What is radioactivity?

Right now, in the rocks under your feet, in the air you breathe and even inside your own body, a few atoms are changing. Every second, thousands of atoms inside a grown-up’s body turn into atoms of other elements. You can’t see it or feel it, and it is completely natural. This is radioactivity.

Nuclei that can’t stay the same

Every atom has a nucleus made of protons and neutrons. In most atoms, the nucleus stays the same forever. We say it is stable.

But some nuclei have an unbalanced mix of protons and neutrons, and they are unstable. Sooner or later, such a nucleus changes all by itself. As it changes, it shoots out a tiny particle or a burst of energy. What comes out is called radiation, and an atom that does this is radioactive.

Often the atom becomes a different element! When the nucleus of a carbon-14 atom changes, for example, the atom turns into nitrogen. Carbon-14 is one of the radioactive isotopes: a version of carbon with extra neutrons.

Three kinds of radiation

Radiation comes in three main kinds, named after the first three letters of the Greek alphabet:

Alpha radiation is a clump of 2 protons and 2 neutrons, shot out of the nucleus. A sheet of paper stops it, and so does the outer layer of your skin. But it is dangerous inside the body, if someone breathes in or swallows something that gives it out. That is why we watch out for radon, a radioactive gas from the ground.
Beta radiation is a fast electron, shot out of the nucleus. It goes further, but a thin sheet of aluminium stops it.
Gamma radiation is a burst of energy, like very strong light that you can’t see. It goes through most things, and only thick lead or concrete stops most of it.

You can’t see, hear, smell or feel any of them. To find radiation, scientists use special detectors. A Geiger counter, for example, clicks each time it catches a bit of radiation.

Alpha radiation is stopped by paper, beta radiation by a thin sheet of metal, and gamma radiation only by thick lead or concrete.

Half-life: waiting for half

Nobody can tell when one particular nucleus will change. It might happen in the next second, or in a million years.

But with lots of nuclei, something amazing happens. You can predict very well how long it takes for half of them to change. This time is called the half-life.

After one half-life, half of the radioactive atoms are left. After two half-lives, a quarter are left, and after three, an eighth. The amount keeps halving, again and again.

The same 16 radioactive atoms, again and again. After each half-life, half of the ones that are left have changed into another element. Nobody can tell which ones will change next.

Every radioactive isotope has its own half-life:

Isotope	Half-life
technetium-99m (used in hospitals)	about 6 hours
radon-222 (a gas from rocks)	about 4 days
tritium (a kind of hydrogen)	about 12 years
carbon-14	about 5,700 years
uranium-238	about 4.5 billion years

The half-life of uranium-238 is about as long as the Earth has existed. So about half of the uranium-238 that the young Earth had is still around today.

Try it: the coin game

You can play at radioactivity with coins. Collect about 30 coins. Each coin is a radioactive atom.

Throw all the coins onto a table.
Take away every coin that lands heads up. Those atoms have changed.
Count the coins that are left, and throw them again.

Each throw is one half-life. After every throw, about half of the coins are left. You can’t tell which coin will go next, but you can tell roughly how many will be left.

Radioactivity is all around us

Radioactivity is not only in laboratories. It is a natural part of the world:

Rocks and soil contain small amounts of radioactive uranium, thorium and potassium.
Radon is a radioactive gas that seeps out of the ground. It can collect in cellars and closed rooms, where we breathe it in, so airing them often is a good idea.
Rays from space, called cosmic rays, reach us all the time. More of them reach high mountains and aeroplanes.
Food contains a little radioactivity too. Bananas are famous for it: they are rich in potassium, and a tiny part of all potassium is radioactive potassium-40.
Your body contains potassium-40 and carbon-14 as well.

Life on Earth has always lived with this natural radiation. The amounts are small, and your body copes with them.

How does radioactivity help us?

Medicine. Doctors use radioactive isotopes to look inside the body and find illnesses. Strong, carefully aimed radiation can also destroy cancer cells.
Smoke alarms. Many smoke alarms hold a speck of radioactive americium. Its radiation turns some of the air inside into ions, so a tiny electric current can flow. Smoke disturbs the current, and the alarm goes off.
Finding out how old things are. Radioactive isotopes tell how old bones, wood and rocks are.
Electricity. Nuclear power stations split the nuclei of uranium, a radioactive element, and this gives out lots of heat. The heat boils water, and the steam drives generators. Slovakia, for example, makes more than half of its electricity this way.
Space travel. Some space probes and Mars rovers make their electricity from the heat of radioactive plutonium, far from the Sun.

Marie Curie and the mysterious rays

In 1896, the French scientist Henri Becquerel found that uranium gives off invisible rays. They could darken photographic plates, even through black paper and in total darkness.

