Uses of Radioactive Isotopes – Cancer, Gamma Rays, and Geological Dating

Isotopes are two forms of an element that have the same atomic number but different masses. This is due to the element having a fixed number of protons but with varying numbers of neutrons. The existence of isotopes can be understood by reviewing the structure of atoms.

All atoms contain three kinds of basic particles: protons, neutrons, and electrons. (Hydrogen is the only exception as most hydrogen atoms contain no neutrons.) The protons and neutrons are found in the atomic nucleus, while the electrons are found in the space around the nucleus and constantly orbit around it.

The sum of the protons and neutrons is the mass number.  For example, Helium exists as 3He (2 protons and one neutron) or as 4He (2 protons and 2 neutrons). The two forms of Helium are called isotopes of Helium. The isotopes of an element have the same chemical properties but different weights. This means it is difficult to separate isotopes from each other by chemical processes. However, the physical properties of the isotopes, such as their masses, boiling points, and freezing points, are all different.

Isotopes are commonly represented in one of two ways: either by writing the name of the element followed by the mass number of the isotope (the two forms of helium are called helium-4 and helium-3) or by using the chemical symbol of the element with a superscript that shows their mass number. The representations for the two isotopes of helium are 4 He and 3 He. The half-life of an isotope is the amount of time it takes for half of the atoms to decay into a more stable form. Naturally abundant isotopes exist around us because their half-lives are longer than the age of the earth. For example, Uranium 238 (238U) has a half-life of 4.5 billion years so it is naturally abundant. Most isotopes have short half-lives and must be produced in the laboratory to study or use. For example, cobalt 60 (60Co) has a half-life of 5.3 years and is made in a reactor.

Types of Isotopes and their uses

Three types of isotopes are known in nature: radioactive, radiogenic and stable. Most elements have both stable and radioactive isotopes. The uses of isotopes have been widespread, in all sectors of life.

1. Radioactive isotopes are more common than stable isotopes. A radioactive isotope spontaneously breaks apart, changing into some other isotope. For example, potassium has a radioactive isotope with mass number 40 (40 K or potassium-40) which breaks down into a stable isotope of potassium (39 K or potassium-39).   Radioactive isotopes decay through time to other radioactive or radiogenic isotopes (e.g Uranium, Plutonium and Thorium are elements built of radioactive isotopes alone). When an unstable isotope decays, it makes a new atom of a different element. Stable isotopes, on the other hand, do not decay.

The radioactive isotopes of an element are commonly used as tracers in medical, biological, and industrial studies to gain information about physical and mechanical processes. Fr example, isotopes are used in medicine in three distinct ways:

a. Radioactive isotopes can be injected into a patient, and their emitted energy can then be captured on film. The resulting image is an important diagnostic tool.
b. Gamma rays emitted from a radioactive source can be directed onto a tumour, destroying the cancerous cells.
c. Radioactive isotopes can be manufactured into drugs. Once injected into a patient, the drug will accumulate in a certain part of the body, such as a tumour. As the isotopes decay and release energy, that energy destroys the tumour.

In archaeology, radioactive isotopes are used to determine the age of a sample, called Carbon Dating.  Since the Carbon-14 isotope is radioactive it has been used to assess the age of matter. While living, a plant or animal renews carbon in its system. Once the object dies, the amount of C-14 only changes as it decays into some other material. Since C-14 decays at a rate where half of it has changed in about 5600 years – known as its half-life – the years since death can be calculated. For example, if the object or fossil has only 1/4 of the C-14 in it as compared with living objects, it died around 11,200 years ago.

2. Radiogenic isotopes are isotopes formed from decay of radioactive isotopes; 87Sr is a commonly used radiogenic isotope.

3. Stable isotopes have no relation to radioactivity. Their numbers and ratios have been constant since the formation of the solar system The term stable in this context means ‘not radioactive’. Twenty elements, including fluorine, sodium, aluminum, phosphorus, and gold, have only one stable isotope. By contrast, Tin has the largest number of stable isotopes of any element, 10 altogether.

Other Uses

Every year more than 30 million medical treatments and over 100 million laboratory tests are carried out using isotopes.  In the health sector, isotopes are used for the diagnosis of heart disease, locomotive disorders and cancer, for therapy and palliative applications. A medical isotope is a safe radioactive substance used primarily to diagnose illness.  The branch of medicine and medical imaging that uses these isotopes is known as nuclear medicine. Popular isotopes used in medicine include Chromium-51, which is used to label red blood cells and quantify gastro-intestinal protein loss; Cobalt-60, used for external beam radiotherapy; Iron-59, used in studies of iron metabolism in the spleen and Xenon-133/Xenon-127, which are used for pulmonary (lung) ventilation studies..

In the environmental field isotopes are also used for the measurement of air and water pollution, and to understand effects and risks to public health and environment from certain management scenarios for radioactive waste. in geology, for example, hydrologists can use isotope signatures to distinguish between different groundwater types. In the field of industrial safety, radioisotopes are used to detect flaws in steel sections used for bridge and jet airliner construction, and to check the welds on pipes, tanks and other structures.

In consumer protection and safety, isotopes are used to study the quality of foodstuffs and their metabolisation by humans. Analysis of the isotopic “fingerprint” or “signature” of elements can reveal many things about the food we eat, human physiology, how carbon dioxide moves in the atmosphere and about the misuse of nuclear materials. Furthermore, isotopic techniques are also used by the European Office for Wine, Alcohol and Spirit Drinks in authentication of wines, and their use for the authentication of organic food is currently being studied.

Additionally, an isotope of Hydrogen – called Deuterium – has a nucleus consisting of one proton and one neutron. When Deuterium combines with oxygen, it forms what is called “heavy water” because the Deuterium component of the water is twice as heavy as if it was simple hydrogen. Although the amount of heavy water molecules in nature is very small, it has been concentrated for use in the development of nuclear weapons and, in that state, can be poisonous to plants and animals. Furthermore, the isotopic fingerprint and the microscopic structure of Uranium, in an unknown nuclear sample, can tell us where it came from as well as its most likely intended use. “The European Commission is able to provide rapid analysis and detailed nuclear forensic investigation, which detects, fingerprints and tracks nuclear materials across Europe and beyond, assessing their possible uses and radiological hazards.”