Dead-time Losses Research Articles

Dead-time losses in buffer systems

The dynamics of data-acquisition devices using a buffer system is studied in detail. Dead-time losses for both an image-transfer charge-coupled device (CCD) and a multi-event buffer system under a flux of randomly incoming events are calculated. Results are given as a function of dimensionless variables and they can be used in other applications dealing with different time ranges. As an example, the results shown in this work are applied to a realistic acquisition system for detection of high-energy gamma rays by means of ground-based air Cherenkov telescopes. Results for this particular system are compared with a Monte-Carlo simulation.

A multiwire proportional counter for very high counting rates.

Preliminary measurements in a proportional counter with two independently counting wires showed that counting rates up to 10(6) counts s(-1) wire(-1) can be reached without critical loss in the 'true versus measured' linearity relation. Results obtained with a detector containing 30 active wires (2 mm pitch) are presented. With each wire is associated a fast pre-amplifier and a discriminator channel. Global counting rates in excess of 10(7) events s(-1) are reported. Dead-time losses are corrected by use of simple mathematical-modelling functions. Data-acquisition systems are described for one-dimensional (real-time) and two-dimensional (off-line) position-sensitive detection systems.

The half-lives of 42Sc, 46V, 50Mn and 54Co

As part of a broad program to determine ft values for superallowed O + → O − β transitions, the half-lives of 42Sc, 46V, 50Mn and 54Co have been measured with isotopically pure samples prepared with an on-line isotope separator. Emitted β + particles were observed with a large-solid-angle, high-efficiency, low-background gas proportional counter, from which signals were multiscaled. The counting electronics had a well-defined, non-extendable dead time. As no exact data analysis is possible, three different procedures were compared. The most stringent method involved regression analysis where a common half-life was simultaneously fitted to the individual decay curves of hundreds of samples and their Poisson statistical weights were distorted to reflect dead-time losses. The analysis procedures were tested on hypothetical decay data that were created by binning a time sequence of events, obtained from interval distributions reflecting the given half-lives and intensities of two decay components, and allowing for series and non-extendible dead times that reflected experimental conditions. On the basis of these tests it is demonstrated that the analysis does not introduce any bias at the 0.01% level. The deduced half-lives are 42 Sc: (680.67 ± 0.28) ms, 46 V: (422.57 ± 0.13) ms, 50 Mn: (283.29 ± 0.08) ms, 54 Co: (193.28 ± 0.07) ms. These results establish important elements in a demanding test of the Electroweak Standard Model.

Validation of a loss-free counting system for neutron activation analysis with short-lived indicators

The use of loss-free counting systems makes possible the exact correction for pile-up and dead-time losses during counting of a mixture of short-lived radionuclides even at very high count rates. However, counting statistics cannot be calculated by taking into account only the Poisson distribution of the incoming γ-quanta, such as is done in existing computer programs for γ-spectrometry. At moderate count rates Muller statistic was found to account for the observed variability between duplicate countings; however, at higher count rates the variability of weighing factors was found to be significant in comparison with the Muller statistic. While counting statistics could not be correctly estimated for short-lived species, experiments showed excellent accuracy for initial dead times up to 90%.

Performance evaluation of the whole-body PET scanner ECAT EXACT HR/sup +/ following the IEC standard

The performance parameters of the whole-body PET scanner ECAT EXACT HR/sup +/ (CTI/Siemens, Knoxville, TN) were determined following the standard proposed by the International Electrotechnical Commission (IEC). The tests were expanded by some measurements concerning the accuracy of the correction algorithms and the geometric fidelity of the reconstructed images. The scanner consists of 32 rings, each with 576 BGO detectors (4.05/spl times/4.39/spl times/30 mm/sup 3/), covering an axial field-of-view of 15.5 cm and a patient port of 56.2 cm. The transaxial FWHM determined by a Gaussian fit in the 2D (3D) mode is 4.5 (4.3) mm at the center. It increases to 8.9 (8.3) mm radially and to 5.8 (5.2) mm tangentially at a radial distance of r=20 cm. The average axial resolution varies between 4.9 (4.1) mm FWHM at the center and 8.8 (8.1) mm at r=20 cm. The system sensitivity for unscattered true events is 5.85 (26.4) cps/Bq/ml (measured with a 20 cm cylinder). The 50% dead-time losses were reached for a true event count rate (including scatter) of 286 (500) kcps at an activity concentration of 74 (25) kBq/ml. The system scatter fraction is 0.24 (0.35). With the exception of the 3D attenuation correction algorithm, all correction algorithms work reliably. The results reveal that the ECAT EXACT HR/sup +/ has a good and nearly isotropic spatial resolution. Due to the small detector elements, however, it has a low slice sensitivity which is a limiting factor for image quality.

