Nuclear Reactions


The New Transmutation

Once it was understood that the atom was made up of smaller particles, which rearranged themselves spontaneously in radioactive transformations, the next step seemed almost ordained.

Man could deliberately rearrange the atomic structure of molecules in ordinary chemical reactions. Why not, then, deliberately rearrange the protons and neutrons of the atomic nucleus in nuclear reactions? To be sure, the protons and neutrons are bound together by forces far stronger than those binding atoms in molecules, and methods that sufficed to bring about ordinary reactions would not suffice for nuclear reactions, but the men who had solved the puzzle of radioactivity were traveling the high road of success.

It was Rutherford who took the first step. He bombarded various gases with alpha particles and found that every once in a while an alpha particle would strike the nucleus of an atom and disarrange it.

In fact, Rutherford was able to demonstrate, in 1919, that alpha particle could knock protons out of nitrogen nuclei and merge with what was left behind. The most common isotope of nitrogen is nitrogen-14, which has a nucleus made up of 7 protons and 7 neutrons. Subtract a proton and add the 2 protons and 2 neutrons of the alpha particle and you end with a nucleus possessing 8 protons and 9 neutrons. This is oxygen-17. The alpha particle can be considered as helium-4 and the proton as hydrogen-1.

It follows then that Rutherford had successfully carried through the first man-made nuclear reaction:

nitrogen-14 + helium-4 --> oxygen-17 + hydrogen-1

This is a true example of transmutation, the conversion of one element to another. In a way, it was the climax of the old alchemical goals but it involved elements and techniques of which the alchemists had never dreamed.

Over the next five years, Rutherford carried through a number of other nuclear reactions involving alpha particles. What he could do was limited because radioactive elements provided alpha particles of only moderate energies. To accomplish more, much more energetic particles were required.

Physicists took to designing devices to accelerate charged particles in an electric field, forcing them to move faster and faster and therefore to possess more and more energy. The English physicist John Douglas Cockcroft (1897-1967) and his co-worker, the Irish physicist Ernest Thomas Sinton Walton (1903-1995), were the first to design an accelerator capable of producing particles energetic enough to carry through a nuclear reaction, accomplishing this in 1929. Three years later, they bombarded lithium atoms with accelerated protons and produced alpha particles. The nuclear reaction was:

hydrogen-1 + lithium-7 --> helium-4 + helium-4

In the Cockcroft-Walton device, and in others that were being planned, the particles were accelerated in a straight line, and it was difficult to build devices long enough to produce extremely high energies. In 1930, the American physicist Ernest Orlando Lawrence (1901-1958) designed an accelerator that forced the particles to travel in a slowly expanding spiral. A relatively small cyclotron of this sort could produce highly energetic particles.

Lawrence's first tiny cyclotron was the ancestor of today's huge instruments half a mile in circumference, which have been used to probe for answers to the fundamental questions concerning the structure of matter.

In 1930, the English physicist Paul Adrien Maurice Dirac (1902-1984) had advanced theoretical reasons for supposing that both protons and electrons ought to have true opposites (anti-particles). The anti-electron ought to have the mass of an electron but be positively charged, while the anti-proton would have the mass of a proton but be negatively charged.

The anti-electron was indeed detected in 1932 by the American physicist Carl David Anderson (1905-1991), in his study of cosmic rays. (Cosmic rays consist of particles entering earth's atmosphere from outer space. The particles (mostly protons) are sped to almost unimaginable energies by acceleration across the electric fields associated with the stars and with the galaxy itself.) When cosmic ray particles strike atomic nuclei in the atmosphere, some particles are produced that curve in a magnetic field just as electrons do, but in the opposite direction. Anderson named the particle of this sort the positron.

The anti-proton defied detection for another quarter century. Since the anti-proton is 1836 times as massive as the anti-electron, 1836 times as much energy is required for its formation. The necessary energies were not created in man-made devices until the 1950's. Using huge accelerators, the Italian-American physicist Emilio Segre (1905-1989) and his co-worker, the American physicist Owen Chamberlain (1920- ), were able to produce and detect the anti-proton in 1955.

