Loss-free Counting Research Articles

Uncertainty of nuclear counting

Nuclear counting is affected by pulse pileup and system dead time, which induce rate-related count loss and alter the statistical properties of the counting process. Fundamental equations are presented to predict deviations from Poisson statistics due to non-random count loss in nuclear counters and spectrometers. Throughput and dispersion of counts are studied for systems with pileup, extending and non-extending dead time, before and also after compensation for count loss. Equations are provided for random fractions of the output events, applicable to spectrometry applications. Methods for loss compensation are discussed, including inversion of the throughput equation, live-time counting and loss-free counting. Secondary effects in live-time counting are addressed: residual interference from pileup in systems with imposed dead times and errors due to varying count rate when measuring short-lived radionuclides.

Performance of digital gamma ray spectrometer for loss free counting and its application to NAA using short-lived radionuclides

A digital gamma-ray spectrometer with loss-free counting system was used for real time correction for dead time (DT) due to pulse processing in the electronics. This spectrometer was used for radioactive assay of samples having constant as well as varying DT conditions. The system was tested by measuring activity of short and medium lived nuclides namely 28Al, 52V and 128I in the DT ranges of 80–2 %. Using this spectrometer and neutron activation analysis (NAA), concentrations of Al, V, Ti, Ca, Dy and Mn were determined in some samples and reference materials.

A new rapid neutron activation analysis system at Dalat nuclear research reactor

An auto-pneumatic transfer system has been installed at the Dalat research reactor for rapid instrument neutron activation analysis based on very short-lived nuclides. This system can be used to perform short irradiations in seconds either in the vertical channel 13-2 or in the horizontal thermal column of the reactor. The transferring time of sample from irradiation to measurement position is approximately 3.2 seconds. A loss-free counting system using HPGE detector has been also setup in compacting with the pneumatic transfer system for measurement of sample’s activity, automatically starting for data acquisition at irradiated sample’s arrival. This new facility was tested and shown to have high potential for the determination of short-lived nuclides with half-lives from 10 ¸ 100 seconds. This work presents the results of timing parameter measurements, characterization of irradiation facilities, and application of this system to determining Selenium concentration in several biological reference materials.

Laudatio for Dr. Richard M. Lindstrom

Dr. Richard M. Lindstrom was born in Ashland, Wisconsin, USA. He received his B.S. (Honors) from the University of Wisconsin, Madison, in 1963, and Ph.D. in 1970 from the University of California, San Diego. His Ph.D. dissertation ‘‘Radionuclides in Meteorites and in the Lunar Surface’’ was one of the first theses to be based on the Apollo 11 lunar samples. He then went to the Tata Institute of Fundamental Research in Bombay as a postdoctoral fellow, and a year later moved to Brookhaven National Laboratory. Dr. Lindstrom joined the National Bureau of Standards (NBS, now NIST) in 1972 to develop a lowbackground counting laboratory. In 1974 he moved to the Inorganic Chemical Metrology Group, concentrating on accurate methods of elemental analysis using a variety of nuclear techniques. Dr. Lindstrom pioneered the development of promptgamma activation analysis (PGAA) and its application to the elemental characterization of substances and certification of Standard Reference Materials (SRMs). He also led a team of specialists to develop the world’s first cold-neutron PGAA system at NIST, and participated in establishing PGAA facilities in Japan and Hungary. With other scientists, Dr. Lindstrom used PGAA to detect neutron standing waves for the first time, and also measured the 200microbarn thermal neutron cross section of Pb, with implications for nucleosynthesis and nuclear models. Dr. Lindstrom has contributed significantly to understanding of several fundamental aspects of gamma-ray spectrometry, namely dead time and pileup effects, lossfree counting methods, and methods for determining of peak areas. He developed an interactive computer program called SUM, originally written to address the incorrect calculation of peak area uncertainty in a commercial gamma spectrometry packages, but which also provides improved accuracy in cases where a photopeak is very large, very small, or non-Gaussian. Dr. Lindstrom has contributed to improved nuclear data, and thus improved accuracy of NAA measurements. He provided a more precise value for half-life of As and several other nuclides. As a direct result of his work, the nuclear data evaluation community has changed its algorithms for combining published values. He was one of the first to identify every feature in the background of a gamma-ray detector. He operates a state-of-the-art lowbackground counting facility, and has recently installed a low-background detector more than 500 m underground. S. Glover (&) University of Cincinnati, Cincinnati, USA e-mail: seg3@cdc.gov

