1940: Physics

Nuclear Physics.

Throughout 1940 the type of physics called 'nuclear' continued to be the most conspicuous; and one of the most portentous events of the year was the Conference on Applied Nuclear Physics held in the autumn in Cambridge. Many a physicist who had thought himself versed in the field was none the less surprised at being confronted with no fewer than one hundred papers, taking an entire week with three simultaneous sessions on most of the days. Very few of the hundred were devoted either to nuclear theory or to description of apparatus, and not a large proportion to the medical uses of radioactivity. Six half-day sessions were absorbed by the study of animal and plant metabolism with radioactive tracers: that is to say, feeding the radioactive isotopes of various elements to animals and plants, and following the wanderings of each through the structure of the organism. As lately as six years ago this was feasible with three elements only, none being of great biological interest (lead, polonium, radium). At this conference were reported researches made with carbon, phosphorus, sulphur, sodium, chlorine, potassium, calcium, manganese, iron, arsenic, bromine, rubidium and strontium, all of these radioactive bodies being recent products of the art of transmutation. Three sessions were devoted to applications of radioactive isotopes in following the trends and measuring the rates of chemical reactions, particularly the 'exchange reactions' which take place when compounds involving chemically-identical atoms are brought into contact with one another in or out of solution, and the atoms of one compound exchange their places with the identical atoms of the other. Radioactive isotopes of magnesium, copper, iodine and mercury were used in these researches, as well as some of the others previously listed. Two sessions were filled with metallurgical applications, especially the study of diffusion of like through like—e.g., the diffusion which occurs when a thin film of radioactive copper is laid down upon the surface of a block of ordinary copper, and the radioactivity spreads gradually through the entire mass. Medical applications though not predominant were not neglected, and the dangers of unwise exposure to radioactive bodies or X-rays or neutrons were the subject of long and grave debate.

Radioactive Isotopes.

Since radioactive isotopes are coming to be so variously useful, deserving mention are two of the most outstanding which were discovered during 1939-1940 and will soon be coming into use: 'hydrogen 3' and 'carbon 14.' The former had been known as a product of transmutation for several years, but was supposed to be stable: in proving it radioactive, Alvarez and Cornog enabled physicists to say now and hereafter that there are radioactive isotopes of every known element. Hydrogen 3 is the lightest and simplest of all radioactive nuclei: it transforms itself into helium 3 by emission of a negative electron. Carbon 14 likewise emits a negative electron, transforming itself into nitrogen. The half-lives of both are remarkably long for artificial radioactive substances (many decades) and the emitted electrons are of feeble energy; but both should be very useful in the study of metabolism and of chemical reactions. Another interesting radioactive nucleus is the first well-attested isotope of 'Element 85,' which requires thirty-million-volt alpha-particles (available only from the newest and greatest cyclotron) to produce it from bismuth. Another is the first well-attested isotope of 'Element 93,' which is prepared by projecting slow neutrons into uranium 238; a fraction of these are caught by uranium nuclei, some of which subsequently emit negative electrons, becoming thus the element in question; McMillan has lately proved that this also is radioactive. The great multitude of radioactive substances engendered by fission of uranium or thorium has still not been fully explored; there are probably a hundred or more, among which certainly no fewer than sixteen different elements are represented. Fission has been produced (at the Westinghouse research laboratory) by gamma-ray photons of great energy. The stable isotope 198 of mercury has been made from gold in sufficient quantity so that its spectrum-lines can be detected, an achievement which may lead to the establishment of a new standard of wavelength.

Neutrons.

The usefulness of neutrons continues to increase. The powerful effects of rapid neutrons upon living tissue are now the object of research in many an institute where physics is combined with medicine or biology. During late 1939 and early 1940 it was announced (by Beyer, Whitaker, Rasetti) that single crystals are much more penetrable to slow neutrons than are polycrystalline masses of the identical substance and equal thickness: in calcite the percentage of transmitted neutrons may be threefold as great. Late in 1940 it was announced (Nix, Beyer, Dunning) that annealed iron-nickel alloys are more penetrable by several per cent than are the same alloys when quenched. The latter difference is ascribed to the fact that the annealed alloys are 'ordered,' the quenched alloys 'disordered,' in the sense in which metallurgists use these words; both differences are traced to the undulatory quality which neutrons, in common with all other forms of matter and of light, possess. The magnetic moment of the neutron has been measured by Alvarez and Bloch. When it is subtracted from the well-known magnetic moment of the proton, the remainder agrees closely with the well-known magnetic moment of the deuteron. Now the deuteron is formed of a proton and a neutron adhering together, so the result agrees beautifully with the simple conception that the two particles adhere with their magnetic moments pointing in opposite directions. Nevertheless the theorists are dissatisfied, for such a simple conception appears untenable to them.

