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%.     

Automatic stabilization of relative counting losses in gamma-ray spectrometry of short-lived nuclides

An electronic circuit for the stabilization of the relative counting losses due to dead time and pile-up effects is described. The circuit consists of two independent channel: for stabilization of dead time and pile-up, respectively. The stabilizer (circuit) receives continously information on temporary dead time and pile-up in a spectrometer and owing to feed back the relative counting losses (in peaks) are constant during the measuring time and can be easily calculated. Patent pending.     

Routine multielement instrumental neutron activation analysis of silicate rocks using short-lived radionuclides

By means of thermal neutron activation and countings on the small planar and large coaxial Ge(Li) detector, 13 elements are determined in various silicate rocks, using short-lived radionuclides with half-lives from 2 min to 15 h. A method of routine analysis with simple dead-time and pile-up correction is described and tested with the standard rocks AGV-1 and G-2.     

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.     

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.     

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%.     

Assessment of various mathematical correction methods of matrix effects in X-ray fluorescence analysis

Several methods for mathematical corrections of matrix effects in X-ray fluorescence spectrometry were studied. A computer program to elaborate spectral data by means of different and selected mathematical algorithms was set up. The results obtained using the proposed methods for the elemental analysis of some series of samples are reported.     

Electrochemical Identification and Categorization of the Protective Quality of Intact and Damaged Coatings

Defective polymeric coatings that are particularly relevant in the conservation of outdoor metalwork, are analyzed using electrochemical impedance spectroscopy (EIS), validated by Kramers? Kronig transformations, and modeled using electrical equivalent circuit models (EECs). Using twenty different coated panels of five different coating types, ten mathematical methods for categorizing the protective qualities of coatings are explored as simpler and faster alternatives to circuit modeling; three methods gave a perfect correlation with the category determined by circuit modeling. Our findings highlight the need for fitting data to EECs before relying upon purely mathematical parameters for evaluating protective coating quality.     

Determination of oxygen and silicon in rocks by 14-MeV neutron-activation analysis and its precision

Oxygen and silicon have been determined in the six new U.S. Geological Survey standard rocks and two standard refractory materials by 14-MeV neutron-activation analysis, followed by both single and multichannel analyser counting of the induced (16)N (4.5-8 MeV gamma radiation) and (28)Al activities (1.78 MeV photopeak). Owing to the long analysing time per pulse, dead-time corrections are necessary in multichannel analyser counting. Four methods were investigated in this work: counting in the live-time mode without additional correction, short counting corrected by an external live-time scaler, the method of Bartoseck, and the method of Schonfeld. Each measurement was controlled by a simultaneous measurement with a single-channel analyser. Silica ignited at 1000 degrees was used as a reference. Correction was made for the interfering elements, F, P, Al, Fe and Mg, using literature data. Attention was paid to neutron, gamma and beta attenuation. For oxygen the mean coefficient of variation for a single determination with a single-channel analyser was 1.7 % for silicon 1.1 %. The mean results for single-channel counting were compared with literature values.     

High-precision energy-dispersive x-ray fluorescence analysis of manganese in ferromanganese

A high-precision x-ray fluorescence method for the determination of manganese in ferromanganese is described. The method involves excitation of the sample with a ¹⁰⁹Cd isotopic source and measurement with a high-resolution Si(Li) detector. To preserve the optimal energy resolution even at high count rates, the system incorporates a pulsed optical feedback preamplifier and a pulse pile-up rejector. The rejected pulses are corrected for by means of an adequate live-time correction circuit. Processing of the spectra is accomplished with the aid of a digital computer. The relative precision of the method is approximately 0.2%.     

Electronic circuit for measuring the dead-time of nuclear detectors

The method described in this paper is based on the fact that the distribution of the periods betwen two signals following one other is affected by the dead-time of the detector. The simple electronic circuit designed can be linked to the output of the pre-amplifier.     

