Several cloudy days in Paris in 1896 “ruined” Henri Becquerel’s experiment, but in the process, the field of nuclear physics was born. Becquerel was out to prove his hypothesis that uranium absorbed sunlight and re-radiated it in the form of X-rays, which had been discovered the previous year.
Nuclear Physics Basics: History and Discovery
Becquerel's plan was to bring the potassium uranyl sulfate into the sunlight and then bring it into contact with photographic plates wrapped in black paper, because while visible light wouldn’t make it through, X-rays would. Despite the lack of sunlight, he decided to go through the process anyway, and was shocked when he discovered images still recorded on the photographic plate.
Further testing showed it wasn’t X-rays at all, despite his assumptions. The path of light isn’t bent by a magnetic field, but the radiation from the uranium was deflected by one, and this – along with the first result – was how radiation was discovered. Marie Curie coined the term radioactivity, and along with her husband Pierre, discovered polonium and radium, pinning down the precise sources of the radioactivity.
Later, Ernest Rutherford came up with the terms alpha particles, beta particles and gamma particles for the radiated material, and the field of nuclear physics really got going.
Of course, people know a lot more about nuclear physics now than they did at the turn of the 20th century, and it’s a crucial topic to understand and learn about for any physics student. Whether you want to understand the nature of nuclear energy, the strong and weak nuclear forces or contribute to fields like nuclear medicine, learning the basics is essential.
What Is Nuclear Physics?
Nuclear physics is essentially the physics of the nucleus, the part of the atom containing the two most well-known “hadrons,” protons and neutrons.
In particular, it looks at the forces operating in the nucleus (the strong interaction that binds protons and neutrons together in the nucleus, as well as holding their component quarks together, and the weak interaction relating to radioactive decay), and the interaction of nuclei with other particles.
Nuclear physics covers topics like nuclear fusion (which relates to the binding energy of different elements), nuclear fission (which is the splitting of heavy elements to produce energy) as well as radioactive decay and the basic structure and forces at play in the nucleus.
There are many practical applications of the field, including (but not limited to) working in nuclear energy, nuclear medicine and high-energy physics.
Structure of the Atom
An atom is composed of a nucleus, which contains the positively-charged protons and uncharged neutrons, held together by the strong nuclear force. These are surrounded by negatively-charged electrons, which form what’s called a “cloud” around the nucleus, and the number of electrons matches the number of protons in a neutral atom.
There have been numerous models of the atom proposed throughout the history of physics, including Thomson’s “plum pudding” model, Rutherford's and Bohr’s “planetary” model and the modern, quantum mechanical model described above.
The nucleus is tiny, at around 10−15 m, containing the bulk of the mass of the atom, while the whole atom is on the order of 10−10 m. Don’t let the notation fool you – this means the nucleus is about 100,000 times smaller than the atom overall, but it contains the vast majority of the matter. So the atom is predominantly empty space!
The atom’s mass isn’t exactly the same as the mass of the constituent parts though: If you add up the masses of the protons and neutrons, it already exceeds the mass of the atom, before you even account for the much smaller mass of the electron.
This is called the “mass defect” of the atom, and if you convert this difference into energy using Einstein’s famous equation E = mc2, you get the “binding energy” of the nucleus.
This is the energy you would have to put into the system in order to split the nucleus into its constituent protons and neutrons. These energies are much, much larger than the energy it requires to remove an electron from its “orbit” around the nucleus.
Nuclear Matter and Nuclear Structure
The two types of nucleon (i.e. particle of the nucleus) are the proton and the neutron, and these are tightly-bound together in the nucleus of the atom.
Although these are generally the nucleons you’ll hear about, they are not actually fundamental particles in the standard model of particle physics. The proton and the neutron are both composed of fundamental particles called quarks, which come in six “flavors” and each carry a fraction of the charge of a proton or an electron.
An up quark has a 2/3 e charge, where e is the charge of an electron, while a down quark has a −1/3 e charge. This means that two up quarks and a down quark combined would produce a particle with a positive charge of magnitude e, which is a proton. On the other hand, an up quark and two down quarks produce a particle with no overall charge, the neutron.
The Standard Model of Particle Physics
The standard model catalogs all of the fundamental particles currently known, and groups them into two main groups: fermions and bosons. Fermions are subdivided into quarks (which in turn produce hadrons like protons and neutrons) and leptons (which include electrons and neutrinos), and bosons are subdivided into gauge and scalar bosons.
The Higgs Boson is the only scalar boson known so far, with the other bosons – the photon, gluon, Z-bosons and W bosons – being gauge bosons.
Fermions, unlike bosons, obey “number conservation laws.” For example, there is a law of conservation of lepton number, which explains things like the particles produced as a part of nuclear decay processes (because the creation of an electron with lepton number 1, for example, has to be balanced with the creation of another particle with lepton number −1, such as an electron anti-neutrino).
