THE physicist has been successfully attacking in the past year the problem which five hundred years ago the alchemist vainly pursued — the transmutation of the elements. The alchemist failed then because he did not know enough about the constitution of the atom. He did not even know that an atom had a nucleus carrying a positive electrical charge, the magnitude of which charge alone determines whether it is an atom of oxygen, nitrogen, silicon, iron, gold, or any other of the ninety-two elements, from the lightest, hydrogen, of nuclear charge one, up to the heaviest, uranium, of nuclear charge ninety-two. To-day we do know this, and by utilizing that knowledge within the past year the number of elements which, with wholly artificial techniques, the physicist has been able to transmute into other elements has risen to more than seventy — more than the total number of known elements forty years ago.
How has he done it? Primarily in two different ways. One way is to shoot electrically charged particles with enormous energies, such as protons (the nuclei of hydrogen atoms), into the nuclei of the atoms of the element which he wishes to transmute. But how can he hit a nucleus — the smallest object on earth, so small, indeed, that if everything were to swell like Alice in Wonderland, so that the smallest point visible to our eyes became as big as the earth, the nucleus of an atom would be only the size of a pinhead? No physicist, with all his skill, is marksman enough to hit that kind of bull’s eye at any single shot, so he adopts the machine-gun principle, loads his gun with a million million bullets, and confidently expects one of the million million to find its mark. The fact that all the rest must fail shows why the transmutation of mercury into gold by this method, while scientifically possible, cannot be commercially profitable.
The machine gun which Madame Curie’s daughter and her husband, M. Joliot, used for spraying the nucleus of the atom of aluminum in their attack upon this problem was the so-called alpha rays of polonium, a radioactive element discovered thirty-seven years ago by Madame Curie herself — an element which shoots out spontaneously an enormous number of highspeed particles which are in fact the positively charged nuclei of helium atoms, each of atomic weight four units. The technique used here was precisely that employed in 1919 by Rutherford in the first successful transmutation experiments. When these charged helium atoms strike one of the aforementioned nuclei of aluminum, they not only transform it into phosphorus, but they impose upon this new nucleus a larger positive charge of electricity than it likes. Consequently it begins to emit positive electrons and thus spontaneously changes itself into silicon, so that both phosphorus and silicon appear as a result of the bombardment of aluminum by charged atoms of helium.
This was the discovery of artificial radioactivity announced early in 1934, for which the Curie-Joliot pair has been most justly awarded the Nobel prize. That the babe who was being rocked in the cradle when radium itself was being discovered by the joint work of her father and mother should thirty-eight years later, with her husband in turn, show that radioactivity — a phenomenon her parents’ generation thought completely beyond the control of man — could actually be produced artificially, demonstrates in a striking way both how much there is in heredity and how man is slowly mastering and controlling nature.
This Curie-Joliot discovery has been followed up by many laboratories in many countries, the machine gun which hurls positively charged bullets — protons, deuterons, alpha rays— being in several cases, as in the W. K. Kellogg Radiation Laboratory at Pasadena, not a natural substance, like polonium, but a purely artificial device. This device, developed five years ago by Professor Charles C. Lauritsen, takes in at one end ordinary hydrogen or helium nuclei or other ions, accelerates them by electrical fields up to an energy of a million, or even one and a half million, electron volts, and then throws them into a target, consisting of the element that is to be transmuted.
Put there is another, a gentler and an even more significant way in which the transmutation of the elements is now being brought about. It has been developed largely this past year through the underlying discovery of the socalled ‘neutron’ made three years ago by Chadwick, in England — a discovery for which he also has justly received the Nobel prize. Chadwick’s discovery consisted in showing that when the nucleus of boron, — more familiar to us as boracic acid, — which weighs 11 atom-weight-units, is bombarded by and captures an alpha ray emitted by polonium, which ray weighs 4 units, the result is not an atom that weighs 11+ 4=15 units. Instead there appears an atom of nitrogen, which weighs 14 units, and the extra unit is thrown out in the form of a neutron — a theretofore undiscovered unit of nature which has practically the same weight as has the nucleus of the atom of hydrogen (a proton), save that it carries no electrical charge whatever. That is why it is called a neutron.
Now these neutrons have since been produced artificially in great abundance. They are a by-product of many of the artificial transmutations already mentioned, for they are probably constituents of all nuclei, except hydrogen, and many nuclear transformations throw them out. They can in their turn be hurled into targets where they change the mass of the nucleus and also often make it unstable, so that it throws out, say, a proton and thus transmutes itself again into a new element. Thus, when silicon captures a neutron, an atom of aluminum is made, and a nucleus of hydrogen — a proton —flies out.
