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.