The Atom in Use
DR. OTTO R. FRISCH, a nuclear physicist who has done research, in Berlin, London, Copenhagen, and Los Alamos, is Jacksonian Professor of Natural Philosophy at Cambridge University’s famed Cavendish Laboratory. In this article he describes some of the beneficial effects of man’s use of atomic energy.
ON DECEMBER 2 it will be fifteen years since the first man-made nuclear fire began slightly to warm a pile of graphite blocks and uranium rods in a secret laboratory hidden under the west stand of Stagg Field, Chicago. Today the fire is burning vigorously in a number of reactors all over the world: in the United States, in Britain, France, Norway, Sweden, and the U.S.S.R. It is a fire that gets its heat not from the chemical interaction of atoms and molecules, but from the innermost cores of the atoms: from the transformation of heavy atomic nuclei into lighter ones. In that process, enormous energy is freed: ton for ton, uranium is a fuel over two million times more potent than coal. The first uranium-fired power station in the U.S.S.R. has been followed by a much bigger one in Britain (at Calder Hall), where other stations are going up as fast as they can be built. Seagoing vessels have been supplied with this novel kind of boiler, and there is talk of atomic railway engines and airplanes. The atomic age is well on its way.
Will uranium eventually supplant all our other sources of energy? Not in the foreseeable future, I think. In the first place, it is not suited for small vehicles. A nuclear reactor throws off neutrons and gamma rays, both of them dangerous to living organisms; even when it runs at a power of only one watt (about what a flashlight uses) the radiation is above the safety margin. A railway engine or a car needs a hundred thousand times that power, so the reactor must be surrounded by a shield thick enough to cut the radiation down to a millionth or less. Even with the most careful and compact construction, such a shield weighs many tons. For ships, of course, that is all right, and it may be just feasible with a railway engine and perhaps in the very largest airplanes. Ordinary planes and motorcars will no doubt continue to run on gasoline as long as there is some; after that they may run on batteries charged from atomic power stations. But that is a long time in the future, and by then technology may have taken a turn we don’t dream of today.
POWER WHERE POWER IS NEEDED
With a stationary reactor supplying the energy for a power station, the weight of the shield is of course no problem even if it is made from 20,000 tons of concrete, as in Calder Hall. Such a power station, big enough to supply an industrial town of one hundred thousand inhabitants, can be built anywhere provided there is a landing field where the required few tons of uranium a year can be flown in and the used uranium flown out for reprocessing. No longer does one have to site a power station within railroad reach of a coal field, or where a large catchment area provides highpressure water for driving turbines.
This ability to place power stations where power is needed, however isolated the place may be, will probably be the first great benefit of uranium power. But apart from that, our power demands are going up all the time (it is estimated that they will have trebled before 1980) and our reserves of fossil fuel — coal and oil — are dwindling rapidly. So we must start to replace them by fissile fuel — uranium and later thorium — even though at present it is still cheaper to get power from fossil fuels. The power at Calder Hall is derived from the fission of the isotope U-235, of which ordinary uranium contains only 0.7 per cent; and only a fraction of that can be burned up before the uranium has to be removed for purification. But at the same time some of the U-238 —which makes up the bulk of the uranium — is converted into plutonium, which is a fuel as good as or better than U-235, and which can be used to fortify uranium that has lost part of its U-235.
More advanced reactors, still in the experimental stage, will produce more plutonium than the amount of U-235 they consume; once that is achieved, uranium can be gradually more and more fortified, and building compact and efficient reactors will become progressively easier. The process can be speeded up by fortifying uranium with U-235 — that is, by removing some of the U-238 it contains; this is done by pumping a gaseous uranium compound many times through porous membranes, which allow U-235 to diffuse slightly more quickly than U-238. Such diffusion plants — designed originally to help us make nearly pure U-235 for the A-bomb — are now being built to speed the day when they will no longer be needed because the reactors which they have made possible produce enough plutonium to make fortified fuel.
Once we have such “breeder" reactors, we shall also be able to use the much more common element thorium, which — like U-238 — is not nuclear fuel itself but can be converted into the fuel U-233 by cooking in a pile, just as U-238 can be transformed into plutonium. Easily workable deposits of uranium will last for a period of time variously estimated at from fifty to several hundred years; thorium ought to extend that period. After that we may be compelled to work the lowgrade ores which contain vastly greater amounts of uranium in very diluted form. Some common rocks, like granite, contain enough uranium to make their energy content comparable to that of coal — a few parts in a million. But extracting such minute amounts of uranium will require enormous factories and will leave mountains of residues.
