Experimental Characterization of ESTHER: Expanded Source-based THERmal neutron field

Thermal neutron fields based on sealed fast neutron sources, such as Am-Be or Am-B, embedded in large (few cubic meters) polyethylene or graphite moderator assemblies, are traditionally used for neutron dosimetry. ESTHER (Expanded Source-based THERmal neutron field) is a new thermal neutron irradiation facility at Politecnico di Milano. By housing a ${ }^{241} \mathrm{Am}$-Be neutron source within a cavity with reflecting polyethylene walls and incorporating a shadowing polyethylene bar to shield the irradiation volume from direct neutron exposure, the facility provides a nearly uniform thermal neutron field across the irradiation chamber. The experimental characterization of ESTHER was performed using gold foil activation applying the Westcott convention to measure the sub-cadmium cut-off fluence rate at different positions along th evertical axis. These were compared to Monte Carlo simulations using the FLUKA code. Results show good agreement between experimental measurements and simulations, demonstrating the facility’s effectiveness in achieving a consistent thermal field with significant fluence rates. The ESTHER facility also allows to create different irradiation scenarios by removing the polyethylene cap. This offers a range of applications in neutron metrology, dosimetry, and detector testing. ESTHER has a simple and effective design that combined with its capability to achieve substantial thermal fluence rates with modest neutron sources, makes it a promising resource for the scientific community. Future work aims to characterize the neutron spectrum within the irradiation chamber using a Bonner Sphere System.