Calibration and Performance of the ENGREN Scintillation Array for Prompt Fission Neutron Spectrometry

In experiments performed at GELINA (IRMM, Belgium) and IREN (FLNP JINR, Dubna, Russia) facilities, variations in the total kinetic energy (TKE) of fission fragments as a function of incident neutron energy were experimentally observed in the resonance region of 235U fission. These observations provided a basis to assume a correlation between the number of prompt fission neutrons and the TKE. To further investigate this dependence, the ENGREN setup was developed. It consists of a twin Frisch-gridded ionization chamber (GIC) with a target at its center, surrounded by 32 detectors based on EJ-309 liquid scintillator arranged in a spherical configuration around the GIC. The setup is designed for simultaneous measurement of the TKE and angular distributions of fission fragments, as well as for registration of prompt fission neutrons. We present its comprehensive calibration and performance validation. Energy calibration was performed by comparing measured gamma-ray spectra (22Na, 137Cs, 60Co) with Monte Carlo simulations, establishing a linear response ( R2 = 0.9977 ) and an energy resolution of ∆E/E = 7.7% at 1.333 MeV. The resolution parameters are consistent with the characteristics of organic liquid scintillators. Optimal neutron-gamma pulse shape discrimination (PSD) was achieved, with Figure of Merit (FoM) values matching standard specifications. The system’s readiness is demonstrated by a geometric efficiency of 4.55% and successful test measurements with a 235U target at the JINR IREN facility, confirming its capability for high-statistics fission studies.