UC Berkeley

The β-delayed neutron (βn) emission decay mode, prevalent in a vast number of neutron- rich nuclei, influences abundances calculated in the r-process nucleosynthesis models, affects nuclear reactor safety analysis calculations, and can illuminate aspects of nuclear structure. This thesis describes a newly developed recoil ion detection technique that was applied for high-precision βn branching ratio and neutron energy measurements of 137−138I and 144−145Cs. The recoil ion measurement approach avoids difficulties associated with direct neutron detec- tion by instead detecting the daughter ion recoiling from neutron emission. The radioactive ions of interest are held in near-rest with the use of an ion trap, from which they leave the trap upon β or βn decay. The detector array surrounding the trap measures the time-of- flight of the recoil ion, as well as several associated decay products. Measuring the recoil ion’s time-of-flight determines the recoil energy, from which the emitted neutron’s energy can be deduced. Detecting other decay products gives rise to three different methods of measuring the βn branching ratio, which helps expose systematic effects. The technique builds upon a previous proof-of-principle experiment, and was expanded for the present measurements to include twice as many improved detectors, an upgraded ion trap, and a stronger source. This thesis also examines various backgrounds and detailed detector characterizations. The experimental campaign presented here serves to probe the limits of applying the recoil ion technique to explore further into the neutron-rich region.