NNDC Beta-delayed Neutron Research Activities

Full text: The NNDC has been active in the field of beta delayed neutrons emitters, mainly due to our role in the production of the ENDF/B decay data sublibrary. Beta delayed neutrons are of importance in the operation of nuclear reactors and in the calculation of abundances following the r-process in stellar environments. Two presentations were made at the first CRP meeting. The first presentation was on a new systematics of beta delayed neutron probabilities (Pn) which not only uses the energy available for the neutron after beta decay (Qbn) but also the half-life of the precursor (T1/2). This work was recently published by E.A. McCutchan et al. in Phys. Rev. C. Plots of Pn/T1/2vs Qbn are more compact than those using the traditional Kratz- Hermann systematics, resulting in smaller chi-square values in the fitting procedure and in the observation of anomalous cases. This work was also extended to light nuclides. The second presentation was on the neutron energy spectra in beta-delayed neutron emitters. The role of the beta strength function, Fermi function, and level densities was discussed. A comparison with the evaluated data in ENDF/B-VII.1 was presented and the presence of sharp peaks near closed shells was discussed. During the discussions, we talked about summation calculations to obtain delayed neutron multiplicities (nu-bars) for different systems. The neutrons are generated following the decay of a relatively small group of fission products, less than 50 and mainly neutron rich Br, Rb, I and Cs isotopes. These calculations need fission yields data, ENDF/B or JEFF, and Pn values, which can be obtained from different decay data sources. A comparison of the different calculations with recommended delayed nu-bars was mentioned and the case of 238U was discussed. We also discussed the delayed neutron activity following fission, which is typically parameterized as a sum of 6 or 8 exponential terms. The ENDF/B library, for instance uses 6 terms, while JEFF uses 8. The delayed neutron activity can also be calculated by solving the corresponding Bateman's equations using fission yields and decay data. One goal of the CRP is to produce recommended group parameters, which will require a careful evaluation of experimental data, comparison with calculations and performing benchmark calculations. Additionally, the calculation of delayed neutron activity can be incorporated into a Monte Carlo scheme to obtain covariance matrices for the delayed neutron activity as well as the parameters in the 6- or 8-group parameterizations. We have done some work on this area in the past and we plan to contribute to this effort. (author)