Dead-time Correction Research Articles

Sources of uncertainty in isotope ratio measurements by inductively coupled plasma mass spectrometry.

A model is presented describing the effects of dead time and mass bias correction factor uncertainties, flicker noise, and counting statistics on isotope ratio measurement precision using inductively coupled plasma mass spectrometry (ICPMS) with a single collector. Noise spectral analysis is exploited to enable estimation of the flicker noise parameters. For the instrument used, the flicker noise component exhibited a fairly weak frequency (t) dependence (is proportional to f -0.33+/-0.12), but was directly proportional to the total number of counts, Q. As white noise, determined by counting statistics, is given by Q0.5, the isotope ratio measurement uncertainties will actually cease to improve when Q exceeds a certain threshold. This would suggest that flicker noise could become the limiting factor for the precision with which isotope ratios can be determined by ICPMS. However, under most experimental conditions, uncertainties associated with mass discrimination and dead time correction factors are decisive. For ratios up to approximately 22 (115In/113In), optimum major isotope count rates are generally below 0.3 MHz, for which precision in the mass discrimination factor is limiting. The model derived could be used as a starting point for determining optimum conditions and understanding the limitations of single-collector ICPMS for precise isotope ratio measurements.

Strontium isotope ratio determination in soil and bone samples after on-line matrix separation by coupling ion chromatography (HPIC) to an inductively coupled plasma sector field mass spectrometer (ICP-SFMS)

In this work we investigate strontium isotopic ratios in human skeletal remains and reference soil samples from the corresponding archaeological sites. Because 87Sr is spectrally overlapped by 87Rb (m/Δm = 300.000), a separation prior to analysis was a prerequisite for accurate isotope ratio determination especially in soil samples. The main objective of our study was, therefore, the development of an on-line ion chromatographic method (HPIC) for routine separation of Rb and Sr prior to strontium isotopic ratio determination by sector field inductively coupled plasma mass spectrometry (ICP-SFMS). Operating parameters of the HPIC, including flow rates and eluents, and instrumental bias factors of the ICP-SFMS, were optimized in an initial step. A shielded torch system (CD1-option) in combination with ultrasonic nebulization increased the strontium signal intensity by 100-fold in the low mass resolution mode (m/Δm = 300). During separation, the reduction of the HPIC flow rate from 3.0 to 0.5 mL min−1 in combination with successive injections generated a stable transient strontium signal of more than 16 min duration. The precision obtained (RSD for n = 5 measurements) on such transient signals was 0.068% for 5 ng g−1 strontium in the measurement solution deviating by 0.064% from the certified 87Sr/86Sr isotopic ratio value of NIST SRM 987 after dead time and mass bias correction. The optimized procedure for the coupled on-line HPIC-ICP-SFMS system allowed non-interfered Sr isotope ratio measurements with no statistically significant differences in precision and accuracy compared to direct nebulization. The setup was successfully applied to the investigation of bone samples in an archaeological finding. We found that diagenetic influences on Sr isotope ratios in prehistoric human bones were negligible, and grouping of individuals could be performed.

An innovative method for dead time correction in nuclear spectroscopy

All nuclear spectroscopy systems, whether measuring charged particles, X-rays, or gamma-rays, exhibit dead time losses during the counting process due to pulse processing in the electronics. Several techniques have been employed in an effort to reduce the effects of dead time losses on a spectroscopy system including live time clocks and loss-free counting modules. Live time extension techniques give accurate results when measuring samples in which the activity remains roughly constant during the measuring process (i.e., the dead time does not change significantly during a single measurement period). The loss-free counting method of correcting for dead time losses, as introduced by HARMS and improved by WESTPHAL (US Patent No. 4,476,384) give better results than live time extension techniques when the counting rate changes significantly during the measurement. However, loss-free counting methods are limited by the fact that an estimation of the uncertainty associated with the spectral counts can not be easily determined, because the corrected data no longer obeys Poisson statistics. Therefore, accurate analysis of the spectral data including the uncertainty calculations is difficult to achieve. The Ortec® DSPECPLUS™ implements an improved zero dead time method that accurately predicts the uncertainty from counting statistics and overcomes the limitations of previous loss-free counting methods. The uncertainty in the dead-time corrected spectrum is calculated and stored with the spectral data (Patent Pending). The GammaVision-32® analysis algorithm has been improved to propagate this uncertainty through the activity calculation. Two experiments are set up to verify these innovations. The experiments show that the new method gives the same reported activity and associated uncertainties as the well-proven Gedcke-Hale live time clock. It is thus shown that over a wide range of dead times the new ZDT method tracks the true counting rate as if it had zero dead time, and yields an accurate estimation of the statistical uncertainty in the reported counts.

