The Large Area Burst Polarimeter (LEAP) Prototype Detector

The Large Area Burst Polarimeter (LEAP) is a wide field-of-view Compton polarimeter designed to study Gamma-ray burst (GRB) polarization in the 50–500 keV energy range. LEAP has been proposed to the recent NASA Astrophysics Explorers Mission of Opportunity solicitation and, if selected, will be deployed as an external payload on the International Space Station in 2027 for a three-year mission. During the baseline mission, LEAP is expected to measure the polarization from >65 GRBs with a Minimum Detectable Polarization (MDP) of <30%. In addition to the polarization measurements, LEAP is capable of simultaneously measuring GRB spectra in the 20 keV – 5 MeV energy range. The LEAP instrument consists of seven independent polarimeter modules, each with a 12×12 array of individual detector elements. Each detector element is comprised of a 17×17×100 mm³ high-Z (CsI(Tl)) or a low-Z (plastic) scintillator coupled to a small Hamamatsu photomultiplier tube. In order to demonstrate LEAP’s ability to measure polarization, a small-scale (5×5) prototype module was assembled using identical detector elements in a similar arrangement to the full scale instrument. The individual detector elements were readout by benchtop front-end electronics. Both spectral and polarization measurements with several radioactive sources were performed and the results compared to simulations using GEANT4. The results of the LEAP prototype studies demonstrate the basic functionality of the LEAP design and are presented here.