The High Energy Telescope (HET) is a wide-field hard X-ray coded-aperture telescope. On the top is the tungsten coded mask with thick (3.0 mm), thin (0.3 mm) and open cells and 2.0m below a large array (4.5m2) of CZT detector modules on the bottom.
The outer side is a "graded Z" passive shield made of lead, tantalum, tin, and copper. Below the CZT detectors are NaI (or BGO: TBD) crystals used for anti-coincidence shielding, part of the HACOS particle and gamma-ray detection system. Scintillation photons from these crystals are detected by photomultiplier tubes (PMTs).
The large FoV (~70° × 90° at 10% coding) of HET is ideal for GRB hunting. The hybrid mask pattern with fine (1.25 mm pitch) and coarse (15mm pitch) elements coupled with fine (0.6mm) detector pixels allow fast, accurate localization of GRBs (<20" in 10 sec), which will trigger a immediate autonomous slew to lock the SXI and IRT FoV on the GRB afterglow in less than 100 sec.
Thick (5mm) CZT crystals mounted on low-noise electronics using Application Specific Integrated Circuit (ASIC) cover a wide energy range from 5 to 600 keV. For GRB, the anti-coincidence shield extends the spectroscopy to 3 MeV. The large energy band width is ideal for discovering obscured (or dormant) AGNs as well as GRBs. It overlaps nicely with the soft X-ray band covered by SXI and is high enough to capture hard X-rays emerging out of the material blocking the central sources.