What Is Radioactivity? Unstable Nuclei and Radiation Explained

Atoms contain a nucleus made of protons and neutrons, surrounded by electrons. Some combinations of protons and neutrons are stable, while others are unstable. An unstable nucleus can transform without being triggered from outside, releasing radiation in the process. The identity of the nucleus may change, depending on the kind of decay.

This spontaneous nuclear change is the heart of radioactivity. Temperature, pressure, and ordinary chemical reactions usually have little effect on the decay rate because radioactivity involves the nucleus rather than the outer electrons that take part in chemistry. That is one reason radioactive decay is useful as a clock in some dating methods.

Background radiation also comes from several natural and human-made sources, so measuring exposure often requires comparing a source with the radiation already present in the environment.

Alpha radiation consists of relatively heavy particles made of two protons and two neutrons. Beta decay involves changes within the nucleus that produce an electron or positron along with other particles. Gamma radiation is high-energy electromagnetic radiation emitted when a nucleus loses excess energy without changing its numbers of protons and neutrons.

The three types interact with matter differently. Alpha radiation is strongly ionising but does not penetrate far. Beta radiation generally penetrates farther than alpha. Gamma radiation is highly penetrating and often requires dense shielding to reduce exposure. When learning about radioactivity, separate penetration from ionising ability because they are related but not the same property.

Radioactivity is measured in several ways. Activity describes how many nuclear decays occur per second and is measured in becquerels, where one becquerel means one decay per second. Absorbed dose describes energy deposited in matter, while other dose measures account for biological effects. These quantities answer different questions, so a large activity value does not by itself tell you the dose a person receives. Distance, shielding, radiation type, and exposure time all matter.

Half-life gives a practical way to describe a random process:

• •Half-life is the time for the number of undecayed nuclei to fall to half its value.
• •After two half-lives, about one quarter of the original nuclei remain.
• •Decay activity also falls as fewer unstable nuclei remain.
• •Half-life is characteristic of a particular radioactive isotope.
• •Random individual events can still produce a predictable overall decay curve.

Suppose a sample contains a very large number of radioactive nuclei with a half-life of 10 days. After about 10 days, roughly half the original unstable nuclei remain. After another 10 days, roughly half of that remainder is left. What is radioactivity at the level of one nucleus cannot tell you the exact moment it will decay, but statistics make the behaviour of a large sample predictable.

Radiocarbon dating puts half-life to work. Cosmic rays striking the upper atmosphere produce a small amount of carbon-14, a radioactive form of carbon. Living plants absorb it along with ordinary carbon, and animals take it in by eating plants, so living things keep a roughly steady proportion. When an organism dies, it stops taking in new carbon, and its carbon-14 slowly decays. The half-life is about 5,730 years. If a piece of wood contains a quarter of the expected carbon-14, two half-lives have passed, so it is roughly 11,500 years old.

The method has clear limits. After around 50,000 years, too little carbon-14 remains to measure reliably. It only works on material that was once alive, such as bone, wood, shell or cloth, not on rock. The amount of carbon-14 in the atmosphere has also varied over time, so raw results are calibrated against records such as tree rings, whose ages are known by counting. For much older samples, geologists use isotopes with far longer half-lives, such as uranium decaying into lead, to date rocks millions or billions of years old.

Radioactivity was discovered almost by accident. In 1896 the French physicist Henri Becquerel was studying whether uranium salts gave off rays after being exposed to sunlight. On overcast days he left the salts in a drawer on top of wrapped photographic plates, and when he developed the plates anyway, they were strongly fogged. The uranium was emitting radiation by itself, with no sunlight needed. Marie and Pierre Curie took up the work, coined the word radioactivity, and discovered two new radioactive elements, polonium and radium. Ernest Rutherford later named alpha and beta radiation.

A common worry is that anything exposed to radiation becomes radioactive itself. For alpha, beta and gamma radiation, that is not true. Being irradiated means radiation passes into or through something, which can damage cells but does not normally make the material radioactive. Food irradiated to kill bacteria, or medical equipment sterilised with gamma rays, does not become a radiation source. Contamination is different: it means radioactive material itself, such as dust, ends up on or inside something, and it then keeps emitting radiation until it is removed or decays. Neutron radiation is an exception, because it can make some materials radioactive.

Nuclear decay can be written as an equation in which two totals must balance: the mass number, which counts protons plus neutrons, and the atomic number, which counts protons. In alpha decay, uranium-238, with 92 protons, emits an alpha particle, which has mass number 4 and atomic number 2. What remains must have mass number 234 and atomic number 90, which makes it thorium-234. In beta decay, a neutron turns into a proton and an electron is emitted. Carbon-14, with 6 protons, therefore becomes nitrogen-14, which has 7 protons and the same mass number.

Which decay a nucleus undergoes depends largely on its balance of neutrons and protons. Light stable nuclei have roughly equal numbers of each, while heavier stable ones need extra neutrons, because neutrons add the strong nuclear attraction without adding electrical repulsion between protons. A nucleus with too many neutrons for its size tends to undergo beta decay, turning a neutron into a proton. One with too few may emit a positron or capture an electron. The heaviest nuclei, beyond lead and bismuth, often shed mass by alpha decay. Plotted on a chart, stable nuclei form a narrow strip called the band of stability.

Many homes contain a radioactive source without anyone noticing it. Ionisation smoke alarms hold a tiny amount of americium-241, which emits alpha particles that ionise the air inside a small chamber so that a weak electric current can flow across it. When smoke drifts in, particles attach to the ions and the current falls, which sets off the alarm. Alpha particles suit the job because they ionise strongly yet are stopped by a few centimetres of air or the alarm's casing. Americium-241 also has a half-life of about 430 years, so the source never needs replacing during the alarm's working life.

Medicine chooses isotopes with the opposite timescale. The most widely used tracer for medical imaging is technetium-99m, which emits gamma rays and has a half-life of about six hours. The gamma rays pass out of the body to reach a camera, while the short half-life means the activity falls to a small fraction within a day or two, limiting the patient's dose. Such a short-lived isotope cannot be stored for long, so hospitals receive generators containing molybdenum-99, which has a half-life of about 66 hours and decays into technetium-99m. Staff draw off fresh technetium each day, a system sometimes nicknamed a moly cow.

Radioactivity is spontaneous change in unstable atomic nuclei, accompanied by emissions such as alpha particles, beta particles, or gamma radiation. You cannot predict exactly when one nucleus will decay, but half-life describes the regular statistical pattern of a large sample. Keep nuclear change, radiation type, and half-life separate, and the topic becomes much easier to organise.