Evaluating the Performance of a Photofission Prompt Neutron Detection System Based on Threshold Activation Scintillation Coupled with a 9 MeV Electron Accelerator

Developments in photofission prompt neutron detection are crucial in nuclear instrumentation, whether for waste characterization and security applications. Bremsstrahlung photons, emitted by a linear electron accelerator (linac), can induce photofission reactions when interacting with actinides. These reactions then emit prompt and delayed neutrons and delayed gammas. However, the intense photon flash from the linac saturates detection systems, commonly He-3 proportional counters, making prompt neutron instantaneous detection challenging. This study evaluates a photofission prompt neutron detection system coupled with a 9 MeV linac to investigate the use of prompt neutrons as a third signature, alongside delayed neutrons and gammas. This to reduce uncertainties in detecting, differentiate and quantify actinide despite an attenuated interrogating flux or absorbed photofission particles by the environment. First, with Monte Carlo codes PHITS and MCNP6 and their respective nuclear data libraries (ENDF/B-VIII.0 and JENDL-5), we assessed the parameter of a photofission setup which consists in detecting the delayed 6.131 MeV gamma radiation emitted by N-16 decay, resulting from (n, α) activation of F-19. This approach avoids the impact of the linac’s photon flash and minimizes the strong photoneutron background. Secondly, we conducted experimental tests with neutron sources and then with a 9 MeV electron accelerator, Linatron® M9A (Varex Imaging Corp.), at the SAPHIR platform (CEA Paris-Saclay, France), using machined polytetrafluoroethylene and a bismuth germanate scintillator to detect prompt neutrons from a depleted UMo sample. This work enhances prompt neutron detection developments and contributes to advancements in characterizing special nuclear materials (SNMs) by photofission for various applications.