The effect of beam time structure on counting detectors in SRS experiments

Counting detector systems are increasingly used in X-ray experiments because of their attractive properties as regards linearity, large dynamic range and simple noise properties. In synchrotron radiation source (SRS) applications of X-ray detectors, counting rates are generally high enough to require dead-time correction. The time structure of an SRS beam interacts with the dead-time characteristics of the detector in a way that the simple stochastic dead-time models cannot always handle. This report generates analytical and Monte Carlo mathematical models which describe the rate performance of any given detector system when used with the typical beam structures encountered in an SRS.

Discovery of Kilohertz Fluctuations in Centaurus X‐3: Evidence for Photon Bubble Oscillations (PBO) and Turbulence in a High‐Mass X‐Ray Binary Pulsar

We report the discovery of kilohertz fluctuations, including quasiperiodic oscillations (QPO) at {approx}330 Hz (260-407 Hz) and {approx}760 Hz (671-849 Hz) and a broadband kilohertz continuum in the power-density spectrum of the high-mass X-ray binary pulsar Centaurus X-3. These observations of Cen X-3 were carried out with the Rossi X-Ray Timing Explorer (RXTE). The fluctuation spectrum is flat from millihertz to a few hertz, then steepens to f{sup -2} behavior between a few hertz and {approx}100 Hz. Above 100 Hz, the spectrum shows the QPO features, plus a flat continuum extending to {approx}1200 Hz and then falling out to {approx}1800 Hz. These results, which required the co-adding three days of observations of Cen X-3, are at least as fast as the fastest known variations in X-ray emission from an accreting compact object (kilohertz QPO in LMXB sources) and probably faster since extension to {approx}1800 Hz is indicated by the most likely parameterization of the data. Multidimensional radiation hydrodynamics simulations of optically thick plasma flow onto the magnetic poles of an accreting neutron star show that the fluctuations at frequencies above 100 Hz are consistent with photon bubble turbulence and oscillations (PBO) previously predicted to be observable in this source. Wemore » show that previous observations of Cen X-3 constrain the models to depend on only one parameter, the size of the polar cap. For a polar cap opening angle of 0.25 radians (polar cap radius {approx}2.5 km and area {approx}20 km2, for a neutron star radius of 10 km), we show that the spectral form above 100 Hz is reproduced by the simulations, including the frequencies of the QPO and the relative power in the QPO and the kilohertz continuum. This has resulted in the first model-dependent measurement of the polar cap size of an X-ray pulsar. The simulations underpredict the overall amplitude of the observed spectrum, which we suggest is the consequence of a two-dimensional axisymmetric simulation of an intrinsically three-dimensional phenomenon. The power-density spectrum of Cen X-3 shows a dramatic decrease above {approx}1000 Hz, which suggests an optical depth {approx}30 across the accretion mound consistent with effects of radiative diffusion in the simulations. We identify this decline at high frequency as the first direct evidence of radiative diffusion near the surface of a neutron star (NS). We suggest the fluctuations observed at frequencies below 100 Hz, whose spectrum has a different form from that of the kilohertz phenomena, reflect intermittency in the mass transfer mechanism that carries plasma from the accretion disk to field aligned flow onto the neutron star's polar caps. Using simple estimates based on Rayleigh-Taylor instabilities, possibly modulated by intrinsic disk turbulence, we show that mass transfer in ''blobs'' forming through Rayleigh-Taylor disruption of the disk's inner edge can explain the large amplitude fluctuations required by the spectrum at frequencies f{approx}1 Hz, but only if magnetic pressure in the disk's innermost regions inflates the disk until its scale height is comparable to the magnetosphere's size {approx}4300 km. The observational results required the development of a procedure for the careful determination of the dead-time effects of the PCA. This procedure is described in Appendix A. As a consequence of the use of observations of Cyg X-1 for the estimation of dead-time corrections for the observations of Cen X-3, we have also demonstrated that the black hole Cyg X-1 shows clear evidence of variability up to a frequency of {approx}280 Hz. Also observations of GX 17+2 were used to validate the procedure for dead-time corrections. This analysis of GX 17+2 clearly indicates the presence of a kilohertz QPO and the absence of any significant simultaneous kilohertz continuum. (c) 2000 The American Astronomical Society.« less

