The basics of radioactivity

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Radioactivity is the spontaneous transformation of one element into another. When radioactive elements decay they emit different types of radiation known as alpha, beta or gamma. You cannot see or smell radioactive alpha, beta or gamma materials and this makes it extremely difficult to trace, locate and quantify. Indeed, for some radioactive elements you may be chasing a 'moving target' because the element may be solid one minute and gas the next e.g. radon. That is why it is important to have some idea of the way radioactive elements decay, the so-called 'decay chain' and their half-lives (time taken for its mass to decrease by 50%) because this gives you some idea of its form (i.e. gas, solid or liquid) as one element decays into another r and how long they remain active in that particular radioactive decay form.

The earth is roughly 4,600 million years old and during this time life on earth has evolved within a radioactive environment. It is present in the air we breathe and the food we eat. However, we now live in what is called the 'nuclear age' which is about 80 years old and some elements such as plutonium (with a few notable exceptions) are almost entirely anthropogenic or 'man-made'. Plutonium is one of the most toxic materials known to mankind as well as being an alpha-emitter. We know little of its effects on the body and much of the data we have is based on occupational exposure incidents, nuclear accidents or military research. We know something of its ability to mutate cells but little knowledge of dose effects unless that dose is very large. We have scant data on the effects for example, of small regular doses over long periods. Interestingly enough, some researchers have data which supports the idea that cell mutations that lead to cancer may not show up immediately but may skip one or two generations.

The upper atmosphere is still polluted with material from the atomic weapons test eras of the 1950's and nuclear reprocessing plants like Sellafield in Cumbria UK and Cap le Hague in France emit radioactive waste to the air and sea on a daily basis. What is very clear to me is that we know very little of the environmental changes this type of material is capable of producing. This is reflected in the constant revisions of what are defined as 'acceptable dose' from bodies such as the International Atomic Energy Agency. The amount of radioactivity absorbed by the body to produce either clinical change or cell mutation i.e. 'dose-effect' seems to be on a downward trend, almost at yearly intervals. The downgrading of the amount of radioactivity to produce adverse health or genetic changes to the body perhaps underlie the limited knowledge we have at present in determining dose cause-effect relationships, particularly chronic small doses over long time periods.

Elements contain atomic nuclei which consist of positively charged protons and neutral neutrons and they are bound together by a special nuclear force. Atoms also contain electrons which are negatively charged which are equal in magnitude to the proton. This means the atom is electrically neutral because the charges on the protons and electrons cancel each other out. Isotopes of an element are defined as such because they have a different number of neutrons For example, stable sodium nuclei contain 11 neutrons, whereas those with 12 and 13 are radioactive. These isotopes are notated as Na12 and Na13, where the right hand subscript indicates the number of neutrons.

Isotopes are also known as nuclides and radioactive nuclides are unstable in that they undergo spontaneous transformation into nuclides of other elements thereby releasing energy in the process. These transformations include alpha (a ) decay which is the emission of two protons and two neutrons from the nucleus, better known as an alpha-particle, alpha-emission or helium atom. Beta-decay-occurs when there are too many neutrons in the nucleus and one of the neutrons decays into a proton with the emission of a high energy electron (also known as an antineutrino). Gamma decay occurs when there are too few neutrons (or too many protons) within a nucleus where a proton is transformed into a neutron and a positively charged electron or positron. Gamma radiation, like light, is electromagnetic radiation, but by virtue of its much higher frequency, gamma rays are enormously more powerful.

When radioactive material decays it can emit matter alpha, beta or gamma decay and each mode of decay has the ability to penetrate into matter differently. Alpha particles although powerful in the sense that they have a high linear energy transfer (LET) into biological tissue can be stopped by the surface of skin cells. Beta particles can penetrate into metal of a few mm thick whilst gamma rays can penetrate tens of mm of lead and concrete.

Radioactivity was discovered in uranium salts by the French physicist Henri Becquerel in 1896 and the unit of radioactivity, the Becquerel is named after him. He noticed that photographic plates placed near uranium compounds became fogged from which he deduced that the uranium was emitting penetrating radiation.

The German nuclear chemists Otto Hahn and Fritz Strassmann (1902-80) provided evidence in 1938 that newer lighter elements were produced when uranium is irradiated with neutrons. This process is known as nuclear fission. Fission releases enormous energy and is used in nuclear fission weapons and reactors. Neutron reactions are studied by placing samples inside nuclear reactors, which produce a high neutron flux (high number of neutrons per unit area).

