absorption spectroscopy (AAS). Its attractive features include its having a low cost and being compact and tunable. Its major limitation is that commercial laser diodes are available only for fundamental wavelengths over the range of 670 to 1550 nm, and the tunability covers only several nanometers. In the AA investigation, a single-mode laser diode was used as the line source. Background correction was achieved by detuning the radiation to an off-resonance position; and the linear dynamic range could be extended by using the shoulder portion of the resonance line. Simultaneous multielement detection and internal standardization could be accomplished by the simultaneous use of several laser diodes. The atom reservoir employed was a graphite furnace and the detection spectrometer was a Fourier analyzer. We report here our observation of a single-mode laser diode used as the line source in flame AAS, with a simple, low-resolution monochromator and photomultiplier tube for detection. A multimode laser diode flame AAS has been reported in a separate paper.

Charge-Coupled Device Fluorescence Detection for Capillary-Zone Electrophoresis (CCD-CZE)

This paper describes a new instrument system combining capillary-zone electrophoresis (CZE) separation and charge-coupled device (CCD) fluorescence detection. The performance of this CCD-CZE system is evaluated and compared with the performance of a photomultiplier tube (PMT) detector, with the use of signal-to-noise (S/N) ratio and acquisition time as measures of performance. The feasibility of on-column fluorescence spectrometric detection is demonstrated, and a three-dimensional electrophorogram is presented to show the spectral and time resolution obtained by injection of 2 femtomoles (5 nanoliters of 4.0 × 10−7 M) of fluorescein into the capillary column. The software uses a “peak sum” method to achieve optimum S/N ration. The limit of optical detection (LOD) for this system is 4 attomoles (4 × 10−18 mole) of fluorescein, with a CCD integration time of only 0.2 s. If the fluorescence signal were integrated for 4 s on the CCD, an LOD of less than 1 attomole could be achieved for fluorescein. A 4-s integration time would still permit three complete spectra during the 30-s elution time of the sample.

Selection of spectral windows for plasma atomic-emission spectrometry

A program has been developed to evaluate the number of spectral windows required to perform elemental analysis by ICP-AES, given a certain spectral window width. The data indicate that photodiode array systems (sequential slew scan) with as few as 7 or 8 acquisitions (windows) might be viable. With smaller windows, approaching in size those used by direct-reading spectrometers, it appears that conventionally designed spectrometers with photomultiplier tube detectors could use between 29 and 37 windows to determine 59 elements. This indicates that a general purpose direct-reading spectrometer may be feasible.