Mesons.

In the cosmic-ray field, it is the mesotron or meson which at present holds the spotlight. The existence of this surprising particle, intermediate in mass between electron and proton (around 150-200 times heavier than the former and 10 times lighter than the latter) is now no longer questioned. Its radioactivity (for like the nuclei just mentioned, it is unstable and resolves its instability by emitting an electron) is now very nearly if not quite beyond question, though as lately as July of 1939 a conference of cosmic-ray specialists at Chicago held it by no means proved. Were it not for this radioactivity, the percentage of mesons absorbed by (say) the three kilometers of air extending between altitudes of 0.5 and 3.5 kilometers in the atmosphere would be no greater than that absorbed by 40 centimeters of lead. Actually the former is considerably greater than the latter, this being proved by comparing the number of mesons underneath a lead plate at the greater altitude with the number in the open air at the lesser altitude (Rossi, Hilberry, Neher, Pomerantz and many others). From the comparisons it follows that the mesons disappear in the air at a rate compatible with an average lifetime of a few microseconds. 'Disappearance' means, in this connection, the separation of the meson into a neutral particle and a free electron; the neutral particles have not been detected, the electrons have been observed but have much less penetrating power than the mesons. Since the lifetime of the meson is so short, one is practically obliged to infer that these particles are created in the upper atmosphere. This has in fact been demonstrated by Jesse, Schein and Wollan of A. H. Compton's school, who sent up sounding-balloons equipped with automatic apparatus for the registration of mesons, and found the number of these particles rising rapidly at first and then reaching a maximum at an altitude where the pressure of the air is less than a tenth what it is at sea-level. Other apparatus demonstrated the productions of mesons at these altitudes by chargeless particles, presumed to be photons. Expansion-chambers taken in airplanes to heights of 30,000 feet exhibit many tracks of indubitable mesons and even of protons, these last now being regarded by some as the true primary cosmic radiation which enters the atmosphere from extra-terrestrial space and engenders all the other kinds of rays.

Pressures.

It would be unfair to yield the whole of this brief notice to nuclear physics. As a counterpoise I quote some of the work of Bridgman with very high pressures, a field in which for many years he has rarely been rivaled and never surpassed. Having three years before extended the range of attainable pressures to 50,000 (kg./cm²) by making the pressure-chamber of tapering form and forcing it like a stopper into a tapering cavity in a thick metal plate, he now in 1940 presents the first large group of data obtained at such pressures, viz., the shrinkages of volume suffered by 6 of the more compressible elements and by 38 compounds. He established new 'fixed points' of the pressure-scale, comparable with the fixed points of thermometry, by measuring the pressure at which mercury freezes at 30°C., and the pressure at which there occurs a 'polymorphic transition' of bismuth marked by a sudden appreciable shrinkage: these are 13,715 and 25,420 respectively. The presence of a piece of bismuth in a compressor acts as the presence of a piece of ice in a system undergoing a gradual heating or cooling: that is to say, when the compressing piston is forced down the pressure ceases to rise as soon as the transition commences, remaining constant at the critical value until all of the lump is transformed. Bridgman then found himself unexpectedly able to extend his pressure-range to and even beyond 200,000, owing to the remarkable increase in the strengths of metals when these are exposed to confining pressure exerted hydrostatically from all sides instead of one side only; this occurs in his newest apparatus, in which advantage is taken of the high confining pressures made available by the construction described above. Sodium chloride experiences a shrinkage of over 20 per cent and sulphur one of 30 per cent, but the long-sought-for transformation of graphite into diamond is still not achieved.