MTR2: a discriminator and dead-time module used in counting systems

In the field of radioactivity measurement, there is a constant need for highly specialized electronic modules such as ADCs, amplifiers, discriminators, dead-time modules, etc. But sometimes it is almost impossible to find on the market the modules having the performances corresponding to our needs. The purpose of the module presented here, called MTR2 (Module de Temps-mort Reconductible), is to process, in terms of pulse height discrimination and dead-time corrections, the pulses delivered by the detectors used in counting systems. This dead-time, of the extendible type, is triggered by both the positive and negative parts of the incoming pulse and the dead-time corrections are made according to the live-time method. This module, which has been developed and tested at LPRI, can be used alone in simple counting channels or in more complex systems such as coincidence systems. The philosophy governing the choice and the implementation of this type of dead-time as well as the system used for the dead-time corrections is presented. The electronic scheme and the performances are also presented. This module is available in the NIM standard.

MAC3: an electronic module for the processing of pulses delivered by a three photomultiplier liquid scintillation counting system

In the field of radioactivity measurement, there is a constant need for high quality electronic modules such as ADCs, amplifiers, high voltage generators, dead-time modules, etc. and sometime there is a need for a specialized module not available on the market. The purpose of the module presented here, called MAC3 (module d'acquisition de coı̈ncidences triples), is to process the pulses delivered by the three detectors (photomultipliers) used in our liquid scintillation counting system. The dead-time generated by this module is of the extendible type and the dead-time corrections are made according to the live-time method. This module, which has been developed and tested at LPRI, can replace the complex interconnection of several independent and costly modules. The philosophy governing the choice and the implementation of this type of dead-time as well as the system used for the dead-time corrections is presented. The electronic scheme and the performances are also presented. This module is available in the NIM standard.

A model for calculating dead time and mass discrimination correction factors from inductively coupled plasma mass spectrometry calibration curves

A model allowing simultaneous determination of the detector dead time and the mass discrimination factor in inductively coupled plasma mass spectrometry (ICP-MS), as well as the corresponding uncertainties, is presented and compared for three representative isotope systems, namely magnesium, indium and thallium. The advantages of using the model presented are firstly that both the detector dead time and the mass discrimination factor can be obtained simultaneously and secondly that the sampling time can be spent entirely on the isotopes of interest.

Extending the Dynamic Range of Photon Counting by Dead-Time Correction Using TLD Glow Curves and I/f Conversion

For measuring low doses using TLD equipment, photon counting is the preferred method of light measurement. However, at high doses pulses get lost and the resulting glow curves become clipped, due to the dead-time which is caused by the pulse width of the PM tube and limitations of the processing electronics. For high doses the photon current of the PM tube must be measured instead, e.g. by counting pulses from an attached current to frequency (I/f) converter. This paper describes a system, which combines the operating ranges of both detection systems into one continuous operating range of approximately nine orders of magnitude.

