Radioactivity

The purpose of this web page is to allow the relative novice to have a better understanding of the relationship of uranium, radioactivity, radon, thorium, radiation and related words often associated with the Yerington Anaconda Mine Site. An excellent discussion of radioactivity in nature can be found at http://www.physics.isu.edu/radinf/natural.htm

All substances are made of atoms. Atoms have electrons around the outside and a nucleus in the middle. The nucleus consists of protons and neutrons, and is extremely small. Atoms are almost entirely made of empty space! In some types of atom, the nucleus is unstable and will decay into a more stable atom. This radioactive decay is completely spontaneous.

When an unstable nucleus decays, there are three ways that it can do so. It may give out:

an alpha particle (we use the symbol a)
a beta particle (symbol b)
a gamma ray (symbol g)

Alpha particles are made of 2 protons and 2 neutrons. This means that they have a charge of +2, and a mass of 4, (the mass is measured in "atomic mass units", where each proton & neutron=1). Alpha particles are relatively slow and heavy. They have a low penetrating power, (you can stop them with just a sheet of paper). Alpha-decay occurs in very heavy elements, such as Uranium. These heavy elements have too many protons to be stable. They can become more stable by emitting an alpha particle. Most alpha particles eventually gain two electrons and become atoms of helium gas. (Go to Minerals, Metals & Chemicals Alpha)

Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton. This means that beta particles are the same as an electron; they are fast, and light. Beta particles are electrons. Beta particles have a medium penetrating power; they are stopped by a sheet of aluminium or plastics such as perspex. Beta particles ionise atoms that they pass, but not as strongly as Alpha particles do. Protons & neutrons are made of combinations of even smaller particles, called "quarks". Under certain conditions, a neutron can decay to produce a proton plus an electron. The proton stays in the nucleus, while the electron flies off at high speed. This means that when a nucleus emits a b-particle, the atomic mass is unchanged and the atomic number increases by 1. (Go to Minerals, Metals & Chemicals Beta)

Gamma rays are waves, not particles. This means that they have no mass and no charge. Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly. Gamma rays (g) are electromagnetic waves, muck like X rays and radio waves. After a nucleus has emitted an a-particle or a b-particle, it may still have too much energy. It is in an "excited state". It can get rid of this energy by emitting a pulse of very high frequency electromagnetic radiation, called a gamma ray.

Gamma rays do not pull electrons off atoms they pass, as a-particles and b-particles do. This means that they do not lose much energy as they travel, as they do not interact as much with the matter they pass. Therefore, gamma rays have a high penetrating power, and a very long range. (Go to Minerals, Metals & Chemicals Gamma)

In some atoms, the nucleus can change naturally. Such an atom is called radioactive. The change in the nucleus may be only in the arrangement of the protons and neutrons. Or the actual number of protons and neutrons may change. When a nucleus changes, it gives off radiation. This radiation consists of alpha or beta particles or gamma rays. Atoms of uranium, radium, and all other elements heavier than bismuth are radioactive. Some isotopes of lighter elements are also radioactive. In addition, physicists can create radioactive isotopes of nearly all elements in a laboratory by bombarding atoms with subatomic particles.

The type of radiation given off by a radioactive nucleus depends on the way the nucleus changes. Gamma rays are given off if only the arrangement of the protons and neutrons in the nucleus changes; but alpha or beta radiation is given off if the number of protons and neutrons in the nucleus changes. The atom then becomes an atom of a different element. This process is called transmutation or radioactive decay.

A Geiger Counter, also called Geiger-Muller counter, is an instrument that detects forms of ionizing radiation. Such radiation includes gamma rays and alpha and beta particles. Prospectors use Geiger counters to find uranium, thorium, and other radioactive elements. A Geiger counter can detect low-energy radiation because even one ionizing particle produces a full pulse on the central wire. However, the instrument cannot measure the energy of a particle because particles of different energies generate pulses of the same size.

Radioactive decay, or radioactivity, is the process by which a nucleus changes into the nucleus of another isotope or element. The process releases energy chiefly in the form of particles and rays called nuclear radiation. Uranium, thorium, and several other elements decay naturally and so contribute to the natural, or background, radiation that is always present on the earth. Scientists measure the rate of radioactive decay in units of time called half-lives. A half-life equals the time required for half the atoms of a particular radioactive element or isotope to decay. Half-lives range from a fraction of a second to billions of years.

