Development of the STEFF detector for the neutron Time Of Flight facility (n_TOF), CERN

Significant work has been performed on the development of STEFF (SpecTrometer for Exotic Fission Fragments), a 2E2V (2-Energy 2-Velocity) spectrometer built by the University of Manchester Fission Group. The majority of this work was in the development of the time-of-flight systems, in particular the stop detector; with the main goals of improving the timing resolution and the detection efficiency of the ssion fragments. Further development of the STEFF spectrometer was done to enable 2E2V measurements of the $^{235}$U(n,f) reaction with coincident measurements using a white neutron spectra of energies ranging from 10 meV to 200 MeV provided by the n_TOF (neutron Time Of Flight) facility, CERN. The STEFF spectrometer was successfully operated twice on the Experimental Area-2 high flux pulsed neutron beam line resulting in 2E2V measurements for ssion events with neutron energies ranging from 20 meV to 10 MeV. The first experiment received 1.36 X 10$^{18}$ POT (Protons On Target) with stable conditions and the second received 1.53 X 10$^{18}$ POT with stable conditions. The development of the stop detector resulted in a replacement MWPC (Multi- Wire Proportional Counter) detector for the second of the two experiments. This allowed direct comparison for the timing coincidence resolution, $\sigma_{c}$, between the start and stop detectors and gave $\sigma_{c}$ = 0.81 $\pm$0:08 ns for the prior PPAC (Parallel Plate Avalanche Counter) detector and $\sigma_{c}$ = 0.40 $\pm$ 0:04 ns for the MWPC. The MWPC gave improved the detection efficiency per fission fragment of $\epsilon$= 0:67 compared to $\epsilon$ = 0:43 for the PPAC. The methods and research described in this work also provided alternate stop detector designs with greater performance. This work produced two large data sets from the two successful deployments of the STEFF spectrometer on the n TOF beam line that will be the future work of many nuclear structure scientists to come.