It has been pointed out that atoms might well exist in which negatively charged nuclei, containing anti-protons, are surrounded by positively charged positrons. Such anti-matter could not exist for long on the earth or, perhaps, anywhere in our galaxy, for on contact both matter and anti-matter would be annihilated in a great burst of energy. However, astronomers wonder if there may not be whole galaxies built of anti-matter. If so, they might be most difficult to detect.

Artificial Radioactivity

The first nuclear reactions carried through successfully produced isotopes already known to occur in nature. This, however, was not inevitable. Suppose a neutron-proton combination not occurring in nature were to be produced, as, a century earlier, organic molecules not occurring in nature had been produced. This phenomenon was, indeed, accomplished in 1934, by the husband-wife team of French physicists, Frederic Joliot-Curie (1900-1958) and Irene Joliot-Curie (1897-1956), the latter being the daughter of the Marie and Pierre Curie of radium fame.

The Joliot-Curies were bombarding aluminum with alpha particles. After they had ceased the bombardment, they discovered that the aluminum continued to radiate particles of its own. They had begun, they discovered, with aluminum-27 (13 protons plus 14 neutrons) and ended with phosphorus-30 (15 protons plus 15 neutrons).

But phosphorus, as it occurs in nature, is made up of one atom variety only, phosphorus-31 (15 protons plus 16 neutrons). Phosphorus-30, therefore, was an artificial isotope, one that did not occur in nature. The reason it did not occur in nature was clear; it was radioactive, with a half-life of only 14 days. Its radioactivity was the source of the continuing particle radiation the Joliot-Curies had observed.

The Joliot-Curies had produced the first case of artificial radioactivity. Since 1934 over a thousand isotopes not occurring in nature have been formed, and every one of them is radioactive. Every element possesses one or more radioactive isotopes. Even hydrogen has one, hydrogen-3 (also called tritium) with a half-life of 12 years.

An unusual radioactive carbon isotope, carbon-14 was discovered in 1940 by the Canadian-American chemist (Martin D. Kamen (1913- ). Some of this isotope is formed by cosmic ray bombardment of the nitrogen in the atmosphere. This means that we are always breathing some carbon-14 and incorporating it into our tissues, as all life-forms do. Once a life-form dies, the incorporation ceases and the carbon-14 already present slowly decays away.

Carbon-14 has a half-life of over 5000 years, so that significant amounts linger on in material (wood, textiles) dating back to prehistoric times. The American chemist Willard Frank Libby (1908-1980) devised a technique for dating archaeological remains by their carbon-14 content as the earth's crust can be dated by uranium and lead contents. Thus, chemistry has come to be of direct use to historians and archaeologists.

Chemicals can be synthesized with unusual isotopes incorporated in place of the ordinary ones. These might be the rare stable isotopes, for instance (hydrogen-2 in place of hydrogen-1, carbon-13 in place of carbon-12, nitrogen-15 in place of nitrogen-14, or oxygen-18 in place of oxygen-16). If animals eat such tagged compounds and are later killed and their tissues analyzed, the compounds in which the isotopes are found yield significant information. It becomes possible to deduce reaction mechanisms within living tissue that might otherwise go undetected. An inventor in this sort of work was the German-American biochemist Rudolf Schoenheimer (1898-1941), who performed important researches on fats and proteins using hydrogen-2 and nitrogen-15 in the years after 1935. The use of radioactive isotopes makes it possible to trace reactions even more delicately, but it was not until after World War II that such isotopes became available in quantity. An example of what can be done with isotopes was the work of the American biochemist Melvin Calvin (1911- ). He used carbon-14 during the 1950's to work out many of the reactions involved in the process of photosynthesis. He did this with a detail that would have been deemed wildly impossible only twenty years earlier.

