Pulse-counting Detector Research Articles

Development of Surface Fluorescence X-Ray Absorption Fine Structure Spectroscopy Using a Laue-Type Monochromator.

Surface fluorescence X-ray absorption fine structure (XAFS) spectroscopy using a Laue-type monochromator has been developed to acquire structural information about metals with a very low concentrate on a flat highly oriented pyrolytic graphite (HOPG) surface in the presence of electrolytes. Generally, surface fluorescence XAFS spectroscopy is hindered by strong scattering from the bulk, which often chokes the pulse counting detector. In this work, we show that a bent crystal Laue analyzer (BCLA) can efficiently remove the scattered X-rays from the bulk even in the presence of solution. We applied the technique to submonolayer (∼1014 atoms cm-2 ) Pt on HOPG and successfully obtained high signal/noise in situ XAFS data in combination with back-illuminated fluorescence XAFS (BI-FXAFS) spectroscopy. This technique allows in situ XAFS measurements of flat electrode surfaces to be performed in the presence of electrolytes.

Signal Characterization and Receiver Design for Visible Light Communication under Weak Illuminance

We characterize the received signals and propose corresponding detection methods for the visible light communication under weak illuminance. Photo-multiplier tube is employed as the receiver, which can amplify weak photon-level signal down to a series of electrons. It is observed that under the extremely low transmission power, the received signals can be characterized by the discontinuous pulses. As the received power increases, the density of the pulses increases, and finally, the received signals form continuous waveforms. We provide the sampling-based and pulse counting-based transmitted symbol detection methods for the continuous signal waveform and the discontinuous pulses, respectively. We further improve the symbol detection performance by combining the two types of detection schemes. Numerical results show that the pulse counting detection can outperform the sampling-based detection with lower sampling rate, and the combined detection can improve the detection performance.

Time-of-flight of He ions from laser-induced processes in ultra-dense deuterium D(0)

Time-of-flight (TOF) energy measurements of ions from pulsed laser-induced processes in ultra-dense deuterium D(0) have been accomplished. The scintillation detector is a fast plastic scintillator preceded by a thin Al foil, with photo-multiplier detection of the scintillations. Signal is normally observed only when the laser focus is moved over the target, which means that the process is critically dependent on the state of the D(0) layer. Ions require up to 1MeVu−1 to penetrate through the Al foil and the observation of a signal in this setup proves directly that nuclear processes take place. Most TOF peaks agree with 4He ions ejected with 3.5–3.6MeV energy in the D+D nuclear fusion process. These ions are further delayed by collisions with deuterium atoms or ultra-dense deuterium clusters. All probable collision processes of 4He and 3He are observed. T emission is not observed, as expected due to the large reaction rate for T+D. To exclude that after-pulses in the photo-multipliers can give a similar signal, two flight lengths, two photo-multipliers with several mounting methods, several optical filters, and both oscilloscope and pulse-counting detection methods have been employed to study the TOF distributions.

A high dynamic range pulse counting detection system for mass spectrometry

In the paragraph below this equation, the sentence “At 1e8 cps we calculate P(x = 1, m = 0.65) = 0.341, P(x = 2, m = 0.65) = 0.110, P(x = 3, m = 0.65) = 0.024, etc., which demonstrates that the probability of extending the dead time is significant.” should read “At 1e8 cps we calculate P(x = 1, m = 0.65) = 0.339, P(x = 2, m = 0.65) = 0.110, P(x = 3, m = 0.65) = 0.024, etc., which demonstrates that the probability of extending the dead time is significant.”

A high dynamic range pulse counting detection system for mass spectrometry

A high dynamic range pulse counting system has been developed that demonstrates an ability to operate at up to 2e8 counts per second (cps) on a triple quadrupole mass spectrometer. Previous pulse counting detection systems have typically been limited to about 1e7 cps at the upper end of the systems dynamic range. Modifications to the detection electronics and dead time correction algorithm are described in this paper. A high gain transimpedance amplifier is employed that allows a multi-channel electron multiplier to be operated at a significantly lower bias potential than in previous pulse counting systems. The system utilises a high-energy conversion dynode, a multi-channel electron multiplier, a high gain transimpedance amplifier, non-paralysing detection electronics and a modified dead time correction algorithm. Modification of the dead time correction algorithm is necessary due to a characteristic of the pulse counting electronics. A pulse counting detection system with the capability to count at ion arrival rates of up to 2e8 cps is described. This is shown to provide a linear dynamic range of nearly five orders of magnitude for a sample of aprazolam with concentrations ranging from 0.0006970 ng/mL to 3333 ng/mL while monitoring the m/z 309.1 → m/z 205.2 transition. This represents an upward extension of the detector's linear dynamic range of about two orders of magnitude. A new high dynamic range pulse counting system has been developed demonstrating the ability to operate at up to 2e8 cps on a triple quadrupole mass spectrometer. This provides an upward extension of the detector's linear dynamic range by about two orders of magnitude over previous pulse counting systems.