Validation of a loss-free counting system for neutron activation analysis with short-lived indicators

The use of loss-free counting systems makes possible the exact correction for pile-up and dead-time losses during counting of a mixture of short-lived radionuclides even at very high count rates. However, counting statistics cannot be calculated by taking into account only the Poisson distribution of the incoming γ-quanta, such as is done in existing computer programs for γ-spectrometry. At moderate count rates Muller statistic was found to account for the observed variability between duplicate countings; however, at higher count rates the variability of weighing factors was found to be significant in comparison with the Muller statistic. While counting statistics could not be correctly estimated for short-lived species, experiments showed excellent accuracy for initial dead times up to 90%.

A pulse generator simulating Ge-detector signals for dead-time and pile-up correction in gamma-ray spectrometry in INAA without distortion of the detector spectrum

Commercially available pulse generators for use in γ-ray spectroscopy to correct for dead-time and pile-up losses share the problem that the fall-time of the output pulses does not match the fall-time of the detector pulse. Upon pulse shaping in the main amplifier the pulser's signals result in a considerable undershoot of the amplifier's output signal, which cannot be restored by theP/Z network. As a result, peaks in the γ-ray spectrum are broadened. Moreover, amplitude stability of the generators is often poorer than the overall gain stability of the spectrometer. To overcome these problems a special pulse generator has been developed at the Interfaculty Reactor Institute, which simulates Ge-detector signals. It generates pulses with a constant frequency of 25 Hz using a crystal oscillator controlled mercury relay. The stability of the generator's amplitude and frequency is extremely high and the peak-shapes in the γ-ray spectrum are not distorted.

Evaluation of moderately cooled pure NaI as a scintillator for position-sensitive PET detectors

A new evaluation of pure NaI has been performed to determine if moderate cooling would lead to better performance than that of existing, activated NaI(Tl) position-sensitive detectors, particularly at high countrates. Using a freezer, an initial effort was performed to cool the crystal assembly to -90/spl deg/C (183/spl deg/K). At this temperature, pure NaI has a decay constant of 35 nsec, a light output which is about 20% that of room temperature NaI(Tl), and an energy resolution of 15%. For the authors' PET applications the signal of room temperature (25/spl deg/C) NaI(Tl) is normally pulse clipped, reducing the light output to 40% of the unclipped signal and yielding an energy resolution of 10.5%. Since the long decay of NaI(Tl) causes it to suffer more significantly than pure NaI from pre-pulse pileup, the difference in energy resolution between the two crystals at high countrates will be reduced. Also, a significantly shorter trigger deadtime with pure NaI will lead to a reduction in coincidence deadtime losses in PET. Computer simulations of large-area crystals operating at high countrates have been performed to quantify their trigger deadtime behavior and position resolution as a function of light output and pulse decay time. Having gained experience with the practical issues of cooling large crystals, measurements of position resolution have been performed with a NaI bar detector of similar geometry to the NaI(Tl) detectors in use in the PENN-PET scanner.

Determination of dead-time in photon counting photometers by a polarimetric method

Experiments are described which demonstrate that the dead-time constant associated with a photon counting stellar photometer/polarimeter is readily determinable by the application of the Malus law. It is also proposed that corrections for dead-time losses should be applied by an iteration technique to the exact equation rather than using the available approximation formulae.

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.

Note on dead-time losses for pulsed radiation

The dead-time induced counting loss in the detection of pulsed radiation, in certain cases, does neither depend on the exact magnitude, type, and possible variations (e.g., with signal amplitude) of the dead time nor on the shape of the radiation pulse. These favourable cases and the resulting simple calculation of the losses are discussed.