Corrections of counting losses in gamma-spectrometry of short-lived radionuclides

A short-lived radionuclide is characterized essentially by the decrease of its activity even while it is counted. The relations used between the number of counts recorded in a given time and the half-life in order to obtain the true counting rate are reviewed. Moreover, in gamma-spectrometry with multichannel analysers having variable dead-time, counting losses are observed. Exact corrections are calculated for pure short-lived radionuclides and for mixtures of one short-lived and one or more long-lived radionuclides. Based on these calculations, the approximations given by the built-in “live-timer” device of the analysers and by a new “actual-time” concept (with external live-time measurement) are evaluated. Curves are drawn as functions of the ratio counting time vs. half-life, to ensure minimal errors of less then 1%.     

Determination of counting losses and pile-up rejection in ionizing radiation spectrometry

The spectrometric system for ionizing radiation measurement with pile-up rejection and counting losses correction has been described. The results for HpGe, Ge(Li), Si(Li) and surface barrier detectors have been presented. The total count rate ranged from 500 to 10⁵ cps and different radioisotopes have been used. The counting losses correction accuracy has been within ±1% with tenfold reduction of background from pile-up pulses. The possibility of the system application for radiation intensity measurement of the mixture of short- and longlived radioisotopes has been discussed.     

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.     

Method of moments of coupled-cluster equations: a new formalism for designing accurate electronic structure methods for ground and excited states

The method of moments of coupled-cluster equations (MMCC), which provides a systematic way of improving the results of the standard coupled-cluster (CC) and equation-of-motion CC (EOMCC) calculations for the ground- and excited-state energies of atomic and molecular systems, is described. The MMCC theory and its generalized MMCC (GMMCC) extension that enables one to use the cluster operators resulting from the standard as well as nonstandard CC calculations, including those obtained with the extended CC (ECC) approaches, are based on rigorous mathematical relationships that define the many-body structure of the differences between the full configuration interaction (CI) and CC or EOMCC energies. These relationships can be used to design the noniterative corrections to the CC/EOMCC energies that work for chemical bond breaking and potential energy surfaces of excited electronic states, including excited states dominated by double excitations, where the standard single-reference CC/EOMCC methods fail. Several MMCC and GMMCC approximations are discussed, including the renormalized and completely renormalized CC/EOMCC methods for closed- and open-shell states, the quadratic MMCC approaches, the CI-corrected MMCC methods, and the GMMCC approaches for multiple bond breaking based on the ECC cluster amplitudes.     

Short-lived nuclide radioactive decay compensation by counting geometry variation in NAA

In short-lived nuclide activation analysis, short-time counting periods, due to rapid radioactive decay, and limited count rates, to avoid dead-time corrections, lead to low counting statistics. This can be improved by cyclic activation, being limited by the sample container strength and by radioactivity build-up. With the new device, longer counting periods and thus better counting statistics, leading to higher accuracy and sensitivity, are achieved by changing the geometry factor during counting for radioactive decay compensation.     

Analysis of DSC Data Relating to Proteins Undergoing Irreversible Thermal Denaturation

New approaches to the analysis of differential scanning calorimetry (DSC) data relating to proteins undergoing irreversible thermal denaturation have been demonstrated. The experimental approaches include obtaining a set of DSC curves at various scanning rates and protein concentrations, and also reheating experiments. The mathematical methods of analysis include construction of a linear anamorphosis and simultaneous fitting of a theoretical expression for the dependence of the excess heat capacity on temperature to a set of experimental DSC curves. Different kinetic models are discussed: the one-step irreversible model, the model including two consecutive irreversible steps, the Lumry and Eyring model with a fast equilibrating first step, and the whole kinetic Lumry and Eyring model. This revised version was published online in July 2006 with corrections to the Cover Date.     

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 -quanta, such as is done in existing computer programs for -spectrometry. At moderate count rates Müller 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 Müller statistic. While counting statistics could not be correctly estimated for short-lived species, experiments showed excellent accuracy for initial dead times up to 90%.     

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).     

Use of very short-lived nuclides in nondestructive activation analysis with a fast shuttle rabbit

The use of 0.80 sec^207m Pb for the nondestructive determination of lead is described. The 0.570 MeV gamma-ray activity is measured, and corrections are made to allow for the rapid decrease of the dead-time during the measurements. A sensitivity of about 1400 cps/mg Pb and a precision of ±4 % were obtained with a neutron flux of 5·10¹² n·cm^?2·sec^?1. The influence of possible interferences was studied. The fast shuttle rabbit used in this type of analysis is described.