The paper is "Helium-like ions as powerful X-ray plasma diagnostics"
Proceedings of “X-ray Astronomy 2000”, Palermo, Sep. 2000
ASP Conference Series, Vol. TBD (in press), 2001
R. Giacconi, L. Stella, S. Serio
[Mentors note: arXiv link at https://arxiv.org/abs/astro-ph/0012218v2]
By the way, why...