When Ernest Rutherford was working under J.J. Thomson (discoverer of the electron), he found that uranium gave off two distinct types of radiation. The first, which he called "alpha radiation," could only travel a few centimeters through air and could not penetrate a sheet of aluminum mere hundredths of an inch thick. The second type traveled through matter much more easily, and he termed it "beta radiation." Later research found that alpha radiation consisted of particles that are clusters of two protons and two neutrons. Alpha particles have the symbol $↖4↙2$α2+. Beta particles, like cathode rays, were found to be electrons. (This is a newly formed electron, not one of the electrons in the cloud surrounding the nucleus.) A beta particle is represented by the symbol 0β1-. A third type of radiation from uranium was found in 1900 and termed "gamma radiation." Gamma rays are a type of high-energy electromagnetic wave like X-rays. They have no mass and no charge, and are represented by the Greek letter gamma: γ.
Different types of radiation penetrate substances to different degrees.
Note that the mass numbers (in superscripts) balance on both sides of the arrow, similar to how the number of atoms balance in a regular chemical equation. In a beta decay, one of the neutrons in the nucleus releases a beta particle, which results in the neutron becoming a proton. So, carbon-14 became nitrogen-14; this is the main reaction that is involved in the removal of 14C from living material that is exploited for carbon dating. Even bananas contain a lot of naturally occurring potassium-40, which undergoes a beta decay; for this reason, bananas will set off a Geiger counter, a tool used for detecting radiation.
Because a gamma ray has no mass and no charge, the mass and charge numbers on the technetium atom don't change. The m indicates that the protons and neutrons in the technetium nucleus are in a metastable, high-energy arrangement; as the particles return to a stable state, that excess energy is released as a gamma ray. Technetium-99m is used in many radio-imaging facilities in hospitals; it is the main component of Cardiolyte™, which is used to take 3D images of the heart.
Three decades later, in the 1930s, physicists reported two additional kinds of nuclear decay: electron capture and positron emission.
The captured electron leaves a vacancy in a low-level orbital. A higher-energy electron will fall into its place, and this transition will release an electromagnetic wave. Because this is a large drop in energy, the wave is a high-energy wave: an X-ray.
Because each proton in the nucleus carries a +1 charge, there are tremendous repelling forces constantly trying to force the protons apart—like a highly compressed spring just waiting for a chance to uncoil. So what holds the protons together?
The range of strong force determines whether the nucleus is more or less stable. © Science Media Group.
One of the four fundamental forces of nature, the strong force, stabilizes the protons. (The other three are gravity, electromagnetism, and the weak force.) The strong force only works on objects that are within 1.5 x 10-15 m of each other—about the diameter of an atomic nucleus. Because the strong force has such a limited range, any nucleus that is too big will be unstable; its diameter will be larger than the range of the strong force.
The band of stability starts at a ratio of about 1-to-1 (neutrons-to-protons) and increases with the atomic number to about 1.5-to-1. © Science Media Group.
Neutrons are attracted to protons and other neutrons by the strong force, and so they help to hold a nucleus together. Neutrons thus act as a "glue," and with the right amount of neutrons, a nucleus can contain a lot of protons. The glue analogy is not perfect, however, because more neutrons only add stability up to a point; the ratio of protons to neutrons is what matters most. For small nuclei, the most stable ratio of neutrons to protons is about 1-to-1. For larger nuclei, the stable ratio tends toward 1.5-to-1. The line on the graph represents a 1-to-1 ratio. The dots represent stable nuclei. As the graph shows, small stable nuclei lie close to the line. Larger stable nuclei lie above the line. This means that any isotopes that are not in this band or sea of stability are unstable and will want to decay radioactively. Depending on where they are relative to the band, they can undergo alpha decay, beta decay, positron emission, or electron capture to move closer to or into that band of stability.
Though it sounds like something out of science fiction, a positron is an example of antimatter. It has the same mass as an electron, but the opposite charge: +1 instead of -1. And when a positron collides with an electron, they annihilate each other, releasing energy in the form of gamma rays. In hospitals, positron-emitting elements are used in PET (or positron emission tomography) scanning. In this technique, the gamma rays caused by the annihilation of the positrons are captured by a camera, and an image of where these elements were in the body can be created.
A substance made of antiparticles, such as positrons and antiprotons, which have the same mass but opposite charge as their matter counterparts. When a particle and its antiparticle collide, both are annihilated and energy is released.
A high-energy electron produced in the process of nuclear decay (β-).
A type of nuclear decay in which a nucleus captures an electron from a low energy level.
A type of nuclear decay in which a rearrangement of nuclear particles releases a gamma ray.
An antimatter particle equal in mass to the electron but with a positive charge.
A type of nuclear decay that releases a positron.
The nuclear force between protons and neutrons that holds a nucleus together.