The alpha particle is emitted by certain radioactive elements as they decay to a stable element. It consists of two protons and two neutrons; it is positively charged. The element that undergoes "alpha decay" changes into a new element whose atomic number is down two and atomic mass is down four from the original element. Alpha decay occurs when a nucleus has so many protons that the strong nuclear force is unable to counterbalance the strong repulsion of the electrical force between the protons. Because of its mass, the alpha particle travels relatively slowly (less than 10% the speed of light), and it can be stopped by a thin sheet of aluminum foil.
When Henri Becquerel first discovered the property of radioactivity in 1896, he did not know that the radiation consisted of particles as well as energy. Ernest Rutherford began experimenting to determine the nature of this radiation in 1898. One experiment demonstrated that the radiation actually consisted of 3 different types: a positive particle called "alpha," a negative particle, and a form of electromagnetic radiation that carried high energy. By 1902 Rutherford and his colleague Frederick Soddy were proposing that a different chemical element is formed whenever a radioactive element decays, a process known as transmutation. Rutherford was awarded the Nobel Prize in chemistry in 1908 for discovering these basic principles of radioactivity. In Rutherford's classic "gold foil" experiment to determine the structure of an atom, his assistants Hans Geiger and E. Marsden used positively charged high speed alpha particles that were emitted from radioactive polonium to bombard a gold foil. The results of this experiment showed that an atom consisted of mostly empty space, with essentially all its mass concentrated in a very small, dense, positively charged center called the nucleus. In fact, the alpha particle itself was identified as the nucleus of the helium atom! Soddy, working with William Ramsay, who in 1895 had discovered that the element helium was a component of earth minerals, verified that helium is produced when radium is allowed to decay in a closed tube.
The first production in the laboratory of radioisotopes—as opposed to the naturally occurring radioactive isotopes—was achieved by bombarding stable isotopes with alpha particles. In 1919, Rutherford succeeded in producing oxygen-17 by bombarding ordinary nitrogen-14 with alpha particles; a proton was set free in the reaction. The 1935 Nobel Prize in chemistry was awarded to Irène and Frédéric Joliet-Curie for their work in producing radioisotopes through the bombardment of stable elements with alpha particles from polonium decay. After bombarding aluminum with alpha particles, they found that when the nucleus absorbed an alpha particle, it changed into a previously unknown radioisotope of phosphorus; an unidentified neutral particle was set free in the reaction. James Chadwick repeated their experiment using a beryllium target, then captured the particle that was emitted and used principles of classical physics to identify its mass as being the same as that of the proton. This particle was named the neutron, and Chadwick received the 1935 Nobel Prize in physics for its discovery. Subsequently, scientists began accelerating alpha particles to very high energies in specially constructed devices like the cyclotron and linear accelerator before shooting them at the element they wanted to transmute.
A number of radioisotopes, both natural and man made, have been identified as alpha emitters. The decay chain of the most abundant uranium isotope, uranium-238, to stable lead-206 involves eight different alpha decay reactions. Uranium has a very long half life (4.5 billion years) and because it is present in the earth in easily measured amounts, the ratio of uranium to lead is used to estimate the age of our planet Earth. More recently, Guenther Lugmair at the University of California at San Diego introduced age-dating using samarium-147, which undergoes alpha decay to produce neodymium-143.
Most of the transuranic radioactive elements undergo alpha decay. During the 1960s when an instrument was needed to analyze the lunar surface, Anthony Turkevich developed an alpha scattering instrument, using for the alpha source curium-242, a transuranic element produced by the alpha decay of americium-241. This instrument was used on the moon's surface by Surveyors 5, 6, and 7. Plutonium has been used to power more than twenty spacecraft since 1972; it is also being used to power cardiac pacemakers.
Ionization smoke detectors detect the presence of smoke using an ionization chamber and a source of ionizing radiation (americium-241). The alpha particles emitted by the radioactive decay of the americium-241 ionize the oxygen and nitrogen atoms present in the chamber, giving rise to free electrons and ions. When smoke is present in the chamber, the electrical current produced by the free electrons and ions (as they move toward positively and negatively charged plates in the detector) is neutralized by smoke particles; this results in a drop in electrical current and sets off an alarm.
Because of their low penetrability, alpha particles do not usually pose a threat to living organisms, unless they are ingested. This is the problem presented by the radioactive gas radon, which is formed through the natural radioactive decay of the uranium (present in rock, soil, and water throughout the world). As radon gas seeps up through the ground, it can become trapped in buildings, where it may build up to toxic levels. Radon can enter the body through the lungs, where it undergoes alpha decay to form polonium, a radioactive solid that remains in the lungs and continues to emit cancer-causing radiation.