Nuclear physics studies the properties and behaviour of atomic nuclei. It has many useful applications and is important in a wide variety of situations.
Everything we can see in the night sky is made of nuclear matter. Nuclear physics describes:
- how the Sun generates the energy we need for life on Earth
- how all the atoms in your body were made in stars
- what happens in stars when they die.
Nuclear physics research tries to answer the fundamental questions:
- where do we come from?
- what are we?
- where are we going?
Atoms, nuclei, elements and isotopes
Atoms are made up of a positively charged nucleus surrounded by a cloud of negatively charged electrons. Nuclei are very dense and extremely small. They contain more than 99.9% of the mass of an atom and are ten thousand times smaller than an atom!
The nucleus is a collection of particles called protons, which are positively charged, and neutrons, which are electrically neutral. Protons and neutrons are in turn made up of particles called quarks.
The chemical element of an atom is determined by the number of protons, also known as the ‘atomic number (Z)’ of the nucleus. The element oxygen has an atomic number Z=8, while carbon has Z=6. The ‘atomic mass’ of the nucleus is given by A=Z+N, where N is the number of neutrons in the nucleus.
Different isotopes of an element have different numbers of neutrons in their nucleus. For example:
- the stable isotope carbon-12, the most common type of carbon in the human body, has Z=6 and N=6
- carbon-14, the radioactive isotope used in carbon dating, has Z=6 and N=8.
There are less than 300 stable nuclei and over 3,000 unstable radioactive nuclei.
Radioactivity is the spontaneous decay of an unstable atom through the emission, from the atomic nucleus, of a particle of ionising radiation.
The different types of radiation can be identified by their ability to pass through matter. In 1899 Ernest Rutherford named alpha (α), beta (β), and gamma (γ) radiation, after the first three letters of the Greek alphabet.
When an atomic nucleus transforms into a different element by emitting an α-particle. An α-particle is the nucleus of a helium atom, helium-4, which consists of two protons and two neutrons.
The atomic number of an α-particle is Z=2, so the atomic number of the decaying nucleus is decreased by two during an α-decay and a different element is created. α-radiation can be stopped by a sheet of paper.
When a nucleus decays spontaneously by emitting an electron or a positron (the electron’s positive antimatter partner). An electron is emitted when a neutron is converted to a proton inside the decaying nucleus.
This tends to occur in proton-deficient nuclei, where the neutron number is much greater than the proton number. A positron is emitted when a proton is converted into a neutron. During a β-decay, a proton is either created or lost, so the atomic number of the decaying nucleus is changed by one. Most β-particles can be stopped by six millimetres of aluminium.
When a nucleus emits electromagnetic radiation in the form of a high-energy photon or γ-ray. It can be caused by the redistribution of protons and neutrons in a nucleus, or by the de-excitation of an energetic nucleus.
The atomic number is not changed during a γ-decay, so the chemical element of the decaying nucleus does not change. γ-rays can be stopped by several millimetres of lead.
Nuclear astrophysics is the study of nuclear processes in stars. It aims to understand how energy is generated in stars and the origin of the chemical elements.
When the universe formed, the Big Bang only created isotopes of the light elements hydrogen (Z=1) and helium (Z=2), with traces of other light elements like lithium (Z=3).
One of the goals of nuclear astrophysics is to show how all the other elements, up to uranium (Z=92) and beyond, were created in stars.
The birth, life and death of stars
Stars are born in huge clouds of gas and dust, which are mostly made up of hydrogen and helium. In regions of higher density the gas and dust begins to gravitationally attract. If enough material is attracted this can result in an infall of matter, releasing gravitational energy and raising the temperature of the gas and dust.
As more and more material from the surrounding cloud is attracted, the temperature and density at the centre becomes so high that nuclear reactions begin to occur and a new star is born. If the energy generated by the nuclear reactions balances the gravitational attraction of all the matter, the star becomes stable.
For most of a star’s life it will generate energy by hydrogen fusion to form helium. For stars like our Sun, the dominant nuclear energy process is called the proton-proton chain. This is a sequence of nuclear reactions which take two hydrogen nuclei (that is, two protons) and produces helium-4.
Stars like our Sun can generate energy this way for about 10 billion years. We estimate that the Sun has used about half the hydrogen in its centre and has almost reached middle age.
When the hydrogen at the centre of a star runs out, gravity finally wins and the star begins to collapse. This drives up the temperature and pressure in the centre of the star, allowing helium fusion to occur. This requires more energy than hydrogen fusion as there are more positively charged protons in helium nuclei to repel each other.
Helium fusion produces carbon (Z=6) and oxygen (Z=8), the elements essential for life as we know it. Helium fusion produces enough energy to stabilise the star again and it becomes a red giant.
How a star dies depends on how massive it is. For stars less than eight times the mass of our Sun, when the helium in the centre runs out the star begins to collapse again. The temperatures and pressures cannot get high enough to fuse elements heavier than helium, so the star becomes unstable. The outer layers blow away leaving a dense hot core behind called a white dwarf star.
For stars with masses more than eight times the mass of the Sun, heavier elements can fuse in the centre, and carbon, oxygen, neon (Z=10) and silicon (Z=14) fusion creates elements up to iron (Z=26). Unfortunately for the star, the fusion of iron does not produce energy, so once the star has an iron core it begins to collapse again.