Spatio-temporal characteristics of ion velocity in a Hall thruster discharge

The time evolution of the axial velocity of a metastable Xe+ ion was examined in the crossed-field discharge of a PPS100-ML Hall thruster fired at 250 V by means of laser-induced fluorescence spectroscopy at 834.72 nm. A pulse-counting detection technique was employed to achieve a time resolution of 0.1 µs. A periodic break of 10 µs duration of the anode current is used to stabilize the discharge and allow the investigation of both forced and natural plasma oscillations. Measurements were carried out along the channel axis throughout the region of large magnetic field strength. The mean ion flow velocity was found to oscillate at the discharge breathing mode frequency of about 21 kHz. By contrast, the ion velocity dispersion appears not to depend on time, which suggests a strong correlation between ionization and acceleration processes. The spatio-temporal behavior of the electric field was computed from experimental data using a Lagrangian description of the ion fluid motion. As expected, the field amplitude varies significantly at 21 kHz. More surprisingly, an electric field front seems to propagate periodically from the exterior toward the interior of the discharge chamber with a speed close to the thermal speed of the Xe atom.

Signal linearity of an extended range pulse counting detector: Applications to accurate and precise U‐Pb dating of zircon by laser ablation quadrupole ICP‐MS

Element concentration and isotope ratio measurements by single‐collector mass spectrometry often require the detection system to handle ion beams with very large intensity ratios. In order to obtain accurate and reproducible element concentration and isotope ratio data, the detection system must have a linear response with respect to the intensity of the incident ion beam. An extended range scaling pulse counting detector equipped on a Varian 810 quadrupole inductively coupled plasma–mass spectrometer (ICP‐MS) was tested for linearity across count rates of ∼2000 to 110,000,000 cps with different concentrations of natural U solutions. We also tested detector linearity by the laser ablation analysis of 206Pb/238U, 207Pb/235U, and 207Pb/206Pb ratios in well‐characterized 416–1565 Ma zircon standards. Results indicate that there is no correlation between the measured isotope ratio and ion intensity for the solution tests or the tests of natural zircon standards. The results of these tests confirm the suitability of this instrument for isotope ratio measurements that require a substantial dynamic range without having to switch between pulse counting and analog modes on electron multipliers or switching between electron multiplier to Faraday detectors.

Fast Multiple Electron Capture Dissociation in a Linear Radio Frequency Quadrupole Ion Trap

We developed a fast electron capture dissociation (ECD) device using a linear radio frequency-quadrupole (RFQ) ion trap. The device dissociated peptides and proteins using a focused electron beam with an intensity of 0.5 microA and a diameter of 1 mm. The electron capture rate was 13%/ms for doubly charged peptides, and the total amount of ECD products was identical to the theoretical limit, i.e., 50% of incident precursor ions were observed as maximum ECD products by electron irradiation of 7 ms in a pulse counting detection scheme. Coupling this ECD device to a time-of-flight mass spectrometer, we applied multiple ECD. Protonated ubiquitin precursor ions with a charge state of 10 were repeatedly cleaved by ECD, i.e., charge-reduced species and their highly charged fragments were cleaved again and again, creating lower charged products, leaving only singly to triply charged states among the final products. Meanwhile with the amount of electron irradiated, lower charged products increased. Applying an electron beam for 8 ms, we obtained 96% of the total sequence coverage using a 40 fmol sample except at three proline sites. This fast ECD device should be widely applicable to proteomics including post-translational modification analysis and top-down analysis.

Low energy electron diffraction using an electronic delay-line detector

A low energy electron diffraction (LEED) instrument incorporating a delay line detector has been constructed to rapidly collect high-quality digital LEED images with low total electron exposures. The system uses a position-sensitive pulse-counting detector with high bias current microchannel plates. This delay-line detector combined with a femtoampere electron gun offers a wide range of flexibility, with electron dosing currents ranging from 0.15pAto0.3fA. Using the highest current setting and collecting 1×106 counts per image, individual LEED images can be completed in 4s with an acquisition rate of 250kHz and a total electron exposure of 5×106 electrons. Under the latter conditions, images can be collected in 20min with an acquisition rate of 1kHz with a total electron exposure of 2×106 electrons. An angular width of 0.13° at 108eV is demonstrated, which means that domain sizes as large as 600Å can be resolved, depending on the surface quality of the crystal. The system electronics collect 2048×2048pixel images with a spatial resolution of about 75μm. The dynamic range of this system is 32bits∕pixel (limited only by physical memory). The construction of the detector results in a “plus”-shaped artifact, which requires that, for a given sample orientation, two images be taken at a relative angle of 45°. Identical current-voltage curves from an MgO(111)1×1H terminated sample, taken during several hours of exposure to the low current electron beam, demonstrate minimal electron induced H desorption.

