Cross-Section Ratio of Double to Single Ionization of Helium by Compton Scattering of 40 - 100 keV x Rays L. Spielberger(1), H. Br"auning(1,2), A. Muthig(1), J.Z. Tang(3), J. Wang(4), Y. Qiu(3), R. D"orner(1), O. Jagutzki(1), Th. Tschentscher(5), V. Honkim"aki(5), V. Mergel(1), M. Achler(1), Th. Weber(1), Kh. Khayyat(1), J. Burgd"orfer(3,6), J. McGuire(4), H. Schmidt-B"ocking(1) 1) Inst. f. Kernphysik, August-Euler-Str. 6, 60486 Frankfurt, Germany 2) Dept. of Physics, Kansas State Univ., Manhattan, USA 3) Dept. of Physics, Univ. of Tennessee, Knoxville, USA and Oak Ridge Nat. Lab., Oak Ridge, USA 4) Dept. of Physics, Tulane Univ., New Orleans, USA 5) ESRF, Grenoble, Fance 6) Inst. f. Theoretische Physik, TU Wien, Austria We present a measurement of the cross-section ratio of double to single ionization, Rc, after Compton scattering at helium at 40, 80, 100keV using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS). Comparison with calculations involving highly correlated initial states and approximate final states, with and without final state correlations, are made.