Aplicação Dos Métodos De Espectrometria Alfa E Gama Ao Estudo Do Desequilíbrio Radioativo Das Séries Do U E Th Nos Fosfatos Do Arquipélago De Abrolhos

This paper presents the results of uranium and thorium measurements made in aluminium and iron phosphates that occur<br>in the Abrolhos Archipelago (offshore Brazil) by instrumental neutron activation analysis. The U concentrations were also<br>indirectly determined from the gamma-ray activity of 234Th, 214Pb and 214Bi, while those of Th were obtained from the<br>228Ac, 212Pb, 212Bi and 208Tl gamma-ray emitters. In both cases it was assumed that 238U and 232Th series are in radioactive<br>secular equilibrium.<br>The comparison between the results obtained by neutron activation and gamma-ray spectrometry indicates that the 238U<br>series is in disequilibrium in some of the investigated phosphates. The 234U/238U activity ratios determined by alpha<br>spectrometry also indicate the disequilibrium in this series. For the 232Th decay chain the concentrations obtained by<br>neutron activation and gamma-ray spectrometry show radioactive secular equilibrium in the analysed samples.