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Energy spectrum determination of an epithermal neutron beam for BNCT is always a critical issue to its dose accuracy. The spectrum is generally determined by multiple activation foils with few-channel unfolding codes. This study aims to make improvement to the activation-detector-based unfolding technique via introducing specially designed detector complexes in addition to the commonly applied multiple activation foils. The extra detector complexes can obtain valuable information from planned, artificial perturbations posed on the incident epithermal neutron beam, without the need of modifying the unfolding algorithm. Two different types of BNCT beams, one is based on nuclear reactor and the other one is based on low energy accelerator, with two different conditions were tested: under- and over-moderated cases. The spectra adjusted by using the original foil package containing 11 foils and the improved system are compared based on the square root of the sum of deviation squares. The specially designed detector complexes have shown the great ability to significantly reduce the deviations between the unfolded spectra and standard solutions. For a seriously under-moderated case tested in this work, the improvement can even reach more than 400% in deviation reduction.