And Now the Neutron
In 1932 British physicist Sir James Chadwick discovered the subatomic particle known as the neutron, filling in a key missing piece in science’s understanding of the atom. The neutron has no electric charge, has a mass nearly equivalent to that of a proton, and is a component of the nucleus of an atom. Chadwick received the 1935 Nobel Prize in physics for his discovery. Manchester Guardian Scientific Correspondent J. G. Crowther reported on Chadwick’s work in the following article, which appeared in Scientific American in 1932. At the time the article was written, the atomic nucleus was still being described as containing combinations of electrons and protons, rather than combinations of neutrons and protons.
And Now The Neutron
By J. G. Crowther
With the
discovery of the neutron, science now has one more physical entity to juggle with. Though some of the press appears several weeks ago to have given the impression that the neutron was a brand new lucky strike, physicists have suspected for years that some such thing was “hiding out†and if sought would sooner or later be brought to light by experiment. The accompanying discussion is more mature than it could have been had it been written earlier. It was especially prepared for Scientific American by a writer, himself a Cambridge man, who is in close personal touch with the principals now engaged in the brilliant researches going on in the famous Cavendish Laboratory at Cambridge University, England.
—The Editor
The modern theory of atomic physics has been erected on the results of sporting with atomic particles. By observing what happens when one atom bumps into another the physicist is led to deduce what the structure of atoms is like. Rutherford suggested that atoms are made of an extremely small positively charged nucleus with relatively distant electrons revolving around it as planets revolve around the sun, because particles shot at atoms go right through them except when they pass through a very small volume in the center, when they are deflected.
The positive charge of electricity in the atom appeared to be concentrated in a spot in the center; it was not spread all over the atom. The particles used in this shooting affair were the alpha-particles from radium, which consist of the nuclei of helium atoms. The machine-gun used by the gangsters of experimental atomic physics is the disintegrating atom of radium or a similar radioactive substance. When the radioactive atom explodes it shoots forth alpha-particles at 10,000 miles per second, beta-particles or electrons at nearly 186,000 miles per second, and some very penetrating wave radiations. It may send forth all or only one or two of these three sorts of radiations. Polonium emits alpha-particles only.
In 1912 Dr. James Chadwick, then a young man of 21 working with Rutherford, discovered that if an alpha-particle or a beta-particle shot out from a disintegrating atom were arranged to bump into the nucleus of another atom, this nucleus received such a shock that it vibrated violently and emitted a penetrating radiation or gamma ray. In 1919 Rutherford showed how an atom of aluminum could be disintegrated by arranging that an alpha-particle emitted by an exploding radioactive atom should hit the nucleus of an aluminum atom. The alpha-particle knocked a bit out of the nucleus of the aluminum atom, and this bit proved to be the nucleus of a hydrogen atom, or a proton. Soon afterward Blackett showed that the tracks of alpha-particles and the tracks of struck atoms of nitrogen and the bits knocked out of them, might be revealed by the Wilson cloud chamber expansion apparatus. These experiments showed that the nucleus of the nitrogen atom captures the alpha-particle which strikes it, while a proton is ejected.
By arranging atomic collisions the physicists learned the constitution, structure, and properties of atoms. Today this method of atomic research is much refined and developed. Many workers use it in order to discover more about the finer points in the structure of the atom.
In 1930 Professor Bothe, now of Giessen in Germany, and Dr. Becker discovered that if atoms of the light metal beryllium are bombarded by alpha-particles they emit a radiation of quite exceptional penetrating power, much more penetrating than the radiations usually emitted by atomic nuclei caused to vibrate by impinging alphaparticles. These penetrating rays were apparently an extremely hard or penetrating gamma ray, which consists of waves similar in nature but shorter in wavelength than X rays. The new radiation was exceptionally interesting because it appeared to be a wave radiation intermediate in penetrating power between the ordinary gamma rays emitted by radioactive substances and the famous cosmic rays. It appeared to be due to waves shorter than gamma waves but longer than cosmic waves, so it might represent a near relative of the fascinating cosmic rays. Bothe and Becker had evidently discovered a very interesting radiation, and Dr. Millikan immediately saw how it might help toward the elucidation of the nature of the cosmic rays.
Toward the end of 1931 Mme. Curie-Joliot, the daughter of Mme. Curie, and her husband M. Joliot, started an investigation of the beryllium rays and measured their power of penetration by the thickness of material required to absorb them. They used polonium (which was originally discovered by Mme. Curie and named after her native country) as the radioactive source, because it emits alpha-particles only. Other radioactive substances emit mixtures of rays, and consequently produce complicated effects difficult to interpret. A small piece of beryllium was placed in front of the polonium. The alpha-particles from it shot forward and struck the nuclei of the beryllium atoms and caused the peculiar rays to be emitted. If these rays are allowed to fall on an ionization chamber they produce ionization in proportion to their intensity. If a sheet of lead is placed between the beryllium and the ionization chamber the rays are partly absorbed by the lead and the degree of ionization in the chamber falls.
