Alpha, Beta, Gamma: Types of Ionizing Radiation

Radiation is all around us. From radio waves transmitting information to the visible spectrum that gives our world color. Heck, even talking produces radiation in the form of sound. But what about the more dangerous forms of radiation: alpha and beta particles, gamma and X-rays? What makes these ‘ionizing’ forms of radiation so different from microwaves, for example, which we encounter in our lives on a daily basis?

What is Ionizing Radiation?

Radio waves, microwaves, infrared and ultraviolet. Radiation can take any form across the entire electromagnetic spectrum, most of which are invisible to the naked eye. What might be surprising to you is that the term ‘radiation’ doesn’t just encompass photons of light. Sound waves, for example, are also a form of radiation. In fact, any wave or particle is radiation!

But when we hear the word ‘radiation’, our first thought goes to uranium and plutonium, among other radioactive substances. These sources produce extremely high energy radiation, hence their notoriety for being harmful and dangerous. These radiation products include helium ions (alpha radiation), electrons and positrons (beta radiation), and photons (gamma radiation).

As mentioned, radiation doesn’t have to be electromagnetic. ‘Ionizing radiation’ is the given term for any wave or particle with the potential to cause harm to our bodies. They are ionizing because they possess high enough energy to ‘kick’ electrons out of other atoms and molecules, forming ions. We take a look at the forms of ionizing radiation below.

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Alpha Particles

Alpha (α) radiation is just an alpha particle, consisting of 2 protons and 2 neutrons. Notice that it is charged (2 protons provide a 2+ charge), with no electrons to balance this out. Hence, an alpha particle is identical to the nucleus of a helium atom, or rather a helium ion with a 2+ charge.

Alpha particles are the product of radioactive decay, usually involving heavier atoms (greater than 106 atomic mass). Uranium-238 contains 92 protons and 146 neutrons, and it can release an alpha particle to form a new element. This new atom has 90 protons and 144 neutrons, also known as thorium-234. The half-life of this process is around 4.5 billion years, however, which is the time it takes for half of a sample of U-238 to decay into Th-234.

Now alpha particles, as you’ll see, are relatively large. This means they don’t travel that quickly. The kinetic energy that an alpha particle increases with the size of the parent nucleus, but this doesn’t usually exceed ~10% of the speed of light. It might sound very quick (and it is!), but this is much slower than other forms of radiation.

As mentioned, alpha particles are large and carry a 2+ charge. This means they don’t pass through materials easily, quickly dissipating the energy to surrounding matter. Though they don’t travel far, their ionizing potential is the greatest. This means that although skin contact with alpha particles is relatively safe, ingesting large enough doses spells very bad news for your internal organs. Russian KGB defector Alexander Litvinenko was poisoned in this manner, having been given a lethal dose of polonium-210.

Beta Particles

Beta (β) radiation involves two distinct processes, in which a neutron decays into a proton (or vice versa). The process releases either an electron or a position from the nucleus, in the form of a high-energy beta particle.

A neutron turns into a proton in the first case, releasing an electron in the process. Since the proton stays in the nucleus, the outgoing electron is what we can detect. This is known as beta minus (β^–) decay, in which the atomic number (also known as the proton number) goes up by 1. In the reverse process, a proton decays into a neutron and a positron, with the atomic number going down by 1. This is known as beta plus (β⁺) decay.

However, when measurements were first made, it was found that some energy involved in the process was lost. For some time, physicists were left puzzled. Then Enrico Fermi brilliantly postulated that an almost undetectable particle of no charge and little mass was also created, to account for this missing energy¹. In his 1933 paper—famously rejected by Nature—Fermi named this particle the ‘neutrino’.

Notice that beta decay involves a change in the atomic number, which means that they result in the creation of a new element! An example of β^– decay is the decay of C-14 into N-14 (+1 atomic number) with a half-life of 5,730 years². An example of β⁺ decay is the decay of Mg-23 into Na-23 (-1 atomic number) with a half-life of 12 seconds³.

What’s pretty interesting is that the release of these particles occurs at a certain energy, which makes it easy to identify the exact element doing the emitting. Along with alpha particles, the detection of beta particles in this manner is the principle behind radiometric dating techniques.

