Along with beta particles, gamma rays, and neutrons, alpha particles are a type of radiation, also called alpha radiation. Like most forms of radiation, alpha particles are emitted by radioactive elements.
Alpha particles are relatively heavy and slow compared to their companions, so they pose little danger to humans unless ingested.
However, they are often used in research – alpha particles played a decisive role in Ernest Rutherford’s discovery of the atomic nucleus, which formed the basis of his atomic model.
What are alpha particles?
Alpha particles are positively charged particles composed of two protons, two neutrons, and a zero electron. The mass of an individual particle is 4 amu. (6.642 × 10–4 g). According to the Britannica, alpha particles are emitted by heavy radioactive elements (both natural and artificial), including uranium, radium, and plutonium. Because of this, these elements are also called alpha emitters.
An alpha particle is usually denoted by the symbol α, the Greek letter alpha after which the particle is named. It was the first type of nuclear radiation to be discovered, before beta particles and gamma rays. But since the alpha particle is identical to the nucleus of the helium-4 atom, it is sometimes represented as He2+, that is, a doubly ionized helium-4 atom.
Explanation of Rutherford’s gold foil experiment
Between 1898 and 1899, physicist Ernest Rutherford, who was studying radioactivity at the University of Cambridge in England, determined that there were at least two types of radiation, which he named alpha and beta. The alpha particle would lead him to the discovery of the atomic nucleus and help him develop the Rutherford atomic model, a radical shift in humanity’s understanding of atoms.
In 1911, Rutherford officially published a paper in which he announced the existence of a positively charged nucleus at the center of the atom (although he did not formally call it a nucleus at that time). From 1907 Rutherford, Hans Geiger and Ernest Marsden carried out a series of experiments on Coulomb scattering at the University of Manchester in England. These experiments involved firing alpha particles at thin gold foil and then observing where those particles then hit the foil.
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At the time, J. J. Thompson’s “plum pudding” model of the atom was the dominant theory of atomic structure—it assumed that atoms were perfect spheres of positively charged material, in which negatively charged electrons were relatively evenly distributed.
If this model were correct, alpha particles would have passed through the foil in Rutherford’s Coulomb experiments. But Rutherford and his colleagues noticed that several alpha particles bounce off the foil in different directions. Rutherford then suggested that atoms have a dense nucleus surrounded by orbiting electrons—alpha particles pass through the space between the electrons and bounce off the nucleus.
What is alpha decay and how does it happen?
According to the Britannica, alpha decay is the process of producing alpha particles. Unstable radioactive elements, called radionuclides, emit particles from their nuclei to become more stable, changing from the original element to a new one. These emissions are radiation – in the case of alpha decay, alpha particles are emitted from the nuclei of heavy radioactive elements.
Alpha Radiation Charge and Penetration
Alpha radiation has a positive charge of two. Alpha particles are the heaviest and slowest of the main types of radiation, with a mass of 4 a.m.u., according to the Australian Radiation Protection and Nuclear Safety Authority. and ejection speed of approximately 12,400 miles per second (20,000,000 km per second).
Although high in energy, alpha particles use up most of that energy immediately after being ejected, so they don’t propagate farther than a few inches. They also have extremely low penetrating power – they cannot penetrate the human epidermis or the outer layer of the skin. According to the US Nuclear Regulatory Committee, even a piece of paper is enough to block an alpha particle.
Use of alpha radiation
For commercial purposes, alpha radiation is mainly used in smoke detectors (smoke reduces the amount of alpha particles in the detector, causing alarms) and antistatic devices (alpha ionizers).
Research is also ongoing to develop alpha-particle therapy for the treatment of cancer – clinical trials have shown some success in the treatment of metastatic, castration-resistant prostate cancer. For research purposes, alpha particles are used as projectiles, as in the case of Rutherford’s gold foil experiment.
Dangers of alpha radiation
Alpha radiation is outwardly not dangerous to humans due to its low penetrating power; alpha particles cannot penetrate the skin. However, they can damage the cornea.
The real danger comes from within the body. If an alpha emitter (that is, a radioactive element) enters your body by ingestion, inhalation, through a wound, or by any other route, great internal damage can be done to living tissues.
Learn more about Rutherford’s work with alpha particles in this online exhibition from the American Institute of Physics Center for History. You can also read more on his biography page from the Nobel Foundation – Rutherford was awarded the Nobel Prize in Chemistry in 1908 “for his research on the decay of the elements and the chemistry of radioactive substances.”
Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), Alpha Particles.
Britannica, Alpha Decay.
Britannica, Alpha Particle.
United States Environmental Protection Agency (EPA), Fundamentals of Radiation.
United States Nuclear Regulatory Commission (USNRC), Fundamentals of Radiation.
Center for History, American Institute of Physics, Rutherford’s Nuclear World.