  We present the results of the first high-altitude balloon flight test of a
concept for an advanced Compton telescope making use of modern scintillator
materials with silicon photomultiplier (SiPM) readouts. There is a need in the
fields of high-energy astronomy and solar physics for new medium-energy
gamma-ray (~0.4 - 10 MeV) detectors capable of making sensitive observations. A
fast scintillator- based Compton telescope with SiPM readouts is a promising
solution to this instrumentation challenge, since the fast response of the
scintillators permits the rejection of background via time-of-flight (ToF)
discrimination. The Solar Compton Telescope (SolCompT) prototype was designed
to demonstrate stable performance of this technology under balloon-flight
conditions. The SolCompT instrument was a simple two-element Compton telescope,
consisting of an approximately one-inch cylindrical stilbene crystal for a
scattering detector and a one-inch cubic LaBr3:Ce crystal for a calorimeter
detector. Both scintillator detectors were read out by 2 x 2 arrays of
Hamamatsu S11828-3344 MPPC devices. Custom front-end electronics provided
optimum signal rise time and linearity, and custom power supplies automatically
adjusted the SiPM bias voltage to compensate for temperature-induced gain
variations. A tagged calibration source, consisting of ~240 nCi of Co-60
embedded in plastic scintillator, was placed in the field of view and provided
a known source of gamma rays to measure in flight. The SolCompT balloon payload
was launched on 24 August 2014 from Fort Sumner, NM, and spent ~3.75 hours at a
float altitude of ~123,000 feet. The instrument performed well throughout the
flight. After correcting for small (~10%) residual gain variations, we measured
an in-flight ToF resolution of ~760 ps (FWHM). Advanced scintillators with SiPM
readouts continue to show great promise for future gamma-ray instruments.
