One way to approach the measurement of radiation is to count the number of nuclear transformations or explosions which occur in a given unit of radioactive substance per second. This measure is usually standardised to radium, the first radioactive substance to be discovered and widely used. One gram of radium undergoes 3.7 x 10^10 nuclear transformations or disintegrations per second. The activity of 1 gram of radium is called 1 curie (Ci), named for Madame Marie Curie, a Polish-born French chemist (1867-1934). Marie Curie discovered the radioactivity of thorium, polonium and radium by isolating radium from pitchblende. She and her daughter Irene were among the earliest known radiation victims, both dying of aplastic anaemia.
        In recent radiation protection guides, the curie is being replaced by the becquerel, which indicates one atomic event per second. One gram of radium would equal 1 curie of radium or 3.7 x 10^10 becquerels of radium.
        The energy released in nuclear disintegrations has the ability to do work, i.e. to move matter. In physics, the erg is a very small unit of work done. Lifting 1 gram of radium 1 centimetre requires 980 ergs of work. Any material exposed to the force from nuclear disintegrations at a rate of 100 ergs/gm is said to absorb one rad, i.e. radiation absorbed dose. There is no direct conversion from curies, which is related to the number of atomic events, to the rad dose, which is energy absorbed in tissue. The curie gives one an estimate of the number of microscopic transformations or explosions per second and the rad is an estimate of the energy release, absorbed by the surrounding tissue. On the macro-level, the word `explosion' tells us only of an event in time. A dynamite explosion or hydrogen bomb explosion adds information about the energy released.
        Sometimes radioactivity is measured in counts per minute on a Geiger counter. A nuclear transformation within an energy range measured by the instrument and close enough to the instrument causes a noise or `count'. Most Geiger counters cannot detect alpha particle emitters like plutonium.
        The radioactivity of elements which experience nuclear disintegrations is measured relative to radium. For example, it would take more than 1 million grams of uranium to be equivalent in radioactivity, i.e. to have the same number of nuclear events per second as 1 gram of radium has per second. Both 1 million grams of uranium and 1 gram of radium would be measured as 1 Ci. It has been the custom in the past to limit human exposure to uranium more for its toxic chemical properties (it is a heavy metal) than for its radioactivity. This practice may have underestimated damage caused by the biological storing of uranium in the liver.
        When uranium decays, it passes through about 12 radioactive forms, called daughter products, before reaching a stable chemical form of lead. One of the radioactive daughter products of uranium is radium. Uranium released into drinking water or incorporated into food and human tissue today will eventually plague the world as radium and its other disintegration products: radon gas and the radioactive forms of polonium, lead and bismuth. The environmental and biochemical forces which may tend to reconcentrate these toxic materials in living cells are not well known. Although uranium occurs naturally, it has become much more available for entering into water, food, living cells and tissue since the mining boom which began shortly after the Second World War.
        The activity which takes place in the nucleus of the uranium or radium atom is a `haphazard' event obeying the laws of random probabilities. An atom is characterised by its atomic number, that is, the positively charged particles in its nucleus, and by its atomic mass, expressed in atomic mass units (similar to the concept of weight), which includes both the number of protons (the atomic number) and the number of neutrons in the nucleus. Carbon, the most frequently occurring chemical in living material, is taken as having exactly 12 atomic mass units and other atoms are measured in relation to this. Carbon 14, which is radioactive, has two extra neutrons in its nucleus.
        Hydrogen, another example, has an atomic number of 1 and an atomic mass of 1. Isotopes of hydrogen have the same atomic number (that is, the same number of positively charged particles in the nucleus and electrons in orbit around the nucleus) but a higher atomic mass. Deuterium or hydrogen 2, an isotope of hydrogen, has an atomic number of 1 and an atomic mass of 2. It is not radioactive. The increased atomic mass is due to an added neutron in the nucleus. Deuterium is in the `heavy water' used in the Canadian CANDU nuclear reactor. Hydrogen 3, called tritium, is radioactive, with two neutrons and a proton in the nucleus. It is produced in a nuclear reaction.
        When radium 226 decays, it loses a positively charged alpha particle from its nucleus. An alpha particle has two protons (positive electrical charges) and a mass of 4 atomic units. This means a reduction in both radium's atomic number and atomic mass. Loss of the alpha particle changes radium 226 (transmutes it) into another element, radon 222. While radium 226 is a radioactive solid under normal conditions, radon 222 is a radioactive gas. Loss of one or more protons changes the chemical element into a different chemical. Absorption or loss of a neutron gives an isotope of the same chemical since chemical properties are determined by the number of protons and electrons in an atom.
