We propose to utilize guard ring electrode structures and a new film growth technique to create improved polycrystalline mercuric iodide detectors for background suppression by active anticoincidence shielding in gamma-ray spectrometers. Two proposed NASA missions will require anticoincidence shielding for x-ray and gamma-ray spectrometers: the High Energy Telescope of the EXIST (Energetic X-ray Imaging Survey Telescope) mission, and the Space Science Vision Mission expected to visit Titan, one of Saturn's moons. Shielding improves the performance of x-ray and gamma-ray spectrometers by reducing the effect of charged particle interactions which cannot be distinguished from true x-ray and gamma-ray interactions by the spectrometer. Active shields produce a blanking signal when a charged particle is detected, so that the signal from the spectrometer can be ignored during the spectrometer's charged-particle interaction. While it is well know that this technique produces significant improvement in x-ray and gamma-ray spectrometer performance, the technology to implement it is lacking. The attributes of mercuric iodide make it an excellent candidate for anticoincidence shielding detectors. Because of its detection characteristics, light weight, small size, low cost, robustness, and ease of application to non-planar geometries, this material can replace the costly, heavy, and bulky scintillator/photomultiplier tube (PMT) systems currently in use.