The Super PET 3000-E

Mathematical models for the delineation of regional myocardial perfusion and metabolism with PET require faithful reconstruction of arterial and myocardial time-activity curves following administration of radiotracers. High temporal resolution is often required in such measurements. Many commercially available tomographs exhibit long dead times that limit their count rate capabilities. To overcome these limitations, we developed and tested a whole-body tomographic device (Super PET 3000-E) with high count rate capabilities. The use of cesium fluoride scintillation detectors coupled with a one-to-one detector photomultiplier configuration reduces the system resolving and dead times. The Super PET 3000-E was subjected to a series of tests with phantoms to determine its resolution, sensitivity, linearity, count rate capabilities, dead time, and random coincidence contribution. The system sensitivity is 136 kcounts/s/microCi/ml and its transverse and longitudinal resolutions are 8.5 and 10.5 mm full width at half-maximum, respectively. The system can easily record a total event rate of 2.0 Mcounts/s with minimal dead time loss and excellent linearity. The system fulfills its design goals and allows the very high count rate performance needed for the application of the physiological models used in our cardiac studies.

Attenuation correction in PET using single photon transmission measurement.

The use of single photon transmission measurement with a rotating rod source has been evaluated to measure the attenuation correction factors in positron emission tomography (PET). The singles projections are resampled into the coincidence geometry using the detector positions and the rod source location. A nonparalyzable dead time correction algorithm was developed for the block detectors used in the McMaster PET scanner. This enables accurate attenuation correction factors (ACFs) to be computed using a wide range of source strengths for transmission scanning. Transaxial resolution is approximately 6 mm, which is comparable to emission scanning performance. Axial resolution is about 25 mm, with only crude source collimation. ACFs are underestimated by as much as 10% due to increased cross-plane scatter, compared to coincidence transmission accuracy. The response of the correction factors to object density is within 15%, when comparing singles transmission measurement to current coincidence transmission measurement. The major advantage of using singles transmission measurement is a dramatically increased count rate. A factor of 7 increase in count rate over coincidence scanning is possible with a 2-mCi transmission rod source. Uniformity of 2% in the transmission images is possible with this source strength and a 2-min acquisition. There are no randoms counted in singles transmission scans, which makes the measured count rate vary linearly with source activity. Singles detector dead time losses are approximately 6% in the detectors opposite a 2-mCi rod source.

A simple method to correct for pulse pile-up and dead time losses: Application to cyclic activation analysis with 14-MeV neutrons

A method to correct for pulse pile-up and dead time ‘losses is described for the case where activities decrease during counting. For pulse pile-up, the Wyttenbach method is used and pile-up parameters are first determined. It was assumed that dead time is linearly dependent on activity. The total correction is then carried out by simple integration. In application, sodium is determined in IAEA standard reference materials (Soil-5 and milk powder A11) using cyclic activation analysis with a neutron generator and the reaction 23Na(n,α) 20F.

Performance of QPET, a high-resolution 3D PET imaging system for small volumes

QPET is a positron imaging system developed at Queen's University for high-resolution, 3D imaging of small volumes; it includes a pair of planar gamma-ray detectors 25.4 cm square, which rotate about a central axis with a quasi-cylindrical geometry. The authors describe the performance of this system. Basic characteristics of the detectors are evaluated: a spatial sampling of 1 mm, a quantum efficiency of 9.3% (for 511 keV gamma rays with normal incidence), and a time resolution of 88 ns. Models are developed to characterize the system deadtime and the sensitivity in terms of the noise-equivalent counting rate. With an 8 cm diameter spherical source, the noise-equivalent counting rate reaches a maximum at just over 3 kcps for an activity concentration of 2 muCi/cc; the random coincidence events and the deadtime losses both contribute significantly and the scatter contribution is small. Spatial resolution and uniformity over the field of view are evaluated by imaging short and long line sources; a spatial resolution of 2.7 mm in the transverse directions and 2.0 mm in the axial direction is achieved, with excellent uniformity throughout the field of view. The detector response is amplitude invariant across a 20 cm transverse diameter and a 9 cm axial length with the acceptance angle limited to +/-25 degrees in the axial direction. As an example of the imaging capabilities of QPET, the authors show 3D images of (18)F uptake in the bones of a rat, showing the excellent spatial resolution. This system is best suited to limited-volume applications where high counting rates are not necessary, but where high spatial resolution and uniform detector response are priorities.

