On the limits of low level measurements of137Cs as a natural radiotracer

High rate, high resolution gamma-spectrometry with real-time correction of counting losses is made possible by combining the novel Preloaded Filter (PLF) pulse processor with the Virtual Pulse Generator (VPG) counting loss correction method. A spectrometry system for high-speed activation analysis based on the PLF processor, VPG correction and a high resolution LOAX detector is tested up to 850 kc/s.Dedicated to Professor Vincent P. GUINN on the occasion of his retirement.     

Digital implementation of the preloaded filter pulse processor

Adapting it's processing time to the respective pulse intervals, the Preloaded Filter (PLF_ pulse processor offers optimum resolution together with highest possible throughput rates. The PLF algorithm could be formulated in a recursive manner which made possible it's implementation by means of a large field-programmable gate array, as a fast, pipe-lined digital processor with 10 MHz maximum throughput rate. While pre-filter digitization by an ADC with 12 bit resolution and 10 MHz sampling rate resulted in a poorer resolution than that of an analog filter, a digital PLF based on an ADC with 14 bit resolution and 10 MHz sampling rate, surpassed high-quality analog filters in resolution, throughput rate and long-term stability.     

Instrumental correction of time dependent counting losses in high rate gamma-ray spectrometry

A CAMAC system was installed for pulse height analysis and correction of counting losses due to the dead-time of a multichannel analyzer and the pulse pile-up. A computer program was developed to control the whole system, and to collect and store data in both conventional and cyclic measurement modes.     

An alternative approach to the corrections of deat-time losses

A new approach to the problem of the dead-time of pulse detection systems is proposed. The presented method of the correction for counting losses is based on the evaluation of the shortest time interval between two successive output pulses.     

High rate gamma-spectrometry—A status report

Combined with a new, low dead-time transistor reset preamplifier, the Preloaded Filter pulse processor offers up to 49% higher throughput rates than a conventional gated integrator system, at resolutions which are better by 0.3 to 0.4 keV. Low rate resolution of the Preloaded Filter is better than that of the gated integrator by more than 40%.     

A software counting loss correction method for neutron activation analysis with short-lived radionuclides

A method has been developed for the correction of counting losses in NAA for the case of a mixture of short-lived radionuclides. It is applicable to systems with Ge detectors and Wilkinson or successive approximation ADC's and will correct losses from pulse pileup and ADC dead time up to 90%. The losses are modeled as a constant plus time-dependent terms expressed as a fourth order polynomial function of the count rates of the short-lived radionuclides. The correction factors are calculated iteratively using the peak areas of the short-lived radionuclides in the spectrum and the average losses as given by the difference between the live time and true time clocks of the MCA. To calibrate the system a measurement is performed for each short-lived nuclide. In a test where the dead time varied from 70% at the start of the measurement to 13% at the end, the measured activities were corrected with an accuracy of 1%.     

Empirical method of counting losses correction in gamma-ray spectrometry at elevated counting rate

Empirical method of counting losses correction in -ray spectrometry at elevated /up to 1000 cps/ counting rate is suggested. Using experimental data it was found that a counting losses correction coefficient was a lineare function of true fractional deadtime of spectrometer. It was shown that counting losses in peak area of⁶⁰Co /1332 keV/ corrected by the empirical method did not exceed 1.2% with fractional dead-time up to 35%.     

A correction for dead-time losses in γ-ray spectrometry for a mixture of short-lived radionuclides

A simple procedure for the correction of residual dead-time losses in γ-ray spectrometry of mixtures of short-lived radionuclides is given. It is based on the value of the total deadtime at the beginning of the measurement and three constants which are characteristics for a given matrix. The application to the instrumental neutron activation analysis of fluorine and sodium in bone is given as an exaple.     

Refining a high rate gamma spectrometry system

An overload resistent input stage with PA step response equalizer, automatic self-alignment of previous pulse subtraction gain, and a novel automatic discriminator are valuable improvements for the Preloaded Filter (PLF) pulse processing system. Measurements with an RC-type preamplifier also make clear that the PLF gives superior results not only with transistor reset preamplifiers.     

Real-time correction of counting losses in nuclear pulse spectroscopy

Reviewing the current status of real-time correction of counting losses in nuclear pulse spectroscopy, the pileup problem is identified as the last question not resolved satisfactorily up to now. Correction of pileup losses in provided, at least in principle, by the classical pulse generator method, however, severe limitations in test frequency prohibit its application to real-time correction of counting losses. A solution is offered by the novel principle of the virtual pulse generator which obviates the shortcomings of the classical method simply by not introducing pulses into the spectroscopy system. Instead, the probability for pileup-free pulse processing is determined by suitable tests of the system status at arbitrarily high test frequencies. After a discussion of the principles of the new method and its application to a real-time correction system experimental evidence is provided for the complete correction of counting losses of more than 98% under conditions of stationary as well as variable counting rates up to the limit of stable operation of the underlying spectroscopy system which is 800 000 c/s for an experimental high-rate gamma spectrometer.     

