Bragg cut-off for$$(10\bar 10)$$ plane of polycrystalline beryllium of various lengths of 300 and 116 K has been measured with an energy resolution of 5 μeV. The natural width of the cut-off is 12.5±1.5 μeV, independent of temperature and length of beryllium and also of physical characteristics and certain metallurgical treatments of the powder. Such blocks of beryllium would be suitable for designing a ΔT-window spectrometer with resolution ⩾20 μeV. Bragg cut-offs corresponding to (0002) and$$(10\bar 11)$$ planes of beryllium have been separated for the first time. These can also be used for producing additional energy windows in a ΔT-window spectrometer, thus increasing its efficiency.