Secondary electron emission generated by the impact of positive ions from insulators is generally strongly retarded due unavoidable positive charging of the target hindering the reliable characterization of the materials. Special measurement procedure utilizing a distant single-ion impact is developed and the experimental conditions to avoid the charging are found. The data on secondary electron energy distribution (SEED) in the helium ion microscope for platinum, silicon nitride, and silicon dioxide are presented. It is shown that with the help of the suggested procedure SEED can be determined for any material with a high accuracy.