Present Performances and Potential Applications of Mercuric Iodide Detectors

The need for room temperature detectors capable of high detection efficiency for x-ray and γ-ray spectroscopy has long been recognized. This has resulted in substantial work on potentially suitable high-Z semiconductors. Detectors made from mercuric iodide have proved to be the most attractive candidates, particularly for low-energy x-ray applications. Such detectors are now commercially available. Mercuric iodide exhibits the wide-band gap required for room temperature operation and the high atomic number (Z) required for efficient photons detection. However, in common with other II-VI compound semiconductors, poor hole collection results from deep hole traps. Fortunately, for low-energy x-rays incident on the negatively-biased contact, the signal is dominated by electron collection. Therefore in this case, very good energy resolution can sometimes be obtained, opening up the possibility of wide-ranging applications for x-ray fluorescence spectroscopy. Good resolution, long-term stability, and a lack of polarization effects all represent the objectives of work on these detectors; these factors and their control will be discussed here. The performance of recent detectors has been discussed elsewhere. The paper will focus on three aspects of development of HgI2 detectors: i) Methods of purification and crystal growing. Several methods are used and these will be discussed. ii) Fabrication of detectors. iii) Applications and performance of detectors. Mercuric iodide detectors are described here both from the point of view of fabrication methods and from the user's point of view. The paper highlights the performance achieved at different photon energies.