Nuclear instability is a key concept in explaining how nuclear power plants work and why nuclear radiation is so dangerous. Let’s study the subatomic processes that give rise to this phenomenon and look at a simple way to visualise and find general patterns of the stability of atoms/nuclei.
What is nuclear instability?
Nuclear instability is a feature of certain atoms that are not energetically stable due to their particle content.
This means that these atoms will change their state through different physical processes until they reach energetic stability. This process is the ejection of radiation in the form of protons, neutrons, beta particles, or high energy photons (gamma rays), which we collectively name radioactive radiation.
Be sure to check out our explanations on Alpha, Beta, and Gamma Radiation and Radioactive Decay.
How can nuclear instability be represented?
The process of nuclei becoming atomically stable may involve a variety of radiation emissions in combination with each other. We can represent this graphically to aid our understanding. Below we describe the N-Z curve and the decay series.
Elements and isotopes
Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element. In other words, isotopes have the same atomic number but different mass numbers.
For light elements, instability is usually due to an excess of neutrons, so only some light isotopes undergo decay processes. For heavy elements, instability is due to an excess of protons and/or neutrons, so all heavy isotopes undergo decay processes.
Although an element is defined by the number of protons of its atoms (which specifies a lot of its chemical properties), the number of protons is also responsible for certain features of atoms and substances. For instance, properties determined by the mass of atoms vary significantly. For light elements like hydrogen, we find that the chemical properties of its isotopes (deuterium and tritium) are also very different from each other.
The N-Z curve
The N-Z curve is a graph that represents elements according to their proton number (Z) and their neutron number (N).
As you’ll see in the graph below, the neutron number is shown on the vertical axis and the proton number on the horizontal axis. You can also see a straight line corresponding to N = Z, which gives a quick reference of elements that are below, above, or on the line. This provides information about their decaying properties.
We can also add another method of representation (like a colour code) that indicates which type of decay unstable atoms undergo to achieve energetic stability. This is usually done to extract general, relevant information about the structure of atoms and their decaying properties.
The decaying properties only depend on the number of protons and neutrons, so a representation of decays is feasible since the number of protons and neutrons is enough to specify the element and isotope.
N-Z graph of isotopes by type of nuclear decay, Sjlegg CC BY-SA 3.0
Here are some of the general features of this graph:
- The curve gets steeper (above the N = Z line) as the number of protons increases. This means that for heavier elements, atoms typically have more neutrons than protons because of the complex interactions happening inside nuclei.
- Alpha decay processes occur in atoms that have a high ratio of neutrons to protons. All unstable isotopes subject to alpha decay are over the N = Z line.
- The ratio of neutron number to proton number is higher for beta minus decay processes than for beta plus decay processes. The latter can also occur even if an atom has more protons than neutrons.
- For light elements (less than 20 protons), stability is roughly achieved by having the same number of protons and neutrons.
- For heavier elements, stability is achieved by having a higher number of neutrons than protons.
- For certain elements, there are various stable isotopes (vertical black lines in the graph). The case of tin (N = 50) is remarkable since it has ten stable isotopes.
- For elements with more than 82 protons, there are no stable forms and decay processes are bound to happen.
There are other kinds of decay processes atoms can undergo, like fission or neutron disintegration, which we will not consider here.
There are theoretical predictions regarding the stability of newly created elements and isotopes that could shed light on the validity of quantum models and nuclear physics. For instance, a zone of stability called "island of stability" has been predicted for certain isotopes of very heavy elements that, theoretically, would not decay.
Also, it’s important to note that this graph does not contain the information of the nuclei that result after decaying processes – this is a very complex matter that cannot be described graphically in a simple way. While most unstable atoms decay into a certain atom that will stabilise without further alpha/beta decay, there are four elements that decay in four characteristic sequences that are very well studied. These sequences are what we call radioactive series. The elements that decay following these sequences are thorium (Z = 90), uranium (Z = 92), actinium (Z = 89), and neptunium (Z = 93).
Gamma radiation graphics
A metastable state is a form an atom reaches after a decay process occurs. This terminology only applies to the final stage of the radioactive series.
Metastable is a temporal stable state: it is not as stable as the ground state and not as unstable as an excited state. This instability is caused by excess energy resulting from the decay process/es. Since the particle content corresponds to a stable state, atoms usually release the surplus of energy through gamma radiation.
The diagram below shows two possible decay processes for a cobalt isotope. After the two possible beta decays, there is an emission of gamma radiation that takes the atom to its stable state. The blue lines indicate possible beta minus emissions associated with certain energies (all measured in megaelectron volts). The red lines indicate possible gamma emissions. Depending on the energy of the beta particle emitted, the gamma emission will have more or less energy.
The possible decay process of cobalt-60 into nickel-60, Wikimedia Commons
What are some applications of nuclear instability?
After Becquerel’s discovery of nuclear radiation at the end of the nineteenth century and through Marie Curie’s (and other scientists’) studies, the applications of nuclear radiation have appeared everywhere. Here are some examples.
Nuclear fusion and fission
Nuclear power plants are one of the best-known forms of energy production. Although they are not a perfectly clean or perfectly renewable energy source, the waste they produce is much less than the waste produced by non-renewable sources (however, its waste is extremely dangerous).
Unstable atoms that need to lose neutrons to reach a stable state can be forced to do so in a chain reaction that can release an incredibly massive amount of energy. This chain reaction is based on breaking unstable nuclei, a process called nuclear fission. When controlled, the reaction can produce energy that can be used for everyday purposes. If the production is not controlled, it can be dangerous.
There are, however, some light atoms that need to gain particles to reach a stable state (those below the N = Z line in the N-Z curve). It turns out that the joining of nuclei of this kind (nuclear fusion) into a new stable nucleus releases energy. This is the process that happens inside of stars. No one has been able to use it as a source of energy on Earth, but it would be ideal because the waste would be less dangerous and more abundant than in fission. Also, the nuclei susceptible to being used in fusion occur much more abundantly in nature than those susceptible to being used in fission.
A nuclear power plant
Isotopes used as tracers (metastability)
Metastable nuclei can decay very fast (in a matter of days or hours). This short-time characteristic radiation emission allows us to use these atoms as tracers. By using gamma radiation detectors and by reconstructing the measured path, we can accurately trace specific movements.
Technetium-99 is used in medicine to scan the body (the patient either ingests it or receives an injection before the scan). The tracers emit radiation that can be detected by cameras sensitive to high-energy photons (gamma cameras). The photons detected are then analysed and used to build an image of the inside of a patient’s body. Technetium-99 can be used to scan bones, the brain, thyroid, lungs, liver, blood, and tumours.
In general, different elements and substances are used to scan different areas of a subject in a process known as tagging, which consists of attaching a specific source of radiation to a certain kind of molecule in the body.
Nuclear Instability - Key takeaways
Nuclear instability is a property of certain atoms/nuclei that appears whenever there is an excess of subatomic particles.
The process of nuclei becoming atomically stable may involve a variety of radiation emissions in combination with each other.
The N-Z curve is a graph where all known atoms are represented according to their proton and neutron numbers.
We can extract general features of the N-Z curve regarding the patterns of decay processes and the stability of elements.
Nuclear instability has many applications – for instance, energy production or tracing systems.
ImagesGraph of isotopes by type of nuclear decay. https://commons.wikimedia.org/wiki/File:Table_isotopes_en.svg
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