Scattering of X-ray emission lines by a helium atom
  The differential cross section for scattering of the astrophysically important X-ray emission lines by a helium atom is calculated with an accuracy sufficient for astrophysical applications. For a helium atom an energy ``gap'' (due to the structure of the energy levels) is twice larger than for a hydrogen atom and the ``compton profile'' (due to broader distribution of an electron momentum) is significantly shallower. This opens principle possibility to distinguish helium and hydrogen contributions, observing scattered spectra of X-ray emission lines. With the appearance of a new generation of X-ray telescopes, combining a large effective area and an excellent energy resolution it may be possible to measure helium abundance in the molecular clouds in the Galactic Center region, in the vicinity of AGNs or on the surface of cold flaring stars. 
L.Vainshtein (Lebedev Physical Institute, MPA), R.Sunyaev (MPA, IKI),   E.Churazov (MPA, IKI)