The importance of delayed neutrons in nuclear research—A review

Abstract

This paper presents a comprehensive review of the uses of β-delayed fission neutron information in nuclear research with special emphasis on the energy spectra and reactor applications. A short introduction is followed by a discussion of the applications of delayed neutron properties in nuclear structure and astrophysics. Section 3 gives the delayed neutron requirements for reactor physics purposes and points out the difference between the physical and the mathematical representations of the delayed neutron data. In Sections 4 and 5, respectively, there are discussions of the total delayed neutron yield and the decay constants. Section 6 highlights the importance of and the need for an exact knowledge of delayed neutron energy spectra in high-accuracy criticality calculations and in precise evaluations of reactor kinetic characteristics, particularly the fast breeders. Section 6 also gives a brief description of the principal methods that are commonly used for determining the delayed energy spectra. Section 7 gives a chronological account of the developments in the measurement of delayed neutron energy spectra, both aggregate (composite) and from individual fission product isotopes, and compares the spectra, wherever possible, with the ENDF/B evaluations. A comparison is made of the energy spectra of a few precursors measured at different laboratories. There are discussions on the major spectrometry techniques employed in the measurements of delayed neutron energy spectra as well as on the methods of performing spectral analysis. This covers the response function, the efficiency and the sensitivity of the spectrometers, their merits and demerits, and their applicability. Calculations of delayed neutron energy spectra from precursor data, and decomposition of composite spectra into six-group delayed spectra using summation and/or fitting procedures are described in Section 8. Sensitivity studies of fast reactor kinetic behaviour to delayed neutron energy spectra are reviewed in Section 9. Both direct and adjoint methods are discussed. Section 10 gives a summary of the paper which concludes in Section 11 with a number of recommendations.