Wiktionary
soft x-ray
n. the lowest energy, lowest frequency, longest wavelength end of the X-ray EM band, overlapping with EUV (extreme ultraviolet).
Usage examples of "soft x-ray".
The second alien ship was larger than the first, but estimates of its mass had a high finagle factor, depending on the acceleration (known), fuel consumption (deduced from drive temperature), operating temperature (deduced from the radiation spectrum, whose peak was in the soft x-ray region) and efficiency (pure guesswork).