In this lesson we will learn how unstable isotopes can undergo different types of nuclear decay.
By the end of this lesson you will be able to:
- Describe alpha, beta and gamma decay.
- Predict the products of different types of nuclear decay.
- Define half life and calculate how much of a radioactive substance will remain after a given amount of time.
- Compare the penetrating power of alpha, beta and gamma radiation.
- Define radiation dose and give examples of natural and artificial sources of radiation.
- Most atoms are stable but a small proportion are unstable and undergo spontaneous radioactive decay.
- There are different types of radioactive decay, depending of the type of radiation emitted from the atom.
- The rate of radioactive decay and the intensity of the radiation depends on the particular isotope.
- There are natural and artificial sources of radiation, with both positive and negative effects.
Stable and Unstable Atoms
- Most elements are made up of different isotopes, which means they can have atoms with different numbers of neutrons in their nuclei.
- For example, carbon can exist as carbon-12 (with 6 protons and 6 neutrons), carbon-13 (with 6 protons and 7 neutrons) and carbon-14 (with 6 protons and 8 neutrons).
- Isotopes can be stable or unstable.
- Stable isotopes are made up of atoms with balanced forces between nuclear particles (protons and neutrons).
- For example, carbon-12 and carbon-13 are stable isotopes of carbon.
- Unstable isotopes are made up of atoms with unbalanced forces between nuclear particles.
- For example, carbon-14 is an unstable isotope of carbon.
- Most naturally occurring atoms are stable, but a small proportion are unstable.
- For example, approximately 99% of carbon atoms are the carbon-12 isotope and approximately 1% of carbon atoms are the carbon-13 isotope, which means that virtually all carbon atoms are stable. However, about one in every trillion carbon atoms is the unstable carbon-14 isotope.
- Unbalanced nuclear forces in unstable isotopes are usually the result of too few or too many neutrons in the nuclei of atoms.
- As a result, unstable isotopes emit radiation, to become more stable.
- Radiation is energy in the form of subatomic particles or electromagnetic waves.
- Radiation emitted from unstable nuclei is called nuclear radiation.
- The release of radiation from the nuclei of atoms is known as radioactivity, radioactive decay or nuclear decay.
- Unstable isotopes are also known radioactive isotopes, which is usually abbreviated to radioisotopes.
(Image: ORNL, Wikimedia Commons)
Types of Nuclear Decay
- Nuclear decay may involve the ejection of nuclear particles, to alter the numbers of protons and neutrons, or it may just involve the release of excess nuclear energy.
- There are three main types of nuclear decay, depending on the type of radiation emitted – alpha decay, beta decay and gamma decay.
- Alpha decay and beta decay involve the ejection of nuclear particles, whereas gamma decay involves the release of electromagnetic waves.
- Alpha decay and beta decay result in a change in the number of protons in the nuclei of atoms and therefore a change in the type of atom – a process known as transmutation.
- Alpha decay is the ejection of alpha particles from a nucleus.
- An alpha particle consists of two protons and two neutrons.
- This is equivalent to a helium-4 nucleus, so it is often represented as ⁴₂He²⁺.
- Alpha particles can gain electrons from their environment to become neutral.
- Alpha particles can also be represented using the symbol alpha, as α or ⁴₂α²⁺.
- Alpha decay results in a decrease in atomic number and mass number.
- For example, the radioisotope plutonium-239 undergoes alpha decay to form uranium-235, as shown in the following equation:
- Note that the sum of the atomic numbers (subscripts) and mass numbers (superscripts) is the same on both sides of the equation.
- Alpha decay occurs in large atoms with unstable nuclei.
- Beta decay is the ejection of beta particles from a nucleus.
- A beta particle is the same as an electron, so it is often represented as e⁻ or ⁰₋₁e.
- Beta particles can also be represented using the symbol beta, as β or ⁰₋₁β.
- The beta particle ejected during beta decay is not one of the electrons orbiting the nucleus.
- Instead, it forms when a neutron is converted into a proton and an electron, as shown by the following equation:
- Since beta particles contain no protons or neutrons, they have a mass number of 0.
- They are given an atomic number of -1, so that the sum of the atomic numbers is the same on both sides of the equation.
- Beta decay results in an increase in atomic number but the mass number remains the same.
- For example, the radioisotope carbon-14 undergoes beta decay to form nitrogen-14, as shown in the following equation:
- Gamma decay is the emission of gamma rays from a nucleus.
- Gamma rays are not particles, but a type of electromagnetic wave, similar to X-rays but having more energy.
- Gamma rays are represented using the symbol gamma, as γ or ⁰₀γ.
- The latter symbol is to show that, since gamma rays are not particles, they contain no protons or neutrons and therefore their atomic number and mass number are both 0.
- Gamma radiation results from the movement of protons and neutrons in a high-energy nucleus.
- Since gamma decay does not change the number of protons or neutrons in a nucleus, the atomic number and mass number stay the same.
