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Nuclear Physics

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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:

Nuclear physics and radiation can be applied as tracing methods using metastable isotopes. It is what we commonly know as PET scans, which are used by doctors to detect certain diseases in our bodies. In a PET scan, we use radioactive elements that decay in a short amount of time to create internal body images. The radioactive elements injected into the system emit radiation. The emitted radiation is later detected by special instruments. Finally, images are created by observing the emission.

Radiation is also used for energy generation. That is what happens in nuclear plants. We use the heat produced by radiation to power steam turbines. Then, the heat is conducted to a fluid (water) to produce steam. Finally, the steam is directed towards a turbine connected to an electrical generator.

Nuclear physics. Nuclear power plant. StudySmarterFigure 1. Nuclear power plant. Source: Lukáš Lehotský, Unsplash.

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’.

Nuclear Physics. Disintegration. Beta particles. Alpha Particles. StudySmarterFigure 2. Radiation processes emit alpha and beta particles. Alpha particles are formed by two protons and two neutrons with a relative charge of +2. Beta particles are usually electrons with a relative charge of -1 but can also be positrons with a charge of +1. Source: Manuel R. Camacho, StudySmarter.

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 transforms into a later element .

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

Rutherford and Soddy put it like this:

A fixed amount of the radioactive element will decay in each unit of time”.

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

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

Nuclear Physics. Decay rate. StudySmarterFigure 3. The half-life provides information about the decay of a substance, in this case, Cobalt 60. N0 is the initial amount of substance when t=0. The decay constant λ for Cobalt 60 is 0.1314. Source: Manuel R. Camacho, StudySmarter.

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.

Nuclear Physics. Strong nuclear force. Electrostatic force. StudySmarterFigure 4. The balance of the electrostatic force and the strong nuclear force keeps the nucleus glued together. Radioactive decay occurs when that balance breaks down. Source: Manuel R. Camacho, StudySmarter.

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:

Here, is the initial mass or the mass of all the particles, while 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 Physics. Disintegration. Radiation. Fission. StudySmarterFigure 5. Radiation processes emit energy as photons (left), whereas fission processes emit particles (right). Source: Manuel R. Camacho, StudySmarter.

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.

Nuclear Physics - Key takeaways

  • Nuclear physics is the branch of physics that studies atomic structures and their reactions.
  • Nuclear physics has produced many practical applications. It is used for developing new technologies regarding energy, medicine, monitoring tracing methods, and many more.
  • Many applications from nuclear physics are related to the emission and detection of radiation.
  • During the emission of energy or mass, the laws of conservation apply. In some cases, energy is transformed into mass or vice versa.

Frequently Asked Questions about Nuclear Physics

The energy produced by the strong force in the nucleus is called ‘binding energy’. The binding energy is responsible for keeping the atom together.

Ernest Rutherford is known as the father of nuclear physics. However, it is indisputable that the knowledge produced by Marie Curie and her husband Pierre Curie, makes them also progenitors of nuclear physics.

Nuclear physics can be used in areas such as energy tracing methods, sterilisation, monitoring, medicine, and many more.

Final Nuclear Physics Quiz

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What is nuclear physics?

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Nuclear physics is the branch of physics that studies atomic structures and their reactions.

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Does medicine use nuclear physics?

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Yes, for example, in PET scans. 

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The knowledge contributed by nuclear physics has been used to produce energy.

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True, it is currently used in the nuclear industry.

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Does nuclear physics study molecules and the chemistry of macroscopical changes, such as fluids changing into gasses?

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No, it studies the atomic structure and its reactions.

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Which of the following areas use nuclear physics?

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All of them.

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Does the conservation of mass and energy apply to nuclear reactions?

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Yes, the mass and energy after and before a reaction must be the same.

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Is density conservation important in nuclear reactions?

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No, density does not play any role, as particles are very small.

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What is the binding energy?

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The mass transformed into energy in the atom.

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Is the mass of the particles larger when they form an atom?

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No, the mass is the same, as it is conserved.

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Energy is preserved in nuclear reactions.

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True

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Mass is preserved in nuclear reactions.

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True, it follows the laws of conservation.

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Is mass transformed into energy in some reactions?

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Yes. In this case, mass is lost, or it might be better to say that the energy contained in the mass is lost and transformed into energy.

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Which are the three particles released by an atom when it decays?

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Alpha particles, beta particles, and gamma rays.

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What are the basic constituents of alpha radiation?

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Two neutrons and two protons.

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What are the basic constituents of beta radiation?

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An electron or a positron.

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What are the two types of beta decay?

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Beta plus decay and beta minus decay.

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Who was the main discoverer of radioactivity?

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Marie Curie.

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What did Paul Villard discover?

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Gamma radiation.

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What is the effect of ionising radiation on cells?

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Ionising radiation breaks chemical bonds and affects structures like DNA.

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What are the basic constituents of gamma radiation?

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High-energy photons.

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Which of the following is correct?

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Alpha and beta radiation are particle-like radiation and gamma radiation is wave-like radiation.

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There are conservation laws associated with disintegration processes.

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The more ionising a radiation, the less penetrating.

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We are exposed to non-damaging radiation every day.

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What is the name of the detectors of radioactivity used in nuclear power plants? 

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Geiger detectors.

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Is radiation used to fight tumours?

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Yes.

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What kind of damage can gamma radiation cause?

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Gamma radiation can cause burns.

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What is radioactive decay?

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A random process that occurs when an atom is unstable and wants to achieve stability.

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What is an unstable atom?

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An atom with an excess of particles and/or energy.

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Give two types of decay processes that emit massive particles.

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Alpha decay and beta decay.

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How many types of beta decays are there?

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There are two types: beta minus and beta plus decay.

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The law of exponential radioactive decay is derived from quantum mechanics.

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Radioactive decay occurs faster at earlier times than at later times.

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The rate of percentual decay is constant and depends only on the decay constant.

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Into which element does uranium-238 decay?

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Uranium-238 decays into lead. 

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What is the name of the time it takes the unstable nuclei of an element to decrease in half?

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Half life.

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Which element is used for estimating the age of organic structures?

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Carbon-14.

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Why does the exponential decay law work for samples of unstable elements with a high number of atoms?

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The exponential decay law works for samples of unstable elements with a high number of atoms because it is a statistical feature.

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Which decays do elements usually undergo to gain stability?

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Alpha and beta decay.

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Is the constant of radioactive decay universal?

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No, the constant of radioactive decay is specific to each element.

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Do electrons determine the instability of an atom?

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No, only neutrons and protons determine the instability of an atom. 

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Which particles can beta radiation emit?

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Electrons and positrons.

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Nuclear instability is caused by an excess of particles (instability) and energy (metastability).

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Alpha and beta radiation are forms of particle radiation and gamma radiation is a form of wave radiation.

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Alpha decay takes place for unstable heavy atoms.

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Gamma radiation usually takes place after other forms of nuclear decay.

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Nuclear instability allows us to produce energy when controlled.

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What is the N-Z curve?

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It is a curve where nuclei are represented according to their numbers of protons and neutrons.

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After which proton number do elements lose any form of stability?

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After 82 protons. 

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What is the stability island?

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It is a predicted set of heavy nuclei that are not unstable.

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