Correction of counting losses of the preloaded fil ter pulse processor

By adapting noise filtering to individual pulse intervals, the Preloaded Filter (PLF) pulse processor (1) combines high resolution with optimum throughput efficiency. As a consequence, its output pulse interval distribution contains strong non-random components which render conventional ADC dead-time correction an impossibility. Quantitative correction of dead-time and pileup losses of the PLF processor may be achieved, however, with the Virtual Pulse Generator (2), together with a new, distribution-independent method of measuring ADC losses which is based on a pulse counting technique.

Improved neutron activation analysis with short-lived nuclide decay compensation

A novel technique in neutron activation analysis has been developed, particularly for short-lived nuclides. The purpose is to optimize the count rate by keeping it relatively constant so that the counting period can be prolonged, in order to increase the counting statistics and consequently, increase the sensitivity and accuracy of the method. The rapid radioactive decay of the short-lived nuclides, resulting in rapid decrease of the count rate, is compensated for by changing the distance between the irradiated sample and the detector and hence the geometry factor and the efficiency of the system during the counting period. After several trials with air- and oil-driven piston systems, the slow approach of the sample holder to the detector was implemented by means of a mechanical device consisting of a slow d.c. motor with variable rotation speed reduction, combined with a rotary-to-linear motion converter. In order to avoid pulse pile-up effects, dead-time losses and background interferences, it is necessary to keep the count rate within certain limits and hence to partially compensate the exponential radioactive decay by a linear approach of the sample to the detector, although the radiation increase on the detector is inversely proportional to the square of the sample–detector distance. For complete decay compensation, the sample motion speed should decrease exponentially. The relatively high background at long sample–detector distances, particularly in trace element analysis, necessitates good shielding. As the system is calibrated by use of standards, repeatable experimental conditions have to be ensured. Promising preliminary results have been obtained. However, automation of the short-lived nuclide analytical system has been improved by the recent installation of a programmable logic controller to facilitate the operation of the system.

A Technique for the Measurement of Cloud Structure on Centimeter Scales

Abstract A system has been developed for measuring the distance between cloud droplets along the flight path with an airborne optical particle counter. This technique resolves separation distances as small as 0.2 mm at a typical flight speed of 100 m s−1, a measure of cloud structure at scales much smaller than previously possible. This technique also provides an important alternative measure of droplet concentration that is negligibly affected by coincidence and dead-time losses. This new measurement technique may lead to a better understanding of cloud processes by providing the means of examining clouds at scales that are important in determining rates of droplet growth and evaporation. The capability of measuring droplet concentrations that are unbiased by coincidence and dead-time losses is a major offshoot of this technique and removes much of the uncertainty that has hampered previous measurements with optical particle probes.

Liquid scintillation applied to alpha counting of plutonium

A liquid scintillation counter has been modified with fast electronic modules to extend its range for α-counting rate measurements. The practical upper limit for accurate counting is about 30 X 10 6 cpm, at which 5% dead-time loss correstions are applied. Solutions of 239Pu up to 200 g l −1 can be aliquoted directly with capillary pipets, thereby obviating the need for dilutions. The aliquots are mixed with 250 μl of cocktail in 400-μl plastic centrifuge tubes for counting so that waste generation is minimized. Although the counting set-up includes no coincidence circuitry, blanks show less than 10 cpm. Therefore, low-activity samples can also be measured accurately.

The Correction of the Dead Time Loss of the Ge Detector in X-ray Absorption Spectroscopy

The effects of dead time loss of the 13 element Ge detector (Canberra, GL0110), which result from the associated electronics, were studied. Distortions and noise attributable to dead-time losses are found to be substantial even for dilute metalloprotein samples and modest count rates. The dead time loss can introduce two types of non-linear problems into the X-ray Absorption Fine Structure (XAFS) data collection. First, the I0 fluctuation cannot be fully divided out; second, XAFS spectra for concentrated samples can be substantially reduced. The dead time loss calibration showed that the response of the system can be better approximated by the first order approximation of the paralysable and the non-paralysable models rather than the two models themselves. The dead time correction using this algorithm produces satisfactory results with the correction error generally within the statistical error of photon counting. This will permit reliable XAFS data collection at high Incident Count Rate greatly improving the counting efficiency. The dependences of the dead time and the energy resolution on the shaping time of the shaping amplifier were also examined.