  We present up to 24-photon Bragg diffraction as a beam splitter in
light-pulse atom interferometers to achieve the largest splitting in momentum
space so far. Relative to the 2-photon processes used in the most sensitive
present interferometers, these large momentum transfer beam splitters increase
the phase shift 12-fold for Mach-Zehnder (MZ-) and 144-fold for Ramsey-Borde
(RB-) geometries. We achieve a high visibility of the interference fringes (up
to 52% for MZ or 36% for RB) and long pulse separation times that are possible
only in atomic fountain setups. As the atom's internal state is not changed,
important systematic effects can cancel.