The outer layers fall rapidly into the star and bounce back off the dense iron core in a powerful shock wave. The resulting explosion is called a supernova and leaves behind a neutron star or a black hole.
In the explosive environments found in a supernova, much more dramatic nuclear reactions can occur to produce the heavy elements. One such process is called the r-process, which is a sequence of rapid neutron captures by nuclei like iron in the star. The resulting neutron-rich nuclei then beta-decay to produce a new element.
This process is thought to occur in supernova and to be responsible for the creation of about half the neutron-rich nuclei heavier than iron. The supernova explosion also blows these newly made elements out into space where they can get caught up in the next generations of stars, perhaps forming planetary systems and life forms.
Research into nuclear physics has enabled the development of science and technology that directly benefits us. Here are just a few examples of how nuclear processes and ionising radiation are being used to improve our lives.
Medical diagnosis and treatment
X-rays are the most common form of ionising radiation used in medicine. In the body, calcium (Z=20) in bones absorb x-rays very efficiently, while soft tissue and fat absorb x-rays less efficiently. The difference in absorption efficiency creates the contrast in an x-ray picture, allowing doctors to see inside the body without the need for surgery. In this way, x-rays are useful for checking broken bones, but they can also be used to identify other medical problems. A chest x-ray can be used to diagnose lung diseases such as pneumonia and mammograms use x-rays to screen for breast cancer.
X-rays are also used in a type of computed tomography, more commonly known as a CT scan. This imaging technique builds up a highly detailed picture by taking x-ray images from different angles to give a series of image cross-sections (or slices) through the part of the body being scanned.
Radiotherapy is the use of ionising radiation to treat cancer. Almost half the people in the UK with cancer have radiotherapy as part of their treatment. Radiotherapy can be carried out externally, usually with a high-energy beam of x-rays, or internally. Internal radiotherapy is when a radioactive liquid is swallowed or injected, or a small piece of radioactive material is placed temporarily in or close to the cancerous cells. This is known as brachytherapy.
PET scan or positron emission tomography (PET) is an imaging technique that also shows how processes in the body are functioning. A small amount of radioactive tracer is injected into the body, most commonly fluorine (Z=18), which is a radioactive sugar. This tracer undergoes beta-decay emitting a positron, the antimatter partner of the electron. When a positron interacts with an electron they annihilate and give off a pair of γ-rays travelling in opposite directions. The scanner detects these pairs of γ-rays and builds up a sequence of 2D images of the tracer concentration. A 3D image can be built up with the help of an x-ray CT scan, which is performed by the same machine at the same time.
Nuclear power stations generate energy through nuclear fission, the splitting apart of heavy atomic nuclei. When elements like uranium (Z=92) fission, the large nucleus splits into smaller ‘daughter’ nuclei releasing a lot of energy, which can be harnessed to produce electricity.
Nuclear fuel is very energy dense, with 1 tonne of uranium equal to 20,000 tonnes of coal, and it is a low-carbon method of producing electricity. There are 16 operational nuclear reactors in the UK and they provide approximately 15% of the UK’s electricity.
Nuclear batteries use the decay of radioactive nuclei to generate electricity. They are very expensive, but have a high energy density and last an extremely long time.
Nuclear batteries are therefore extremely useful as power sources for equipment where there is no opportunity to ‘change the batteries’, such as pacemakers and spacecraft.
Art and archaeology
Using nuclear techniques to identify different stable and radioactive isotopes in archaeological relics and works of art enables the histories of these artefacts to be discovered.
As different radioactive isotopes decay at different rates, there are a lot of different methods that can be used to reliably date an object from the creation of the Earth, right up to the present day.
One of the most well-known forms of radioactive dating is carbon (Z=6) dating. The radioactive isotope of carbon used in dating is carbon-14, which has 6 protons and 8 neutrons. This isotope exists naturally on Earth and is taken up by plants and animals while they are alive.
During an organism’s life the ratio of carbon-14 and the stable carbon-12 is roughly constant, but once it dies, the amount of carbon-14 is reduced by radioactive decay at a known rate and is not replaced. By measuring the ratio of carbon isotopes in a sample the date of death can be estimated.
Similar nuclear methods can also be used to determine the origin of artefacts, helping archaeologists better understand the trade, cultural contacts and influences between ancient settlements.
The most common domestic smoke alarms use a radioactive isotope of the element americium (Z=95) to detect smoke. In a smoke detector a very small americium-241 source emits alpha particles into an ionisation chamber that is open to the air.
The air in this chamber becomes ionised, allowing a very small electrical current to flow. If smoke is present this current drops and the alarm sounds.
A wide variety of household items are sterilised using ionising radiation, from plastic materials, cables, wires and car parts to food packaging and even gemstones. The radiation, usually γ-rays, destroys bacteria, viruses, fungi, mould, insects and eggs using much less energy than sterilisation through heating.
Medical equipment and supplies are also sterilised using ionising radiation, as sterilisation can occur after packaging, reducing the risk of contamination further.
Some foods can be sterilised using γ-rays. In the UK fruit, vegetables, cereals, bulbs, tubers, dried aromatic herbs, spices, vegetable seasonings, poultry, fish and shellfish can be irradiated, but they must be clearly labelled.