  A plastic scintillator paddle detector with embedded fiber light guides and
photomultiplier tube readout, referred to as the Reaction Plane Detector
(RXNP), was designed and installed in the PHENIX experiment prior to the 2007
run of the Relativistic Heavy Ion Collider (RHIC). The RXNP's design is
optimized to accurately measure the reaction plane (RP) angle of heavy-ion
collisions, where, for mid-central $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions,
it achieved a $2^{nd}$ harmonic RP resolution of $\sim$0.75, which is a factor
of $\sim$2 greater than PHENIX's previous capabilities. This improvement was
accomplished by locating the RXNP in the central region of the PHENIX
experiment, where, due to its large coverage in pseudorapidity
($1.0<|\eta|<2.8$) and $\phi$ (2$\pi$), it is exposed to the high particle
multiplicities needed for an accurate RP measurement. To enhance the observed
signal, a 2-cm Pb converter is located between the nominal collision region and
the scintillator paddles, allowing neutral particles produced in the heavy-ion
collisions to contribute to the signal through conversion electrons. This paper
discusses the design, operation and performance of the RXNP during the 2007
RHIC run.
