A type of radioactivity that occurs when matter that is not naturally radioactive is bombarded with particles such as protons, neutrons, and alpha particles. It was discovered by Frédéric and Irène Joliot-Curie in 1934, when they found that bombarding aluminium with alpha particles caused the aluminium to emit positrons for several minutes after the source of the alpha particles was removed. This occurred because some of the nuclei of the aluminium atoms absorbed alpha particles to become a radioisotope of phosphorus, with this radioisotope then decaying by emitting a positron to become silicon. Radioisotopes produced by induced radioactivity are very useful in research, industry, and medicine.