Alpha, Beta, and Gamma Particles

Core Concepts

In this article, we explore the properties of radioactive alpha particles, beta particles, and gamma rays, as well as how to stop them.

Topics Covered in Other Articles

• Rutherford’s Gold Foil Experiment
• Rutherford’s Atomic Model
• Bohr’s Atomic Model
• How to find protons, neutrons, and electrons in an atom
• What is an Isotope?

What is “Radiation”?

Nuclear scientists use the term “radiation” to refer to the emissions given off by radioactive substances. Substances are known for being radioactive if they frequently participate in nuclear decay reactions such as alpha, beta, and gamma decay. If you’d like to learn more about the specifics of radioactive decay reactions, check out this article.

Each of these decay reactions involves the nucleus of an atom emitting or radiating a radioactive particle. Nuclear scientists name these particles based on the decay reaction that produces them. For instance, alpha decay produces alpha particles, beta decay produces beta particles, and so on.

As you likely have heard, radiation is considered to be dangerous when exposed to human bodies. Nuclear scientists classify alpha, beta, and gamma radiation by two metrics to characterize their biological harm: ionization potential and penetration depth.

Ionization Potential of Radioactive Particles

When a decaying nucleus is close to a human body, radioactive particles affect exposed biological molecules. Specifically, alpha, beta, and gamma particles have the potential to extract or displace a molecule’s electrons. This “ionizes” the molecule, which makes it more unstable, often causing it to break down. Radiation delivers the most harm to DNA, which disrupts and mutates genes, which tends to lead to cancer and birth defects in later generations. 

Different radioactive particles interact with other molecules differently. As charged particles, alpha and beta particles ionize other molecules through remarkably strong Coulombic forces. Gamma particles, meanwhile, interact through the photoelectric effect, which can still ionize but have a much lower potential.

Penetration Depth of Radioactive Particles

However, if some layer of protection separates the human and the radioactive substance, not all particles will reach the human. Velocity serves as the most important factor for determining the penetration depth of a radioactive particle. When a particle collides with an absorbent, its ability to penetrate the material depends on the magnitude of its impulse. Importantly, lower velocities correspond to greater impulses.

As we know from kinetic molecular theory, a particle’s velocity depends on its mass and kinetic energy:

As we discuss in later sections, mass varies considerably between alpha, beta, and gamma particles. Kinetic energies, meanwhile, remain much closer between alpha (5-5.5MeV), beta (0.5-2.27MeV), and gamma (0.04-8MeV) particles. Thus, mass serves as the most important factor in determining the difference in velocity between radioactive particles. Specifically, high velocities correspond to low masses, resulting in smaller impulses and higher penetrations.

The presence of Coulombic forces additionally decreases penetration depths.

What are Alpha Radioactive Particles?

When an atom undergoes alpha decay, its nucleus emits a cluster of two protons and two neutrons. 

This cluster, the alpha particle (ɑ), sometimes goes by the name “twice ionized helium” (He²⁺), since it’s technically the nucleus of a helium atom. 

Alpha particles weigh 4 atomic mass units, making them the heaviest of the common decay particles. Thus, they have the lowest velocities and largest impulses relative to beta and gamma particles. Further, their +2 charge gives alpha particles the strongest Coulombic force. Due to the high impulse and Coulombic force, alpha particles have the highest ionization potential and lowest penetration depth.

To stop alpha radiation, all you need is a few micrometers of material as protection. Indeed, paper is more than enough to block virtually all alpha particles. Even when alpha radiation contacts human skin, only about 40 micrometers of skin cells experience alpha particles. In many cases, alpha particles don’t even get past the dead cells on the outer layer of skin.

However, if you breathe or eat an alpha particle-emitting substance, nothing protects your cells from the alpha particles. For this reason, nuclear scientists consider the ingestion or inhalation of alpha radiation much more serious than simple exposure.

What are Beta Radioactive Particles?

When an atom undergoes beta decay, it emits a high-energy electron, the beta particle (β). 

The beta particle, consequently, has a much smaller mass of 5.48 x 10^-4 atomic mass units. Further, the -1 gives beta particles Coulombic force, but with a lower magnitude of charge than alpha particles. Thus, beta particles have greater penetration depth but lower ionization potential than alpha particles.

To stop beta radiation, only a few millimeters of aluminum are required. While minimal protective layers protect humans from ionization, like alpha particles, the real danger of beta particles comes from ingestion or inhalation. 

Interestingly, beta particles produce an eerie blue glow in water called Cherenkov radiation. This only occurs with large quantities of beta particle-producing substances. Thus, Cherenkov radiation tends to be observed in nuclear reactors, where radioactive nuclear fuel is submerged in a pool of water.

What are Gamma Radioactive Particles?

When an atom undergoes gamma decay, it emits a high-energy photon, the gamma particle or gamma ray (γ). 

Physicists generally consider photons to have negligible mass or to be completely massless. Additionally, as mentioned before, the lack of charge of gamma particles means they have no significant Coulombic force. Thus, gamma rays have the highest penetration depth but the lowest ionization potential.

Importantly, the kinetic energy, and thus the magnitude of impulses, can vary considerably between different sources of gamma rays. For instance, nickel-60 releases gamma rays at 1.33 MeV, while antimony-125 emits gammas at 1/50th the energy at 35.3 keV.

Because they technically classify as light rather than a stream of massed particles, a certain percentage of gamma rays will always penetrate a given protective layer. Thus, nuclear scientists quantify the penetration of gammas by HVL, or “half value layer”, which is the thickness of a given substance required to stop half of all gamma radiation.

For energies of roughly 1.0 MeV, gamma rays have an HVL of 1cm of lead. Other HVLs include 2.1cm of steel and 6.0cm of concrete. Due to the significant penetration of gamma rays, external exposure presents the biggest health risk to humans.

Summary Table of Radioactive Particles

The following table summarizes the properties of alpha, beta, and gamma radioactive particles.

Radioactive Particles Practice Questions

Question 1

If you have a sample of radioactive material encased in a few inches of metal, which radiative particle would pose the most danger?

Question 2

Between the two physical characteristics of mass and charge, which characteristic determines the particle’s ionization potential and which determines it’s penetration depth?

Radioactive Particles Practice Question Solutions

1: Gamma radiation

2: Mass determines penetration depth, charge determines ionization potential.