The combination and optimization of the cyclic modes in neutron activation analysis (NAA) at the Dalat research reactor were performed to determine short-lived nuclides. This work focused on determination of selenium in biological materials using 77m Se (T 1/2 = 17.4 s) as a case in point. The detection limit was significantly improved by a factor of 3-4 times in comparison with the conventional NAA. The precision was typically within the range of 4-6 % at the third or fourth cycle. It is a reliable technique for determination of selenium in biological materials at sub-ppm concentration levels.

A rapid neutron activation analysis technique for determination of the concentration of selenium in biological materials using short-lived radionuclide 77mSe (half-life = 17.4 seconds) has been developed at Dalat Nuclear Research Institute (DNRI). The technique is very simple and rapid. It involves irradiation of a sample for 20 s, decay for 20 s and counting for 20 s. The accuracy of the method has been evaluated by analyzing a number of biological standard reference materials of varied selenium levels. An agreement between measured and certified values was acceptable in regarding to the deviation of the above mentioned two values within 8 percent. The result shows that the utilization of short-lived radionuclide 77mSe is more useful in comparison with long-lived radionuclide 75Se (half-life = 120 days). In addition, it is suggested that a further study for cyclic irradiations should be done in order to enhance the detection limit of the determination of the short-lived radionuclide 77mSe.