Usefulness of thermal and epithermal cyclic activation analysis with a reactor system

Abstract

A new cyclic activation system (CAS) has recently been installed at the University of London Reactor and is briefly described. It has the unique feature that it allows choice as to whether irradiations are to be carried out under cadmium or in a bare tube and thus both epithermal and thermal cyclic activation analysis can be performed. Measurements in this work, have been carried out using a Ge(Li) detector but the facility allows other detectors to be used and an additional return line from the central distributive mechanism of the system (not yet installed) will allow samples to be cycled to and from a neutron detector thus providing the possibility of measuring radionuclides emitting delayed neutrons. By way of illustration of the theory of cyclic activation analysis, since the paper is to serve as an introduction to other papers in the conference using the technique, the results of a calculation for the detection of uranium under thermal and epithermal conditions are presented and compared to conventional activation. The usefulness of cyclic neutron activation analysis (CNAA) is also demonstrated in cases where identification of an isotope through its gamma-ray energy alone is not sufficient and conformation is achieved through determination of the half-life of the activity of interest, without resort to further experimental work or additional instrumentation. This leads on the thorny problem as to whether Poisson statistics should continue to be used in cases where the period of observation is of the same order as the decay constant of the activity of interest and it is pointed out how CNAA can be employed to extract the information required if a binomial-type statistical density function is used. Finally in order to prove the system and the cyclic method and as a preliminary to a larger project, prepared single element standards and standard reference materials [NBS Orchard Leaves, NBS Bovine Liver, IAEA Animal Blood, IAEA Hair standard and Bowen's Kale] are analysed under the two cyclic irradiation conditions. Sensitivities and cadmium ratios are obtained for the twelve elements from the elemental standards (Na, Al, Sc, Ge, Se, Rh, Pd, Ag, Ce, Er, Hf and Pb) and detection limits for a number of elements of interest presented for the standard reference materials.