Biological effects can be observed when ionising radiation interacts with living tissue by transferring energy to molecules of cellular matter. Cellular function may be temporarily or permanently impaired as a result of such interaction, or the cell may be destroyed. The severity of the injury depends on the type of radiation, the absorbed dose, the rate at which the dose was absorbed, and the radio-sensitivity of the tissues involved. The effects are the same, whether from a radiation source outside the body or from material within. It is also important to point out that an a -particle from natural sources such as granite has the same biological effect on living tissue as that produced when a -particles from say nuclear weapons interact with human tissue.

The biological effects of a large dose of radiation delivered rapidly differ greatly from those of the same dose delivered slowly. The effects of rapid delivery are due to cell death, and they become apparent within hours, days, or weeks. Protracted exposure is better tolerated because some of the damage is repaired while the exposure continues, even if the total dose is relatively high. If the dose is sufficient to cause acute clinical effects, however, repair is less likely and may be slow even if it does occur. Exposure to doses of radiation too low to destroy cells can induce cellular changes that may be detectable clinically only after some years.

High whole-body doses of radiation produce a characteristic pattern of injury. Doses are measured in rads, 1 rad being equal to an amount of radiation that releases 100 ergs of energy per gram of matter. Doses of more than 4000 rads severely damage the human vascular system, causing cerebral oedema, which leads to profound shock and neurological disturbances; death occurs within 48 hours. Whole-body doses of 1000 to 4000 rads cause less severe vascular damage, but they lead to a loss of fluids and electrolytes into the intercellular spaces and the gastrointestinal tract; death occurs within ten days as a result of fluid and electrolyte imbalance, severe bone-marrow damage, and terminal infection. Absorbed doses of 150 to 1000 rads cause destruction of human bone marrow, leading to infection and haemorrhage; death, if it occurs, can be expected about four to five weeks after exposure. Currently only the effects of these lower doses can be treated effectively; but if untreated, half the persons receiving as little as 300 to 325 rads to the bone marrow will die.

Exposure of small areas of the body-the most frequent kind of radiation accident-leads to localised tissue damage. Damage to the blood vessels in exposed areas causes disturbed organ function and, at higher doses, necrosis (localised tissue death) and gangrene.

Injury from internally deposited radiation sources is not likely to cause acute effects but, rather, delayed phenomena, depending on the target organ and on the half-life, radiation characteristics, and biochemical behaviour of the radiation source. Consequences may include degeneration or destruction of the irradiated tissue and the initiation of cancer.

Non-malignant delayed effects of ionising radiation are manifested in many organs-particularly bone marrow, kidneys, lungs, and the lens of the eye-by degenerative changes and impaired function; these are largely secondary to radiation-induced damage to blood vessels. The most important late effect of radiation exposure, however, is an increased incidence of cancers and leukaemia of the types that occur naturally in non-exposed individuals. Statistically significant increases in leukaemia and of cancers of the thyroid, the lung, and the female breast have been demonstrated unequivocally only in populations exposed to relatively high doses (greater than 100 rads). Non-specific life-shortening effects suggested by animal experiments have not been demonstrated in humans as yet, however.

Are explosive devices, designed to release nuclear energy on a large scale, used primarily in military applications. The first atomic bomb (or A-bomb), which was tested on July 16, 1945, at Alamogordo, New Mexico, represented a completely new type of artificial explosive. All explosives prior to that time derived their power from the rapid burning or decomposition of some chemical compound. Such chemical processes release only the energy of the outermost electrons in the atom. See Atom and Atomic Theory.

Nuclear explosives, on the other hand, involve energy sources within the core, or nucleus, of the atom. The first A-bomb gained its power from the splitting, or fission, of all the atomic nuclei in several kilograms of plutonium. A sphere about the size of a baseball produced an explosion equal to 20,000 tons of TNT.

Subsequently, other types of bombs were developed to tap the energy of light elements, such as hydrogen. In these bombs the source of energy is the fusion process, in which nuclei of the isotopes (see Isotope) of hydrogen combine to form a heavier helium nucleus (see Thermonuclear, or Fusion, Weapons below).

