Short-time analysis by delayed neutrons and gamma-spectrometry

This work deals with the identification of fissionable materials by analyzing neutrons and gammaspectra after irradiation in a TRIGA MARK II reactor. Samples of U-235, U-233, Pu-239 and Th-232 are irradiated using a fast transfer system. So the transfer time is reduced to about 200 ms. This fact allows to identify even the short living groups of the delayed neutron emitters. The measurement device of the transfer system is substituted by a sample catcher containing 16 BF3 neutron counters. They are surrounded by a plexiglass-moderator to reduce Bremsstrahlung, too. To record gammas a high-rate, high-resolution system is used, where the gamma-detector is positioned outside the catcher. It is possible to distinguish the fissionable materials by analyzing the counting rates of some fission products of the different samples. The next step is to observe the time-dependence of the neutron-activity. The resulting curves are fitted with the Levenberg-Marquardt method. A sum of five exponential functions containing different half-lives was used. The weight of these functions leads to the possibility of distinguishing the fissionable materials, too. Summarizing, the simultaneous measurement in combination with a fast transfer system is a powerful tool for analyzing fissionable materials. (author)