Beta-Delayed Neutron Studies of Fission Fragments Using VANDLE

Beta-delayed neutrons emitted from the decay of fission fragments play an important role in the r-process and nuclear reactor physics. Neutron energy spectra provide information necessary to accurately reconstruct a level scheme that reflects nuclear structure. Decay cascades where neutron emission populate excited states in the (N-1) daughter complicate level scheme construction. These excited states in the (N-1) daughter often emit gamma-rays. These situations necessitate measurement of three-fold coincidence between the beta, neutron and gamma-ray. However, there is a scarcity of measurements providing these data.

To address this need, we designed the Versatile Array of Neutron Detectors at Low Energy (VANDLE), which measures neutron energies via the time-of-flight technique. VANDLE is instrumented with a digital data acquisition system. The digital system provides a neutron detection threshold as low as 70 keV and time resolutions better than 1 ns. A high-efficiency gamma-ray detection system enables complete calorimetry of the decay by detecting gamma-rays that follow neutron emission. High purity Ge clovers present isotope identification, but suffer from poor geometric efficiency. A high efficiency array of NaI and LaBr₃ detectors boost neutron-gamma coincidences.