Weapons research since 1945 has resulted in the production of bombs that range in power from a fraction of a kiloton (1000 tons of TNT equivalent) to many megatons (1 million tons of TNT equivalent). Furthermore, the physical size of the bomb has been drastically reduced, permitting the development of nuclear artillery shells and small missiles that can be fired from portable launchers in the field. Although nuclear bombs were originally developed as strategic weapons to be carried by large bombers, nuclear weapons are now available for a variety of both strategic and tactical applications. Not only can they be delivered by different types of aircraft, but rockets and guided missiles of many sizes can now carry nuclear warheads and can be launched from the ground, the air, or underwater. Large rockets can carry multiple warheads for delivery to separate targets.

In 1905 Albert Einstein published his special theory of relativity. According to this theory, the relation between mass and energy is expressed by the equation E = mc2, which states that a given mass (m) is associated with an amount of energy (E) equal to this mass multiplied by the square of the speed of light (c). A very small amount of matter is equivalent to a vast amount of energy. For example, 1 kg (2.2 LB) of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNT.

In 1939, as a result of experiments by the German chemists Otto Hahn and Fritz Strassmann (1902-80), who split the uranium atom into two roughly equal parts by bombardment with neutrons the Austrian physicist Lise Meitner, with her nephew, the British physicist Otto Robert Frisch (1904-79), explained the process of nuclear fission, which placed the release of atomic energy within reach.

When uranium or other suitable nuclei fission, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits a number of fast neutrons, the same type of particle that initiated the fission of the uranium nucleus. This makes it possible to achieve a self-sustaining series of nuclear fission; the neutrons that are emitted in fission produce a chain reaction, with continuous release of energy.

The light isotope of uranium, uranium-235, is easily split by the fission neutrons and, upon fission, emits an average of about 2.5 neutrons. One neutron per generation of nuclear fission is necessary to sustain the chain reactions. Others may be lost by escape from the mass of chain-reacting material, or they may be absorbed in impurities or in the heavy uranium isotope, uranium-238, if it is present. Any substance capable of sustaining a fission chain reaction is known as a fissile material.

A small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, would not sustain a chain reaction. Too many neutrons escape through the surface area, which is relatively large compared with its volume, and thus are lost to the chain reaction. In a mass of uranium-235 about the size of a baseball, however, the number of neutrons lost through the surface is compensated for by the neutrons generated in additional fission taking place within the sphere. The minimum amount of fissile material (of a given shape) required to maintain the chain reaction is known as the critical mass. Increasing the size of the sphere produces a supercritical assembly, in which the successive generations of fission increase very rapidly, leading to a possible explosion as a result of the extremely rapid release of a large amount of energy. In an atomic bomb, therefore, a mass of fissile material greater than the critical size must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. A heavy material, called a tamper, surrounds the fissile mass and prevents its premature disruption. The tamper also reduces the number of neutrons that escape.

If every atom in 0.5 kg (1.1 LB) of uranium were to split, the energy produced would equal the explosive power of 9.9 kilotons of TNT. In this hypothetical case, the efficiency of the process would be 100 percent. In the first A-bomb tests, this kind of efficiency was not approached. Moreover, a 0.5-kg (1.1-lb) mass is too small for a critical assembly.

Various systems have been devised to detonate the atomic bomb. The simplest system is the gun-type weapon, in which a projectile made of fissile material is fired at a target of the same material so that the two weld together into a supercritical assembly. The atomic bomb exploded by the United States over Hiroshima, Japan, on August 6, 1945, was a gun-type weapon. It had the energy equivalent of about 20 kilotons of TNT.

A more complex method, known as implosion, is utilised in a spherically shaped weapon. The outer part of the sphere consists of a layer of closely fitted and specially shaped devices, called lenses, consisting of high explosive and designed to concentrate the blast toward the centre of the bomb. Each segment of the high explosive is equipped with a detonator, which in turn is wired to all other segments. An electrical impulse explodes all the chunks of high explosive simultaneously, resulting in a detonation wave that converges toward the core of the weapon. At the core is a sphere of fissile material, which is compressed by the powerful, inwardly directed pressure, or implosion. The density of the metal is increased, and a supercritical assembly is produced. The Alamogordo test bomb, as well as the one dropped by the U.S. on Nagasaki, Japan, on August 9, 1945, were of the implosion type. Each was equivalent to about 20 kilotons of TNT.

