Radioactivity was discovered by Henri Becquerel in 1896. He noticed that photographic plates wrapped in light-tight paper were blackened when held close to uranium salts. Uranium emits radiation with the power to penetrate paper and affect the photographic emulsion.
Becquerel took to carrying a vial of uranium salt in his waistcoat pocket. He later noticed a red patch on his chest directly behind this pocket, the first illustration that radiation can also damage biological tissue.
The universe is composed of matter, matter is composed of elements, and an atom is the smallest part of an element that can exist. Every atom has a central nucleus that has a very tiny volume but contains almost all the mass of the atom. Surrounding the nucleus is a cloud of electrons that has very little mass but occupies most of the volume of the atom.
The nucleus contains two types of particles, protons and neutrons. Each has a mass of 1 atomic mass unit. Also, the proton has a positive charge, whereas the neutron has no charge.
The electron has a negative charge and a mass of 1/1840 the mass of a proton. An atom contains equal numbers of electrons and protons and therefore has no net charge. There are 92 elements in nature, ranging from the lightest, hydrogen, to the heaviest, uranium. The elemental identity of an atom is determined by the number of protons (atomic number) in its nucleus. Thus, if an atom contains 92 protons, it must be uranium.
The number of protons plus the number of neutrons in an atom is the atomic mass. Elements are denoted by letter symbols, often followed by the atomic mass, e.g. C12 for carbon. Atoms of the same element can have different numbers of neutrons, and therefore different masses. These are called isotopes, e.g. C12 and C14.
Atoms of every element except hydrogen contain neutrons. Atoms of higher atomic number have a greater ratio of neutrons to protons. A proton is positively charged and repels other positive charges. How then are many protons squeezed into the tiny volume of the nucleus? This packing is helped by the neutrons, which act like nuclear cement.
However, for the nucleus to be stable, it must have the right ratio of protons to neutrons. If this ratio is wrong, the nucleus disintegrates, spitting out fragments in an attempt to achieve a stable ratio of protons to neutrons. This spontaneous disintegration is radioactivity. Quantity of radioactivity, called activity (A), is measured as number of disintegrations per second. The unit of activity is the Becquerel (1 disintegration per second).
Each element has several isotopes. All isotopes of atomic number greater than 83 are naturally radioactive. Every element below atomic number 83 has one or more stable isotopes.
Alpha and beta particles are emitted during radioactive decay. An alpha particle contains 2 protons and 2 neutrons. A beta particle is an electron. How can an electron originate in the nucleus? We can look on the neutron as a close combination of a proton and an electron. This is the origin of beta emission, and, in the process, a neutron changes into a proton.
Also, in almost all cases, after a particle is emitted, the nucleus emits a gamma ray. Gamma rays are not particles. They are electromagnetic radiation similar to visible light, but having much more energy.
When a nucleus emits an alpha or beta particle it changes its atomic number. When it emits an alpha particle, its atomic number drops by 2. When it emits a beta particle a neutron turns into a proton and the atomic number increases by 1. In both cases the elemental nature of the disintegrating atom changes into a daughter element.
Several successive disintegrations may be required before a stable nucleus is reached. This is called a radioactive decay scheme - the initial radioactive isotope decays into radioactive daughter number 1, which in turn decays into radioactive daughter number 2, and so on, until a stable non-radioactive isotope is produced.
Several natural radioactive decay schemes are present in the earth, e.g. the uranium decay scheme whose parent radioisotope is Uranium 238 which decays through 13 radioactive daughters, including the gas radon, until eventually stable Lead 206 is reached.
Radioactive atoms decay exponentially, i.e. the number present is halved after each successive passage of some fixed interval of time. This time interval is characteristic of the particular radioisotope and is called the half-life (T 1/2). Some radioactive isotopes break down quickly, others break down slowly, and T 1/2 can vary from fractions of a second to thousands of millions of years. The T 1/2 of uranium 238 is 4.5 billion years. The T 1/2 of radon gas is 3.8 days. If you have a jar containing 100 atoms of radon, in 3.8 days time you will find that only 50 radon atoms are present, and 3.8 days later only 25 atoms of radon will be present, and so on.
Radioactive decay occurs with such precision that it is often used as a clock. Carbondating has been invaluable to archaeologists, historians and anthropologists. It was used recently to date the Shroud of Turin. The method is based on the measurement of C14, a radioactive isotope of carbon with a T 1/2 of 5,730 years. C14 occurs to a small extent in the atmosphere together with the much more common non-radioactive isotope C12. Living organisms constantly exchange carbon with the atmosphere and the ratio of C14 to C12 in living tissue is the same as it is in the atmosphere.
This ratio is assumed to have remained the same since prehistoric times. When an organism dies, it stops exchanging carbon with the atmosphere, and its C14 nuclei keep disintegrating while the C12 in the dead tissue remains undisturbed.
The ratio of C14 to C12 falls exponentially from its initial benchmark value and, knowing the ratio, one can calculate the time of death of plants and animals and the age of materials derived from plants or animals, e.g., clothing.
William Reville is a senior lecturer in biochemistry and director of microscopy in UCC.