  We describe the performance of a $\mathrm{23\times 23\times30 ~mm^3}$ low
background cerium bromide, CeBr$_3$(LB), scintillator crystal coupled to a
Hamamatsu R11265U-200 photomultiplier. This detector will be the building block
for a gamma-ray detector array designed to be the payload for a CubeSat to be
launched in 2020. The aim of the mission is to study flashes of gamma rays of
terrestrial origin. The design of the detector has been tuned for the detection
of gamma rays in the 20 keV$-$3 MeV energy range.