Regardless of the method used to attain a supercritical assembly, the chain reaction proceeds for about a millionth of a second; during this process vast amounts of heat energy are liberated. The extremely fast release of a very large amount of energy in a relatively small volume causes the temperature to rise to tens of millions of degrees. The resulting rapid expansion and vaporisation of the bomb material causes a powerful explosion to occur.

Much experimentation was necessary to make the production of fissile material practical.

The fissile uranium-235 isotope accounts for only 0.7 percent of natural uranium; the remainder is composed of the heavier uranium-238. No chemical methods suffice to separate uranium-235 from ordinary uranium, because both uranium isotopes are chemically identical. A number of techniques were devised to separate the two, all of which depend in principle on the slight difference in weight between the two types of uranium atoms.

A huge gaseous-diffusion plant was built during World War II in Oak Ridge, Tennessee. This plant was enlarged after the war, and two similar plants were built near Paducah, Kentucky, and Portsmouth, Ohio. The feed material for this type of plant consists of extremely corrosive uranium hexa-fluoride gas, UF. The gas is pumped against barriers that have many millions of tiny holes, through which the lighter molecules, which contain uranium-235 atoms, diffuse at a slightly greater rate than the heavier molecules, containing uranium-238 (see Diffusion). After the gas has been cycled through thousands of barriers, known as stages, it is highly enriched in the lighter isotope of uranium. The final product is weapon-grade uranium containing more than 90 percent uranium-235.

Although the heavy uranium isotope uranium-238 will not sustain a chain reaction, it can be converted into a fissile material by bombarding it with neutrons and transforming it into a new species of element. When the uranium-238 atom captures a neutron in its nucleus, it is transformed into the heavier isotope uranium-239. This nuclear species quickly disintegrates to form neptunium-239, an isotope of element 93 (see Neptunium). Another disintegration transmutes this isotope into an isotope of element 94, called plutonium-239. Plutonium-239, like uranium-235, undergoes fission after the absorption of a neutron and can be used as a bomb material. Producing plutonium-239 in large quantities requires an intense source of neutrons; the source is provided by the controlled chain reaction in a nuclear reactor.

During World War II nuclear reactors were designed to provide neutrons to produce plutonium. The U.S. Atomic Energy Commission established reactors in Hanford, Washington, and near Aiken, South Carolina, capable of manufacturing large quantities of plutonium each year.

Even before the first atomic bomb was developed, scientists realised that a type of nuclear reaction different from the fission process was theoretically possible as a source of nuclear energy. Instead of using the energy released as a result of a chain reaction in fissile material, nuclear weapons could utilise the energy liberated in the fusion of light elements. This process is the opposite of fission, since it involves the fusing together of the nuclei of isotopes of light atoms such as hydrogen. It is for this reason that the weapons based on nuclear-fusion reactions are often called hydrogen bombs, or H-bombs. Of the three isotopes of hydrogen the two heaviest species, deuterium and tritium, combine most readily to form helium. Although the energy release in the fusion process is less per nuclear reaction than in fission, 0.5 kg (1.1 lb) of the lighter material contains many more atoms; thus, energy liberated from 0.5 kg (1.1 lb) of hydrogen-isotope fuel is equivalent to that of about 29 kilotons of TNT, or almost three times as much as from uranium. This estimate, however, is based on complete fusion of all hydrogen atoms. Fusion reactions occur only at temperatures of several millions of degrees, the rate increasing enormously with increasing temperature; such reactions consequently are known as thermonuclear (heat-induced) reactions. Strictly speaking, the term thermonuclear implies that the nuclei have a range (or distribution) of energies characteristic of the temperature. This plays an important role in making rapid fusion reactions possible by an increase in temperature.

Development of the hydrogen bomb was impossible before the perfection of A-bombs, for only the latter could yield that tremendous heat necessary to achieve fusion of hydrogen atoms. Atomic scientists regarded the A-bomb as the trigger of the projected thermonuclear device.