Radiation levels in cyclotron-radiochemistry facility measured by a novel comprehensive computerized monitoring system

Radiation levels in a cyclotron-radiochemistry facility were measured during the production of commonly used PET radiopharmaceuticals by a comprehensive computerized monitoring system. The system consists of three major components: on-line radiation monitoring channels, an area control unit, and a gas waste management unit. During production the radiation levels were measured in the cyclotron vault, inside automatic chemistry production and research shielded cells, in the radiochemistry room, in the gas waste decay tank, in the chimney filters, and at the top of the cells chimney. Each detector was calibrated in a known radiation field, and a special detector dead time correction was performed in order to achieve detected signal-to-radiation linearity for the Geiger tubes located in the radiochemistry production and research cells. During production of C-11 and O-15 PET radiopharmaceuticals, high radiation levels were measured in the gas waste decay tank (240 and 80 mR/h, respectively). In contrast, the radiation levels at the chimney filters and at the top of the cells chimney did not exceed the International Atomic Energy Agency (IAEA) Drive Air Concentration (DAC) recommended for C-11 or O-15. During production of FDG, high radiation levels were measured at the chimney filters, however the radiation level at the top of the chimney (3.7 μCi/m 3) did not exceed the F-18 DAC recommendation (27 μCi/m 3). Low radiation levels of approximately 0.5–1 mR/h were measured in the radiochemistry room during production of PET radiopharmaceuticals. In the cyclotron vault, 2 min after bombardment the radiation levels at 2 m from the cyclotron decreased to 1–2 mR/h. The addition of a gas waste decay system to computerized monitoring channels located near each strategic point of the site allows for a comprehensive survey of the radiochemical processes.

Investigation of deadtime characteristics for simultaneous emission-transmission data acquisition in PET

Modern PET systems generally utilize rotating rod sources and sinogram windowing for transmission scan and are also usually capable of simultaneous emission-transmission data acquisition. The count rate capability of a PET system using BGO is primarily limited by the deadtime of the block detectors. This deadtime is directly related to the singles event rate of the system. In a simultaneous emission-transmission acquisition, the moving rod sources generate a spatially varying singles distribution which rotates synchronously with the rods. The emission activities tend to create a nearly uniform singles distribution which is static or varies slowly depending on the half life of the radioisotope. In this study, the system deadtime is characterized for different sources of activity in a simultaneous acquisition. Typically, there are 60 to 150 MBq Ge-68 in each of the three rods causing a 26% peak-to-peak variation in the instantaneous detector deadtime. The time-averaged deadtime in a transmission sinogram has a variation of 30% peak-to-peak. Rod activities create a 4 to 10% peak-to-peak variation in the time-averaged deadtime in the emission sinogram of a simultaneous scan. The current deadtime correction method results in a 10% error in a simultaneously acquired emission sinogram with 60 MBq in each rod and 74 MBq in a plane source. A spatially variant deadtime correction technique was developed for simultaneous scans. The error was reduced to 4% for the same study.

Performance testing of two dual-energy pulsers

The Idaho National Engineering Laboratory (INEL) has been developing a dual energy pulser for dead time and random summing corrections, and for the assessment of the quality of the data for many years. This effort now includes several revisions of the original concept. Recently, Canberra has developed a version of this dual energy pulser as part of a technology transfer from INEL to Canberra Industries. This new design includes many of the same characteristics as the original INEL design. In this paper, we will present the results of a series of tests performed at NIEL with both the latest INEL pulser design and the Canberra design. These test results include measurement results of pulser peak width, energy equivalence and dead time and random summing correction capability as a function of count rate and temperature. It is demonstrated that both designs perform comparably with count rate and temperature when operated over a limited temperature range. In applications where extreme temperature variations are likely during the measurements, the INEL design is the better choice.

A portable, photon analysis spectrometer for the assay of X- and gamma-ray emitting radionuclides

A portable, battery-powered, multichannel analyzer (MCA) for use with Ge spectrometers has been developed for in-field use for the assay of x-and γ-ray emitting radionuclides. The spectrometer is capable of operating to rates greater than 150,000 counts per second. The analyzer is a Canberra InSpector MCA, that is equipped with the INEEL ultra-stable dual-energy pulser, and pulse injection with subsequent removal (PISR) circuitry. PCGAP, a set of MCA control and spectral analysis programs, was developed for use on a PC with a Windows NT Operating System. It includes an interactive peak analysis program as well as automatic spectral analysis programs for the X- and γ-ray regions, and a number of utility programs. The pulser peaks are calibrated with radioactive sources in terms of energy (i.e., their energy equivalents are measured) using the PCGAP spectral analysis package so that energy shifts, including those due to changes in temperature or count rate, do not cause a loss of energy calibration. The number of injected low- and high-energy-equivalent pulses is known so the stored pulser pulses can be used for a dead-time and random summing correction. The pulser peaks are also used to monitor any deterioration in spectral quality caused by noise, ground loops, etc. The results of performance tests to demonstrate the capabilities of this pulser-equipped InSpector are reported.

