What Is a Half-Life and How Is It Used to Measure Radioactivity?

what is a half-life

What Is a Half-Life and How Is It Used to Measure Radioactivity?

Key Points

  • Half-life is the amount of time it takes for an object to reduce its quantity by fifty percent.
  • Radioactivity is the release of energy from the decay of radioactive nuclei.
  • There are four primary types of radiation: alpha, beta, gamma, and X-ray.
  • Exposure to radiation can be harmful to our health, and different devices have varying half-life values.

Half-Life and Radioactivity: An Exact Definition

Half-life is a value that represents the amount of time it will take for an object to reduce its quantity by fifty percent. The nuclei of specific types of isotopes and atoms release their energy through radiation.

The radiation emission may last for a few seconds, a few million years, or somewhere in between. Half-life defines the duration of time it takes (the rate of decay) for fifty percent (half) of the radioactive atoms to decay.

What Is Radioactivity: A Complete Explanation

To understand radioactivity and half-life, we need to have a rudimentary understanding of the structure of atoms. Rest assured, in our prior life, we worked with protons and electrons daily. We’ve gotcha covered! Ready? Let’s go!


An atom contains three microscopic subatomic particles: electrons, neutrons, and protons. The nucleus is the center of the atom. It contains protons and neutrons.

Electrons fly in a circle around the nucleus and have a negative electrical charge. The protons carry a positive electrical charge. The neutrons have no electrical charge. In a neutral atom, the number of protons and the number of electrons are equal.

what is a half-life
An atom is the smallest part of a substance that cannot be broken down chemically.


Negative electrons attract positive protons (just like a magnet). The opposite forces between the electron and the proton hold the atom together. If the number of electrons is the same as the number of protons, we have a stable nucleus.

Unstable Nucleus

If the protons and neutrons in the nucleus aren’t in balance, the atom is considered an “unstable nucleus.” And an unstable nucleus occurs when the neutron count exceeds the proton count. The instability in the nucleus causes a neutron to convert into a proton.

This conversion releases a beta particle. The unstable nucleus will continue to shed or decay as it releases photons, electrons, neutrons, or alphas. An alpha is two neutrons bound to two protons.


The unstable nucleus releases energy as the nuclei decay. Radioactivity is the released energy from the nucleus.

In short, radioactivity is the release of energy from the decay of radioactive nuclei. The decay only relates to certain kinds of atoms and isotopes, though.

The nucleus seeks a state of balance, so it sheds whatever it needs to in order to bring about balance. Let’s take a look at a few different types of energy releases:

  1. If the nucleus has too many neutrons, it will emit a negative beta particle, which changes one neutron into a proton.
  2. If there are too many protons in the nucleus, it will create a positively charged electron (positron), which changes one proton into a neutron.
  3. Too much energy causes the nucleus to generate a gamma ray, which discharges a lot of energy without modifying any particles in the atom’s nucleus.
  4. If there’s too much mass in the atom, the nuclei emit an alpha particle, which ejects four particles in total: two neutrons and two protons.

Different Types of Radiation

There are four primary types of radiation that we’ll touch on. Some types of radiation aren’t dangerous, while others are deadly.


An alpha particle (α) has a positive charge. It’s created from two neutrons and two protons from the nucleus of an atom. Heavy elements, like radium and uranium, create alpha particles that have radiation.

Alpha particles are heavy and don’t travel very far. They also lack the energy to pierce through the outer layer of our skin.

If an alpha particle is inhaled, swallowed, or passed into our bodies through a cut, it will cause significant damage. You’ll find alpha particles in smoke detectors, industrial mechanical static electricity removal, and probes used in telescopes in space.


A beta (β) particle has a negative charge and is generated from the nucleus during radioactive decay. Beta particles are generated when a neutron in the nucleus converts into a proton and electron. The electon is ejected as a beta emission.

what is a half-life
The four major types of radiation differ in mass, energy, and how deeply they penetrate people and objects.


Hydrogen-3, strontium-90, and carbon-14 generate beta particles. A beta particle penetrates further than an alpha particle.

A T-shirt, jacket, or pants will stop a beta particle (or aluminum, if you want to wear a tin foil hat). Similarly to alpha particles, beta particles are most dangerous if inhaled or ingested.

Medical applications such as PET scans, eye cancer, and bone cancer treatment use beta particles. Beta particles are used to calculate material thickness.

A stream of beta particles is shot toward a thin film (like a piece of paper). A measurement of the amount of particles that reach a collection unit on the far side of the paper is used to calculate the thickness of the paper.


We’re getting serious now! Gamma radiation is pure photon energy. And you’ll find gamma rays together with beta or alpha particles during the radioactive decay process.

Gamma rays are dangerous to humans. They will pass through clothing, skin, and your tinfoil hat. A gammy ray is capable of passing through walls, dirt, underground bunkers, and lead. It will also cause severe tissue and DNA damage.

If you take a quick look at the electromagnetic spectrum (EMS), you’ll see that gamma rays are all the way on the far right side of the EMS. Gamma rays have the smallest wavelength and pack the most significant penetrating power.

