What Are Alpha, Beta & Gamma Particles?

Alpha, beta, gamma rays: It almost sounds like the tagline of an old-school motion picture about aliens from outer space, newly arrived on Earth with their ultra-high-tech gadgets (and hopefully a warm disposition). In reality, this isn't too far off. Alpha, beta and gamma radiation are all real entities in the physics world and are worth avoiding when you can manage it.

You probably know that different kinds of atoms can join together via the process of chemical bonding to create molecules. For example, two hydrogen atoms (H on the periodic table of elements) and one oxygen atom (O) can combine to form a water molecule (H₂O). This molecule can be broken into the ions H+ and OH– by breaking one of the O-H bonds.

In chemical bonds, electrons of different atoms interact, but their nuclei (the plural of nucleus) remain intact. This is because the force holding the protons and neutrons together is extremely strong compared to the electrostatic forces underlying chemical bonding between atoms.

Nevertheless, atomic nuclei do decay, usually spontaneously and often at an incredibly low rate, depending on what the element is. This radioactivity comes in the three basic flavors introduced in the first sentence of this article: Alpha, beta and gamma radiation, also called alpha, beta and gamma particles (except, technically, in the last instance).

The atom was once described somewhat impetuously as "the smallest indivisible thing" even by people in the know. This definition is true in some ways: Take any single element, or substance made of a single irreducible component, and the atom is the smallest whole unit of that substance. There are 118 elements on the periodic table as of 2020, 92 of them naturally occurring.

Atoms consist of a nucleus, which has one or more protons and, except hydrogen (the smallest element), at least one neutron. They also have one or more electrons, found at some distance from the nucleus in specific energy levels.

Protons are positively charged and electrons negatively charged, with the magnitude of the charge the same in each. Since an atom in the ground state has the same number of protons as it does electrons, atoms are electrically neutral unless ionized (i.e., their electron number changes).

An atom's proton number is its atomic number on the periodic table and determines the identity (name) of the element. Some atoms can gain or lose neutrons while continuing to happily exist, but if a nucleus loses or gains a proton instead, it's a game-changer, because now whatever the element was has a brand new name and new attributes to go with it.

The force that holds protons and neutrons together is, not for nothing, called the strong nuclear force. The nuclei of atoms can be regarded, in a sense, of sitting at the center of all matter, so their extreme stability makes sense in a cosmos rife in organization and capable of sustaining life on at least one humble planet.

But nuclei are not perfectly stable, and over time, they decay, emitting particles and energy. Each element that undergoes radioactive decay, or more specifically the isotope of the element being studied, has its own characteristic half-life, which can be used to predict how many nuclei will decay over time while offering no information about any one nucleus. It is thus similar to a risk, essentially a probability statistic.

The half-life of a radioactive species is the time it takes for half of the unstable nuclei in a sample to decay into a different form. This number can go very high, into the billions of years, although for carbon-14 it is about 5,730 years (a blip in geological time, if not in human civilizations).

The various kinds of radioactive decay are given the first three letters of the Greek alphabet. Thus alpha radiation emits a particle often represented by a lowercase version of this letter, α. It would be unconventional, however, to write "α-radiation."

This kind of particle is tantamount to the nucleus of a helium (He) atoms. Helium is the second element on the periodic table, and with an atomic mass of 4.00, it has two protons and two neutrons. The entire atom also has two electrons that balance out the charge of the two protons, but these are not part of an alpha particle, only the nucleus.

These particles are massive with respect to other kinds of radiation; the beta particle, for example, is some 7,000 times smaller. This on the surface might make it seem to be especially dangerous, but in fact the opposite is true: The size of the α-particles means that they penetrate things, including biological barriers such as skin, very poorly.

Beta particles (β-particles) are actually just electrons, but they retain their name because their discovery predates the formal identification of electrons as such. When an atom emits a beta particle, it also emits another subatomic particle at the same time called an electron antineutrino. This particle shares in the momentum and the energy of the particle emission, but it has almost no mass (even compared to an electron, itself only about 9.1 × 10^–31 kg in mass).

Beta particles, being a lot smaller than alpha particles, can penetrate deeper than can their far more massive counterparts.

Another type of beta particle is the positron, which occurs as a result of the decay of neutrons in the nucleus. These particles have the same mass as electrons, but have the opposite charge (hence their name).

Gamma rays, or γ-rays, represent the most dangerous result of radioactivity to humans. They are massless because they are not particles at all. "Rays" is actually short for the general term electromagnetic radiation (EM radiation), which travels at the speed of light (denoted c, or 3 × 10⁸ m/s) and comes in a variety of combinations of frequency and wavelength values whose products are c.

Gamma rays have very short wavelengths and hence very high energy. They are similar to X-rays, except that X-rays originate outside the nucleus. They typically pass through human bodies without touching anything, but because they are so penetrating, a lead shield two inches thick is required to ensure their stoppage.

Alpha particles can be safely ignored, to the extent that this is true of anything classified as radiation. They can travel only about 4 to 7 inches (10 to 17 cm) in air, and their energy is lost when striking the protons and neutrons of whatever material they encounter, preventing them from penetrating further.

Most of the damage from beta particles comes from ingesting, or swallowing, them. (This can be true of alpha particles, too.) Drinking or eating radioactive material is the major source of damage from this kind of radiation, though prolonged exposure to the skin can produce burns.

Gamma rays can pass through bodies without striking anything, but there is no assurance that they actually will do so, and they can travel about a mile in air. Because they can penetrate practically anything in addition to traveling long distances, they can damage all body systems and their presence in environments with living systems must be carefully monitored.