Nor was it merely artificial isotopes that were formed. Artificial elements were formed also. In 1937, Lawrence, the inventor of the cyclotron, had bombarded a sample of molybdenum (atomic number 42) with deuterons (nuclei of hydrogen-2). He sent the bombarded sample to Segre in Rome. (Later Segre was to come to the United States and in his new home was to discover the anti-proton.)

Segre, on close study, found the sample to contain traces of a new radioactive substance, which turned out to be atoms of the element with atomic number 43. At the time that element had not been discovered in nature (despite some false alarms) and so it was named technetium, from a Greek word meaning "artificial".

Eventually the three remaining gaps in the periodic table were filled. In 1939 and 1940, elements number 87 (francium) and 85 (astatine) were discovered, and in 1947, the last gap, that of element number 61 (promethium) was plugged. All these elements are radioactive.

Astatine and francium are formed from uranium only in most minute quantities, the scarcity explaining why they were not discovered earlier. Technetium and promethium are formed in even smaller quantities, and are unusual in that they are the only elements of atomic number less than 84 which possess no stable isotopes at all.

Transuranium Elements

The first particles used to bombard atomic nuclei were positively charged - the proton, deuteron, and alpha particle. Such positively charged particles are pushed away by the positively charged atomic nuclei, since among electric charges like repels like. It takes considerable energy to force the speeding particles to overcome the repulsion and strike the nuclei, and so nuclear reactions were rather hard to bring about.

Once the neutron was discovered a new possibility offered itself. Since neutrons were uncharged, the atomic nuclei did not repel them. A neutron could easily strike an atomic nucleus, without resistance, if the neutron happened to be moving in the right direction.

The first to investigate neutron bombardment in detail was the Italian physicist Enrico Fermi (1901-1954). He began his work almost immediately upon hearing of the discovery of the neutron. He found that a beam of neutrons was particularly effective in initiating nuclear reactions if it passed through water or paraffin first. The light atoms in these compounds absorbed some of the neutron's energy with each collision and did so without absorbing the neutrons themselves. The neutrons were therefore so slowed down that eventually they moved with only the normal speed of molecules at room temperature. Such thermal neutrons stayed in the vicinity of a particular nucleus a longer fraction of a second and were more likely to be absorbed than fast neutrons were.

When a neutron is absorbed into an atomic nucleus, that nucleus does not necessarily become a new element. It may simply become a heavier isotope. Thus, if oxygen-16 gained a neutron (with a mass number of 1) it would become oxygen-17. However, in gaining a neutron an element might become a radioactive isotope. In that case, it would generally break down by emitting a beta particle, and by Soddy's rule that would mean it would become an element one place higher in the periodic table. Thus, if oxygen-18 gained a neutron, it would become radioactive oxygen-19. That isotope would emit a beta particle and become stable fluorine-19. Thus oxygen would be converted (one atomic number higher) by neutron bombardment.

In 1934, it occurred to Fermi to bombard uranium with neutrons to see whether he could produce atoms more massive than uranium (transuranium elements). At that time uranium had the highest atomic number in the periodic table, but this could mean merely that elements of higher atomic number had half-lives too short to have survived the earth's long past history.

At first Fermi actually thought he had synthesized some of element number 93, but the results he obtained were confusing and led to something else far more dramatic, as will shortly be described. These other developments distracted attention for a few years from the possible formation of transuranium elements.

In 1940, however, the American physicist Edwin Mattison McMillan (1907-1991) and his colleague, the chemist Philip Hauge Abelson (1913- ), in their work on neutron bombardment of uranium, did indeed detect a new type of atom. On investigation, it proved to be one with an atomic number of 93, and they named it neptunium. Even the longest-lived neptunium isotope, neptunium-237, had a half-life of only a little over two million years, not enough to allow it to survive across earth's long history. Neptunium-237 was the ancestor of a fourth radioactive series.