        The time required for half of any amount of radium 226 to transmute to radon 222 by these small explosions which emit alpha particles is 1,622 years. This is called the physical half-life of radium. Half of the radium literally disappears in that length of time, but radon gas is produced to replace it. Radon gas is radioactive and more mobile in air and water (it dissolves) than the solid radium. The half-life of radon is 3.82 days, after which half the gas will have disintegrated, again releasing alpha particles and transmuting into radioactive polonium 218, which is a solid. With a wind of 10 mph (or kph), the radon gas could travel 1,000 miles (or kilometres) from the point of origin before half of it would have decayed into its solid daughter products and been deposited on soil, leafy vegetables, tobacco, groundwater, human skin, lung tissue, etc. If the material receiving the radioactive daughter product is living, then it can carry the particles into its cells. Such contamination cannot be washed off.
        When a negatively charged beta particle is released, there is a transmutation in which a neutron in the nucleus of the atom splits into a proton and an electron, the proton remaining in the nucleus and the electron given off as a fast-moving microscopic bullet. Beta particles are extremely small. The mass of an alpha particle is about 7,400 times that of a beta particle. Thorium 234 decays to uranium 234 (with a short-lived radioactive intermediary) by losing beta particles. Uranium and thorium are different elements, but have the same mass (atomic weight) since a neutron and proton have about the same mass. The thorium neutron becomes the uranium proton. The half-life of thorium 234 is 24.1 days, while the half-life of uranium 234 is 2.50 x 10^5, or 250,000 years. As was pointed out earlier, uranium nuclear events are not as frequent as those in radium, although they are destructive when they occur.
        Given 12 grams of thorium 234, we would have 6 grams after 24.1 days, 3 grams after 48.2 days, 1.5 grams after 72.3 days, 0.75 grams after 96.4 days, etc. At the same time, the stock of uranium 234 would be increasing as the thorium decays into the new radioactive chemical.
        There is no simple physical or chemical process such as temperature change or chemical bonding which can prevent these radioactive elements from decaying. Their nucleus is unstable and because all elements seek a stable low-energy state, they must at some time release particles in an effort to reach a resting state. The decay takes place in the nucleus of the atom regardless of whether the atom exists singly or is part of a molecule; is in the solid, liquid or gaseous state; is within the body or outside, and so on. The decay product after a radioactive disintegration may itself be radioactive, so disintegration does not put an end to the biological problems generated by these small explosions. This decay process must be taken into account when estimating the biological effects of internal exposure to radioactive material. Inhaled radon gas quickly becomes radioactive lead, bismuth or polonium in the bloodstream.
        One should not confuse physical half-life with biological half-life, i.e. the time required to eliminate half of the material from the body through exhalation, urine or faeces. Cesium 137 and strontium 90 both have physical half-lives of almost thirty years, but cesium 137 is normally excreted from the body within two years while strontium 90 can be incorporated in bone for a lifetime.
        One more measure needs to be introduced before radiation protection guides can be understood. Since the various kinds of radiation exposures need to be evaluated for biological impact and not just for the amount of energy absorbed by the tissue, the term rem, roentgen equivalent man (or woman), was introduced. The rem dose is the rad dose times a quality factor Q. For external radiation Q is usually taken as 1, and rads and rems are used interchangeably. However, to reflect the greater biological damage done by alpha particles when inside the body, the rad dose may be multiplied by 20 to give the rem dose. This is another way of saying that the alpha particle does damage of an order of magnitude (20 times) greater when lodged within a tissue, bone or organ. For example, alpha particles giving a 2 rem (or rad) dose to skin would give a 40 rem dose to sensitive lung tissue when inhaled.
        Theoretically, the rem dose measures equivalent biological effect, so that damage from X-rays, for example, would be the same as damage from alpha particles, when the dose in rem was the same. Unfortunately, living systems are too complex for such an approach to provide anything more than a good guess.
        Sometimes references are made to a `fifty-year effective dose equivalent'. This is the full dose that would be received from an internal radionuclide if the dose were given at one time instead of being spread over two to fifty years.