Abstract: Mercuric Iodide X-RAY detectors are made to exhibit high energy resolution characteristics, reduced background level in the x-ray spectra, enhanced sensitivity to low energy x-rays and significantly improved long term stability and reliability. The above improvements has been achieved through specific changes in the detector construction and passivation/encapsulation techniques. Specifically, the introduction of a guard ring structure and collimating shield improved the electric field distribution in the detector, reduced the surface leakage current, lowered effective capacitance and eliminated current injections. A special Parylene-C and Parylene-N deposition onto the detector surface insured excellent long term detector stability and reliability.