McMillan was then joined by the American physicist Glenn Theodore Seaborg (1912- ), and together they formed and identified plutonium, element number 94, in 1941. Under the leadership of Seaborg, a group of scientists at the University of California, over the next ten years, isolated a half-dozen more elements: americium (number 95), curium (number 96), berkelium (number 97), californium (number 98), einsteinium (number 99), and fermium (number 100).

There seemed no reason to suppose that any atomic number represented an absolute maximum. However, each succeeding element is harder to form and is produced in smaller quantities. What's more, the half-lives grow shorter so that what is formed vanishes more and more quickly. Nevertheless, in 1955, mendelevium (number 101) was formed; in 1957, nobelium (number 102), and in 1961 lawrencium (number 103). In 1964, Russian physicists reported the preparation of element number 104, in trace amounts.

Seaborg and his group recognized that the transuranium elements resembled each other much as the rare earth elements do, and for the same reason. New electrons are added to an inner electron shell, leaving the outermost electron shell with a three-electron content throughout. The two sets of similar elements are distinguished by calling the older one, which begins with lanthanum (atomic number 57) the lanthanides, while the newer one, which begins with actinium (atomic number 89), is the actinides.

With the discovery of lawrencium, all the actinides had been formed. Element number 104 is expected to have chemical properties quite different from the actinides.

Nuclear Bombs

But what of Fermi's original work on the bombardment of uranium with neutrons? His suspicion that element number 93 had been formed could not be confirmed at that time, for the physicists who labored to isolate it all failed.

Among those joining the investigation were Hahn and Meitner, the discoverers of protactinium twenty years before. They treated the bombarded uranium with barium, which carried down in precipitation a certain fraction of strongly radioactive material. This reaction made them suspect that one of the products of the bombardment was radium. Radium is very similar, chemically, to barium and would be expected to accompany barium in any chemical manipulations. However, no radium could be obtained from those barium-containing fractions.

By 1938, Hahn began to wonder if it were not a radioactive isotope of barium itself that had been formed from the uranium in the course of neutron bombardment. Such radioactive barium would merge with ordinary barium and the two could not then be separated by ordinary chemical techniques. Such a combination seemed impossible, however. All nuclear reactions known up to 1938 had involved changes in elements of only 1 or 2 units in atomic number. To change uranium to barium meant a decrease, in atomic number, of 36! It was as though the uranium atom had broken more or less in half (uranium fission). Hahn hesitated even to speculate on such a possibility - at least, not in public.

In 1938, Nazi Germany invaded and annexed Austria. Lise Meitner, an Austrian, was forced into exile because she was Jewish. From her place of exile in Sweden, the dangers she had undergone must have made those involved in making a scientific error seem small indeed. She published Hahn's theory that uranium atoms when bombarded with neutrons underwent fission.

This paper created great excitement because of the horrendous possibilities it evoked. If a uranium atom, upon absorbing a neutron, breaks into two smaller atoms, those smaller atoms will need fewer neutrons than were originally present in the uranium atom. (In general, the more massive an atom the greater the number of neutrons it requires in proportion to its mass number. Thus calcium-40 contains 20 neutrons, 0.5 its mass number; while uranium-238 contains 146 neutrons, 0.65 its mass number.) These superfluous neutrons would be emitted, and if they were absorbed by other uranium atoms, those would also undergo fission and emit still more neutrons.

Each splitting uranium atom would bring about the splitting of several more in a nuclear chain reaction, with a result similar to that of an ordinary chemical chain reaction in the case of hydrogen and chlorine. But since nuclear reactions involved far greater energy exchanges than chemical reactions did, the results of a nuclear chain reaction would be far more formidable. After beginning with just a few neutrons, involving only the most trifling investment of energy, colossal stores of energy could be released.

World War II was on the point of starting. The United States Government, fearful that the deadly energies of the atomic nucleus might be unleashed by the Nazis, launched a research program to achieve such a chain reaction and place the weapon in its own hands.

The difficulties were many. As many neutrons as possible had to be made to collide with uranium atoms before escaping out of the uranium altogether. For this reason the uranium had to be quite large in bulk (the necessary size is the critical mass) in order to give the neutrons the needed chance. Yet when research began there was very little uranium available, for there had been almost no use for the substance prior to 1940.