On the use of a dead-time meter for pile-up corrections

Correction for pile-up losses in the amplifier is possible by the dead-time fraction indicator of the ADC in case of long-lived radionuclides. If the dead-time meter has been calibrated, an accuracy of 1.5% is feasible up to a dead-time fraction of 25%. The precision decreases from 1.5% at 10% dead-time fraction to 3% at a deadtime fraction of 30%.     

Simple methods for dead-time and pile-up corrections in analytical gamma-ray spectrometry

In the paper the mathematical methods for dead-time and pile-up corrections are discussed. The dead-time correction formulae for a system of two short-lived isotopes and constant component (background) are reported which mathematical problem has not been solved so far. The new electronic circuit for simultaneous measurements of clock- and live- (or dead-) times is described. It is shown that using this circuit one can correct the counting loss for both effects simultaneously. Finally, the advantages and disadvantages of mathematical methods for dead-time and pile-up corrections are discussed based on the authors' several-year laboratory practice.     

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).     

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.     

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.     

Instrumental dead-time and its relationship with matrix corrections in x-ray fluorescence analysis

The relationship between instrumental dead-time and the self-absorption coefficients, α_ii, in x.r.f. matrix correction by means of influence coefficients, is not generally recognized but has important analytical consequences- Systematic errors of the order of 1% (relative) for any analyte result from experimental uncertainties in instrumental dead-time. Such errors are applied unevenly across a given range of concentration because the error depends on the calibration standards and on the instrumental conditions used. Refinement of the instrumental dead-time value and other calibration parameters to conform with influence coefficients determined elsewhere assumes exact knowledge of dead-time of the instrument used originally, and quite similar excitation conditions and spectrometer geometry for the two instruments. Though these qualifications may not be met, adjustment of any of the parameters (dead-time, reference concentration, background concentration, self-absorption and other influence coefficients) can be easily achieved.     

Half-life of <Superscript>97</Superscript>Zr,re-measured

Summary The half-life of ⁹⁷Zr, used for the calculation of thermal/epithermal neutron flux ratio in k₀-NAA, is re-determined using three measurement systems with different pulse processing principles. The result of 16.755±0.013 hours clarifies the discrepancy between two widely used literature values, 16.744±0.011 and 16.90±0.05 hours. Different dead-time correction methods used on various measurement systems are evaluated. Factors influencing precise measurement of relative peak counting rates are discussed in time-series measurements over a dynamic range of 1000-fold radioactive intensities (10 half-lives).     

Experimental comparison of multi-channel analyzer. Dead-time corrections for short-lived radionuclides

The accuracy of the live-time circuit of a 400-channel analyzer was studied in detail, and was found to be unsatisfactory even for long-lived radionuclides. It was found that automatic live-time correction with the multi-channel analyzer gave rise to increasing positive errors with increasing count rate; this overall positive error was composed of a positive error due to the slowness of the electronic circuitry, and a smaller negative error due to the finite pulse-width. Adequate correction could be performed by feeding the information from the dead-time output of the multi-channel analyzer to an external live-time circuit with variable oscillator frequency and pulse-width. Four methods for dead-time correction were compared experimentally in the case of short-lived radionuclides (T as low as 7 sec): the method of Bartošek et al., the method of Schonfeld, the use of a sufficiently short counting time as compared to the half-life, and the live-time mode of counting without additional correction. These four methods were applied to the determination of oxygen and silicon in rocks by 14 MeV neutron activation analysis. Results are given for USGS standard rock G-2. Research associate of the I. I. K. W.     

Determination of End-to-End Distances in a Series of TEMPO Diradicals of up to 2.8 nm Length with a New Four-Pulse Double Electron Electron Resonance Experiment

A four-pulse version of the pulsed double electron electron resonance (DEER) experiment has been applied to a series of TEMPO diradicals with well-defined interradical distances ranging from 1.4 to 2.8 nm (see picture). The new pulse sequence allows broad distributions of electron–electron distances to be measured without dead-time artifacts.     

Determination of Al,Ca, V and Mn by INAA: Application to the Reference Material IAEA-395 “Urban Dust”

Instrumental NAA based on short-lived radionuclides implies high initial total count rates which change appreciably over the counting period. This in turn necessitates corrections for three negative biases: losses due to differences in counting time between samples and standards; pile-up losses, and (residual) influence of dead-time. The procedure is demonstrated for the determination of Al, Ca, V and Mn in the IAEA Reference Material 395 Urban Dust. The obtained data are in good agreement with the reference values for this material. By limiting the total relative dead-time to 25%, statistical uncertainties are below 5%.