- For example, the radioisotope cobalt-60 undergoes gamma decay as shown in the following equation:
- The rate (speed) of nuclear decay is measured by a radioisotope’s half-life.
- The half life of a radioisotope is the time it takes for half of its nuclei to undergo nuclear decay.
- Depending on the isotope, this can range from a fraction of a second to billions of years.
- The radioisotope sodium-24 is often used for detecting leaks in underground pipes.
- It undergoes beta decay to form magnesium-24, as shown in the following equation:
- The half-life of sodium-24 is 15 hours.
- If there were originally 100 sodium-24 atoms in a sample, after 15 hours there would be 50 sodium-24 atoms and 50 magnesium-24 atoms.
- After another 15 hours there would be 25 sodium-24 atoms and 75 magnesium-24 atoms, and so on.
Penetration and Absorption of Radiation
- As radiation moves through substances, it is absorbed by them.
- As a result, the further radiation travels, the less intense it becomes.
- Thicker, heavier substances are more effective at absorbing radiation.
- Each of the three types of radiation penetrate materials to different extents.
- Alpha radiation is the least penetrating. It can only travel a few centimetres in air and can be stopped by a sheet of paper.
- Beta radiation can travel a few metres in air and can be stopped by a sheet of aluminium foil.
- Gamma radiation is the most penetrating. It can travel several hundred metres in air and requires several centimetres of lead or a thick wall of concrete to be stopped.
- All types of radiation are damaging to living things.
- The more penetrating the radiation is, the more dangerous it is.
- Beta and gamma radiation can cause severe burns, sickness and mutation.
- Alpha radiation is relatively safely, unless inhaled or ingested – then it becomes a serious health hazard.
(Image: Ehamberg, Wikimedia Commons)
- The impact of radiation on living things depends on both the type of radiation and the amount of radiation.
- The amount of radiation absorbed by a substance is referred to as a radiation dose.
- There are several different ways dose can be measured or estimated. These use the units grays (Gy) or sieverts (Sv).
- Each day we are exposed to small amounts of natural radiation, including:
- Solar radiation and cosmic radiation – radiation from the sun and stars.
- Terrestrial radiation – radiation from rocks and soil (uranium, thorium, radium), air (radon) and water (uranium, thorium).
- Internal radiation – radiation from within our bodies or other living things (potassium, carbon).
(Image: NASA/JPL-Caltech/SwRI, Wikimedia Commons)
- We are also exposed to many sources of man-made radiation, including:
- Medical sources – X-rays, nuclear medicine (iodine, caesium).
- Consumer products – televisions, smoke detectors (americium), luminous watches (tritium).
- Building materials and fuels.
(Images: jaytaix, Pixabay; Mikael Häggström, Wikimedia Commons)
- Natural sources account for the majority of radiation that the average person is exposed to.
- Of the man-made sources, medical radiation, in particular X-ray radiation, is the biggest source.
- The total dose of radiation that the average person receives, from both natural and man-made sources, has not been shown to cause harm.
- However, excessive exposure to UV radiation (sunlight) or exposure to nuclear radiation (from nuclear weapons or nuclear accidents) has been shown to cause cancer.
(Images: National Cancer Institute, Wikimedia Commons; US National Archives and Records Administration, Wikimedia Commons)
- Some professions, such as nuclear science and radiography, involve exposure to higher levels of radiation than the average person.
- People who work in these industries have their radiation dose constantly measured, and there are strict limits on the amount of radiation they can be exposed to.
(Image: Fluor, Wikimedia Commons)
- Some atoms are unstable due to unbalanced forces between nuclear particles – protons and neutrons.
- These isotopes, known as radioisotopes, undergo spontaneous nuclear decay.
- Nuclear decay involves the emission of nuclear radiation from the nuclei of radioisotopes.
- There are three main types of nuclear decay – alpha, beta and gamma.
- Alpha decay is the ejection of alpha particles (helium nuclei) from atoms.
- It decreases the atomic number and therefore results in the formation of a new element (transmutation). The mass number also decreases.
- Beta decay is the ejection of beta particles (electrons) from atoms.
- These electrons form when a neutron is converted into a proton and an electron.
- Beta decay increases the atomic number and therefore results in the formation of a new element. The mass number remains the same.
- Gamma decay is the emission of gamma rays (electromagnetic waves) from atoms.
- It does not change the atomic number (or mass number), therefore the type of element stays the same.
- The half life of a radioisotope is the time it takes for half of its nuclei to undergo nuclear decay.
- All types of radiation are damaging to living things, but each penetrates materials to different extents.
- Gamma radiation is the most penetrating and alpha radiation is the least penetrating.
- Radiation dose is a measure of how much radiation is absorbed by a substance or individual.
- Sources of radiation can be natural or artificial, with natural sources accounting for the majority of radiation absorbed by humans.
(Header image: bluedesign, Adobe Stock)