  We are constructing a Silicon Vertex Tracker detector (VTX) for the PHENIX
experiment at RHIC. Our main motivation is to enable measurements of heavy
flavor production (charm and beauty) in p+p, p+d and A+A collisions. Such data
will illuminate the properties of the matter created in high-energy heavy-ion
collisions. The measurements also will reveal the distribution of gluons in
protons from p+p collisions. The VTX detector consists of four layers of barrel
detectors and covers |eta|< 1.2, and almost a 2pi in azimuth. The inner two
silicon barrels consist of silicon pixel sensors; their technology accords with
that of the ALICE1LHCB sensor-readout hybrid. The outer two barrels are silicon
stripixel detectors with a new "spiral" design, and a single-sided sensor with
2-dimensional (X, U) readout. In this paper, we describe the silicon stripixel
detector and discuss its performance, including its response to electrons from
a beta source (90Sr), muons from cosmic-rays, and a 120 GeV proton beam. The
results from the proton beam demonstrate that the principle of two-dimensional
position sensitivity based on charge sharing works; the signal-to-noise value
is 10.4, the position resolution is 33.6 um for X-stripixel (35.2 um for
U-stripixel), and the tracking efficiencies in the X- and U-stripixels are,
over 98.9 +/- 0.2%. The stripixel detector within the VTX project is in the
pre-production phase.