Improved performance in germanium detector gamma-spectrometers based on digital signal processing

In an HPGe spectroscopy system, Digital Signal Processing (DSP) replaces the shaping amplifier, correction circuits, and ADC with a single digital system that processes the sampled waveform from the preamplifier with a variety of mathematical algorithms. DSP techniques have been used in the field of HPGe detector gamma-ray spectrometry for some time for improved stability and performance over their analog counterparts. Recent developments in HPGe detector construction and new liquid nitrogen-free cooling methods have resulted in HPGe detectors which are better adapted to the needs of the application. Some of these improvements in utility have degraded the spectroscopy performance. With DSP, it is possible to reduce the changes, in real time, in several aspects of detector performance on a pulse-by-pulse basis, which is not possible in the old analog environment. In the past, in designing for the analog regime, flexibility was limited by issues of component size, number and cost. In the digital domain, the problem translates to the need for a DSP with enough speed and an efficient algorithm to achieve the desired transformation or correction to the digitally determined pulse shape or height, event-by-event. The use of DSP allows the peak processing to be tuned to the preamplifier peak shape from the detector rather than being set to an average value determined from several detectors of the type in question. The selection of the filter can be automatic or manual. The following corrections are now possible: ballistic deficit correction, peak resolution improvement by reducing the impact of microphonic noise, increase throughput by reducing pulse processing time, and loss-free (zero dead time) counting.

The perfection of loss-free counting

Pileup losses in nuclear pulse spectrometry also depend on energy as lower energies produce narrower pulses which in turn have better chances to avoid pulse pileup. Consequently, in our present system individual energy-dependent pileup correction factors are calculated for all events, making it what very probably may be called the first perfect implementation of Loss-Free Counting. Temporal response and quantitative performance of the new system are tested over the whole range of counting rates (up to 106 c/s) and counting losses (up to 99%) by means of short-lived isomeric transitions and a fast rabbit system.

Review of loss-free counting in nuclear spectroscopy

This paper is a review of techniques for real-time correction of counting losses in nuclear pulse spectroscopy which became known under the name of loss-free counting (LFC).

Automatic activation analysis

Automatic activation analysis (AAA) is rendered possible by a unique neutron activation analysis facility for short-lived isomeric transitions based on a fast rabbit system with sample changer and sample separation, and an adaptive digital gamma-spectrometer for very high counting rates of up to 106 cps. The system is controlled by a computer program performing irradiation control, neutron flux monitoring, and gamma-spectrometry with real-time correction of counting losses, spectra evaluation, nuclide identification and calculation of concentrations in a fully automatic procedure. As spectrometry is done by means of hundreds of sequentially measured pairs of concurrently recorded loss-corrected and non-corrected spectra, concentrations are derived from an optimally weighted average of all individual occurrences in this sequence of spectra which also enable the separation of isomeric transitions with coinciding energies but different half-lives such as 116m2In (162.4 keV, T1/2 = 2.2 s) and 77mSe (162.2 keV, T1/2 = 17.4 s). To clear up repeatedly voiced misconceptions concerning the errors of loss-free counting our findings of 1978 and 1981 are reiterated, namely that the counting error of a peak in a corrected spectrum may be derived consistently from the error of the same peak in the respective non-corrected spectrum and from the error of weighting factors in the corresponding region of interest, according to the principle of propagation of errors. Experimental proof is provided for conditions of stationary as well as rapidly varying counting rates and spectral shapes.

A neutron activation analyzer

Dubbed “Analyzer” because of its simplicity, a neutron activation analysis facility for short-lived isomeric transitions is based on a low-cost rabbit system and an adaptive digital filter which are controlled by a software performing irradiation control, loss-free gamma-spectrometry, spectra evaluation, nuclide identification and calculation of concentrations in a fully automatic flow of operations. Designed for TRIGA reactors and constructed from inexpensive plastic tubing and an aluminum in-core part, the rabbit system features samples of 5 ml and 10 ml with sample separation at 150 ms and 200 ms transport time or 25 ml samples without separation at a transport time of 300 ms. By automatically adapting shaping times to pulse intervals the preloaded digital filter gives best throughput at best resolution up to input counting rates of 106 cps. Loss-free counting enables quantitative correction of counting losses of up to 99%. As a test of system reproducibility in sample separation geometry, K, Cl, Mn, Mg, Ca, Sc, and V have been determined in various reference materials at excellent agreement with consensus values.

