It depends on the thickness of the Be window. Those low-energy x-rays are strongly absorbed in the window, so if it is thick, the intensity of those x-ray lines in the spectrum will be small. This means the detection efficiency is low.

Usually it is convenient to look at the specifications given by the manufacturer of the detector.

Thank you for the answer. But I could detect carbon and oxygen in my sample. Is there any specific reason for this?

My sample included lithium nitrate.

The nitorgen peak should appear between the C and O peaks. I wonder if the substrate x-rays are affecting this area of the spectra.