Then, too, the neutrons had to be slowed down so as to increase the probability of their being absorbed by uranium. This meant the use of a moderator, a substance with light atoms against which the neutrons would bounce. That moderator might be graphite blocks or heavy water.

As a further difficulty, it was not just any uranium atom that underwent fission on absorbing a neutron. It was the rather rare isotope uranium-235. Methods had to be devised to separate and concentrate uranium-235. This was an unprecedented task, for the separation of isotopes on a large scale had never before been carried through. One successful method made use of uranium hexafluoride, which required a massive advance in the handling of fluorine compounds. The man-made element plutonium was found to undergo fission also, and, after it was discovered in 1941, efforts had to be made to produce it in large quantities.

Fermi, who had left Italy in 1938 and had come to the United States, was placed in charge of the task. On December 2, 1942, an atomic pile of uranium, uranium oxide, and graphite "went critical". A chain reaction was maintained and energy was produced through uranium fission.

By 1945, devices were prepared in which, when a small charge of explosive was set off, two pieces of uranium were driven together. Each piece by itself was below critical mass, but together they were above it. Thanks to cosmic ray bombardment, the atmosphere always contains stray neutrons, so a nuclear chain reaction starts at once in the critical mass of uranium, which explodes with fury hitherto unimagined.

In July 1945, the first such "atomic bomb" or "A-bomb" (more properly called a fission bomb) was exploded in Alamogordo, New Mexico. By the next month, two more bombs were manufactured and were exploded over Hiroshima and Nagasaki in Japan, ending World War II.

Uranium fission is not sed exclusively for destruction, however. When the energy production is maintained at a constant, safe level, fission can be put to constructive use. Atomic piles, renamed, more appropriately, nuclear reactors, had been built in great numbers during the 1950's and 1960's. They are used to propel submarines and surface vessels, and also to produce energy, in the form of electricity, for civilian use.

Energy can be obtained not only through the fission of massive atoms, but also through the union of two light atomic nuclei into a somewhat heavier one (nuclear fusion). In particular, colossal energies can be obtained if hydrogen nuclei are fused to helium.

In order to force hydrogen atoms together, past the shielding of the electron which circles the nucleus, tremendous energies must be given them. Such energies are attained in the centers of the sun and of other stars. The radiation of the sun (reaching the earth in undiminished quantities for billions of years) is the energy produced by the nuclear fusion of millions of tons of hydrogen every second.

In the 1950's the necessary energy could also be reached by exploding a fission bomb, and methods were devised for using a fission bomb to spark off a still greater and more destructive variety of nuclear bomb. The result was what is variously called a "hydrogen bomb", and "H-bomb", a "thermonuclear device" but, most properly, a fusion bomb.

Fusion bombs have been constructed and exploded with thousands of times the destructive potential of the first fission bombs that destroyed two cities in Japan. A single large fusion bomb could destroy utterly even the greatest city, and if all the fusion bombs known to exist were exploded over various cities, it is possible that all life would be destroyed by direct blast and fire, and by scattered radioactivity (fallout).

Even the fusion bomb, however, may have uses above and beyond destruction. Among the most important experimental work having been conducted is the attempt to produce extremely high temperatures of hundreds of millions of degrees in a controlled fashion (and not in the center of an exploding fission bomb) and to maintain those temperatures long enough to spark a fusion reaction.

If such a fusion reaction can then be kept going at a controlled rate, fantastic quantities of energy may be produced. The fuel would be deuterium, or heavy hydrogen, which is present in tremendous quantities in the ocean quantities vast enough to last us for millions of years.

Never before has mankind had to face the possibility of extinction in an all-out fusion bomb war, nor has it had occasion for hope of unexampled prosperity in the taming of that same fusion bomb.

Either fate could result from a single branch of scientific advance. We are gaining the knowledge. Are we also gaining wisdom?