The efficiency of channel electron multipliers (CEM) falls drastically at energies of the incident electrons greater than a few KeV. For conversion Electron Mo¨ssbauer Spectroscopy (CEMS) the conversion electron energies are far greater than a few KeV, and the CEM efficiency at 7 KeV, for example, is about 10% determined in our laboratory. The efficiency can be improved by increasing the secondary electron yield of the detector cone. To find a coating material, a study of relative secondary electron yields was carried out using a scanning microscope. The results of the study indicate that the detector efficiency should improve by more than a factor of 3 when the cone is coated with MgO. Experimentally, the efficiency of the coated detector is in fact more than 3 times that of the uncoated detector.