On the statistical control of 'loss-free counting' and 'zero dead time' spectrometry

The assessment of the statistical counting uncertainty is discussed for two pulse loss correction methods in nuclear spectrometry: the 'loss-free counting' technique based on the virtual pulse generator method and the 'zero dead time' technique with 'variance spectrum'.

Experimental tests of zero dead time gamma-ray spectrometry of rapidly decaying sources with the DSPECPLUS ®

In the year 2000, at the MARC V conference, the first results obtained at constant count rates with so-called "zero dead time counting" (ZDT) as implemented in ORTEC's DSPECPLUS® were presented. In this paper, further experiments are described that were performed to establish how the DSPECPLUS® performs at varying count rates. At the same time, the experiments were designed to demonstrate the possible inadequacy of the dual spectrum approach sometimes used to solve the problem of non-Poisson counting statistics encountered in loss-free counting, and to test the "variance spectrum" alternative offered by the DSPECPLUS® . It is concluded that the DSPECPLUS® performs with good accuracy at dead times lower than 90%, even when count rates vary. It is also concluded that the dual spectrum approach indeed is inadequate. Finally, it is shown that the "variance" spectrum approach provides the correct uncertainties to be used in the treatment of LFC or ZDT data.

Low-cost activation analysis at small research reactors

A software implementation of a loss-free counting multichannel analyzer, storing immediately into the multimegabyte memory of a low-cost 486 or Pentium type PC, enables the real-time control of a rabbit system as well as the collection of up to 1000 pairs of simultaneously recorded loss-corrected and non-corrected spectra of 16 k channels each, in a true sequence without time gaps in between, at throughput rates of up to 200 kc/s.1 Intended for activation analysis of short-lived isomeric transitions, the system renders possible peak to background optimizations and separations of lines with different half-lives without an a priori knowledge of sample composition by summing up appropriate numbers of spectra over appropriate intervals of time. By automatically adapting the noise filtering time to individual pulse intervals, the Preloaded Digital Filter (PLDF) combines low- to medium-rate resolutions comparable to those of high-quality Gaussian amplifiers with throughput rates of up to 100 kc/s, and high-rate resolutions superior to those of state-of-the-art gated integrator systems. In contrast to commercially available digital filters, the PLDF in its new implementation performs pulse shortening as well as pole zero cancellation in the analog domain. This not only results in a simpler digital core but also, for the first time, makes possible the use of a low-cost ADC in a spectrometric application.2 A simple but highly effective rabbit system is made from an aluminum incore part, inexpensive plastic tubing and an industrial pressurized air generator. Large sample containers have a volume of 25 cm3. Smaller containers of 5 cm3 are automatically separated from a transport capsule. A transport time of below 0.5 s enables activation analysis of short-lived isomeric transitions. The combination of rabbit system, PLDF and software based multichannel analyzer provides a low-cost but powerful solution for NAA at Triga reactors in developing countries, a forthcoming research project of the IAEA, Vienna.

Adaptive digital filter for high-rate high-resolution gamma spectrometry

By automatically adapting the noise filtering time to individual pulse intervals, the preloaded digital filter (PLDF) combines low- to medium-rate resolutions comparable to those of high-quality Gaussian amplifiers with throughput rates of up to 100 kc/s and high-rate resolutions superior to those of state-of-the-art gated integrator systems. In contrast to commercially available digital filters, the PLDF in its new implementation performs pulse shortening as well as pole-zero cancellation in the analog domain. This not only results in a simpler digital core, but also, for the first time, makes possible the use of a low-cost analog-digital converter in a spectrometric application. Combined with real-time correction of counting losses according to the loss-free counting method, the PLDF is the core of a novel multichannel analyzer system for neutron activation analysis of short-lived isomeric transition.

