Bonner sphere spectrometers (BSS) are being used widely in neutron spectrometry since 1960. The response to neutrons of these moderating detectors spans over a broad energy range. This work discusses the measurement of the sensitivity of an extended range BSS hosting a CR39 nuclear track detector coupled to a boron converter. Two kinds of boron converters were tested, the first made by natural boron and a second one enriched in (10)B. The BSS response functions were calculated with Monte Carlo simulation using the MCNPX 2.4.0. and the FLUKA codes.