Selective radioactivation and multiple coincidence spectrometry in the determination of trace elements in biological material

Abstract

Abstract The capacity of activation analysis to detect trace constituents in samples solely by radio-analytical means can be advanced dramatically by enhancing the specificity of both the activation and the detection methods. Coincidence spectrometric techniques frequently can resolve the radiations from complicated mixtures of radioisotopes by capitalizing on differentiating characteristics of the pertinent decay schemes. When isotopes with distinctive β-γ-decay chains are selected for detection, scintillation spectrometry may be employed. Then the γ-spectrum from a NaI(Tl) crystal can be recorded by a multichannel pulse-height analyzer that is gated by a coincidence circuit when it simultaneously receives signals from the γ-counter and from a β-scintillation spectrometer. When used in this way, the power of a two-crystal β-γ-detector to resolve a single β-γ-decay branch is limited by the γ-response of the β-sensing element, which thus records interfering γ-γ-cascades. A marked improvement in selective β-γ-discrimination is provided by a three-crystal coincidence geometry, where an additional plastic or anthracene crystal (which is so thin as to have negligible γ-sensitivity) is placed in front of the thicker crystal of the β-spectrometer to distinguish β- from γ-rays. When radiometric analysis is applied to neutron-activated sources, detection of certain nuclides with low-energy (n, γ) resonances (e.g. Mn 55 , Co 59 ) may be further enhanced relative to other isotopes without such resonances (e.g. Na 23 , Cl 37 ) by irradiating with resonance neutrons. In an example of neutron activation analysis of manganese, selective activation with a filtered reactor neutron spectrum and triple coincidence spectrometry were combined. The former enhanced the ratios of Mn 56 /Na 24 and Mn 56 /Cl 38 activation to about seven times and fifteen times, respectively, the values produced by thermal neutrons; while the latter suppressed by a factor of about 100 the interference due to Na 24 in the detection of Mn 56 by γ-spectrometry. In this way, manganese in blood plasma may be measured without recourse to radiochemical separations.