Gamma and X-Ray Radiation

Gamma (γ) radiation takes the form of high-energy photons; photons of light are simply packets of energy! Gamma rays have the shortest wavelength in the electromagnetic spectrum, and hence the highest amount of energy. They are ionizing simply because they have the energy to ionize atoms, thereby causing chemical reactions.

Sometimes, the loss of an alpha or beta particle through radiation leaves the ‘daughter’ nucleus in an excited state. Since the nucleus likes to be as low in energy as possible, it emits this excess energy in the form of a gamma photon. In fact, many processes are able to excite the nucleus of an atom in this manner, enabling it to produce gamma radiation.

X-rays are identical to gamma rays, although they originate from a different source. Just as gamma radiation is the excess energy of an excited nucleus after losing an alpha particle, x-ray radiation is the result of excited electrons. High-speed electrons are able to ‘knock off’ electrons from a material, usually a metal.

This creates a hole where the electron once was, which renders the entire atom unstable. It goes back to its relaxed state by filling this hole using another electron (of a higher energy orbital), with the excess energy released as x-ray photons.

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Comparing Health Hazards

Now that we have a better picture of the different types of ionizing radiation, we can compare their health hazards. The dangers of each radiation can be compared on two scales, its ionizing power, and its penetration potential. What this means is that both the energy, as well as its ability to disperse through the air and other barriers, play a role in determining the health hazards of radiation.

Alpha and Beta Particles

As mentioned earlier, alpha particles have great biologic effectiveness as they have the capacity to ionize surrounding cells, driving chemical reactions. Beta particles, being electrons with a negative charge, also share this property. However, because they are charged particles, they encounter a greater interaction with matter.

This reduces their penetrative power; alpha and beta radiation are stopped by a thin sheet of paper and aluminum, respectively. Their hazards are readily apparent when alpha and beta emitters are ingested, as the particles can then interact directly with body tissue and genetic material. This disrupts cell structures, killing them, or even causing cancers to develop⁴.

Gamma Rays

Gamma rays are less ionizing than alpha and beta radiation, as they don’t possess a charge. However—being massless particles with high energy—they are able to penetrate further through matter. They can pass through the skin and damage DNA inside of cells, leading to cancers and genetic diseases.

Their long-term health effects are also widely studied, due to large populations having been exposed to high-energy photons (Japanese atomic bomb survivors and other nuclear accidents). The survivors of the 1945 atomic bombing of Japanese cities Hiroshima and Nagasaki had a 32% increased risk of dying from cancer in the years that followed⁵.

Neutron Radiation

There is also neutron radiation: unstable free neutrons from sources like nuclear reactors and particle accelerators. While neutron radiation has even lower ionizing power, the lack of a formal charge greatly increases its penetration potential. In living tissue, they can cause severe health effects and are up to 10 times more damaging than gamma radiation⁶.

Video and cover graphic: artwork of the types of ionizing radiation by Melanie (@nanoclustering)

Reference

1. Wilson, F. L. (1968). Fermi’s Theory of Beta Decay. American Journal of Physics, 36(12), 1150-1160.
2. Stuiver, M., & Polach, H. A. (1977). Discussion Reporting of 14 C Data. Radiocarbon, 19(3), 355-363.
3. Phipps, P., & Zaffarano, D. J. (1953). The Half-lives of Some Short-lived Low Z Nuclei Formed by Photonuclear Reactions.
4. Barendsen, G. W. (1962). Dose-survival curves of human cells in tissue culture irradiated with alpha-, beta-, 20-kV. x-and 200-kV. x-radiation. Nature (London), 193(4821).
5. Cardis, E., Vrijheid, M., Blettner, M., Gilbert, E., Hakama, M., Hill, C., … & Yoshimura, T. (2005). Risk of cancer after low doses of ionising radiation: retrospective cohort study in 15 countries. BMJ, 331(7508), 77.
6. International Commission on Radiological Protection, & Valentin, J. (2007). The 2007 Recommendations of the International Commission on Radiological Protection (pp. 1-333). Oxford: Elsevier.