SEA WATER AS FUEL
However, before we come to face that unattractive prospect we shall probably have learned to extract energy from an even more abundant source: from water, by fusion of the nuclei of heavy hydrogen contained in it. If we ever master the controlled release of energy by that process (its explosive release is the basis of the H-bomb), our fuel problem would be solved for good and all: sea water would become a fuel a hundred times more potent than gasoline. But the difficulties are vastly greater than with fission: the fusion reaction — as far as we can see at present — will have to be operated at a temperature of many million degrees, and the production, control, and containment of such temperatures — thousands of times hotter than the white heat of an arc lamp — is a stupendous problem. It will probably be a long, hard pull, and I do not expect energy from fusion to gain any large-scale importance during this century.
In the meantime fission is our only source of atomic energy, and here we must not forget the radioactive residues, the ashes of the nuclear fire. Fission means that the heavy nuclei of uranium or plutonium break into fragments about half the weight, and those fragments are unstable — that is, radioactive. While they are in the reactor their radiations are stopped by the shield; but every few months or so the uranium must be taken out and purified. Some of the radioactive nuclei formed from fission are highly unstable and will have changed into a more stable form by the time the uranium is out; they present no problem. Others last for some days, and it is well to leave the uranium for some weeks in a safe location — usually at the bottom of a water-filled ditch — until those atoms too have lost most of their activity.
There are some which last for months or years, and one can’t wait that long; so the chemical processing has to be done by remote control, some stages fully automatic, others done by operators who use complex long-handled tools and watch what goes on through thick lead-glass windows. And the whole plant has to be built with multiple safeguards so that no conceivable accident could lead to an escape of radioactive stuff into the air, the water, or the ground. When the uranium returns to the reactor, purified and fortified, we are left with a large amount of dangerous radioactive residue which has to be taken into protective custody for an indefinite time since some of the substances contained in it take centuries to become harmless. And the prison where they are kept must be guarded so that no accident — including military bombardment — could release those silent murderers from their jail.
X-RAYING WITH COBALT
It is a jaundiced view that sees merely the menace in fission products; viewed from a different angle, they are perhaps the greatest boon of the atomic age. Those radioactive substances allow us to do things we couldn’t do before.
A radioactive substance is one whose atomic nuclei are unstable. While it lasts, such an unstable nucleus behaves just like a stable one; but all the time there is a definite chance that it may suddenly break up, sending out an electron and usually a brief flash of radiation as well. Once that has happened we have a new nucleus which in many cases is stable; but with some substances the new nucleus is again unstable and we must be prepared for another sudden outburst. The radiation which is sent out is called gamma radiation and is of the same nature as X rays. This immediately indicates one use of radioactive substances: to supplant X-ray tubes, which, with their associated electrical equipment, are expensive and clumsy. A piece of radiocobalt, as big as a pea, can replace an X-ray set weighing several hundred pounds. X-raying a boiler to look for flaws in the welding used to be a laborious business; now, with a radiocobalt source lowered into the boiler, X-ray photos of the welds can be obtained very simply by placing X-ray films in the required places on the outside. And there is nothing that can go wrong with a radiocobalt source. Its main disadvantage is that the radiation can’t be switched off, and hence a heavy lead shield must be provided to absorb the radiation when it is not needed. Indeed, that shield is apt to weigh more than the X-ray set which the little cobalt source replaces! But there is still the advantage of intrinsic simplicity, reliability, and independence of electrical supplies.
Let me give you another example in which the advantage of a radioactive source over an X-ray tube is more apparent. Oil pipelines have to be cleaned from time to time, and a convenient way to do that is to insert a so-called go-devil, a scraper that is carried along by the oil flow and scrapes the wall of the pipe as it goes. Occasionally the go-devil jams, and it used to be a laborious business to locate it. Now a small amount of radiocobalt is attached to the go-devil before it is pushed into the pipe; the gamma rays easily penetrate the wall of the pipe and even a reasonable thickness of soil above it, and if the go-devil jams, it can be found quickly by running a radiation detector along the pipeline.
This is perhaps the simplest example of the use of radioactive materials as tracers—that is, for tracing the movement of something that is otherwise hard to trace. And from a go-devil some feet in size there is only a small step to the tracing of sand particles in tidal waters. The movement of sand at the bottom of a harbor is of great interest for the planning of future dredging requirements, and this is now studied with the help of artificial, radioactive sand having grains of the same mean size and density as the sand whose movements we wish to follow. This tracer sand is put down at selected points, and some days, weeks, or months later, samples are taken up in various places. By measuring how strongly radioactive they are, one can find out how much of the artificial sand they contain and hence how the sand has moved.
The most important and dramatic application of radioactive tracers is their use in the study of the metabolism of living beings, underlying the growth of living tissues. What we need here is not counterfeit grains of sand but counterfeit atoms, chemically identical with those whose metabolism we want to study and yet recognizable by some simple test.