Following developmental tests in the spring of 1951 at the U.S. Eniwetok Proving Grounds in the Marshall Islands during Operation Greenhouse, a full-scale, successful experiment was conducted on November 1, 1952, with a fusion-type device. This test, called Mike, which was part of Operation Ivy, produced an explosion with power equivalent to several million tons of TNT (that is, several megatons). The Soviet Union detonated a thermonuclear weapon in the megaton range in August 1953. On March 1, 1954, the U.S. exploded a fusion bomb with a power of 15 megatons. It created a glowing fireball, more than 4.8 km (more than 3 mi) in diameter, and a huge mushroom cloud, which quickly rose into the stratosphere.

The March 1954 explosion led to worldwide recognition of the nature of radioactive fallout. The fallout of radioactive debris from the huge bomb cloud also revealed much about the nature of the thermonuclear bomb. Had the bomb been a weapon consisting of an A-bomb trigger and a core of hydrogen isotopes, the only persistent radioactivity from the explosion would have been the result of the fission debris from the trigger and from the radioactivity induced by neutrons in coral and seawater. Some of the radioactive debris, however, fell on the Lucky Dragon, a Japanese vessel engaged in tuna fishing about 160 km (about 100 mi) from the test site. This radioactive dust was later analysed by Japanese scientists. The results demonstrated that the bomb that dusted the Lucky Dragon with fallout was more than just an H-bomb.

The thermonuclear bomb exploded in 1954 was a three-stage weapon. The first stage consisted of a big A-bomb, which acted as a trigger. The second stage was the H-bomb phase resulting from the fusion of deuterium and tritium within the bomb. In the process helium and high-energy neutrons were formed. The third stage resulted from the impact of these high-speed neutrons on the outer jacket of the bomb, which consisted of natural uranium, or uranium-238. No chain reaction was produced, but the fusion neutrons had sufficient energy to cause fission of the uranium nuclei and thus added to the explosive yield and also to the radioactivity of the bomb residues.

As is the case with explosions caused by conventional weapons, most of the damage to buildings and other structures from a nuclear explosion results, directly or indirectly, from the effects of blast. The very rapid expansion of the bomb materials produces a high-pressure pulse, or shock wave, that moves rapidly outward from the exploding bomb. In air, this shock wave is called a blast wave because it is equivalent to and is accompanied by powerful winds of much greater than hurricane force. Damage is caused both by the high excess (or overpressure) of air at the front of the blast wave and by the extremely strong winds that persist after the wave front has passed. The degree of blast damage suffered on the ground depends on the TNT equivalent of the explosion; the altitude at which the bomb is exploded, referred to as the height of burst; and the distance of the structure from ground zero, that is, the point directly under the bomb. For the 20-kiloton A-bombs detonated over Japan, the height of burst was about 550 m (about 1800 ft), because it was estimated that this height would produce a maximum area of damage. If the TNT equivalent had been larger, a greater height of burst would have been chosen.

Assuming a height of burst that will maximise the damage area, a 10-kiloton bomb will cause severe damage to wood-frame houses, such as are common in the U.S., to a distance of more than 1.6 km (more than 1 mi) from ground zero, and moderate damage as far as 2.4 km (1.5 mi). (A severely damaged house probably would be beyond repair.) The damage radius increases with the power of the bomb, approximately in proportion to its cube root. If exploded at the optimum height, therefore, a 10-megaton weapon, which is 1000 times as powerful as a 10-kiloton weapon, will increase the distance tenfold, that is, out to 17.7 km (11 mi) for severe damage and 24 km (15 mi) for moderate damage of a frame house.

The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot incandescent mass of gas called a fireball. For a 10-kiloton explosion in the air, the fireball will attain a maximum diameter of about 300 m (about 1000 ft); for a 10-megaton weapon the fireball may be 4.8 km (3 mi) across. A flash of thermal (or heat) radiation is emitted from the fireball and spreads out over a large area, but with steadily decreasing intensity. The amount of heat energy received a certain distance from the nuclear explosion depends on the power of the weapon and the state of the atmosphere. If the visibility is poor or the explosion takes place above clouds, the effectiveness of the heat flash is decreased. The thermal radiation falling on exposed skin can cause what are called flash burns. A 10-kiloton explosion in the air can produce moderate (second-degree) flash burns, which require some medical attention, as far as 2.4 km (1.5 mi) from ground zero; for a 10-megaton bomb, the corresponding distance would be more than 32 km (more than 20 mi). Milder burns of bare skin would be experienced even farther out. Most ordinary clothing provides protection from the heat radiation, as does almost any opaque object. Flash burns occur only when the bare skin is directly exposed, or if the clothing is too thin to absorb the thermal radiation.