But the major advance in 1935 consisted in the discovery that neutrons not only do not need to have large energy to get into a nucleus and transmute it into something else, but they slip in — or better, fall in — more easily and oftener when they have slow speeds than when they try to force their way in with violence. Indeed, when any proton, neutron, or alpha ray is thrown with large energy into a given nucleus, it generally smashes up that nucleus and hence often produces several substances of smaller atomic weight than the struck nucleus itself. But when a slow neutron moving no faster than the atoms of the surrounding air falls into a nucleus, — as it can easily do, since it has no charge and is therefore not repelled by the charged nucleus as a proton would be, — it simply adds its mass to that of the nucleus, so that the process results in the quiet building of a heavier atom. There are ordinarily no neutrons floating about to produce such transmutations, since unattached neutrons tend to get back home into some nucleus whenever they find themselves out in the cold world. But the radioactive processes going on all the time in the earth, and probably also the effects of cosmic rays, are such as occasionally to set free a neutron from some nucleus, in which event a new nucleus can be formed by the falling of the wandering neutron into somebody else’s house.
This flood of new light that has recently been shed upon the processes by which all the atoms not only have once been built up out of a unit having the weight of the hydrogen atom, — for a neutron is essentially an uncharged hydrogen nucleus, — but are now being so built up, marks the past year as one of much significance for the history of physics. All this work really started with the Bothe and Becker experiments at Giessen, Germany, and the chief centres in which the new results have been obtained have been at Rome, Cambridge, Paris, New York, Berkeley, and Pasadena.
The artificial beta ray radioactivity described above can be readily exhibited even to a large audience by exposing a plate of silver to bombardment by neutrons. A bit of beryllium mixed with an infinitesimal amount of radium emanation makes a good neutron source, and when one of these neutrons enters the nucleus of an atom of silver the latter, of course, has its atomic weight increased by one unit, — the weight of the neutron, — but it still remains silver, since the chemical properties of an atom are all determined by the charge on its nucleus, and this has not yet changed. But this new, extra-heavy atom of silver is actually unstable, and so it proceeds at once to throw out a negative electron, thus changing the charge on its nucleus and transforming itself, by so doing, into cadmium. Now a few years ago the physicist devised an instrument called a ‘counter-tube’ which responds audibly whenever an electron shoots through it. Bringing the plate of silver, after exposure to neutrons, close to such a counter causes a number of clicks to be heard, and these are a good measure of the number of the aforementioned negative electrons coming out of the silver plate. Anyone who has heard these clicks produced by a silver plate thus rendered artificially radioactive has been a witness of this new man-made, man-controlled process of changing silver into cadmium.
But the counter-tubes, used to exhibit transmutation, also are useful for exhibiting the existence of the cosmic rays, the most powerful nuclear disintegrating agency thus far discovered. Such a tube placed anywhere on the surface of the earth clicks slowly but continuously, and day in and day out perfectly steadily, thus revealing the constancy of the bombardment of everything on earth by the cosmic rays, which originate we know not where or how, but which certainly come to us from beyond the Milky Way. What arc these cosmic rays, and what have we learned about them this past year?
In the first place, 1935 marked the completion of a precision world survey of the sea-level intensities of cosmic rays — which means merely the number per hour of such cosmic-ray shots per square centimetre of the earth’s surface. This survey started in 1932 and has been quietly going on ever since, through the sending of accurate self-recording electroscopes, developed by Dr. H. Victor Neher, on ten different ships all over the world. No observer has accompanied these ships. The captains of Cunard, Dollar, Matson, and Grace Line ships, of English and Peruvian freighters, and of Captain Allan Hancock’s yacht Valero III have taken these automatic instruments on board, agreeing to wind up the driving clock occasionally, and the photographic records of a three months’ voyage have then been worked up in each case after the return of the instruments to the home laboratory. This precision survey has furnished a means of gaining new and exact information about the properties both of the cosmic rays and of the earth’s magnetic field far above the earth’s surface. For this and the following reasons 1935 was a notable year both in the study of cosmic rays and in that of terrestrial magnetism.
But the cosmic-ray advance that has most to do with transmutation is found in the fact that the first direct measurement has been made, with an enormous magnet, of the energies of the cosmic rays on a mountain top 14,000 feet high, where the rays are five and one-half times as intense as at sea level. The huge two-ton magnet, which since 1931 has provided most of our direct knowledge about the energies and the other properties of cosmic rays that can be revealed by cosmic-ray cloudchamber tracks, was mounted on a trailer, pulled up to the top of Pikes Peak, and a fifty-kilowatt power plant also pulled up to run it. The most noteworthy finding is that there is no evidence as yet of any heavy particles, like protons or other nuclei, coming in from outside. More than 98 per cent of all the cosmic-ray tracks found there appear to be the tracks of either positive or negative electrons, the great majority of which are secondaries produced by the collision of the incoming primary cosmic rays with the nuclei of the atoms constituting our atmosphere.
