The alpha particle is a type of ionizing radiation. With its partners the gamma particles and beta particles, alpha particles are one of the most prevalent forms of radiation. Each particle is essentially a helium nucleus, which consists of two neutrons and two protons, without electrons, giving it a net positive charge. Due to their relatively high mass, these particles are the most destructive form of ionizing radiation, but the trade-off is that their penetration is low. A piece of paper stops alpha particles, whereas the lighter beta particles require a aluminum barrier.
Alpha particles are emitted from various radioactive substances. Unlike beta decay, alpha decay is mediated by the strong force. According to classical Newtonian laws, the attraction of the nucleus should be too strong to let alpha particles leave it under any circumstances. However, quantum tunneling permits it anyway. Quantum tunneling is the instantaneous teleporting of the particle to a place outside the nucleus.
Because alpha particles have such low penetrating force, they are stopped by human skin, presenting little danger unless the source is swallowed. This was the sad fate of Russian ex-spy Alexander Litvinenko, thought to be the first person to die from acute radiation poisoning as a result of ingesting the alpha emitter polonium. Other known alpha emitters include americium (found in smoke detectors), radium, radon gas, and uranium. When coupled together with certain other radioactive substances, alpha emitters can agitate neutron emitters to release the neutrons. Neutron emission is a critical part of nuclear reactor and nuclear weapons design.
In investigations into the health effects of smoking, it was found that tobacco leaves contain small amounts of polonium, which emits alpha particles. It is theorized that this could be partially responsible for lung cancer among smokers. In evolution, alpha emitters play a critical role — their likelihood of causing a chromosomal mutation is over 100 times greater than with other types of radiation. Most of the time, this produces less-fit mutants, but when combined with selection over thousands or millions of years results in adaptive biological designs.