Atomic Number and Gamma-ray Measurements from Neutron-induced Fission at the ILL and n_ToF

The STEFF spectrometer was used at the Neutron Time-of-Flight facility (n_ToF) at CERN in 2016 to perform a 30-day long experimental campaign of measurements of fission fragments and gamma rays produced in $^{235}$U fission for a wide range of incident neutron energies. A pipeline for reading, correlating and database deposition of the experimental data from this experimental campaign as well as for future STEFF campaigns at n_ToF has been constructed. The pipeline resulted in 70-fold data size reduction to an experimental database that can be fully processed in ~7 hours. The collected gamma-ray data acquired using NaI and LaBr3 detectors have been analyzed in the <1 eV neutron energy range and compared to prior STEFF 235U fission gamma-ray measurements. A method for correcting NaI signal amplitudes for n_ToF-specific effects, such as rates and pulse types, based on fission gamma-ray spectrum shape has been developed. The correcting factors were the greatest for the dedicated proton pulses at neutron energies of ~0.06 eV, increasing signal amplitude by approximately a factor of 2. Corrections to LaBr$_{3}$ signals have also been considered and performed based on count rates, with the the largest correcting factors reducing signal amplitude by ~15%. The corrected and calibrated energy spectra and calculated fold distributions have been prepared for extraction of gamma-ray multiplicity, average energy and total energy in thermal and epithermal fission of $^{235}$U. An experiment was conducted at the Lohengrin mass spectrometer at Institut Laue-Langevin, France, using a FiFI spectrometer for measurement of masses and atomic numbers of selected $^{235}$U fission fragments. The details of the experiment and the data analysis are presented, and a method for calibrating Bragg detectors for atomic number extraction is proposed. The method is based on amplitudes, derivatives and risetimes of signals produced by fission fragments in isobutane fill gas. The extracted signal properties were used in conjunction with known fragment masses and energies to produce functional forms based on powers of fragment velocities and average atomic numbers. Furthermore, a comparison with simulations produced in SRIM-2013 was performed, assessing the accuracy of the simulations.