Heavier nuclear particles (mostly protons) do occasionally appear on Pikes Peak as they do at sea level, and their frequency of occurrence is considerably greater than at sea level, but the evidence seems to be valid that they represent merely the fragments of the nuclei of atoms thrown out in all directions by nuclear collisions of the incoming cosmic-ray photons or electrons. This direction of emission, as well as the shortness and thickness of the tracks, distinguishes these heavy particles sharply from electrons, for electron secondaries, whether positive or negative, practically always move straight on in the direction of the incoming rays which produce them, whereas these heavier products of nuclear hits show no tendency to do so. Although these heavier nuclear fragments are mostly protons, there seems to be good evidence that neutrons also occasionally appear, possibly also some heavier nuclei. The infrequency of occurrence, however, of any kind of heavier particle is seen from the fact that, out of 10,000 photographs of cosmic-ray tracks taken on Pikes Peak last summer by Professor Carl D. Anderson and Dr. Seth Neddermeyer, only about 100 showed evidence of any heavy particles at all. Secondary electrons, both positive and negative, are, then, the normal results of the collisions of cosmic rays with nuclei both at sea level and on Pikes Peak.
Last year new sea-level energy measurements were made by Blackett and Brode in London, which carry the highest cosmic-ray energies directly measured from the stupendous value of about ten thousand million electron volts, where the measurements made with the foregoing magnet left them, up to the value of fifteen thousand million electron volts. These are the highest particle energies yet known through any direct measurement or any dependable theories. Where such enormous energies come from no one as yet knows. If the super-alchemy of the instantaneous transmutation of a whole atom of oxygen or silicon or iron into radiation is assumed to be possible, then the Einstein quantitative relation between mass and energy (E = mc2) is sufficient to account for the highest cosmic-ray energy that has been observed. But we have as yet no other evidence that such an event can take place. The nearest approach to it is found in the case of the falling together of a positive and a negative electron. Here we have ample evidence that such complete transmutation of mass into radiation actually occurs, the whole mass of the two electrons being transmuted into two half-million electronvolt photons which fly apart in opposite directions.
Even if we leave the origin of the cosmic rays for the future physicist, the mere fact discovered in our day that we are thus being continuously bombarded from all directions by celestial bandits equipped with ordnance of such unearthly power continues to be one of the wonders of the modern world. Not only how these bullets are formed, but whether they travel through space in the main as light waves (photons) or as electrons — both these questions are as yet completely undetermined. Widely circulated opinions favoring the latter theory have been shown, by airplane flights with Neher self-recording electroscopes in Peru and the Philippines up to altitudes of more than 26,000 feet, to rest on incorrect assumptions as to the experimental situation.
Another achievement of 1935 brought particles into a position more nearly at rest than has ever before been attained. Here is another of the wonders of the modern world which may sometime throw new light upon the nature of both the electron and the atom, perhaps even upon the problem of transmutation. At Berkeley, Leiden, Cambridge and Oxford, Berlin, Toronto, and Pasadena are laboratories which are striving to attain temperatures as near as possible to absolute zero, and at Leiden there has actually been attained this year a temperature .005° C. above absolute molecular rest. The method was suggested by Debye of Leipzig as early as 1918, but was first successfully used independently in Berkeley and in Leiden. The equipment of both these laboratories consists of extraordinary compressors and liquefiers to produce liquid air, liquid hydrogen, liquid helium. Then by evaporation of the liquid helium a temperature only a degree or so above absolute zero is obtained. Then enormous magnets are used to force the atoms and electrons to line themselves up in systematic array against their own inclination. Then the exciting magnetic field is suddenly thrown off and the rebellious molecules and electrons at once spring back to where they were before they were forced into line. In so springing, however, they expend all the energy they have and then drop utterly exhausted, almost without motion, and therefore almost at absolute zero. The principle is the same as that used in all refrigeration machines. For, as everyone knows, to cool a room we first compress a gas into the liquid state, taking away by running water the heat of compression; then we let the compressed molecules evaporate or fly apart as their willful nature prompts them to do. In so doing they get over their ‘mad’ and cool themselves. Thus, by utilizing nature’s ‘mads,’ mere man has tricked her until she has fallen down limp and exhausted practically at the absolute zero of temperature.
These three achievements — transmutation, cosmic rays, absolute zero — have been a part at least of the progress of physics in the past year.