Development of an X-ray surface analyzer for planetary exploration

An ultraminiature X-ray fluorescence spectrometer was developed which can obtain data on element composition not provided by present spacecraft instrumentation. The apparatus employs two radioisotope sources (Fe-55 and Cd-109) which irradiate adjacent areas on a soil sample. Fluorescent X-rays emitted by the sample are detected by four thin-window proportional counters. Using pulse-height discrimination, the energy spectra are determined. Virtually all elements above sodium in the periodic table are detected if present at sufficient levels. Minimum detection limits range from 30 ppm to several percent, depending upon the element and the matrix. For most elements, they are below 0.5 percent. Accuracies likewise depend upon the matrix, but are generally better than plus or minus 0.5 percent for all elements of atomic number greater than 14. Elements below sodium are also detected, but as a single group.