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If the nuclei of atoms are bombarded by beams of particles, such as protons, the atoms can be transformed into different variants of the element with different masses, so-called isotopes. The likelihood that particles can penetrate the atomic nucleus and bring about a nuclear reaction increases if the bombarding particles have a high velocity. In 1929 Ernest Lawrence developed an apparatus known as a cyclotron that used electrical and magnetic fields to accelerate protons to high velocities in a spiral-shaped path before they collide with their target.