Neutrons emitted following the beta decay of fission fragments play an important role in many fields of basic and applied science such as nuclear energy, nuclear astrophysics, and stockpile stewardship. However, the fundamental nuclear data available today for individual nuclei is limited – for the vast majority of neutron emitters, the energy spectrum has not been measured and some recent measurements have uncovered discrepancies as large as factors of 2-4 in beta-delayed neutron branching ratios. Radioactive ions held in an ion trap are an appealing source of activity for improved studies of this beta-delayed neutron emission process. When a radioactive ion decays in the trap, the recoiling daughter nucleus and emitted radiation emerges from the ~1 mm3 trap volume and propagates through vacuum without scattering. Information about particles that are difficult or even impossible to detect can be obtained using conservation of momentum/energy from the determination of the nuclear recoil and beta particle kinematics. For the first time, beta-delayed neutron spectroscopy is being performed using trapped ions by identifying neutron emission from the large nuclear recoil it imparts and using this recoil energy to reconstruct the neutron branching ratios and energy spectra. Results from a recent proof-of-principle measurement of the beta-delayed neutron spectrum of Iodine-137 and plans for future experiments at Argonne National Laboratory using significantly higher intensity fission-fragment beams will be presented.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.