Standardization problems in INAA of large organic samples

Abstract

For the practical application of large sample activation analysis, an extensive characterization of the irradiation facility and of the measurement system -of sample size, flux-gradient, maximum count rate, size of irradiation and counting containers, estimation of created activity after irradiation and dose rate -is required.The term, "large samples" is not unequivocal. Here the consideration is restricted to samples from several grams to 100 or 200 grams, which can be irradiated in existing irradiation facilities. Calculations were performed for about 1200 organic samples divided into 18 groups. The results are in about 400 tables. The effective Z number, Z/A ratio, mass attenuation for several gamma lines, the relative contribution of the main absorbing elements in the sample for effective cross sections and for gamma transmission are calculated for each sample. All these data have been calculated for various sizes of irradiation and measuring containers. Macroscopic thermal, epithermal, scattering and transport cross sections were used for calculation of self-shielding by five different methods. Owing to the limited space only a selection of the most pertinent values characterizing each group of samples is presented and discussed.The main differences of Large Sample NAA to other nuclear analytical methods are self-shielding and gamma attenuation in the sample. The main object of this paper is to show up problems that may arise in LS-NAA through self-shielding and gamma attenuation.

Qualidade nutricional de rações para cães: Influência de Zn na absorção de Cu e na prevenção de doença hepática

“…Assim como a NAA convencional (pequenas amostras), a LS-NAA é basicamente dividida em irradiação com neutrôns das amostras e subsequente medição da atividade induzida por espectrometria de raios-gama (BODE; ACHARYA et al, 2010). Apesar do alto poder penetrante dos nêutrons e dos raios-gama, tornam-se necessárias correções do efeito de auto blindagem dos nêutrons e da auto atenuação de raios-gama nas amostras OVERWATER;HOOGENBOOM, 1994) sendo estas as principais diferenças entre a LS-NAA e NAA convencional (BODE; GWOZDZ;GRASS, 2006). Contudo, a análise de grandes amostras pode ser especialmente vantajosa quando homogeneização é complicada ou quando grandes objetos não podem ser danificados para fins de amostragem (VASILOPOULOU et al, 2011).…”