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  • Author or Editor: R. Dams x
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Abstract  

During the last decade the use of short-lived isotopes in activation analysis has exploded, owing to the application of high resolution gamma-ray spectrometry. Complex spectra can in fact be resolved without the need for chemical separations. The application of cyclic activation-counting enables the use of isotopes with half-lives below 10 seconds. These short-lived isotopes can be produced by different activation processes. As neutron sources one can distinguish reactors, generators for 14 MeV or high energy machines such as cyclotrons and isotopic neutron sources. High energy photons can be produced by interaction of an energetic electron beam with a target, giving rise to an intense bremsstrahlung spectrum with maximum energy from 10 up to 70 MeV. While these photons induce several types of threshold reactions, lower energetic photons are used for resonance activation producing metastable isomers. More and more also charged particles (p, d,3He, α) are being used as projectiles to produce radioactive isotopes that can be measured in activation analysis. In the present paper a concise compilation is made of the nuclear reactions, applicable in activation analysis of minor or trace constituents using gamma emitting isotopes with half-lived smaller than 1 hour. Activations of all naturally occurring elements, except the noble gases, with Z values ranging from 9 to 92 have been considered. The reactions resulting in the most sensitive gammaspectroscopic determinations have been selected and are tabulated per element, together with the appropriate cross section and the resonance integral if significant for neutron reactions, the cross section at the giant resonance energy for the photon and at the maximum of the excitation function for the charged particle reactions and the threshold energy for all threshold reactions. For each isotope produced the half-life and the major gamma-ray energy is given. Finally calculated or experimental sensitivities have been compiled from a number of references, as the emission rate per second of the most intense gamma-ray, at the end of a 1 minute irradiation under well-defined circumstances for 1 microgram of the element. When cyclic activation has been applied it is indicated. These data are provided for more than 200 nuclear reactions. It appears that the majority of the 70 elements constacred can in some way be determined with a high sensitivity after such a short irradiation. Only for the elements Tm and Tb no reaction yielding a short-lived isotope with a reasonable sensitivity could be found. Both elements can however very sensitively be determined after a longer neutron irradiation. For a number of elements activation analysis by means of the middle-long-lived isotopes (2 hours to 3 days) is the most sensitive even after a 1 minute irradiation and immediate count (Mn, Ga, As, Sr, Ru, La, Eu, Ho, Lu, Os and Au). But for all other elements the highest gamma emission rate results from isotopes with half-lives shorter than 1 hour. Extremely high counting rates are obtained after thermal reactor neutron activation for Na, Sc, V, Co, Se, Rh, Ag, In, Eu, Dy, Er and Hf. For a number of elements the selectivity of the analysis can however largely be enhanced by irradiation under Cd-cover. High σ0 ratios exist in fact for activation of medium and high Z elements such as Nb, Rb, Rh, Sn, Sb, Ba, Ce, Lu, Ta, Os, Hg and U. For some elements activation with reactor fission neutrons or 14 MeV neutrons provides an interesting sensitivity for a threshold reaction or the production of an isomer (F, Si, P, Cl, Ca, Cr, Se, Y, Ba, Ce, Pr, Bi, Pb). Cyclic activation and counting of very short-lived isotopes has been applied advantageously in the ng to μg range for a number of elements. Also cyclic activation with a 14 MeV generator has recently been studied. Low energy photon activation analysis allows selective production of metastable isomers of Se, Br, Ag, Er, Hf, Ir and Au, while high energetic photon production analysis yields high specific activities for elements such as K, Cr, Se, Br, Mo, Pr, Nd, Ho and allows interesting determinations of Mg, Cl, Si, Cr, Fe, Zr and Pb. Also charged particle activation can be used with surprisingly high sensitivity for some medium and high Z elements (Cr, Y, Se, Br, Zr, Mo, La, Ta, V). The intensity of the irradiation is in these cases mostly only limited by the properties of the sample itself, such as heat transfer and matrix activity. Examples of the compilation will be discussed and applications shown.