Evaluation of detector dead time calculation models for ICP-MS

Four methods of pulse counting detector dead time calculation were evaluated on two quadrupole ICP-MS instruments, using the criteria of ease of application and accuracy (i.e., reliable measurement uncertainty). Three of the methods were based on measurements of the 204Pb/208Pb isotope ratio in the NIST Standard Reference Material NBS-982, while the fourth method was based on the comparison of pulse counting signals versus the corresponding analog signals across a range of signal intensities. The uncertainty in the dead time result derived from each method was evaluated and found to vary considerably, with some approaches underestimating this parameter, while others yielded large uncertainties. Overall, a method based on studies of the 204Pb/208Pb isotope ratio measured for different Pb concentration solutions, recalculated using different dead time settings (herein referred to as Method 2), was found to be the most suitable in terms of the specified criteria, particularly with respect to its uncertainty evaluation.

Determination of fission products and actinides by inductively coupled plasma-mass spectrometry using isotope dilution analysis: A study of random and systematic errors

The theory of the propagation of errors (random and systematic) for isotope dilution analysis (IDA) has been applied to the analysis of fission products and actinide elements by inductively coupled plasma-mass spectrometry (ICP-MS). Systematic errors in ID-ICP-MS arising from mass-discrimination (mass bias), detector non-linearity and isobaric interferences in the measured isotopes have to be corrected for in order to achieve accurate results. The mass bias factor and the detector dead-time can be determined by using natural elements with well-defined isotope abundances. A combined method for the simultaneous determination of both factors is proposed. On the other hand, isobaric interferences for some fission products and actinides cannot be eliminated using mathematical corrections (due to the unknown isotope abundances in the sample) and a chemical separation is necessary. The theory for random error propagation in IDA has been applied to the determination of non-natural elements by ICP-MS taking into account all possible sources of uncertainty with pulse counting detection. For the analysis of fission products, the selection of the right spike isotope composition and spike to sample ratio can be performed by applying conventional random propagation theory. However, it has been observed that, in the experimental determination of the isotope abundances of the fission product elements to be determined, the correction for mass-discrimination and the correction for detector dead-time losses contribute to the total random uncertainty. For the instrument used in the experimental part of this study, it was found that the random uncertainty on the measured isotope ratios followed Poisson statistics for low counting rates whereas, for high counting rates, source instability was the main source of error.

RELAX: An ultrasensitive, resonance ionization mass spectrometer for xenon

RELAX is a resonance ionization, time-of-flight mass spectrometer to which a cryogenic sample concentrator has been added. This has resulted in an increase in sensitivity by a factor greater than 100. The sample concentrator consists of a localized cold spot in the ion source, onto which the sample condenses, and a heating laser to release the condensed sample into the ionization region. The lifetime against detection of a sample atom is close to 20 min, which corresponds to a count rate of 1 cps from a sample of 1000 atoms, while the mass resolution is 300 (10% peak height). Sensitivity depends on the return time of sample atoms to the cold spot (10 s) and the fraction of these atoms subsequently ionized (∼1%). The minimum sample size which can be measured is limited only by blank, which is currently 2×10−15 cc STP total xenon and isotopically atmospheric (this can be attributed to the large aliquots of xenon admitted to the instrument during development, and so may be expected to decrease with time). The precision of abundance measurements has been improved by the incorporation of pulse height discrimination and pulse counting detection for the less abundant isotopes. The design, construction, and operation of the spectrometer in its new configuration are described with particular attention to abundance extraction. The effects of the sample concentrator on ionization efficiency and discrimination are discussed in detail, as are interferences from nonresonantly ionized hydrocarbons and the means of accounting for them.

Measurement of iron isotopes (54Fe, 56Fe, 57Fe, and 58Fe) in submicrogram quantities of iron

Iron isotopes offer an excellent opportunity for detailed tracing of natural processes because of the range of stable isotopes (54, 56, 57, 58) and the importance of iron redox transitions in biochemical and inorganic processes. In this paper, we describe a significantly improved mass spectrometric technique for iron isotopic analysis that utilizes ion pulse counting detection and is calibrated with an absolute iron isotope abundance standard. All four stable iron isotopes are quantified in a single analysis to approximately 0.1% precision and accuracy using a low-temperature (1200[degrees]C) silica gel/boric acid technique on an improved source filament assembly composed of platinum and ceramic. Filament loads of 100-1000 ng can be utilized through implementation of an ultraclean work environment, improved chemistry, and a source designed specifically to minimize isobaric interference in the mass 50-60 region. 30 refs., 5 figs., 4 tabs.

