[en] Active thermal neutron detectors are used in a wide range of measuring devices in medicine, industry and research. For many applications, the long-term stability of these devices is crucial, so that very well controlled neutron fields are needed to perform calibrations and repeatability tests. A way to achieve such reference neutron fields, relying on a 3 MV Tandem Pelletron accelerator available at the CNA (Seville, Spain), is reported here. This paper shows thermal neutron field production and reproducibility characteristics over few days. - Highlights: • Monodirectional neutron beam (<1 eV, ~2×10"3 cm"−"2 s"−"1) was tuned up in a 3 MV tandem. • Reproducibility of the thermal neutron field at test point was estimated in ±4%. • New setup represents a good tool for stability control of neutron detectors.