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Résumé  

On peut déterminer, grâce à l'analyse multi-éléments par activation neutronique et à un réseau de stations multiples, les concentration et les répartitions de plus de 30 éléments dans un milieu de particules véhiculées dans l'air. Cette étude tente tout d'abord d'établir la composition du “blanc” local dans une zone industrielle, l'échantillonnage se faisant lorsque les stations ne sont pas sous l'influence immédiate d'émissions locales. On compare ces niveaux à ceux du “blanc” réel et on soustrait les contributions naturelles. On choisit les jours d'échantillonnage en fonction des paramètres météorologiques; un essai de superposition des émissions des industries locales sur ce “blanc” local est alors réalisé. La comparaison des facteurs d'enrichissement permet l'identification des polluants jouant un rôle significatif dans la zone. On localise les sources principales en comparant pour différentes stations d'échantillonnage, les concentrations en fonction de la direction du vent.

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Abstract  

In the workplace of an iron foundry total and respirable suspended particulate matter was daily collected with a network of stationary filtration systems, with Andersen cascade impactors and with personal samplers. The performances of the different sampling systems are evaluated. All samples were analysed by instrumental neutron activation analysis. More than 30 elements were determined. The composition of the particulate samples is compared to that of the major emission sources. For visualising and interpretation of the data computer programs for contour plotting, classification and clustering of the elements and the samples are applied. On the basis of their distribution and particle size the elements can be divided in a number of groups, for which easily determined elements are proposed as indicator elements. The results allow an estimate of the impact of the major emission sources on the air quality in the entire workroom. Suggestions for representative sampling techniques and locations are made.

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Abstract  

The gamma ray energies of nearly all radionuclides formed by reactor neutron irradiation have been determined using high resolution Ge(Li) spectrometry. The re-producibility of the determinations is demonstrated and, to estimate the accuracy of the measurements, the results are compared with those of other investigators. In the energy range from 80 to about 1400 keV the accuracy for the most abundant gamma rays is better than 0.2 keV. The energies of gamma transitions above 1400 KeV may be less accurate. The data are compiled in an atomic number and in a photon energy sequence. A table of characteristic X-rays is also included. The tables are intended to be helpful in the identification of isotopes in neutron activation analysis.

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Abstract  

Three crude oil samples and 49 commercial petroleum products are analysed using instrumental neutron activation analysis for the following elements: Na, Mg, Al, S, Cl, K, Sc, V, Cr, Fe, Co, Ni, Cu, Zn, As, Se, Br, Mo, Sb, Ba, La and Th. For most elements the concentration ranges are considerable, even within the same product class. The ratios of the concentrations in the heavy distillates of crude oil to those in the light fractions vary from 2 up to 400. On the basis of these ratios the elements are subdivided in 4 groups. The elemental concentrations are in liquid fuels still a few orders of magnitude lower than in coal. Exceptions are the elements V, Se, S, Ni in fuel oil and Br in gasoline. For some elements the potential contribution to heavy metal air pollution in Belgium is estimated.

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Abstract  

NAA and ICP-MS are both highly sensitive methods for multi-element trace-and ultra-trace element determination. A comparison between analytical figures of merit of both methods is made. Both methods have specific advantages that put them beyond competition for certain applications. It is concluded that ICP-MS can replace NAA for many routine analyses. NAA remains essential as a highly reliable and accurate reference method.

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Abstract  

A computer program is described to perform the identification of isotopes in neutron activated samples. The γ-ray energies as obtained from a Ge(Li) γ-ray spectrum are compared with those of a library, containing data for about 250 isotopes. Isotopes whose γ-ray energies match closely with the unknowns are selected as possible constituents. Unlikely attributions are then eliminated by a careful inspection of the γ-rays found. Further exploitation of half-life, the way of production and the sensitivity for the given irradiation and measurement conditions, allow the selection of the most likely constituents in the source. The results of the automated identification agree closely to those obtained by an experienced investigator. The program is written in FORTRAN IV for a PDP-9 computer with a 16 K word memory.

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Abstract  

A tabulation of about 2000 precise gamma-ray energies of about 250 isotopes formed through neutron activation has been used for the computer-assisted identification of the individual gamma emitters in complex gamma spectra. The identification consists in the calculation of the gamma-ray energies from the peak maximum position by taking into account the deviation from linearity of the spectrometer. Successive comparison of these energies with the energies of the tabulation allows then identification. The identification procedure was tested on a number of samples of varying complexity and satisfactory results were obtained.

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