Abstract
The COmpton Spectrometer and Imager (COSI), known as the NuclearCompton Telescope (NCT) before, is a balloon-borne soft gamma-ray (0.2-5MeV) telescope. COSI consists of twelve germanium strip detectors that are 3D position-sensitive. NCT had a successful balloon flight in 2009, and the upgraded instrument as COSI will be launched for an ultra-long duration balloon (ULDB)flight (60-100 days) in Antarctica in December, 2014. The most important change is the con guration of germanium detector array, which becomes 2x2x3 instead of 2x6 in NCT. In the 2014 ULDB ight, our main targets are Gamma-ray bursts (GRB), which are expected to be observed more than once in our observation depending on its fluence and the flight duration. As a Compton telescope, COSI is intrinsically sensitive to polarization. To assess COSI's polarization performance, I estimated the minimum detectable polarization (MDP) of these GRBs with Monte-Carlo simulations, considering a revised mass model as well as the contribution of background.