Yes, the X-ray lines of some elements such as iron are rather easy to see in the X-ray spectrum, but I have not heard that isotopic differences can be discerned because, I think the energy differences may not be great enough to resolve closely-spaced isotopic lines given the resolution of typical X-ray spectrometers.
On the new Chandra Xray Observatory there are two transmission grating spectrometers, formed by sets of gold gratings placed just behind the mirrors. One set is optimized for low energies (LETG) and the other for high energies (HETG). Spectral resolving powers (E/deltaE) in the range 100-2000 can be achieved with good efficiency. The above spectrum shows an image of Capella and reveals many different xray emission lines from the element iron, ionized 15 - 16 times, but many are shifted due to Doppler motion so it is not obvious which ones are isotopic lines. Generally, isotopic lines are only a few percent or less different in energy, and this would be difficult to discern even in the above spectra. However, it might be possible for some objects, and supernovae would be among them because of their brightness. We will have to wait and see what Chandra is able to produce!
Copyright 1997 Dr. Sten Odenwald
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