Characterization and optimization of the photoneutron flux emitted by a 6 or 9 MeV electron accelerator for neutron interrogation measurements
Résumé
Active neutron interrogation is a nondestructive active method that consists in inducing fission reactions on actinides (e.g., $^{235}$U and $^{239}$Pu) 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 × 10$^{10}$ neutrons per second, which is on the order of 10× 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/cm$^3$ iron matrix and a uranium sample at the center of the drum, 44 mg of $^{235}$U can be detected in 5 min of irradiation time using a single detection block housing five 150NH100$^3$He detectors.
Mots clés
Active neutron interrogation
electron accelerator
linear electron accelerator (LINAC)
nuclear waste packages
photoneutron
uranium
ionizing radiation
radioactivity
instrumnetation
nuclear instrumentation
Photonics
Neutrons
Detector
Radiation effects
Tungsten
Bremsstrahlung
linear accelerator
Monte Carlo
neutron detection
neutron flux
neutron sources
neutron interrogation measurements
active neutron interrogation
nondestructive active method
fission reactions
prompt particles
delayed particles
radiological characterization
neutron interrogation facilities
deuterium-tritium neutron generators
photoneutron generator
secondary target
heavy water
neutron production
photoneutron flux emitted
Linatron-M9 VARIAN LINAC
maximum average emission intensity
traditional deuterium-tritium neutron generator
Linatron-M9 LINAC
prompt neutron signals
9-MeV electron accelerator
Origine : Fichiers produits par l'(les) auteur(s)