The Curie-Joliots made the remarkable discovery that if a layer of paraffin wax is placed between the beryllium and the ionization chamber the degree of ionization is actually increased, under certain conditions. This seemed strange, for it was difficult to understand how an obstacle to the rays could increase their ionizing power. They showed that this effect was due to protons, or nuclei of hydrogen atoms, struck out of the wax by the rays. The protons were ejected at a high speed, about one-tenth the velocity of light, as was proved by deflecting them in a powerful magnetic field. When the protons entered the ionization chamber they dissipated their energy within a shorter distance than the beryllium rays had done, and so enabled the beryllium rays to communicate their energy to ions in a smaller volume than they could have done directly, which caused the illusion of an increase of ionizing power from the interposition of obstacles.
The Curie-Joliots assumed that the beryllium radiation was a wave radiation. When waves strike particles there is an exchange of energy, according to the law of the Compton effect. By measuring the energy of the ejected protons the energy of the beryllium rays could be calculated. Assuming that the rays were waves, the Curie-Joliots found that they must have an energy of about 50,000,000 electron-volts. The energy of the alpha-particles which excited them in the beryllium was not more than 6,000,000 electron-volts. 44,000,000 electron-volts of energy seemed to have appeared from nowhere, so the Curie-Joliots assumed they had discovered a new mode of interaction between waves and matter.
While the Curie-Joliots had been making these interesting researches, Dr. James Chadwick and colleagues in the Cavendish Laboratory at Cambridge, England, had also been studying the peculiar rays from beryllium. They were the sort of phenomenon with which he had been familiar from the beginning of his career as a research physicist. He found that the beryllium rays would eject particles from other substances besides paraffin wax and materials containing hydrogen. He found that particles were ejected from helium, lithium, carbon, air, argon, and beryllium itself. When the rays were directed on to a chamber containing hydrogen, protons were knocked forward with speeds up to one-tenth the velocity of light. The particles ejected from the other substances were, in general, nuclei of their atoms which had recoiled after being struck by the radiation.
Dr. Chadwick measured the energy of some of the recoiling nuclei of nitrogen atoms and found that they were capable of releasing 30,000 ions in the ionization chamber, which means that they were able to detach 30,000 electrons from atoms they had bumped into. Now if these recoiling nitrogen nuclei had been struck by a wave radiation of 50,000,000 electron-volts energy they would not have been able to produce more than 10,000 ions. Dr. Chadwick explained that if the nitrogen nuclei had been struck by a particle of mass about equal to that of the proton, and moving with one-tenth the velocity of light, it would be able to produce about 30,000 ions. But the Curie-Joliots had noticed that the beryllium radiation could cause protons to move with about one-tenth the velocity of light. Of course this is what would happen if the beryllium radiation consisted of particles of mass about equal to those of protons, for the struck particle would bounce forward with the velocity of the particles of equal mass which had hit it. Thus Dr. Chadwick explained the high energy of the particles struck by the beryllium rays exactly, and without departing from the law of the conservation of energy. This was very strong evidence in favor of the beryllium rays being particles of mass 1.
There were several other consistent pieces of evidence which could be understood if the rays were particles, but not if they were waves. In conjunction with Webster, Chadwick had found that the beryllium rays were much more penetrating in the direction of movement of the alpha-particles which had excited them. This would be expected if the rays were particles, but not if they were waves. Webster found that alpha-particles from polonium excited rays in boron and fluorine which resembled the beryllium rays in having much greater penetrating power in the forward than in the backward direction. Another point: Dee discovered that the beryllium rays decreased in ionizing power after they had passed through obstacles. This is a property of particle radiations and not of wave radiations. When rays of waves pass through an obstacle there is a reduction of intensity but not of energy. Fewer waves get through, but those which do are as energetic as they were before they came to the obstacle. This result suggested that the beryllium rays were particles.
The original belief that the rays were waves arose from the experiments of Bothe and Becker, which showed that they were not measurably deflected by magnetic or electric fields. Rays of particles, such as alpha-rays or beta-rays, are deflected by strong fields. The failure to obtain such deflections with magnetic and electric fields seemed to show that the beryllium rays were waves. Particles such as protons, electrons and alpha-particles are deflected in virtue of their electric charges. A proton has a positive charge, and an electron a negative charge. These charges enable them to be deflected by a magnetic or electric field. Thus if the beryllium rays were particles, these must be without electric charge. Chadwick required a particle of mass 1 and without electric charge. Such a particle would explain all the observed results.
He had not to look far for the idea of such a particle. If a proton and an electron are held extremely close together, almost in contact, they will form a particle of mass 1, for the mass of an electron is negligible, and without charge, because the positive charge on the proton neutralizes the negative charge on the electron. Such a particle had been clearly conceived by Rutherford in 1920. Rutherford's remarkable foresight in this matter will be discussed presently. Chadwick saw that a close combination of a proton and an electron was required. Such a particle could be named a 'neutron.'
