the nuclear microprobe's beam is usually composed of protons and alpha particles. some of the most advanced nuclear microprobes have beam energies in excess of 2 mev. this gives the device very high sensitivity to minute concentrations of elements, around 1 ppm at beam sizes smaller than 1 micrometer. this elemental sensitivity exists because when the beam interacts with the a sample it gives off characteristic x-rays of each element present in the sample. this type of detection of radiation is called pixe. other analysis techniques are applied to nuclear microscopy including rutherford backscattering(rbs), stim, etc.