STUDY OF GREEK NEOLITHIC CERAMIC SHARDS FOR ELEMENT CONCENTRATION CHANGES IN RELATION TO THE AGE

At the reactor of the NCSR “Demokritos” epithermal irradiation was used in connection with the loss-free counting technique to investigate Neolithic ceramic shards from the Alepotrypa cave of Diros, Greece. The samples were subjected to a 10 min irradiation and a combination of two measurements, a first short one after a cooling time of 3 h, using loss-free counting and a second long one after 18 h in a well-type detector, allowing the study of 17 elements. Pottery of local origin can show depth-depending changes of composition caused by chemical influences from the geological environment, especially the groundwater, which may be or may have been in direct contact with the objects. It may occur also that, because of the progress of mining, various strata of a clay deposit differing slightly in chemical composition, transfer these changes to the ceramic. To study these possible effects determinations were performed with samples from three different depths and dated by the carbon-14 method, using carbonised objects of the same strata. Finally, changes in the concentrations of Mn, K, and Sb in relation to the age should be the effect of a reductive environment and of ion exchange processes, and could be assessed by statistical evaluation. This estimated age dependence of the concentrations should be taken into consideration when provenance studies are carried out.

Loss-free counting with a digital spectrometer and a software program

The computer program LFREE was written to do loss-free counting with a digital spectrometer. It runs in parallel with the normal data acquisition software and corrects the counting losses once per second without interrupting data acquisition. The spectrometer's live time clock is used to measure the live time fraction. Tests showed that losses are accurately corrected at variable count rates which cause dead times as high as 80%. For half-lives of the order of 10 seconds, the accuracy is limited by the response of the live time clock to very rapidly changing count rates.

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.

A gamma-spectrometry system for activation analysis

Based on a Preloaded Digital Filter operating on a 20% n-type HpGe detector with transistor reset preamplifier, and on a software implementation of a loss-free counting MCA, storing immediately into the multi-megabyte main memory of a low-cost 486-type PC, the new system enables the collection of up to 120 pairs of simultaneously recorded loss-corrected and non-corrected spectra of 16 k channels each, in a true sequence without time gaps in between, at a throughput rate of up to 140 kc/s. Intended for activation analysis of short-lived isomeric transitions, the system for the first time makes possible peak to background optimizations and even separations of lines with different half-lives without an a priori knowledge of sample composition by summing up appropriate numbers of spectra over appropriate intervals of time. The speed of this programmable system also made possible the direct comparison of two methods of real-time correction of counting loss, by simultaneous recording of LFC-corrected, ZDTtm -corrected and non-corrected spectra together with their corresponding weighting factor distributions.

Maintaining Accuracy in Gamma-Ray Spectrometry at High Count Rates

Gamma-ray spectrometry losses through pulse processing dead time and pile-up are best assayed with an external pulse technique. In this work, the virtual pulse generator technique as implemented commercially with the Westphal loss free counting (LFC) module is set up and tested with four high resolution gamma-ray spectrometers. Dual source calibration and decaying source techniques are used in the evaluation of the accuracy of the correction technique. Results demonstrate the reliability of the LFC with a standardized conventional pulse processing system. The accurate correction during high rate counting, including during rapid decay of short lived activities, has been the basis for highly precise determinations in reference materials studies.

Comparison of the Precision and Accuracy of Digital Versus Analogue Gamma-Ray Spectrometric Systems Used in INAA for Evaluation of Homogeneity and Certification of Reference Materials of the IAEA

Performance of three commercial gamma-ray spectrometric systems was evaluated for precision and accuracy prior to use in characterization of reference materials. Two of the systems were based on fast processing of the analogue signal from the amplifier (EGG Ortec model 672) using a loss free counting module (Canberra model LFC 599) interfaced to one of two analog-to-digital converters (Canberra models 8713 or 8715). The third system was based on a digital signal processor (Canberra model DSP 9660). Performance of the systems was tested over a range of count rates up to a maximum of 70,000 counts per second (dead time up to 90%) using 60Co and 137Cs sources. Best resolution was achieved with an analogue system with ADC 8713. The analytical results obtained with the digital system show the lowest and well-quantified uncertainty.

Is it Safe to Use Poisson Statistics in Nuclear Spectrometry

The boundary conditions in which Poisson statistics can be applied in nuclear spectrometry are investigated. Improved formulas for the uncertainty of nuclear counting with deadtime and pulse pileup are presented. A comparison is made between the expected statistical uncertainty for loss-free counting, fixed live-time and fixed real-time measurements.