It has been hypothesized that the impactors that created the majority of the observable craters on the ancient lunar highlands were derived from the main asteroid belt in such a way that preserved their size-frequency distribution. A more limited ver
sion of this hypothesis, dubbed the E-belt hypothesis, postulates that a destabilized contiguous inner extension of the main asteroid belt produced a bombardment limited to those craters younger than Nectaris basin. We investigate these hypotheses with a Monte Carlo code called the Cratered Terrain Evolution Model (CTEM). We find that matching the observed number of lunar highlands craters with Dc~100 km requires that the total number of impacting asteroids with Di>10 km be no fewer than 4x10-6 km-2. However, this required mass of impactors has <1% chance of producing only a single basin larger than the ~1200 km Imbrium basin; instead, these simulations are likely to produce more large basins than are observed on the Moon. This difficulty in reproducing the lunar highlands cratering record with a main asteroid belt SFD arises because the main belt is relatively abundant in the objects that produce these megabasins that are larger than Imbrium. We also find that the main asteroid belt SFD has <16% chance of producing Nectarian densities of Dc>64 km craters while not producing a crater larger than Imbrium, as required by the E-belt hypothesis. These results suggest that the lunar highlands were unlikely to have been bombarded by a population whose size-frequency distribution resembles that of the currently observed main asteroid belt. We suggest that the population of impactors that cratered the lunar highlands had a somewhat similar size-frequency distribution as the modern main asteroid belt, but had a smaller ratio of objects capable of producing megabasins compared to objects capable of producing ~100 km craters.
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 RXNPs 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 PHENIXs 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.
