  Fast timing detectors are an essential element in the experimental setup for
time-of-flight (ToF) mass measurements of unstable nuclei. We have upgraded the
scintillator detectors used in experiments at the National Superconducting
Cyclotron Laboratory (NSCL) by increasing the number of photomultiplier tubes
that read out their light signals to four per detector, and characterized them
in a test experiment with $^{48}$Ca beam at the NSCL. The new detectors
achieved a time resolution ($\sigma$) of 7.5 ps. We systematically investigated
different factors that affect their timing performance. In addition, we
evaluated the ability of positioning the hitting points on the scintillator
using the timing information and obtained a resolution ($\sigma$) below 1 mm
for well-defined beam spots.
