Nuclear Physics

Nuclear physics is the branch of physics that studies atomic structures and their reactions. This area covers a wide range of topics, some of which are listed below:

• Atomic structure.
• Physical and energy processes that happen inside the atom, such as disintegration.
• Physical and energy processes in atoms, like fusion and fission.
• Physical and energy processes that result from the interaction between atoms and radiation.

The common thread in all these processes is the study of the atom and its behaviour and responses.

Uses of nuclear physics

Nuclear physics is used widely in areas such as medicine, food, energy generation, monitoring, tracing processes, and many areas of chemistry. All these use the results found by nuclear physics to develop practical applications.

Many of these applications come from physical processes related to the emission of radiation. Radiation emission occurs during the decay of radioactive elements.

The discovery of radiation and the development of new technologies

Henri Becquerel discovered the radioactivity caused by an unstable element as he saw how salts of Uranium emitted penetrating radiation. Later, Marie Curie, along with Pierre Curie, studied two radioactive elements: Radium and Polonium.

Ernest Rutherford analysed the early discoveries about emission. His studies made him realise how radioactive material decays at an exponential rate. Rutherford and his student Frederik Soddy later discovered how this decay transformed a heavy element into a lighter one.

The comprehension of the process of decay and of radiation emission opened the gates for many new technologies. These processes act as key elements in technologies that are able to sense the emission or use the emitted energy.

Examples of the use of nuclear physics and radiation

Let’s consider two examples of practical applications of nuclear physics and radiation:

What is radioactive decay in nuclear physics?

The starting point of nuclear physics is the phenomenon called radioactive decay. This is the process in which an atom releases radiation in the form of particles or electromagnetic waves (photons). During decay, atoms transform into lighter elements. As time progresses, the decay process converts every radioactive substance into another one. The moment (in time) when the substances have decayed to half their original quantity is called ‘half time’.

Radioactive decay is a spontaneous and stochastic (random) process. The process follows a specific rate. Even if we don’t know which atom will decay, we can approximate how much substance will be left after some time, and the average time it will take for it all to decay.

Radiation emitted during decay

During radioactive decay, the atoms release particles, which can be alpha or beta particles. The atom can also emit high-energy photons.

• Beta particles are high-energy electrons emitted from the atom. They can be negative like an electron or positive like a positron.
• Alpha particles are the nucleus of helium (2 neutrons and 2 protons).
• Gamma rays are high-energy photons with frequencies higher than 1019 hertz.

As the nucleus emits particles, it reduces its particle number and loses mass. A nucleus with a mass of A1 thus finishes with a lower mass, A2. The atomic mass is given by the mass number with the symbol ‘A’.

Half-life

The decay of any element depends on the time ‘t’. Initially, there is an amount of substance mass or M₀. As the decay advances, this substance M₀ transforms into a later element N₀.

The decay means that at any time larger than t, the initial mass M₀ decreases. Ernest Rutherford observed that this reduction follows what is called an ‘exponential decay’.

Rutherford and Soddy put it like this:

The decay is modelled using the half-life equation below:

\[N(t) = N_0 e^{-\lambda t}\]

In this equation, N₀ is the initial amount of substance, t is the time, and λ is a constant that depends on the isotope in question.

Nuclear instability

The process of radioactive decay is caused by an instability inside the atom’s nucleus, which is produced by unbalanced forces inside the atom.

The balance of forces in the nucleus

An atom has protons and neutrons in its nucleus. Protons are positively charged particles, while neutrons have no charge. Protons repel each other due to their electrostatic force. However, there is another force, called the ‘strong nuclear force’, which counterbalances the electrostatic force and works to keep the neutrons and protons glued together.

The strong nuclear force attracts the protons and neutrons but keeps them at a fixed distance. It attracts particles at a certain distance but repels them when they come too close.

Fission processes and the mass–energy relationship in nuclear physics

If we were to measure the mass of the individual components that make up most of the atom’s mass (the protons and neutrons), we would find that the sum of them has a larger mass than the atom itself. The mass difference and the energy released from this difference play a key role in the fission processes.

The binding energy

The energy produced by the strong force in the nucleus is called ‘binding energy’. The binding energy is responsible for keeping the atom together. If the binding energy is not strong enough, the atom will start to split due to the unbalanced forces in the nucleus. These unbalanced forces cause radioactive decay and radiation emission. There are, therefore, two possible scenarios:

If the binding energy is enough to keep the atom together, the atom is stable and will not decay.

If the binding energy is not enough to keep the atom together, it is a non-stable atom that will break. This then causes it to emit radiation.

The binding energy also plays an important role in the mass–energy conversion. The mass of an atomic nucleus is lower than the mass of all its parts. This is due to a mass loss that happens when the atom’s nucleus forms. Mass, therefore, is not truly lost but converted into energy. The energy released is calculated using Einstein’s famous equation below:

\[E = (m_i-m_f)c^2\]

Here, m_i is the initial mass or the mass of all the particles, while m_f is the final mass or the mass of the atomic nucleus formed by all these particles. E is the binding energy.

Fission processes

The binding energy can be released when the atom splits in a process known as ‘fission’. In that process, the binding energy becomes the energy required to split the atom.

Elements, such as Uranium, pass through a fission process where they become a lighter and more stable element. This process is exploited as a controlled source of energy in nuclear reactors.

Nuclear processes and the mass–energy balance

The energy and particle emissions by an atomic nucleus follow the conservation of energy and mass rule, according to which and the total mass and energy before and after the process will be the same.

We can summarise this by stating that:

• The charge must be conserved.
• The mass must be conserved.
• The energy must be conserved.

Notice that mass can transform into energy and vice versa. In both cases, energy and mass conservation hold, as they transform into each other.

Photon–photon interaction and matter creation

The conservation of mass and energy regulates other processes that appear strange to us, like photon pair production. This is a process where two high energy photons interact, creating two particles that have the following characteristics:

• The mass of the produced particles depends on the energy of the photons. Low energy photons produce electrons, while high energy photons produce protons.
• The produced particles are matter particles and their antiparticles.
• There is a threshold energy level for particles to be produced.
• The energy of the photons must be higher than the ‘at rest’ energy of the produced particles.

As you might suppose, this process is also included in Einstein’s equation, where E is the photon’s energy, and the mass-produced can be calculated as \(E = mc^2\).