Radioactivity helps
study of universe

J.J. Thompson identified the then-mysterious "cathode rays" as a stream of particles with negative electric charge, and named the particles "electrons." He determined that the particles came from within the atom, and proposed a "fruit cake" model for the atom, with negative electron "fruits" embedded in a positive "cake."

In 1898, while working in Thompson's lab, Ernest Rutherford discovered that radioactivity is produced by the disintegration of atoms and determined that there were three types of energy emanating from radioactive atoms, which he named alpha, beta and gamma.

During the first decade of the 20th century, Rutherford made many other discoveries relating to radioactivity and the structure of the atom, including the discovery that the positive charges are contained within a nucleus, which is minuscule even when compared with the size of the atom.

Following 20 years of struggling with the quantum nature of atomic-scale process after that, a breakthrough came that would lead to understanding the finer points of radioactivity: the discovery of the neutron in 1932.

Protons and neutrons make up the nucleus of every atom and are collectively called nucleons. The neutron has no electric charge and is slightly more massive than the proton, which carries a single positive charge.

The number of protons in the nucleus determines which chemical element a substance composed of those atoms will be.

Two nuclei can have the same atomic number but different number of nucleons, constituting different isotopes of the same element. Some of those isotopes are radioactive, but most are stable.

There are about 270 stable isotopes and 50 naturally occurring radioactive isotopes, but thousands of other radioisotopes have been made in the laboratory.

It is not immediately clear how there can be an atomic nucleus at all since positively charged protons repel each other. In the nucleus, as many as 100 or more protons are packed into a space that is only one-trillionth of a millimeter across.

An attractive force holds the nucleus together, called the strong force, that affects both protons and neutrons and overpowers the electromagnetic repulsion between protons -- most of the time.

The strong force is much stronger than the electromagnetic force, but its effect diminishes more quickly with distance. The effective range of the strong force is, not coincidentally, just the size of the largest atomic nucleus.

The strong force alone is not sufficient to hold protons together.

Neutrons act as mediators, which explains why there are no nuclei that contain more than one proton that do not also contain neutrons.

Neutrons are attracted only by the strong force, while protons are both attracted by the strong force and repelled by the electric force. Neutrons in the nucleus mediate the repulsion that protons "feel" for one other.

More protons require proportionally more neutrons in the nucleus to maintain stability. The more nucleons, the larger the nucleus, and the strong force weakens quickly with distance.

There are certain combinations of protons and neutrons that are stable and others that are not. Atoms with unstable combinations eject particles, sometimes in a series of steps, eventually producing a stable nucleus of a different chemical element.

The particular way that the nuclei "choose" to attain that stability depends on incompletely understood characteristics of the strong and weak nuclear forces. A particular radioisotope always decays by the same process.

In alpha decay a nucleus ejects alpha particles that consist of two protons and two neutrons. It is regulated by the strong force and occurs most often in massive nuclei that have too few neutrons to mediate the proton repulsion.

A proton at the edge of a big nucleus feels the strong force pulling it in, but only from the particles closest to it. The electromagnetic force, having a much greater range, pushes out on it from the other side of the nucleus.

We typically visualize the nucleus as a static lump of protons and neutrons, but it is a seething mass in which individual protons and neutrons lose their individual identities. A more appropriate model of the nucleus is that of a drop of viscous, energetic liquid inhabited by subnuclear particles known as quarks, and constantly deforming through its internal energy.

As the nucleus distorts, the balance between the strong and electromagnetic forces changes. If the electric force wins, a part of the nucleus gets ejected.

The ejected mass is always an alpha particle because the particular combination of two protons and two neutrons is the most stable and least energetic configuration of nucleons.

The resulting nucleus will be that of a different chemical element, its atomic number reduced by two and atomic mass reduced by four.

For example, radium-226 (atomic number 88, atomic mass 226) decays to radon-222 (atomic number 86, atomic mass 222).

In beta decay, a nucleus ejects electrons, which are not found in the nucleus, thus requiring a different kind of process to occur.

Beta decay is mediated by the weak force. It occurs when a proton or a neutron is transformed into the other, typically in lightweight nuclei, less in heavy nuclei.

The weak force is more difficult to visualize since it involves quarks. In beta decay a quark changes "color." A "down" quark changes to an "up" quark.

So a neutron (two down quarks and one up quark) "becomes" a proton (one down quark and two up quarks). In the process an electron and a particle called a neutrino are ejected.

The outward appearance is that a neutron "changes" into a proton and ejects an electron, which we observe as "beta radiation." Radioactivity counters do not observe the elusive neutrino.

In the process, the atomic number of the nucleus increases (the additional proton), but the atomic mass remains the same (neutron "becomes" a proton). The new nucleus is now a different atomic element.

For example, carbon-14 (six protons, eight neutrons) becomes nitrogen-14 (seven protons, seven neutrons), which is a stable isotope of nitrogen.

In little more than a century since its discovery, we have learned much about the nature of the fundamental forces that govern the physical universe by studying radioactivity. We have also learned to use radioactivity in a variety of ways, from determining the age of the earth and its rock layers, diagnosing and treating diseases, and understanding the workings of the universe at every level from the smallest subatomic particle to the largest and most distant celestial objects.

It is just one more example of the unity of physical laws that span the entire range of the physical universe from the smallest to the largest, and affect us in every manner from the most subtle to the obvious.