Measurement of Independent Fission Product Yields with the SPIDER Detector System at LANSCE *

Independent fission product yields (FPY), i.e., yields of fission products right after the prompt neutron emission, but before beta decay, are an important piece of data for nuclear fission modeling and fission applications. Presently, measurements of FPYs are performed using various techniques, such as the 2E method (gridded ionization chambers), the 2E-2v method, radiochemical separation, and magnetic separators. Among these techniques, the 2E-2v is considered the most robust in terms of combining high resolution (< 3 amu), accuracy, and relatively high efficiency. In the 2E-2v method, the mass of fission fragments is determined by measuring the kinetic energy and velocity (via time-of-flight) of both fragments in coincidence. To accurately extract the mass, however, the energy loss of the fission fragments inside the conversion foils of the time-of-flight detectors and the windows of the energy detectors must be well known. In the past, energy loss corrections have been based on theoretical stopping power models, inserting systematic uncertainties in the extracted mass distribution. The Spectrometer for Ion Determination in fission Research (SPIDER), was developed at Los Alamos Neutron Science Center (LANSCE) for measuring FPYs from neutron-induced fission using the 2E-2v method, eventually spanning from thermal up to 20 MeV in incident neutron energy. SPIDER has recently undergone various improvements for increasing the fidelity and resolution of the extracted data. In particular, a gamma-ray tagging system has been implemented for improving the accuracy of the mass calibration by measuring strong gamma-ray transitions from isotopes of known mass. In this presentation, an overview of the upgraded SPIDER system and preliminary results from FPY measurements on ²⁵²Cf(sf), ²³⁵U(n_th,f), and ²³⁹Pu(n_th,f) will be discussed.