In 1907 two Chicago chemists, Dr. Herbert N. McCoy and Dr. William 11. Ross, reported that after mixing two different radioactive substances, thorium and radiothorium, it was impossible to separate them again by any of the chemical methods they had tried. Similar reports soon came in from other laboratories. Most chemists were slow to accept those reports, based on the study of unweighable amounts of substances that could be detected only by their radiation; “chemistry of phantoms” they derisively called it. But by 1913 enough evidence had accumulated to make the British chemist Soddy coin a new word, isotopes, for denoting those variants of a chemical element. About the same time the physicists discovered supporting evidence, based on quite different techniques, and within a short time all doubts had disappeared; the “chemistry of phantoms” had become respectable, and workers crowded into this new and exciting field.
While most of them were trying to understand what isotopes meant for the inner structure of atoms, the young Hungarian chemist Georg von Hevesy saw them as a potential tool for the research chemist and biologist. He acted somewhat like the legendary businessman who made a fortune by selling an ineffective Hair-Growth Promoter in bottles labeled Hair Remover: he saw that the very impossibility of separating isotopes could be turned to good account.
Around 1920, scientists were arguing about the question: did atoms in solid matter stay put, or did they occasionally change places? There was some indirect evidence (for example, from electrical conductivity) that they did move about, but the question couldn’t be decided directly unless you could tell one atom from another of the same kind. Hevesy placed a layer of a lead compound in intimate contact with a layer of the same compound of his mixture of lead and radium D (a radioactive isotope of lead). At room temperature the layers remained sharply defined, but at higher temperature diffusion took place as expected: when the specimen, after a period of heating, was sliced up some radioactivity was found in the previously inactive parts.
By studying many specimens, heated to different temperatures for different times, it was possible to elucidate how the atoms worked their way through the solid. Since all attempts to separate lead and radium E chemically had failed, one could feel sure that their atoms would move in the same way. That was one of the first instances when atoms of radium E were used deliberately as counterfeit lead atoms.
At first the method was limited to lead and few other heavy atoms, but in 1934 the discovery of “artificial radioactivity” made radioactive isotopes of many light elements available. The amounts were small, but very sensitive measuring instruments such as the Geiger counter had come into existence at the right time; indeed, without that invention (about 1928) artificial radioactivity might not have been spotted for a long time, since only minute amounts of radioactive isotope are formed when light elements are bombarded with the alpha rays from radium or its like. Much greater amounts were soon made with the help of the newly invented cyclotron, and a much wider selection could be made through the intermediary of neutrons, also discovered in 1932.
Several years of intense development culminated in the discovery of nuclear fission in 1939, and the war hastened the construction of the first atomic pile. With that, the output of radioactive material was raised a millionfold, and the tracer method became available to any scientist who wanted to try it.
The discovery of isotopic tracers might be compared, in its character and importance, with the invention of the microscope. In each case there was no immediate benefit to health or happiness. But the microscope showed us the existence of small living organisms whose existence had not been suspected; and from there it was not far to the recognition that some of those organisms were causing illness, and to the systematic attack on the enemy once recognized. Isotopic tracers have increased our range of observation by another large factor: though we can’t see atoms, we can now follow their motion through a living organism and obtain entirely new insight into its functioning. Today, germs have ceased to be the principal enemy; the chief causes of death and suffering now are those diseases—like rheumatism or cancer — in which the organism’s inner balance is upset. To understand and prevent those upsets we must first understand how the balance is kept in the healthy organism, and here isotopic tracers will be one of the principal tools.
As always with the first steps in a new direction, some of the experiments now in progress seem disappointingly remote from the cure of human illness. You may for instance find a scientist engaged in growing plants in an atmosphere containing carbon dioxide in which some of the carbon atoms are of the radioactive isotope C-14 (C is the chemical symbol for carbon; 14 is the atomic weight, rounded off to the nearest whole number). After a certain time he will dissect the plant and from its various parts extract a number of chemicals which he will then test with a Geiger counter. In this way he can learn a great deal about photosynthesis, the process by which the plant builds up many of its chemicals from carbon dioxide and water, using light to supply the energy needed. He can find out which chemicals are formed first and which later, and which processes can go on in the dark and which require light. Much of what we now know about this most fundamental life process has been discovered in this manner. He can also regard his plants not only as an object for study but as a factory for the production of labeled chemicals. For instance, the sugar which his plants produce will contain C-14 and can in turn be used to study the sugar metabolism of other organisms, including humans. This might eventually lead us to a more satisfactory cure for diabetes than we have now, when the insulin which the patient has lost the ability to produce must be supplied to him at frequent intervals, and usually for the rest of his life.
