| The nuclear data on the physics of delayed neutrons coming from fission events are of key importance in reactor kinetics and safeguards applications. The accuracy of reactor kinetics calculations, reactor physics validation studies and techniques for non-destructive assay of special nuclear material are all limited by the quality of these data.;The uncertainties on the delayed neutron group abundances of the longest-lived delayed neutron groups are particularly large - up to 13 % for thermal fissions of 235U, 16 % for fast fissions of 238U, and 38 % for fast fissions of 239Pu [1]. There are also several competing data sets with significant variation in values. Recent work indicates that these already large uncertainties are underestimated due to numerical instabilities in the parametric fitting methods [2].;A novel approach to experimentally measure delayed neutron group yields has been proposed by Jordan and Perret [3]. This approach combines gamma-ray scanning and delayed neutron fission rate measurement techniques. With two independent estimates of the same fission rate, a higher uncertainty delayed neutron parameters can be linked to lower uncertainty delayed gamma parameters.;An experimental apparatus implementing these techniques has been designed, optimized, and built. The apparatus consists of a D-D neutron generator, a detector bundle, a sample handler, and related electronics. Existing neutron generator technology was unsuitable for these measurements. A D-D neutron generator (model DD-109X) was newly designed and installed to run at a maximum intensity of up to 4x109 neutrons per second, with modifications to the accelerator beamline allowing very large solid angle irradiations.;A pelletized UO2 sample was irradiated by a D-D generator, and delayed neutrons and delayed gamma-rays at 1032 keV from 89Rb from the sample were simultaneously measured. The time was respectively set up 20 min, 170 sec, and 360 sec for irradiation, cooling, and acquisition. The neutron generation yield during the experiment was 1.1x109 neutrons per second at 115 kV.;The relative uncertainty of the longest-lived delayed neutron group for fast fission of 238U using the optimized experimental measurements has been reevaluated and reduced to 8.6 %. This reevaluated uncertainty value is almost a factor of two lower than the Spriggs and Campbell recommended data set (16 % [1]).;The system built has shown considerable potential to reduce the large uncertainties of the longest-lived delayed neutron group abundances. The neutron generator technology has more widespread applications for nuclear security and safeguards. More accurate fission yield measurements can be carried out by comparison of multiple fission rate measurement models. A new prompt neutron measurement technique with a prototype 4He scintillator detector is proposed as a third experimental technique for future experimental evaluation of nuclear data. |
