A moderator device to produce a uniform thermal neutron field has been designed by Monte Carlo methods using the MCNP6.1 code. It uses a 241Am/9Be neutron source of 111 GBq activity and high-density polyethylene (HDPE) moderator. The main tasks developed were to evaluate the geometry of the moderator and to select the neutron source position, in order to optimize the thermal neutrons flux in the irradiation area, and to assess the dose rate. In the system, named FANT (Fuente Ampliada de Neutrones Térmicos), the neutron moderation and backscattering processes are effective to obtain quite uniform thermal fluence rates above 1000 cm−2 s−1 in a cylindrical irradiation chamber of 32 cm diameter and 34 cm length. The device has been built in the neutron measurements hall of the Energy Engineering Department of Universidad Politécnica de Madrid (UPM), performing several measurements to characterize the neutron field and validate the calculations. FANT can be employed hereafter in different applications requiring a neutron field with a substantial thermal component, like testing and calibration of neutron detectors and neutron dosimeters, or the use NAA (Neutron Activation Analysis) methods for detection of trace substances or materials.
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