The heat radiation can initiate fires in dry, flammable materials, for example, paper and some fabrics, and such fires may spread if conditions are suitable. The evidence from the A-bomb explosions over Japan indicates that many fires, especially in the area near ground zero, originated from secondary causes, such as electrical short circuits, broken gas lines, and upset furnaces and boilers in industrial plants. The blast damage produced debris that helped to maintain the fires and denied access to fire-fighting equipment. Thus, much of the fire damage in Japan was a secondary effect of the blast wave.

Under some conditions, such as existed at Hiroshima but not at Nagasaki, many individual fires can combine to produce a fire storm similar to those that accompany some large forest fires. The heat of the fire causes a strong updraft, which produces strong winds drawn in toward the centre of the burning area. These winds fan the flame and convert the area into a holocaust in which everything flammable is destroyed. Inasmuch as the flames are drawn inward, however, the area over which such a fire spreads may be limited.

Besides heat and blast, the exploding nuclear bomb has a unique effect-it releases penetrating nuclear radiation, which is quite different from thermal (or heat) radiation (see Radioactivity). When absorbed by the body, nuclear radiation can cause serious injury. For an explosion high in the air, the injury range for these radiations is less than for blast and fire damage or flash burns. In Japan, however, many individuals who were protected from blast and burns succumbed later to radiation injury.

Nuclear radiation from an explosion may be divided into two categories, namely, prompt radiation and residual radiation. The prompt radiation consists of an instantaneous burst of neutrons and gamma rays, which travel over an area of several square miles. Gamma rays are identical in effect to X rays (see X Ray). Both neutrons and gamma rays have the ability to penetrate solid matter, so that substantial thickness of shielding materials are required.

The residual nuclear radiation, generally known as fallout, can be a hazard over very large areas that are completely free from other effects of a nuclear explosion. In bombs that gain their energy from fission of uranium-235 or plutonium-239, two radioactive nuclei are produced for every fissile nucleus split. These fission products account for the persistent radioactivity in bomb debris, because many of the atoms have half-lives measured in days, months, or years.

Two distinct categories of fallout, namely, early and delayed, are known. If a nuclear explosion occurs near the surface, earth or water is taken up into a mushroom-shaped cloud and becomes contaminated with the radioactive weapon residues. The contaminated material begins to descend within a few minutes and may continue for about 24 hours, covering an area of thousands of square miles downwind from the explosion. This constitutes the early fallout, which is an immediate hazard to human beings. No early fallout is associated with high-altitude explosions. If a nuclear bomb is exploded well above the ground, the radioactive residues rise to a great height in the mushroom cloud and descend gradually over a large area.

Human experience with radioactive fallout has been minimal. The principal known case histories have been derived from the accidental exposure of natives and fishermen to the fallout from the 15-megaton explosion that occurred on March 1, 1954. The nature of radioactivity, however, and the immense areas contaminable by a single bomb undoubtedly make radioactive fallout potentially one of the most lethal effects of nuclear weapons.

Besides the blast and radiation damage from individual bombs, a large-scale nuclear exchange between nations could conceivably have a catastrophic global effect on climate. This possibility, proposed in a paper published by an international group of scientists in December 1983, has come to be known as the "nuclear winter" theory. According to these scientists, the explosion of not even one-half of the combined number of warheads in the United States and Russia would throw enormous quantities of dust and smoke into the atmosphere. The amount could be sufficient to block off sunlight for several months, particularly in the northern hemisphere, destroying plant life and creating a subfreezing climate until the dust dispersed. The ozone layer might also be affected, permitting further damage as a result of the sun's ultraviolet radiation. Were the results sufficiently prolonged, they could spell the virtual end of human civilisation. The nuclear winter theory has since become the subject of enormous controversy. It found support in a study released in December 1984 by the U.S. National Research Council, and other groups have undertaken similar research. In 1985, however, the U.S. Department of Defense released a report acknowledging the validity of the concept but saying that it would not affect Defense policies.

