A review of the delayed fission neutron technique

Abstract Neutron activation analysis (NAA) has recently become an extremely valuable and sensitive tool for the determination of many elements in a wide variety of matrices. Samples are usually irradiated in a neutron flux, after which the gamma ray emission rate is measured with an appropriate detector. Some nuclides, in addition to being able to undergo a neutron capture reaction, also have the capability of fissioning after absorbing a neutron. In this case, not only are gamma rays emitted by the fission products, but also delayed neutrons as well. These delayed neutrons arise from the decay of certain neutron-rich fission products or precursors which decay by direct neutron emission, rather than the more common beta decay. As the name implies, the delayed fission neutron (DFN) technique is based upon measuring these delayed neutrons emitted. Although limited to fissionable nuclides, the DFN technique nevertheless has several outstanding advantages. A DFN counting system can be made insensitive to gamma ray interference. Furthermore, minimum levels of detection are in the nanorgram range. Analysis is usually quite rapid, typically no more than several minutes per sample. In addition to a description of the theory of the DFN technique itself and a cursory examination of the historical development of this technique, a fairly exhaustive collection of data concerning different DFN systems is presented. The purpose of this compilation is to give a present or prospective user of this technique a resource tool to evaluate state-of-the-art procedures and equipment.