Natural sources of radiation include the sun, other stars and naturally radioactive elements. There are also many artificial sources of radiation. Most naturally radioactive substances belong to one of three sequences of change called radioactive decay series: (1) the uranium series, (2) the thorium series, and (3) the actinium series. In each of these series, heavy isotopes (forms of the same element that have different numbers of neutrons) decay into various lighter isotopes by giving off radiation until they eventually become stable.

The uranium series begins with uranium 238, (the heaviest isotope of uranium), which has 92 protons and 146 neutrons, (totalling 238). After losing an alpha particle, which consists of 2 protons and 2 neutrons, the nucleus has 90 protons and 144 neutrons (234). It is no longer uranium but a radioactive isotope of thorium. Scientists call this process of changing into another element transmutation. The thorium, in turn, breaks down in several steps to radium 226. The radium 226 decays into radon, a naturally occurring radioactive gas. Radon may become a health hazard if it accumulates in certain buildings, especially poorly ventilated ones. The series continues until the isotope becomes a stable form of lead.

The thorium series begins with thorium 232, an isotope of thorium. The actinium series begins with uranium 235, also called U-235, another isotope of uranium. These two series also end with lead.

Scientists use two systems for measuring the amount, or dose, of radiation absorbed by a substance. The older system, still commonly used, measures doses in units called rads. Rad stands for radiation absorbed dose. One rad is produced when 1 gram of material absorbs 100 ergs. (An erg is an extremely small unit of energy.) The newer system, introduced in 1975, measures dosage in units called grays, named after Louis H. Gray, a British radiation biologist. One gray is equal to 100 rads or 1 joule per kilogram of material. A joule is a unit of energy equal to 10 million ergs. A typical dental X ray, for example, exposes the patient to about 0.25 rad (0.0025 gray).

Large doses cause a combination of effects called radiation sickness. Doses above 100 rems damage red and white blood cells. This damage is known as the hematopoietic effect. At doses above 300 rems, death may follow in several weeks. Above 1,000 rems, the cells lining the digestive tract die and bacteria from the intestines invade the bloodstream. This effect, known as the gastrointestinal effect, may lead to death from infection within a week. At doses of several thousand rems, the brain is injured and death can come within hours.

Deaths from radiation sickness are extremely rare. People have only suffered such large doses in reactor accidents, in a few cases where radioactive material was mishandled, and in the 1945 bombings of Hiroshima and Nagasaki, Japan, during World War II. The worst reactor accident in history was a 1986 explosion and fire at the Chernobyl nuclear power plant in Ukraine, then part of the Soviet Union. Thirty-one workers died.

The doses received in daily life, sometimes called background doses, are much smaller. Some scientists believe that the average background dose is 0.3 to 0.4 rem per year. About half of this amount comes from breathing radon gas released by radioactive rocks and soil. Medical and dental X rays add another 0.04 rem per year. Other sources, such as nuclear power plants and waste disposal sites, typically account for less than 0.01 rem per year. Smokers take in much higher doses from radioactive isotopes in smoke. Ionizing radiation affects people by depositing energy in body tissues, and can cause cell changes or cell death. In some cases, there is no ill effect; in others, the cell may become abnormal or malignant.

An accumulation of small doses of radiation increases the risk of developing a condition, but not the severity of the condition. The chief effects of repeated small doses of radiation are cancer and birth defects.

To protect people from the effects of radiation, the International Commission on Radiological Protection, a panel of experts from many countries, sets guidelines for exposure. This group recommends that nuclear workers receive a maximum permissible dose (MPD) of no more than 5 rems per year. The commission also urges that the general public receive no more than 0.5 rem in any year. Other agencies, including the National Council on Radiation Protection and Measurements in the United States and the Atomic Energy Control Board in Canada, set similar guidelines.

Radium, pronounced RAY dee uhm, is a highly radioactive, metallic element. It occurs chiefly in uranium and thorium ores. Before the mid-1950's, radium was widely used for treating cancer. It also was a key ingredient in fluorescent paint used for watch and instrument dials. Today, safer and cheaper sources of radiation have replaced radium for most medical and industrial uses. These sources include the isotope cobalt 60, particle accelerators, and X-ray machines.

Radium releases large amounts of high-energy radiation, which can be harmful to human health. The radiation given off by radium bombards the bone marrow and destroys tissue that produces red blood cells. It also can cause bone cancer.

Back to Top

Previous Page

Next Page

Minerals, Metals, Chemicals Page