  Diamond has been developed as a material for the detection of charged
particles by ionization. Its radiation hardness makes it an attractive material
for detectors operated in a harsh radiation environment e.g. close to a
particle beam as is the case for beam monitoring and for pixel vertex
detectors. Poly-crystalline chemical vapor deposition (CVD) diamond has been
studied as strip and pixel detectors so far. We report on a first-time
characterization of a single-crystal diamond pixel detector in a 100 GeV
particle beam at CERN. The detectors are made from irregularly shaped single
crystal sensors, 395mm thick, mated by bump bonding to a front-end readout IC
as used in the ATLAS pixel detector with pixel sizes of 50 x 400 mm2. The
diamond sensors show excellent charge collection properties: full collection
over the entire detector volume, clean and narrow signal charge distributions
with a S/N value of >100 and a hit detection efficiency of (99.9 +- 0.1)%. The
measured spatial resolution for particles under normal incidence in the shorter
pixel direction is (8.9 +- 0.1) um.