Supernova explosions, black holes, neutron stars, pulsars, lightning, nuclear explosions, and radioactive decay emit gamma rays.


We’ve all heard of X-rays! Like a gamma ray, an X-ray is a photon of pure energy. An X-ray, generated outside the nucleus, has lower energy than gamma rays. CAT scans, images of bones, soft tissues, and dental work all use X-rays.

Why Do We Care About Radiation?

Exposure to radiation can be harmful to our health. Have you noticed how careful the dental hygienists are when shooting X-rays at the doctor’s office? They’ll leave the room when it’s time to “snap the picture” of your X-ray. It’s all about radiation!

Below is a chart of typical and less commonly used devices that emit radiation. As we progress further down the chart, we’ll note that the half-life increases quite a bit.

Dental X-ray4 nanoseconds
PET scan110 minutes
Nuclear Fusion: U-2384.5 billion years
Nuclear Fusion: Pu-24480 million years
Nuclear Weapon: U-235704 million years
Nuclear Weapon: Pu-23924,100 years

How Does Half-Life Measure Radioactivity?

As mentioned above, half-life is the amount of time it will take for fifty percent (half) of the radioactive elements of an atom to decay. As an example, take seven half-lives of radioactive material and you’ll end up with less than one percent of the original radioactive material. (It works, we checked the math!)

Radioactive isotopes break down through gamma, beta, and gamma decay. Decay occurs in the parent source over time. Once the parent has released the child, the child is no longer radioactive, nor will the child produce further radioactive material.

Scientists can’t predict the quantum behavior of a single atom, but they can predict the statistical behavior of millions and billions of atoms. The half-life values of radioactive materials used in medicine allow physicians to determine the proper safe dosage levels in the treatment or exploration of the heart, liver, thyroid, etc. organs.

Commercial applications use half-life calculations every day. Radioactivity and half-life values are of paramount concern in industrial applications such as in thickness, moisture, density, specific gravity, and density measurements.

Frequently Asked Questions

What is half-life, and how is it used?

Half-life measures how long it takes for half of an atomic nuclei radioactive particle to decay or lose fifty percent (half) of the particle’s original radioactivity.

Half-life is also a term used in medicine, most often in conjunction with how long it will take for a prescription medicine to reduce by fifty percent in our bodies.

The least important but the most fun application of Half-life is the first-person shooter computer or gaming console with Gordon Freeman. Long live Freeman!

What is a half-life in simple terms?

Half-life defines the time it takes for a substance to reduce its original value by half. The term Half-life is common when discussing radioactive decay and medicine.

Why is radioactivity measured in half-life?

The half-life of radioactivity allows us to predict the amount of radioactive material remaining after a specified period.

Is the half-life value the same for all radioactive materials?

Not at all. Some radioactive materials may have a half-life of a few thousand a second, a few years, or billions of years.

Why is radioactivity measured?

It’s all about safety. Radiation can be harmful or deadly. Radiation exposure may be short-term or long-term. Both short-term and long-term radiation exposure may be very hazardous to your health.

Do you need to use Personal Protective Equipment (PPE)? Can you be near the radiation source? Should you have five inches of lead between you and the radiation source?

Scientists measure radiation in different ways. Sometimes, scientists estimate the dose a person received or could receive from a radioactive source.

In other situations, scientists measure the amount of radioactivity in water, soil, or air. The water, soil, or air measurements are taken to determine if safety actions are needed.

Why do we measure half-life and not full-life?

Built into the term “half-life” is an understanding that we’re looking at a large group of data. We’re not evaluating how long it takes for decay to occur in one atom, we’re looking at how long it takes for the decay to occur in much larger samples of atoms. The decay timeline of one atom is unpredictable.

The decay pattern of one atom compared to another atom may be wildly different. But statistically, the decay pattern of a large group of atoms is the same. Half-life allows us to look at the statistical values of all of the atoms, not at the decay rate of one atom.

Think of a 3-sig probability chart. Most decay may occur near the center of the chart (1-sigma), but some may appear at the outskirts of the diagram (3-sigma).

It’s the same for half-life calculations. We need to look at all the data, not only the full life decay (or lack of decay!) of a single atom’s nuclei.

How and why is carbon-14 used in half-life measurements?

Carbon-14 is radioactive and slowly decays (half-life!) over time. Carbon-14 is absorbed into biological organisms (bones, plant fibers, wood) throughout their “lives.” When the organism dies, the carbon-14 absorption ceases.

Carbon-14 has a half-life of roughly 5,730 years. Scientists can measure the amount of carbon-14 in the samples they dug up and then calculate the object’s age. This is performed by calculations; you’re ingesting or absorbing both carbon-12 and carbon-14 while you’re walking the earth.

When you die, the carbon-14 in your body begins to decay, but the carbon-12 in your body doesn’t. Archeologists calculate the ratio of carbon-12 to carbon-14 and determine how long ago you kicked the bucket. Pretty cool!

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