  A strong anti-correlation between ionization and scintillation signals
produced by MeV gamma-rays in liquid xenon has been measured and used to
improve the energy resolution by combining the two signals. The improvement is
explained by reduced electron-ion recombination fluctuations of the combined
signal compared to fluctuations of the individual signals. Simultaneous
measurements of ionization and scintillation signals were carried out with
Cs-137, Na-22 and Co-60 gamma rays, as a function of electric field in the
liquid. A resolution of 1.7%(sigma) at 662 keV was measured at 1 kV/cm,
significantly better than the resolution from either scintillation or
ionization alone. A detailed analysis indicates that further improvement to
less than 1%(sigma) is possible with higher light collection efficiency and
lower electronic noise.