THE STRUCTURE OF LIVING TISSUE
From such studies we have already learned that a living organism is far from being a static structure. It is not, so to speak, a house to which a wing may be added in time of plenty, to be torn down again when there is a shortage; it may rather be compared to a shop with a rapid turnover where each tin or package stays on the shelf for a short time only, to be replaced by an identical one as soon as it is sold. This had been expected for the chemicals contained in the body fluids, but it came as a surprise that even the proteins, those complex chains of carbon and nitrogen from which, for instance, our muscles are made, are undone and replaced every few days.
A great deal about the mechanism of this process has been found out by the use of isotopic tracers. The tracer used here, the nitrogen isotope N-15, is not radioactive but is present in small amounts in ordinary nitrogen, which consists mostly of N-14. It is not made in a fission reactor and has indeed been known for a number of years. But the need to separate the uranium isotopes U-235 and U-238 on a large scale stimulated the construction of equipment which can now be used to separate N-15 and other isotopes in practical quantities from their natural mixtures. In that sense even the use of non-radioactive tracers such as N-15 for nitrogen and O-18 for oxygen is a product of the effort that went into the exploitation of nuclear fission. The reason we have to use non-rad inactive tracers which require for their detection a fairly expensive instrument, a mass spectrometer, is that all radioactive isotopes of oxygen and nitrogen are so unstable that they decay in a matter of minutes, too fast for use in most biological experiments.
It is often asked whether the introduction of a radioactive material into an organism doesn’t affect it through its radiation. The answer is that usually there is no difficulty in keeping that effect negligibly small. This is so because a Geiger counter records single quanta of radiation and is thus many thousand times more sensitive than an organism. True, one radiation quantum can modify the nucleus of a germ cell sufficiently to cause a mutation, an inheritable change of character; but only one in many million quanta will score that kind of bull’s-eye. With rare exceptions the experimenter has very comfortable room for maneuver between the smallest radioactivity that he can detect and the biggest that the organism will tolerate.
That margin is often used to employ photographic methods which are less sensitive but much more informative than Geiger counters. A leaf grown, say, from soil containing radiophosphorus (P-32) is pressed onto a photographic plate and will then print its own picture, blackening the plate where the radioactivity has penetrated along the veins in its structure. It is even possible to embed cells in a special photographic emulsion and, after developing, to trace the fine crooked tracks of the individual electrons back to the cell they came from, under a microscope, and thus to discover which part of a cell that is invisible to the naked eye had accumulated the tracer.
HUNTING FOR CANCER
Certain chemical compounds have been found, by the use of tracer techniques, to become attached preferentially to cancerous tissue, and they can be used as bloodhounds: introduced into the bloodstream of a patient, they will seek out any cancerous growth and thus render it liable to detection with a counter. For finding deepseated growths in that manner, one can use one of the tracers that emit, not ordinary negatively charged electrons, but positrons; such a positron, after traveling a short distance, will undergo mutual annihilation with one of the ordinary electrons present in all matter, and the result is two quanta of gamma radiation, flying apart in exactly opposite directions. Two counters, say one above and one below the patient, will then record simultaneous arrivals of quanta in both counters, but only if their source — the looked-for growth — lies on a straight line with the counters, and by tilting that line the growth can be accurately located even deep inside the patient.
There is some hope that those bloodhounds may be trained not merely to spot the game but also to kill it: that strongly radioactive preparations could be made which would become almost exclusively attached to the cancer and would then destroy it by their radiation. So far, the efficiency of only one organ (the thyroid) in concentrating one element (iodine) has been found high enough to allow destruction of that tissue without damage to the rest of the body.
The destruction of cancer can of course be done from the outside, by irradiation with X rays or gamma rays. This well-established radiation therapy brings us back to the beginning: X-ray tubes and radioactive sources compete on roughly equal terms, as seen from the fact that hospitals all over the world continue to buy both kinds of radiation sources. The same competition exists in other applications of strong radiation — for instance, for killing germs in the sterilization of food or medical supplies, for the toughening of plastics, the improvement of transistors, or the removal of electrostatic charges from moving belts, paper, or cloth.
Radiation can also be used to fight certain insect pests where the female only mates once: males are reared in the laboratory, sterilized by radiation, and then released before their wild brethren, when they will mate with many females, causing them to lay eggs that won’t hatch. New applications are being invented all the time; X-ray tubes will be most economical for some of them, radioisotopes for others. Here is medical and technological advance on the move right now.
But in the long run, I think, it is the large-scale use of isotopic tracers that will come to be regarded as the main scientific advance of our time. It will not supplant but supplement the earlier tools of the scientist. Already it is used not only in biology and medicine but also in agriculture and engineering. But in particular it will give us vastly greater insight into the functioning of life processes, and thereby we shall be able to make the earth more healthy, fertile, and beautiful.