Could the beryllium rays reasonably be supposed to be neutrons? Could the beryllium atom be expected to produce neutrons when bombarded by alpha-particles? Chadwick saw that this was reasonable. The nucleus of the beryllium atom contains nine protons and five electrons, that is, eight protons, four electrons, and a proton plus electron or neutron. When the alpha-particle, which contains four protons and two electrons, strikes a beryllium nucleus it is captured and a neutron is emitted. Thus
beryllium nucleus (eight protons, four electrons, one neutron) + alpha-particle (four protons, two electrons) = one neutron (one neutron) + one carbon nucleus (twelve protons, six electrons)
The bombardment produces one ordinary carbon atom nucleus and one neutron. If waves instead of neutrons were emitted, the atomic nucleus formed would be of a much less probable type. There are also energy properties described by what are termed 'packing-fractions' which are fitted by the neutron hypothesis but not by the wave hypothesis.
These are the evidences which led Chadwick to suggest that the beryllium rays are streams of neutrons. There is not one but several converging lines of evidence.
The real birth of the idea of a neutron is contained in Rutherford's Bakerian Lecture to the Royal Society of London in 1920. He was reviewing knowledge of the structure of atoms, after the achievement of artificial disintegration in the previous year, and passed on to speculations concerning possible sorts of matter which might exist though not yet discovered. He said:
'The idea of the possible existence of an atom of mass 1 which has zero nucleus charge [is involved]. Such an atomic structure seems by no means impossible. On present views the neutral hydrogen atom is regarded as a nucleus of unit charge with an electron attached at a distance, and the spectrum of hydrogen is ascribed to the movements of this distant electron. Under some conditions, however, it may be possible for an electron to combine much more closely with the hydrogen nucleus, and in consequence it should move freely through matter. Its presence would probably be difficult to detect by the spectroscope, and it may be impossible to contain it in a sealed vessel. On the other hand, it should enter readily the structure of atoms, and may either unite with the nucleus or be disintegrated by its intense field, resulting possibly in the escape of a charged hydrogen atom or an electron or both.'
Rutherford went on to suggest possible neutrons of mass 2, 3, 4, and so on. His remarks on the property of passing freely through matter are interestingly illustrated by absorption measurements which indicate that neutrons penetrate more than one mile of air, compared with the few inches which is the maximum range of the most energetic alpha-particle from radium.
Recently the study of the cosmic rays had focused attention on the possible properties of neutrons. Dr. Millikan and his colleagues had discussed the theory of their properties and some American physicists had published theoretical papers. In May, 1931 Langer and Rosen of the Massachusetts Institute of Technology published an interesting paper in the Physical Review. They suggested how neutrons might have a fundamental rôle in the evolution of matter. They conceived the neutron as a special form of the hydrogen atom, in which the electron and proton had come very close together. Assuming that this could happen, the evolution of the ordinary elements such as oxygen and iron out of the primordial protons and electrons became easier to understand:
In the beginning the universe consisted of protons and electrons, the two ultimate particles of electricity. Some of these came together and formed ordinary neutral atoms of hydrogen. Some of the atoms of hydrogen condensed into neutrons. As neutrons have no electric charge they have no difficulty in packing together, because they do not repel each other, as particles with like electric charges. Thus neutrons might be capable of making extremely dense material, millions of millions times denser than water. If a few odd electrons or protons happened to be ejected from such a conglomeration there might be condensations into little groups of protons and electrons which formed nuclei of the various heavy elements.
This suggestion made by Langer and Rosen is more credible than the usual view that the next element after hydrogen to be formed was helium, and that helium was formed by the accidental coming-together of four protons and two electrons. This would involve the simultaneous collision of six particles, which is extremely improbable. Besides, the possibility of dense matter being formed out of neutrons might help to explain the nature of the 'white dwarf' stars. These are thousands of times denser than water and are supposed to be at extremely high temperatures. If they had a core of neutrons they could be dense and yet cool. The neutron may have remarkable possibilities in astrophysical theory.
A first-rate development in the theory of the neutron has already occurred. I was fortunate enough to be in Copenhagen, Denmark, in April and to attend the discussions on theoretical physics held there by Professor Niels Bohr. Bohr opened the discussions this year with a brilliant paper of his own on the theory of collisions between neutrons and other particles. His argument was based on the wave theory of matter. Matter itself is supposed to be made of waves, though of entirely different character from ether waves such as radio waves. Protons, electrons, neutrons, atoms, the earth, or any other material object, are supposed to be made of bundles of these peculiar waves. The size of the waves varies inversely as the mass, which means that electron waves are bigger than proton waves, and vastly bigger than earth waves.
When two electrons collide, two tiny bundles of waves of the same size collide. As the waves are of the same size they bounce apart elastically. When a neutron and an electron collide something quite different happens. The electron waves are much bigger than the neutron waves because the electron is so much lighter than the neutron. Consequently, the neutron bundle of waves behaves as a particle while the electron bundle of waves behaves as waves when a neutron and an electron collide. Thus a neutron may actually go through an electron.
Professor Bohr calculates that the chances of an electron and a neutron interacting when they collide are proportional to the square of the ratio of their masses. As the neutron is over 1000 times as massive as the electron, this means that the chances are less than a million to one. Thus electrons are scarcely ever disturbed when a neutron hits them, so no wonder they have been difficult to discover, and then only from their effects on protons and larger particles!
Source: Reprinted with permission. Copyright © August 1932 by Scientific American, Inc. All rights reserved.
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