Growth of Single Crystals of Mercuric Iodide on the Ground and in Space
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GROWTH OF SINGLE CRYSTALS OF MERCURIC IODIDE ON THE GROUND AND IN SPACE*
L. van den Berg, EG&G Energy Measurements, Inc., Santa Barbara Operations, 130 Robin Hill Road, Goleta, California
ABSTRACT A short review will be given of the methods by which mercuric iodide is prepared and purified to obtain material suitable for the growth of single crystals. The method used in our laboratory to grow single crystals up to 1,000 grams in weight from the vapor will be discussed. The effects of gravity on the growth process will be described. A crystal growth system suitable for operation in the reduced gravity environment of space was designed, and crystal growth experiments were performed during the flights of Spacelab 3 (April 1985) and the International Microgravity Laboratory (IML-1) (January 1992). The structural quality and electronic properties of the ground-based and spacegrown crystals were compared, and the results will be presented.
INTRODUCTION The purpose of the crystal growth effort at EG&G Energy Measurements, Inc., Santa Barbara Operations (EG&G/EM SBO), is to provide high-quality crystalline material of mercuric iodide for use in a variety of radiation detection instruments. The advantages of mercuric iodide are its high-bandgap value (2.1 eV) which makes operation of the devices possible at ambient temperatures and its operation as a direct solid-state detector which avoids the use of cumbersome scintillator-photomultiplier combinations. The major negative aspects of mercuric iodide are the rather unique processes used to purify the material and grow the crystals and the relatively low values of the transport properties of the charge carriers which sometimes limit the spectral resolution of the devices. This paper will review the methods presently used to prepare pure material and grow crystals. In addition, the results of the latest crystal growth in space experiment will be presented. The structural and electronic analysis of the space crystal indicates that improved process control on the ground may yield better detector material.
"The submitted manuscript has been authored by a contractor of the U.S. Government under Contract No. DE-AC08-88NV10617. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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MATERIAL PREPARATION AND PURIFICATION The material presently used for crystal growth is synthesized in-house. This procedure guarantees a consistent product, and improvements caused by changes in the process can be tracked and identified. The synthesis process itself is a very simple reaction of solutions of mercuric chloride and potassium iodide. The precipitated mercuric iodide can easily be separated from the remaining solution by filtration. The concentrations of the solutions and the temperature at which the reaction takes place can be varied over a wide range without noticeable e
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