As part of Boron Neutron Capture Therapy (BNCT) studies, a research program has been carried out to design an epithermal neutron beam (1 eV – 10 keV range) inside the ISIS experimental reactor at Saclay. Aluminium beam tubes were specially manufactured to be fitted with different screens. The main goal of this study is to compare different filters in order to reach at the beam exit the best epithermal selectivity. A calculation scheme was validated to simulate the device response by a Monte Carlo calculation code (TRIPOLI 3) and required a measurement campaign to compare experimental spectra with calculated ones. An experimental method of spectra determination was performed to provide quickly an approximate spectrum shape in the epithermal range. This method confirmed the calculation scheme at each neutron beam interface with eight different foils (five in the epithermal range). Therefore, filters research was performed exclusively by calculations and led us to define a final set of screens.