Propagation of uncertainties in isotope dilution mass spectrometry using pulse counting detection

Propagation of uncertainties in isotope dilution mass spectrometry using pulse counting detection

Diffraction imaging of polycrystalline materials

A new diffraction imaging technique for the characterization of polycrystalline materials is proposed and applied to obtain direct information about individual grains and their size and shape distributions and, in turn, strains in these materials. Unlike traditional powder diffractometry, where divergent and focusing x-ray optics are essential to collect information from an ensemble of grains, the nearly parallel and monochromatic beam available from a synchrotron x-ray source is employed to observe and measure diffraction images from individual grains and component particles in consolidated materials prepared by various processes. Images can be recorded by traditional methods, such as film and pulse counting detectors, but modern image detectors, such as charge coupled device (CCD) detectors and image analyzers, make the proposed imaging technique more practical. Unlike traditional diffractometry, this new technique provides the ability to measure shape, size, and strain without model based analyses. The spatial distribution of strain within individual grains, displayed as a diffraction image (topograph), indicates the presence of defects, such as dislocations, subgrain boundaries, and precipitates, and sheds new light on the origins of residual strains (stresses) in industrial materials. The resolution of the imaging system used is limited to grains ∼10 μm or larger due to diffraction broadening (∼20” from the size effect) and the resolution of the recording medium.

Application of a Dual-Mode Detection System for ICP-MS. Part I: Determination of Majors, Minors, and Traces in Geostandards

The high-gain pulse-counting detection systems currently used in ICP-MS, although exhibiting excellent characteristics for trace analysis, have only a finite gain. At count rates of the order of 106 cps, gain suppression occurs, thus effectively limiting the upper measurable concentration ranges to a few μg mL−1. It is possible to utilize such detections in an analogue mode, whereby the output signal is a direct function of the applied voltage. In this paper, the applicability of a detection system employing both analogue and pulse-counting detection is evaluated, with particular emphasis on the determination of majors, minors, and traces in geological reference materials. In addition, two sample preparation modes are compared; in the first, a conventional lithium metaborate fusion is utilized, while alternatively, a rapid slurry technique is found to be particularly effective for some more volatile elements.

Further investigation of CsI-coated microchannel plate quantum efficiencies

Previously we have reported pulse-counting detection efficiencies measured for CsI-coated microchannel plate (MCP) detectors (two-stage chevron configuration with a single collecting anode) to be 15-20% near Lyman-alpha (1216 A), compared to typical 65% quantum yields of opaque CsI photocathodes. To investigate the possibility that an improvement in quantum yield could result from use of MCPs with a bias angle of ~25 degrees , instead of 8 degrees as used previously, we repeated our measurements with new MCPs having the larger bias angle. No significant improvement in detection efficiency was achieved; the new detector tests still yielded maximum efficiencies of the order of 20% near 1216 A.

Imaging extreme ultraviolet spectrometer employing a single toroidal diffraction grating: the initial evaluation

A high-efficiency extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically deformable submaster grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a 2-D pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The results of these initial tests are described in detail, and the basic designs of two instruments which could employ the imaging spectrometer for astrophysical investigations in space are briefly described, namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona and an EUV spectroscopic telescope for studies of nonsolar objects.

Applications of a Transportable Spent-Fuel Measurement System

A portable tool for making measurements on irradiated fuel has been developed to where in-plant installations having a 1-2% measurement uncertainty of relative exposure are feasible. The measurement uses a passive gross neutron signal and data from a gross gamma measurement as a consistency check of the neutron result and the operators declaration of cooling time. The uncertainties are about the same as those obtained using HRG techniques without the instrumentation being as obtrusive. The battery-operated microprocessorbased electronics package used with the irradiated fuel measurement system can also be used with single channel pulse counting detectors for other applications. This feature together with the large dynamic range of its current-mode ion chamber channel makes ION-I a good building block to be used in emergencies with an arsenal of detectors at a variety of nuclear plants.

Multianode Microchannel Array Detector Systems: Performance Characteristics

Multianode microchannel array detector systems with formats as large as 256 x 1024 pixels are currently under evaluation in the laboratory. Performance data recorded with sealed ultraviolet and visible-light detector tubes have shown that the detector systems have unique characteristics that make them complementary to photoconductive ar-ray detectors, such as charge-coupled devices, and superior to alter-native pulse-counting detector systems employing high-gain microchannel plates.