Quark number is also conserved, and there are other conserved quantities as well.
Bosons are force-carrying particles, and so interactions of the fundamental particles are mediated by the bosons. For example, the interaction of quarks is mediated by gluons, and electromagnetic interactions are mediated by photons.
Strong Nuclear Force and Weak Nuclear Force
Although the electromagnetic force does apply in the nucleus, the main forces you need to consider are the strong and weak nuclear forces. The strong nuclear force is carried by gluons, and the weak nuclear force is carried by W± and the Z0 bosons.
As the name suggests, the strong nuclear force is the strongest of all fundamental forces, followed by electromagnetism (102 times weaker), the weak force (106 times weaker) and gravity (1040 times weaker). The huge difference between gravity and the rest of the forces is why physicists essentially neglect it when discussing matter at the atomic level.
The strong force needs to be strong to overcome the electromagnetic repulsion between the positively-charged protons in the nucleus – if it had been weaker than the electromagnetic force, no atoms with more than one proton in the nucleus would have been able to form. However, the strong force has a very short range.
This is important because it shows why the force isn’t noticeable even on the scale of whole atoms or molecules, but it also means that electromagnetic repulsion becomes more relevant for heavy nuclei (i.e. bigger atoms). This is one of the reasons unstable nuclei are often those of the heavy elements.
The weak force also has a very short range, and it essentially causes quarks to change flavor. This can cause a proton to become a neutron and vice-versa, and so it can be thought of as the cause of nuclear decay processes like beta plus and minus decay.
There are three types of radioactive decay: alpha decay, beta decay and gamma decay. Alpha decay is when an atom decays by releasing an “alpha particle,” which is another term for a helium nucleus.
There are three sub-types of beta decay, but all of them involve a proton turning into a neutron or vice-versa. A beta minus decay is when a neutron becomes a proton and releases an electron and an electron anti-neutrino in the process, while in beta plus decay, a proton becomes a neutron and releases a positron (i.e. an anti-electron) and an electron neutrino.
In electron capture, an electron from the outer parts of the atom is absorbed into the nucleus and a proton is converted into a neutron, and a neutrino is released from the process.
Gamma decay is a decay where energy is released but nothing in the atom changes. This is analogous to the way a photon is released when an electron makes a transition from a high-energy to a low-energy state. An excited nucleus makes a transition to a low-energy state and emits a gamma ray as it does.
Nuclear Fission and Nuclear Fusion
Nuclear fusion is when two nuclei fuse and create a heavier nucleus. This is the way energy is generated in the sun, and getting the process to occur on Earth for power generation is one of the biggest goals for experimental physics.
The problem is that it requires extremely high temperatures and pressures, and therefore very high energy levels. However, if scientists achieve it, fusion could become a vital power source as society continues to grow and we consume increasing amounts of energy.
Nuclear fission is the splitting of a heavy element into two lighter nuclei, and this is what powers the current generation of nuclear reactors.
Fission is also the operating principle of nuclear weapons, which is one of the main reasons it’s a controversial area. In practice, fission works through a series of chain reactions. A neutron that creates the initial split in a heavy element like uranium, generates a further free neutron after the reaction, which can then go on to cause another split and so on.
Essentially, both of these processes gain energy through the E = mc2 relation, since fusing or splitting atoms involves a release of energy from the “missing mass.”
Applications of Nuclear Physics
There are a huge range of applications of nuclear physics. Notably, nuclear reactors and nuclear power plants are operational in many countries around the world, and many physicists are working on new and safer designs.
For instance, some nuclear reactor designs are aiming to ensure that the source material cannot be used to create nuclear weapons, which require a much more enriched source of uranium (i.e. a “purer” uranium) to operate.
Nuclear medicine is another important area for nuclear physics. Nuclear medicine involves very small amounts of radioactive material being administered to the patient, and then detectors are used to capture images from the radiation given off. This helps doctors diagnose kidney, thyroid, heart and other conditions.
Of course, there are many other areas where nuclear physics is essentially, including high-energy physics and particle accelerators like CERN, and astrophysics, where many of the dominant processes in stars depend strongly on nuclear physics.
- The Physics Hypertextbook: The Standard Model
- Georgia State University Hyper Physics: Quarks
- Particle Adventure: Quarks
- Georgia State University Hyper Physics: Nuclear Binding Energy
- Boston University: Nuclear Binding Energy and the Mass Defect
- Duke University: High Energy Physics
- CERN: The Standard Model
- Berkeley Lab: The Discovery of Radioactivity
- The Star Garden: Nuclear Physics
- Duke University: The Four Forces
- Johns Hopkins Medicine: Nuclear Medicine