A young scientist in Paris, Marie Curie, wanted to know more. She measured these rays with great care. Together with her husband, Pierre, she named the effect radioactivity.

In 1898 they discovered two new radioactive elements. She named the first one polonium, after Poland, the country where she was born. The second was radium, which glows faintly in the dark.

Marie Curie was the first woman ever to win a Nobel Prize, and later she won a second one. She is still the only person with Nobel Prizes in two different sciences: physics and chemistry.

Back then, nobody knew how dangerous strong radiation can be. Marie Curie carried tubes of radioactive material in her pockets and kept them in her desk drawer. She died in 1934 of an illness that radiation most likely caused. Her notebooks are still so radioactive that they are kept in boxes lined with lead.

Staying safe

The small amounts of natural radiation all around us are part of everyday life. But strong radiation can damage the tiny cells your body is built from. That is why places and containers with strong radioactive material carry a warning sign.

The radiation warning sign: three dark blades around a dot, on yellow. It can be on a triangle, a square or a round label.

Go deeper: The maths of half-life for grown-ups and the extra curious

If a sample starts with $N_0$ radioactive nuclei, the number left after a time $t$ is

N = N_0 \cdot \left(\tfrac{1}{2}\right)^{t/T}

$N$ is the number of nuclei still left, and $N_0$ is the number at the start.
$t$ is the time that has passed, and $T$ is the half-life, in the same unit of time.

Example: the radium on Marie Curie’s notebooks has a half-life of about 1,600 years. After 100 years, $\left(\tfrac{1}{2}\right)^{100/1600} \approx 0.96$ , so about 96% of it is still there.

The number of decays each second is called the activity, measured in becquerels (Bq): one becquerel is one decay per second. The activity is proportional to the number of nuclei: $A = \lambda N$ , where the decay constant is $\lambda = \frac{\ln 2}{T}$ . No nucleus “ages”: an unstable nucleus that has already waited a million years is no more likely to decay in the next second than a brand-new one. That is also where the wobbly tower and the coins fall short: a nucleus needs no push and no throw.

Go deeper: Kinds of decay, precisely for grown-ups and the extra curious

Alpha decay: the nucleus throws out a helium-4 nucleus. Its atomic number drops by 2 and its mass number by 4. Uranium-238 becomes thorium-234 this way.
Beta-minus decay: a neutron turns into a proton, and the nucleus throws out a newly made electron and an antineutrino. The atomic number rises by 1: carbon-14 becomes nitrogen-14.
Beta-plus decay and electron capture: a proton turns into a neutron. The nucleus either throws out a positron, the electron’s antiparticle, or swallows one of the atom’s own electrons. The atomic number drops by 1. PET scans in hospitals use a beta-plus emitter, fluorine-18.
Gamma decay: a nucleus with extra energy calms down by giving out a gamma ray, a photon of very high energy. It stays the same element.

Nuclear changes involve huge energies: often millions of electronvolts per nucleus, while a chemical reaction involves only a few electronvolts per atom. Splitting one uranium-235 nucleus in a reactor releases about 200 million electronvolts. That is why a little nuclear fuel gives so much heat.

Go deeper: Doses: how much is too much? for grown-ups and the extra curious

The effect of radiation on the body is measured in sieverts (Sv). One sievert is a lot, so everyday doses are given in millisieverts (mSv, thousandths) or microsieverts (µSv, millionths).

Natural radiation gives people an average of about 2.4 mSv a year, usually between 1 and 10 mSv, depending on where they live. About half of that average comes from radon, whose level varies hugely from home to home.
Eating a banana gives roughly 0.1 µSv. Even that overstates it: the body keeps its potassium level steady, so extra potassium from food soon leaves again.
A CT scan of the chest gives several millisieverts. Doctors order scans when the benefit is worth it.
Radiation sickness comes only from a very large dose to the whole body, received in a short time, usually within minutes. It takes more than about 700 mSv, although mild signs can appear from about 300 mSv.

Go deeper: Why a second warning sign was added for grown-ups and the extra curious

The three-bladed sign was first sketched in 1946 at the University of California’s Radiation Laboratory in Berkeley. It became the international standard ISO 361, printed in black or magenta on yellow. But studies found that few people without training knew what it meant.

So in 2007, the IAEA and ISO added a second sign for the most dangerous sources: a red triangle with radiation waves, a skull and a running person. It sits on the part that holds the source inside a machine, where someone taking the machine apart would see it. One reason was accidents like the one in Goiânia, Brazil, in 1987. Scrap collectors took a metal capsule from an abandoned hospital machine, and it was later opened. The glowing powder inside was radioactive caesium-137. Four people died, and 249 were contaminated.

Check yourself

A sample of radioactive atoms has a half-life of 1 day. What fraction is left after 2 days?

Show the answer

A quarter — After 1 day, half is left. After another day, half of that half is left: a quarter.