[en] The subject is the computerized analysis of the gamma-ray spectra in INAA. This analysis can be separated in three parts: The conversion of the spectra to information on γ-ray energies and their relative intensities (spectrum reduction), the determination of the relation between the intensity of a γ-ray and the amount of the corresponding element present in the sample (standardization) and the attribution of the γ-ray energies to the elements, including the subsequent computation of the amounts of the elements (interpretation). A γ-ray spectrum can be considered to be the linear sum of the γ-ray spectra of the individual radionuclides present in the sample. Knowing the relative activities of the different radionuclides that may be produced by activation of a single element, a γ-ray spectrum in INAA can also be considered to be the linear sum of the spectra of the elements. This principle has hitherto not been used in INAA to analyze the spectra by linear least squares methods, using all γ-ray energies observed in the spectrum. The implementation of this 'holistic' approach required that attention be paid to both spectrum reduction, standardization and interpretation. The thesis describes the methods developed for the holistic analysis of γ-ray spectra in INAA, and present results of experimental comparisons between the holistic and other approaches. (orig./HP)

[en] The k₁-method for standardization in INAA specifically tackles the problem of the interpretation of gamma-ray spectra as obtained with highly efficient detectors, as opposed to the k₀-method. Results obtained from three NIST reference materials, measured after neutron activation with a gold-lined well-type detector, are presented. It is concluded that the accuracy of the method is better than 1%. (author)

[en] It is sometimes stated that a gamma-ray spectrum analysis program need not produce absolute peak area estimates as long as the program is consistent and the area estimate is indeed proportional to the true peak area. In this paper, it is demonstrated both theoretically and experimentally that this is not true in the presence of true coincidence summing. The experiment described provides a method to test the correctness of peak areas determined by any method. An alternative, simpler method is also proposed. It is concluded that the ANSI N42.14-1991 standard is incomplete in this respect. The results are relevant to gamma-ray spectrometry with either semiconductor or scintillation detectors

[en] By employing neutron (or X-ray) diffraction the structure of crystalline materials can be determined. However, if an impurity in the crystal is present in concentrations below, say, 1 x 10^-4, its influence cannot be observed in the diffraction patterns. If the impurity present at low concentrations is to be localized, a signal uniquely attributable to the impurity must be obtained. Two such methods, based on the same principles as the 'X-ray standing wave' technique, are proposed for neutrons. (author)

[en] The feasibility of the INAA of samples in the kg range has been demonstrated in 1994 by OVERWATER et al. In his studies, however, he demonstrated only the agreement between the 'corrected' γ-ray spectrum of large samples and that of small samples of the same material. In this paper, the k₀-calibration of the IRI facilities for large samples is described, and some of OVERWATER'S results for homogeneous materials are presented again, this time in terms of (trace) element concentrations. It is concluded that large sample INAA can be as accurate as ordinary INAA. (author)

[en] There are several cases in which large volumes containing radioactivity have to be measured by gamma-ray spectroscopy such as in-situ measurements, measurements of very low specific activity or measurement of composed materials with components of strongly different activities. In the latter case, not just the total radioactivity may be required but also an indication of the spatial distribution of the radioactivity may be needed, both as source of information as such and as a degree of the quality of the measurement. Determination of the radioactivity distribution by emission tomography is only possible in those cases in which all emitted gamma-rays have the same energy whereas also the amount of radioactivity should be sufficiently high. In more complex cases, such as if dealing with natural radioactivity or neutron-activated large samples, a different approach is needed. Such a method has been developed at the IRI in Delft in the frame of the large sample INAA research. In this method, a balance had to be found for detection efficiency, spatial resolution and energy resolution. The activated samples (cylindrical, with typical dimensions of 12 cm diameter and 20-100 cm in length) with total induced radioactivities in the order of 10² kBq, are measured with a collimated 97 % Ge detector, operated using a longitudinal and rotating scanning device. For collimation a 10 cm thick variable size slit collimator was used. The improvement of spatial resolution versus the decrease of detection efficiency with increasing thickness and decreasing opening will be discussed. For calculation of detection efficiencies for varying gamma energies and 3D relative source positions, a semi-empirical first order approximation was determined as an improvement of the traditional method. This method uses attenuation factors corrected for escape of interaction products from the crystal. These corrections are determined only as a function of the crystal and energy. In the here presented improvement, these correction factors are determined as a function of energy, position and direction of intersection with the crystal front end. In Monte Carlo simulations, line beams of a range of gamma energies are intersecting the crystal at positions and in directions as would also occur in sample measurements. The correction factors were fitted on polynomials of the above mentioned dependencies. The final detection efficiency function was checked on measured detection efficiencies. (author)

