Water window

The water window consists of the soft x-rays between the K-absorption edge of oxygen at a wavelength of 2.34 nm and the K-absorption edge of carbon at 4.4 nm (x-ray energies of 530 and 280 eV, respectively). Water is transparent to these x-rays while nitrogen and other elements found in biological specimens are absorbing. These wavelengths could be used in an x-ray microscope for viewing living specimens.

...
Wikipedia