On the average, about 50 percent of the power of an H-bomb results from thermonuclear-fusion reactions and the other 50 percent from fission that occurs in the A-bomb trigger and in the uranium jacket. A clean H-bomb is defined as one in which a significantly smaller proportion than 50 percent of the energy arises from fission. Because fusion does not produce any radioactive products directly, the fallout from a clean weapon is less than that from a normal or average H-bomb of the same total power. If an H-bomb were made with no uranium jacket but with a fission trigger, it would be relatively clean. Perhaps as little as 5 percent of the total explosive force might result from fission; the weapon would thus be 95 percent clean. The neutron bomb, which has been produced and tested by the U.S. and several other nuclear powers, does not release long-lasting radioactive fission products; however, the large number of neutrons released in thermonuclear reactions is known to induce radioactivity in materials, especially earth and water, within a relatively small area around the explosion. Thus the neutron bomb is considered a tactical weapon because it can do serious damage on the battlefield, penetrating tanks and other armoured vehicles and causing death or serious injury to exposed individuals, without producing the radioactive fallout that endangers people or structures miles away.

Chemical elements of atomic number greater than 92, that of uranium in the periodic table (see Periodic Law). At least 15 such elements have been identified. These elements consist of more than 100 radioactive isotopes (see Isotope), which are characterised by radioactive instability (see Radioactivity). These radioisotopes are produced artificially by bombarding heavy atoms either with neutrons produced in nuclear reactors or in specially designed nuclear explosives, or with charged particles accelerated to high energy in such devices as cyclotrons or linear accelerators. The first 11 transuranium elements, together with actinium, thorium, protactinium, and uranium, constitute the actinide elements chemically analogous to the lanthanide or rare earth elements (see Actinide Series; Lanthanide Series). They are, in order, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium.

Between 1964 and 1984, scientists in the United States, Europe, and the Soviet Union announced the definite or tentative production of six further transuranium elements beyond lawrencium in the periodic table, and hence beyond the actinide series. The first of these, element 104, was reportedly produced in a heavy ion cyclotron at Dubna, near Moscow, in 1964, by irradiating a plutonium target with neon ions. A team at Lawrence Berkeley Laboratory led by the American scientist Albert Ghiorso (1915- ) could not reproduce these results, but instead produced element 104 by bombarding californium with carbon atoms in 1969.

Element 105 was produced at Dubna in 1968 by bombarding americium with neon ions; Ghiorso's team achieved a similar result in 1970 by bombarding californium with nitrogen ions. In 1974 the Dubna group produced element 106 by bombarding lead isotopes with a beam of chromium; the American team produced it that same year by using californium and oxygen.

The production of element 107 was announced in 1977 by the Dubna research team, using a bismuth target and a beam of chromium, but has not yet been confirmed elsewhere. Elements 108 and 109, unlike these earlier discoveries, were synthesised in West Germany (now part of the united Federal Republic of Germany) in 1984 and 1982, respectively, by a team of researchers using the Unilac accelerator at Darmstadt.

For several years some international competition existed for naming these later entries in the periodic table. In 1980 the International Union of Pure and Applied Chemistry ruled that element 104 and all those beyond would henceforth simply be called the Latin equivalent of their atomic numbers. Thus 104 is named unnilquadium, 105 is unnilquintium, and so forth.

The radioactive decay rates of the transuranium elements tend to increase with increasing atomic number; the very heavy transuranium nuclei, such as californium, tend to fission spontaneously. As a result, it is extremely difficult to manufacture large quantities of the elements heavier than plutonium. This problem is being attacked by bombarding uranium and plutonium with very intense streams of neutrons in reactors such as the High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee. In the mid-1970s this reactor was producing several milligrams per year of berkelium, californium, and einsteinium, and small amounts of fermium. In addition, nuclear explosions, which release very high neutron fluxes, can be designed specifically to encourage the instantaneous production of the heavy elements einsteinium and fermium. Once sufficient quantities of the heavy elements are available, it should be possible to use isotopes such as plutonium-238 and curium-244 as extremely compact and dependable, although somewhat expensive, sources of power, with the radioactive-decay heat converted directly to electricity by thermoelectric devices. Other transuranium isotopes such as americium-241 and californium-252 have medical and industrial uses.

The search for still heavier elements continues. Many such superheavy elements, however, will have such short half-lives that positive identification will likely be very difficult. Theoretical studies suggest that the hypothetical superheavy element with atomic number 114 and a number of neighbouring elements may have comparatively stable nuclear arrangements. Producing such heavy nuclei will require accelerating much heavier ions than have been accelerated to date.