[en] The effects of coincidence summing in γ-ray spectrometry were used in a novel method to obtain absolute photopeak and total efficiency curves of efficient Ge detectors. Without precise knowledge of source activity, both curves were obtained from a single ⁸² Br spectrum. The efficiency curves were tested by computation and experimental verification of the γ-ray spectrum of ¹⁵²Eu. The method makes it possible to calibrate efficient Ge detectors in any counting geometry without the use of calibrated sources. As a result, the feasibility of the use of well-type detectors for single comparator INAA is greatly enhanced. (orig.)

[en] Epithermal neutron activation analysis is a well-established approach to improve the sensitivity for certain elements by suppressing the activation of interfering elements. If epithermal neutrons of a given energy could be selected, the signal-to-noise ratio might be further improved by taking advantage of resonance capture. This reaction occurs mainly by intermediate and heavy nuclei. Moreover, most of these reactions take place with epithermal or fast neutrons. Intense epithermal neutrons are available as ''white'' beams at accelerator-driven neutron sources. Neutron resonance capture offers interesting analytical opportunities. Low-Z elements have little capture of epithermal neutrons and are thus virtually absent in the time-of-flight spectrum. Relatively large objects can be placed in the neutron beam and analyzed nondestructively. The induced radioactivity is relatively low. If an element has several stable isotopes, each of these isotopes can be recognized by its specific resonances. This would allow for multitracer studies with several isotopically labeled compounds. Different from mass spectrometry, the sample remains intact and can be used for further studies after analysis. Applications may be in the field of archaeology, metallurgy, and certification of reference materials

[en] Most peak-search algorithms for γ-ray spectrometry are based on digital filters, and most of them resemble the Routti-Prussin filter. The statistical properties of this filter must be known to determine detection limits correctly. Its properties were therefore studied as a function of peak width, background level and differential nonlinearity of the ADC using simulated spectra, and they were found not to be as expected from simple statistical considerations. These deviations were modelled, resulting in formulas yielding Currie's critical level L_C and detection limit L_D as a function of peak width and background level. (orig.)

[en] The well-type semiconductor detector has been around almost as long as the coaxial semiconductor detector. Even though it has important advantages over the coaxial detector, the drawbacks have been severe enough to very much limit the use of these detectors. The high sensitivity (e.g. 50 % at 100 keV, 10 % at 1000 keV) of a well-type could be achieved with a modern, large coaxial crystal with the sample on the end cap, but in that set-up the geometry becomes a crucial source of error for the coaxial detector, whereas the well-type's efficiency hardly depends on the counting geometry at all. The severity of the coincidence summing effects has rendered it impossible in the past to calibrate a well-type detector with a standard calibration source and use the efficiency curves obtained for other radionuclides. The popular approach to this problem, where the detector is characterised with a measurement in a 'reference position' and efficiencies for other geometries are obtained through computation is invalid for the well-type case. Since 1992, a method has been under development at IRI to characterise a well-type detector using a peak efficiency and a peak-to-total efficiency curve, and compute radionuclide specific counting efficiencies from those curves for 'real' peaks as well as for 'sum' peaks. Recent improvements to this method have demonstrated that now, a well-type can be as easy to calibrate and operate as a coaxial detector, once the software is in place. In this paper, the calibration method will be described and examples of results given. It will also be shown just how severe the errors are that can be made when coincidence summing is neglected, for a few typical INAA and environmentally relevant radionuclides. (author)