Dead-time correction of a multi-element SSD for fluorescent XAFS.

The correct evaluation of the live time of a detection system is found to be important for correcting the counting loss of a multi-element detection system for fluorescent XAFS experiments. Synchronous resetting of preamplifiers and the suspension of electronics during the reset period is an effective method. The dead time of the incoming count rate should also be corrected, since its non-linearity cannot be neglected. A 19-element solid-state detector system which can count up to 3.7 x 10(5) counts s(-1) channel(-1) with <270 eV FWHM for Mn Kalpha has been realized; the dead time was independent of the incoming photon energy.

Nineteen-element high-purity Ge solid-state detector array for fluorescence X-ray absorption fine structure studies

We describe a 19-element high-purity Ge solid-state detector array developed for high-throughput fluorescence X-ray absorption fine structure (XAFS) experiments in ultra-high vacuum (UHV). The packing ratio is 57%, a significant improvement over that of standard 13-element arrays (37%). The average energy resolution in the 19 channels is ∼170 eV at 5.9 keV with an amplifier shaping time of 6 μs. The detector array can be used up to a count rate of 200 kcps per channel using a shaping time of 0.5 μs and a dead-time correction. A total count rate of 3.8 × 10 6 cps is obtained with an energy resolution of 240 eV. Capability of the new detector for fluorescence-detected XAFS applications is demon-strated for 1000 Å thick YBa 2Cu 3O 7 on SrTiO 3 substrate and ∼0.1 monolayer of As atoms on an InP(001).

Deadtime correction for two multihead Anger cameras in 131I dual-energy-window-acquisition mode.

Two side-by-side energy windows, one at the photopeak and one at lower energy, are sometimes employed in quantitative SPECT studies. We measured the count-rate losses at moderately high activities of 131I for two multihead Anger cameras in such a dual-window-acquisition mode by imaging a decaying source composed of two hot spheres within a warm cylinder successively over a total of 23 days. The window locations were kept fixed and the paralyzable model was assumed. In addition, for the Picker Prism 3000 XP camera, the source was viewed from three different angles separated by 120 degrees and the final results are from an average over these three angles. For the Picker camera, the fits to the data from the individual windows are good (the mean of the squared correlation coefficient equals 0.98) while for the Siemens Multispect camera fits to the data from head 1 and from the lower-energy, monitor window are relatively poor. Therefore, with the Siemens camera the data from the two windows are combined for deadtime computation. Repeated autopeaking might improve the fits. At the maximum count rate, corresponding to a total activity of 740 MBq (20 mCi) in the phantom, the multiplicative deadtime correction factor is considerably larger for the Picker than for the Siemens camera. For the Picker camera, it is 1.11, 1.12, and 1.12 for heads 1-3 with the photopeak window and 1.10 for all heads with the lower-energy monitor window. For the Siemens camera, the combined-window deadtime correction factor is 1.02 for head 1 and 1.03 for head 2. Differences between the deadtime correction factor for focal activity and for the total activity do not support the hypothesis of count misplacement between foci of activity at these count rates. Therefore, the total-image dead time correction is recommended for any and all parts of the image.

Strontium isotope ratio measurements in prehistoric human bone samples by means of high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS)

This paper explores the suitability and application range of a commercially available high-resolution ICP-mass spectrometer (Finnigan MAT Element) in determining strontium isotope ratios of human bone samples. Any paleoanthropological interpretation of such samples requires a highly accurate isotopic ratio determination with a precision of at least 0.1% relative standard deviation (RSD). Thus, optimum instrument operating parameters such as sampling time, scan duration, and instrumental bias factors including mass bias and deadtime, were investigated in an initial step. The instrument itself was used without modification but equipped with a commercial microconcentric nebulizer. All experiments were performed under clean room conditions with filtered (class 10.000), temperature controlled, and excess pressured air. Complementary mathematical correction methods like deadtime and mass bias corrections, allowed the achievement of a87Sr/86Sr isotope ratio precision below 0.03% (RSD for n=5). Compared with the certified NIST SRM 987 Strontium Carbonate (Isotopic) value (0.71034±0.00026), our 87Sr/86Sr isotopic ratio revealed a 0.7103±0.0002 match. This optimized procedure was performed on 7000-year-old human skeleton samples from a Neolithic settlement in Asparn/Schletz, Lower Austria, to determine their stable strontium isotope ratio. Specific isotope patterns thereby shed light on the provenance of single individuals.

