A British physicist named Ernest Rutherford described alpha particle for the first time in 1899. He also differentiated and named alpha and beta radiation. However, it wasn’t until 1928 that George Gamow solved the theory of alpha decay using quantum tunneling.
In this overview article, we have explained why alpha decay occurs, what actually happens in the process, what are its primary sources, and whether it has any adverse effects. But let’s start with basics.
What Is Alpha Decay?
Definition: Alpha decay (also written as α-decay) is one of the three kinds of radioactive disintegration (other being beta and gamma decay) in which an unstable atomic nucleus dissipates excess energy by spontaneously ejecting an alpha particle.
Since the alpha particle contains a mass of four units and two positive charges, its emission from a nucleus produces a daughter nucleus having a mass of four units less and an atomic number of two units less (than its parent nucleus).
The emitted alpha particle is identical to a helium nucleus, which contains two neutrons and two protons. It also has a mass of 4u and a charge of +2e. The symbol for the helium nucleus is He2+, or sometimes it is written as 42He2+.
In nuclear physics, the formula or equation of alpha decay can be written as:
- ABX is the parent nucleus
- A-4B-2X’ is the daughter nucleus
- 42He is the emitted helium nucleus or alpha particle
In a nuclear equation, the alpha particle is usually shown without considering a charge (however, it does contain a charge +2e).
Alpha decay only occurs in the heavy nuclides. Theoretical calculations show that this type of decay can occur in nuclei slightly heavier than Nickel (atomic number 28). In the real world, however, it has only been detected in nuclides significantly heavier than Nickel.
Tellurium (atomic number 52) is the lightest element whose isotopes (104Te to 109Te) are known to undergo alpha decay. However, there are some exceptional cases, such as an isotope of beryllium (8Be) that decays into two alpha particles.
The most popular example of this sort of nuclear transmutation is uranium decay. Uranium-238 (the most common isotope of uranium found in nature) decays to form thorium-234.
23892Ur → 23490Th + 42He
- 23892Ur is the unstable Uranium-238 parent nucleus
- 23490Th is the thorium-234 daughter nucleus
- 42He is the ejected alpha particle
As you can see, the sum of the subscripts (masses and atomic numbers) remains the same on each side of the equation.
Similarly, Thorium becomes Radium
23290Th → 22888Ra + 42He
Neptunium becomes Protactinium
23793Np → 23391Pa + 42He
Platinum becomes Osmium
17578Pt → 17176Os + 42He
Gadolinium becomes Samarium
14964Gd → 14562Sm + 42He
In summary, three things happen in alpha decay:
- The heavy (parent) nucleus splits into two pieces.
- The alpha particle is ejected into space.
- The (daughter) nucleus left behind has its mass number reduced by four, and its atomic number reduced by two.
Why Does Alpha Decay Occur?
Two fundamental interactions play a major role in alpha decay: nuclear force (short-range) and electromagnetic force (long-range). The strength of attractive nuclear forces (acting between neutrons) is much greater than repulsive electromagnetic forces (acting between protons). Thus, the nuclear force holds an atomic nucleus together.
However, when the total disruptive electromagnetic force overcomes the nuclear force, the atomic nucleus splits into two or more pieces. Studies show that a nucleus containing more than 209 nucleons is so big that the electromagnetic repulsion between its proton often defeats the attractive nuclear force holding it.
This happens because the strength of nuclear force drops rapidly beyond one femtometer, while the electromagnetic force maintains the same strength for longer distances.
The classical physics do not permit alpha particles to escape the strong nuclear force within the nucleus. Quantum mechanics, however, allows alpha particles to escape via quantum tunneling, even though they don’t have sufficient energy to overcome the nuclear force.
Main Source of Alpha Decay
Alpha particles are mostly emitted by heavier atoms (atomic number >106) such as thorium, uranium, radium, and actinium. In fact, almost 99 percent of the helium generated on Earth comes from the alpha decay of underground minerals consisting of thorium or uranium.
Cosmic rays, which originate outside of Earth’s atmosphere, also contain alpha particles. About 90 percent of cosmic ray nuclei are hydrogen (protons), 9 percent are helium (alpha particles), and 1 percent are HZE ions. The fraction varies according to the energy range of cosmic rays.
Some human-made isotopes emit alpha particles: for example, the radioisotopes of curium, americium, and plutonium. They are created in a nuclear reactor through the absorption of neutrons by different uranium isotopes.
High energy helium nuclei can also be artificially created by particle accelerators, such as synchrotron and cyclotrons. However, they aren’t commonly referred to as alpha particles.
Is It Dangerous?
Typically, ejected alpha particles have a kinetic energy of 5 MegaElectronVolt, and they travel at nearly 5 percent of the speed of light. Because they carry +2e electric charge and have a large mass, they can easily interact with other atoms and lose their energy.
Although alpha decay is highly ionizing particle radiation, it has low penetration depth. The forward motion of the alpha particles can be stopped by a piece of paper, a thick layer of air, or outer layers of human skin.
The penetration level of alpha, beta and gamma particles
They are not dangerous to life unless the source is inhaled, ingested, or injected. If a radioactive substance decaying alpha particle enters the body, it could be 20 times dangerous than gamma radiation. Large doses can result in radiation poisoning. Polonium-210, a strong alpha emitter, plays a key role in bladder cancer and lung cancer.
Although alpha particles cannot penetrate human skin, it can harm the cornea. Some alpha sources are also accompanied by beta-emitting nuclei, which in turn are accompanied by gamma photon emission.
Radon is one of the biggest contributors to public radiation dose. When inhaled, some of its particles get attached to the lung’s inner lining and eventually damage cells in the lung tissue.
Working principle of smoke detector
Radioactive sources of alpha particles are used in smoke detectors. Americium-241, for example, releases alpha particles that ionize the air inside the detector. When smoke enters the equipment, it absorbs the radiation, triggering the alarm.
Alpha particles from polonium-210 are used to eliminate static electricity from equipment. Alphas attract free electrons, decreasing the potential for local static electricity. This technique is common is paper mills.
Alpha Particle X-ray Spectroscopy is used to determine the composition of rocks and soils. NASA used this process on Mars Exploration Rover to collect curtal data, weather data, and water activity on the Red Planet.
A pellet of 238PuO2 as used in RTG for space missions. The pellet is glowing red hot because of the heat generated by alpha decay | Image credit: Wikimedia
Space agencies use radioisotope thermoelectric generators (RTG) to power various spacecraft and satellites, including Voyager 1/2 and Pioneer 10/11. These generators use plutonium-238 to operate as a long-lasting battery. Plutonium-238 emits alpha radiation resulting in heat, which is converted to electricity.
Scientists are currently working to utilize the damaging nature of alpha-emitting sources to treat cancer. They are trying to direct small amounts of alpha particles towards tumor cells. Since these particles have small penetration depth, they could stop the growth of the tumor or probably destroy it, without affecting the surrounding healthy tissue. This kind of treatment is known as unsealed source radiotherapy.