Toward the Energy Resolution Limit of Mercuric Iodide in Room Temperature Low Energy X-Ray Spectrometry

Development of mercuric iodide nuclear detectors fabricated from vapor grown single crystals continues with emphasis in our work on their capabilities as room temperature operable low-energy X-ray spectrometers. The influence of trapping in the detector crystal, electronic noise and statistical fluctuations on energy resolution of the detectors are discussed. A room temperature energy resolution value of 490 eV (FWHM) has been measured for Fe-55 (with a corresponding 420 eV value of electronic noise) using a 2mm2 HgI2 detector. The K¿ and Kß lines of Fe-55 at 5.9 keV and 6.5 keV have been separated, to our knowledge, for the first time at room temperature. A new Fano-factor value of 0.46 has been established for mercuric iodide. In the room temperature HgI2 detector - FET preamplifier combination the latter is the largest contributor to the width of the spectral line in the low energy region. Our efforts are therefore directed toward reducing room temperature preamplifier noise. A room temperature preamplifier noise level of 360 eV (HgI2) has been attained. We are currently investigating use of low noise, low capacitance FET devices in a more optimized preamplifier configuration which should further reduce the noise value (H.E. Kern, IEEE Trans. Nucl. Sci. NS-17). Also under investigation is the effect of Peltier cooling of the input stage of the FET preamplifier.