After much searching I found the energy resolution here [1], stated as <2 keV. For comparison, a modern silicon drift diode fluorescence detector will have an energy resolution of ~130 eV.
[1] http://dpnc.unige.ch/seminaire/talks/campbell.pdf
Also: http://iopscience.iop.org/article/10.1088/1748-0221/8/02/C02...