Efficiency of delayed fission neutron system applied at CDTN/CNEN

This work is about the analytical nuclear technique Neutron Activation Analysis applied to determine uranium using the Delayed Fission Neutron method using the TRIGA Mark I IPR-R1 reactor, at CDTN/CNEN, Belo Horizonte. This method is specific for uranium analysis. It is based on a short irradiation followed by measurement of delayed neutrons. Usually an irradiation time of 50 seconds, decay time of 15 seconds and measurement time of 40 seconds are used. A pneumatic transfer system is used for irradiation and the delayed neutrons are counted in a device comprising BF3 detectors in a moderator of paraffin. The system irradiates, measures and calculates the results automatically. In this paper, the efficiency of the system is evaluated basing on the range of dispersion of the uranium standards results. Uranium standards - with large range of concentration, from 20 to 3300 mg kg-1 - were analyzed several times and the results were registered. Using these data, it was obtained a polynomium representing the behavior of the variance in function of the average. Assuming that the distribution is close to the Normal distribution, the intervals of the uncertainty were established with 95% of confidence. An Excel sheet was used as a tool to follow the variation of the uncertainty. (author)