Radioactivity
Atoms are most stable when they have similar numbers of protons and neutrons. An imbalance of these subatomic particles causes atoms to become unstable and undergo radioactive decay. There’s different types of radioactive decay - alpha, beta and gamma - which you’ll learn about on this page.
Atomic structure and isotopes
Atoms consist of a tiny central nucleus surrounded by negatively charged electrons whizzing around the nucleus in electron orbitals. The nucleus is made up of two types of particles: positive protons and neutral neutrons. The atomic number of an atom tells you the number of protons in the nucleus and the mass number tells you the total number of protons and neutrons.
Isotopes are atoms of an element with the same number of protons but a different number of neutrons in their nucleus. This means they will have the same atomic number but different mass numbers. For example, carbon exists in a few different isotopes which all have six protons in their nuclei but different numbers of neutrons. Carbon-12 has six neutrons, carbon-13 has seven and carbon-14 has eight. We show the mass numbers and atomic numbers of isotopes by writing each number beside the chemical symbol of the isotope, with the mass number written on top and the atomic number below.
A unbalanced number of protons and neutrons makes atoms unstable and prone to radioactive decay, which means it breaks down and emits radiation.
Alpha, beta and gamma radiation
Unstable nuclei will randomly emit radioactive particles to become more stable. The emission of radiation causes ionisation by hitting other atoms and knocking electrons off them (turning the atoms into ions). This is why radiation is dangerous - it knocks electrons from atoms in human cells, resulting in damage to living tissue.
Alpha particles are made up of two protons and two neutrons so they are identical to the nucleus of a helium atom. Compared to other types of ionising radiation, alpha particles are fairly big which means that they are more likely to bump into other particles and knock electrons from them - in other words they are highly ionising. Their large mass also means that they can’t travel very far which makes them weakly penetrating. Alpha particles are blocked by paper, skin or a few centimetres of air. The two protons gives alpha particles a charge of +2, which means that they can be deflected by an electric field as the negative electrons will attract the positive alpha particles. The emission of an alpha particle by a radioactive nucleus will reduce the mass number by 4 and the atomic number by 2.
A beta particle is an electron which is emitted from an atom when a neutron turns into a proton. Beta particles are lighter than alpha particles which means they are able to travel further which makes them moderately penetrating. Beta particles are able to travel through paper but will be blocked by a thin metal such as aluminium. We also describe them as moderately ionising since they are smaller than alpha particles so don’t manage to collide into quite as many other atoms. Beta particles have a charge of -1 so they will by deflected away from an electric field due to repulsion between the beta particle and the electrons. The emission of a beta particle by a radioactive nucleus will result in no change to the mass number but increases the atomic number by 1.
Unlike alpha and beta radiation, gamma radiation does not cause the emission of any particles, just energy in the form of an electromagnetic wave. It occurs after a substance has undergone alpha or beta decay - the emission of an alpha or beta particle leaves the nucleus with excess energy which the atom gets rid of by emitting a gamma ray. Gamma radiation can only be blocked by a thick piece of lead or concrete, making it highly penetrating. Gamma rays tend to pass through substances without knocking off any electrons therefore they are weakly ionising. Since gamma radiation has no charge, it will not be deflected by an electric field and since no particles are emitted, there is no change to the mass number or atomic number.
Detecting ionising radiation
Ionising radiation can be detected using a Geiger-Muller tube or photographic film.
Geiger-Muller tubes can measure the activity of a radioactive source by converting the number of radioactive particles into a count rate. Remember to also measure the count rate in the absence of the radioactive source to account for background radiation. Measure the background radiation three times and find the mean. Subtract this from all of the data recorded for the radioactive sources. Make sure you follow the necessary safety precautions, including handling the radioactive source with tongs and storing the source in a lead box when you’re finished with the experiment.
Photographic film works by turning darker the longer it is exposed to radiation. Individuals who work with radiation will wear badges containing photographic film so they know whether they are within the safe limits of radiation exposure.
Background radiation
Weak radiation that can be detected from external sources is called background radiation. The sources of background radiation can be natural or artificial and include:
Cosmic rays
Radioactive rocks
Plants which have absorbed radioactive minerals from the soil
Fallout from nuclear weapons testing
Medical sources (such as X-rays and MRI scanners)
Nuclear power plants
Activity and half life
The ‘activity’ of a radioactive source is the number of decays per unit time and is measured in becquerels. For instance, a highly radioactive sample of polonium will emit more ionising radiation in an hour compared to a stable sample of carbon-12 - this means that the polonium has a higher activity. The activity of a radioactive source decreases over time because as more of the sample is emitted, there are fewer and fewer radioactive particles remaining.
The half-life of a radioactive isotope is the time taken for half of the nucleus to decay or the time taken for the activity to halve. Half-life is different for different radioactive isotopes, for instance a sample of cobalt-60 has a half life of around five years whereas uranium-238 takes around 4.5 billion years for half of its nucleus to disappear.
We can determine the half-life of a sample using a graph showing how the activity decreases over time. We can draw a line on the vertical axis where the activity of the sample is halved and read off the time from the horizontal axis. If you look at the graph on the right, you can see that it takes one day for the activity of the sample to halve from 60 to 30 becquerels.
Uses of radioactivity
Smoke detectors: alpha emitters with a long half-life are used in smoke detectors. Alpha particles are emitted from the source and travel across a small gap to reach a detector, which produces an electric current when it detects the alpha particles. When smoke enters the fire alarm, it absorbs the alpha particles which results in a drop in the current. A drop in the current triggers activation of the fire alarm.
Thickness monitoring: beta emitters are used to measure the thickness of metal sheets. A source and receiver are placed either side of a metal sheet during its production. The number of beta particles detected by the receiver is proportional to the thickness of the sheet. If there is an increase or decrease in the number of beta particles detected then the thickness of the metal sheet has changed and the rollers will adjust their distance.
Diagnosis: gamma emitters such as technetium-99m are used as tracers in medicine as they concentrate in certain parts of the body. Doctors can detect the radiation as it travels through the body and check whether the organs are functioning properly. The half-life of the radioactive source needs to be long enough for the diagnostic procedure to be performed but not so long that the patient is exposed to too much radiation.
Contamination and Irradiation
If a radioactive substance physically touches an object, that object will be contaminated as it will contain radioactive material on its surface. For example, when the ex-Russian spy Alexander Litvinenko drank poisoned tea in 2006, he was unaware that it had been contaminated with the radioactive substance polonium-210.
If a radioactive source is placed near an object and exposed to its radiation without coming into contact with it, the object is irradiated. For example, food can be sterilised by placing near a source of gamma radiation. This kills any microorganisms on the food and makes it safe to eat. The food is not itself radioactive, since it hasn’t physically come into contact with the radioactive source.
Dangers of ionising radiation
When alpha, beta or gamma radiation collides with human cells, it can ionise the molecules inside which causes damage to the cells and tissues. Ionisation can also result in DNA mutation. A mutation is when the code in the DNA changes, causing the cell to rapidly divide and become a cancer.
Some radioactive waste can remain radioactive for thousands of years so it needs to be disposed of carefully. It is usually encased in glass and sealed in metal canisters then buried deep underground. It is tricky to find suitable places for disposal - anywhere prone to earthquakes is unsuitable since the movement can disturb the waste and cause leakages. Even in less earthquake prone regions, governments often have to deal with opposition by residents who, perhaps unsurprisingly, don’t fancy living next to a radioactive dump site.