Pitchblende, or uraninite, is a radioactive mineral that is an oxide of uranium, often with oxides of lead thorium and rare earth elements included. The uranium atoms have an unstable nucleus due to the presence of 92 positively charged protons. To increase their nuclear stability, the uranium nuclei emit alpha particles. These alpha particles, which travel only short distances, can be detected and counted with a Geiger-Muller counter.