Radioactive Decay: Kinds and Properties


By Robert Hazen, Ph.D.George Mason University

Research into radioactive elements for minerals led to the important discovery of three distinct kinds of radioactive decay. These types are called alpha, beta, and gamma—in the order of their discovery. All three kinds of radiation have penetrating power, they have energy, and they can transfer energy, for example, to biological systems.

Illustration of radioactive decay
Radioactive decay is categorized into three main kinds. (Image: gstraub/Shutterstock)

Alpha Decay

In alpha decay, an atom spontaneously releases a fast-moving particle, and that particle is composed of two protons and two neutrons.

An image of Ernest Rutherford.
Ernest Rutherford won the Nobel prize for discovering alpha radiation. (Image: Unknown/Public domain)

Think about a particle that’s composed of two protons and two neutrons: it looks just like a nucleus. In fact, it is the nucleus of a helium-4 atom; two protons are helium, plus two neutrons, which gives helium-4. Basically, you’re emitting a helium nucleus, which then quickly becomes a helium atom and floats around the atmosphere as a gas.

Ernest Rutherford discovered this alpha radiation by sealing a small amount of radioactive material in a tube. When he came back several months later, he found that the tube had been enriched in helium, which wasn’t there before. Rutherford won the Nobel Prize for this work. Interestingly, the alpha particles were his atomic bullets in the experiments where he discovered the nucleus.

After an alpha decay, an atom has two fewer protons and two fewer neutrons; that particle flies out of the nucleus and it must transform to a different element. You lose two protons, so you have to drop back two places in the periodic table. For example, radium-226 decays to radium-222. So you see an element, radium, transforming, dropping back two places, and going all the way across the periodic table to radon.

Alpha particles carry a lot of energy, and this energy is converted to heat as the particles collide with their surroundings. Indeed, much of the earth’s geothermal heat is provided by alpha radiation being released from radioactive isotopes deep in the earth. Alpha particles also carry enough energy to damage biological materials, and so they’re quite dangerous.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Beta Radiation and the Discovery of Neutrino

The second kind of radiation is beta radiation, and beta radiation produces high-energy electrons, which are called beta rays. This is a much lighter kind of radioactive particle.

In this process, a neutron spontaneously changes to a proton, plus this fast-moving electron that speeds away from the atom.  

When this decay process was first studied, researchers found that the energy of the decay products didn’t quite add up, and this led to another important discovery. In other words, the total mass of the proton and the electron afterwards, compared to the neutron and the energy released, just wasn’t quite enough to make up for Einstein’s equation that energy equals mass times the speed of light squared [E=mc2]. There had to be another source of energy; there had to be another particle, and this was how the neutrino was discovered.  The neutrino is an important kind of particle which is emitted mainly by the Sun.

Learn more about the properties of light-matter interactions.

Gamma Radiation: Highest Energy of All

Some isotopes undergo gamma decay; they spontaneously emit high-energy electromagnetic radiation. In electromagnetic spectrum, there are all different wavelengths of electromagnetic waves, and the highest energy waves are called gamma radiation. That’s because nuclear processes are so energetic. 

In this particular case, electromagnetic radiation is emitted as a result of an excited state of a proton. In the Bohr atom, there’s a central, massive, positive nucleus, a nucleus that contains positive protons, and orbiting around that nucleus are electrons. The electron is in a ground state. If you excite that electron to an excited state, and it drops back down, you emit a photon—typically a visible light, which is why many materials in our everyday world are colored. 

The energies of the nucleus are much higher than when a proton is in its ground state, moves into an excited state, and then drops back down to the ground state; the energy of electromagnetic radiation that’s released is much higher, and that’s the gamma radiation. 

The gamma radiation, since it’s just the emission of a photon, doesn’t change the isotope at all. The number of protons is the same, the number of neutrons is the same, but still, gamma radiation is high-intensity, very potentially damaging radiation.

A person in white hazard suit and gas mask holding a radioactive sample in his left hand
Radioactive decay can inflict damage to the body if the energy is absorbed. (Image: Comaniciu Dan/Shutterstock)

Learn more about the periodic table of the elements.

Kinds of Radioactivity

To summarize, there are three kinds of radioactivity. There is alpha radiation; the emission of an alpha particle, two protons and two neutrons, and the element drops down two places in the periodic table. Then there is beta radiation; a transfer of a neutron into a proton plus an electron, so you bounce up one place in the periodic table. And then, finally, gamma radiation, the emission of a photon. 

The alpha particle is a big, massive particle; it’s like slugging something with a big fist, and it doesn’t penetrate very far as a result. The beta radiation electron can penetrate father can go deeper into your tissues before it stops. And the gamma-ray often passes right through your body because it’s so small and energetic.

However, all three can do damage if the energy is absorbed by your body.

Common Questions about Radioactive Decay

Q: How was the neutrino discovered?

While researching beta decay, scientists noticed that the numbers didn’t add up according to Einstein’s equation. Hence, they concluded another particle had to exist that they didn’t know about until then. That was the neutrino.

Q: What kind of radioactive decay has the most energy?

Gamma radiation has the highest energy out of all kinds of radioactive decay. In the electromagnetic spectrum, gamma radiation has the highest energy waves.

Q: How dangerous are the different kinds of radioactive decay?

All three kinds of radioactive decay can damage the body if their energy is absorbed.

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