Abstract
Neutron metrology relies on the establishment of reliable references in terms of energy and fluence, which are crucial for research, industry, and radiation protection. The micro-Irradiation, neutron metrology and dosimetry Laboratory (LMDN), in collaboration with the French National Metrology Institute (LNE), is responsible for the national standards in this field. The LMDN uses several systems as scintillators to determine the energy distribution of the fluence of the neutron fields. However, classical liquid scintillators are limited to energies above 1 MeV.
Recent advances in stilbene crystal growth have enabled the development of new crystals with better energy resolution while extending neutron sensitivity down to the 100 keV – 1 MeV range. Within the SIMONE project, two 2''×2'' stilbene detectors and two 2''×2'' EJ309 scintillators were acquired by the LMDN and coupled to a digital acquisition system in order to enhance measurement capabilities.
This thesis focuses on the complete characterization of these detectors for both photons and neutrons. After optimizing digital acquisition parameters and evaluating neutron/gamma discrimination performances, photon characterization was carried out over the 59 keV – 7 MeV range. An automatic calibration method, based on an adaptation of the so-called second derivative technique, was developed. Monte Carlo simulations (MCNP6.2) were performed to establish the photon response matrix, which was then adjusted with source measurements. Results confirmed the better resolution of stilbene detectors compared to EJ309, while also showing discrepancies between simulations and measurements, particularly in the fluence normalization.
Neutron response matrices in energy and fluence were established over an energy range from 100 keV to 22 MeV. The neutron response matrix was determined using the time-of-flight method on a white neutron spectrum at the NFS facility (GANIL, France). Fluence normalization was performed using measurements in quasi-monoenergetic neutron fields at the PTB (Germany). A Monte Carlo simulation was developed to correct for pile-up and dead-time effects. The responses of stilbenes below 1 MeV were obtained, but fluence normalization could not be carried out due to a technical problem unrelated to the SIMONE measurement system
The results demonstrate that stilbene scintillators represent a significant step in neutron metrology by covering the previously inaccessible 100 keV – 1 MeV range. The EJ309 scintillators provide a complementary isotropic response well-suited for multidirectional neutron fields. Altogether, these detectors will be suitable for establishing metrological references at the LMDN.
Key words
Neutron spectrometry, Time of flight, stilbene scintillator, Neutron characterization, Photon characterization, EJ309
Full text
Full text: https://theses.fr/s374672