Characterization and Optimization of the Photoneutron Flux Emitted by a 6- or 9-MeV Electron Accelerator for Neutron Interrogation Measurements

Abstract

Active neutron interrogation is a nondestructive active method that consists in inducing fission reactions on actinides (e.g., 235U and 239Pu) and then in detecting prompt and delayed particles emitted further to these reactions. This method is required in various application fields such as radiological characterization of nuclear waste packages and homeland security. Neutron sources traditionally used in neutron interrogation facilities are deuterium–tritium neutron generators. However, a linear electron accelerator (LINAC) can also be used as a photoneutron generator. When accelerating electrons with energy lower than 10 MeV, the use of a secondary target such as heavy water is of interest to convert bremsstrahlung photons to neutrons and to improve neutron production. In this paper, we characterize the photoneutron flux emitted by a Linatron-M9 VARIAN LINAC and the photoneutron flux emitted by a secondary target made of heavy water. We show that operating the LINAC at 9 MeV, the average emission intensity reaches $1.08 \times 10^{10}$ neutrons per second, which is on the order of $10{\times}$ higher than the maximum average emission intensity delivered by a traditional deuterium–tritium neutron generator. Then, we carry out neutron interrogation measurements on uranium samples using the Linatron-M9 LINAC and a secondary target made of different volumes of heavy water. We show that prompt neutron signals were enhanced when using 16 kg of heavy water and operating the LINAC at 9 MeV. Performances of the setup were then assessed by carrying out measurements on a mockup of nuclear waste drum containing different types of matrices. Considering a 0.22-g/cm3 iron matrix and a uranium sample at the center of the drum, 44 mg of 235U can be detected in 5 min of irradiation time using a single detection block housing five 150NH100 3He detectors.