Beta particles are electrons or positrons emitted by some radioactive nuclei such as potassium-40 or carbon-14 (which becomes nitrogen-14). The beta particles emitted are a form of ionizing radiation sometimes called beta rays. Henri Becquerel and Ernest Rutherford discovered beta rays in the 1890s. Beta particles are made by beta decay. When scientists write it in an equation, they are use the Greek letter beta (β).