Comparison of mathemical methods for the calculation of retention indices at high temperature in gas chromatography

AbstractThe retention indices of three homologous series (2‐alkanones, 1‐alkanols, cycloalksanones) have been determined at high temperature by the application of two new adaptation methods: A multiparametric least‐squares regressions iterative method based on the dertermination of the adjusted retention times and a cubic interpolation directly using the uncorrected retention times without dead time correction. The two methods were applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane‐87 stationary phases), while the second includes five glass capillary columns (four methyl‐silicons with different film thicknesses and Apolane‐87 stationary phases). The retention indices obtained with a multiparametric and a cubic interpolation methods were compared with each other and with those calculated by Grobler's, Guardino's, Kaiser's and Kovàts' methods. The influence of coating, film thickness, and temperature on them was investigated.

Evidence That the Transfer of Hydride Ion Equivalents between Nucleotides by Proton-translocating Transhydrogenase Is Direct

The molecular masses of the purified, recombinant nucleotide-binding domains (domains I and III) of transhydrogenase from Rhodospirillum rubrum were determined by electrospray mass spectrometry. The values obtained, 40,273 and 21,469 Da, for domains I and III, respectively, are similar to those estimated from the amino acid sequences of the proteins. Evidently, there are no prosthetic groups or metal centers that can serve as reducible intermediates in hydride transfer between nucleotides bound to these proteins. The transient-state kinetics of hydride transfer catalyzed by mixtures of recombinant domains I and III were studied by stopped-flow spectrophotometry. The data indicate that oxidation of NADPH, bound to domain III, and reduction of acetylpyridine adenine dinucleotide (an NAD+ analogue), bound to domain I, are simultaneous and very fast. The transient-state reaction proceeds as a biphasic burst of hydride transfer before establishment of a steady state, which is limited by slow release of NADP+. Hydride transfer between the nucleotides is evidently direct. This conclusion indicates that the nicotinamide rings of the nucleotides are in close apposition during the hydride transfer reaction, and it imposes firm constraints on the mechanism by which transhydrogenation is linked to proton translocation.

Aqueous Solutions of Ethyl (Hydroxyethyl) Cellulose and Hydrophobic Modified Ethyl (Hydroxyethyl) Cellulose Polymer: Dynamic Surface Tension Measurements

The dynamic surface tension of aqueous solutions of ethyl (hydroxyethyl) cellulose (EHEC) and hydrophobic modified ethyl (hydroxyethyl) cellulose (HM-EHEC) were determined using the maximum bubble pressure method. Values were monitored over surface lifetimes ranging from 0.15 to 2.5 s (after dead time corrections). In the low concentration range (<100 ppm) HM-EHEC was shown to be more surface active, and the presence of salt (0.1M) was shown to increase the surface activity of both polymer systems. The results were compared with surface tension measurements carried out over longer time scales, determined using the du Nouy ring technique. From these results, isotherms were constructed relating surface tension to surface aging time. Although the results could not be directly correlated to interfacial diffusion models, the isotherms were found to be comparable to previous data reported for lower-molecular-weight poly(oxyethylene ether) surfactants and a higher-molecular-weight EHEC polymer determined using the pendant drop technique. Essentially, the isotherms could be divided into distinct regions: induction period, fast fall region where surface coverage occurs fairly rapidly, and finally the meso-equilibrium region. From the Gibbs equation, the number of segments of polymer per unit surface area at saturated adsorption levels was calculated over regions of different aging times, and the results are discussed in terms of configuration changes of the polymer at the interface.