JF Ptak Science Books Post 2091
James Chadwick--the discoverer of the neutron and leading Brit investigator in the Manhattan Project--announced his great discovery in the form of a letter-to-the-editor. Of course, this was a letter to the editor of Nature--not exactly the Billboard Breeze of Sumpin', Montana, or the NYT or WSJ for that matter--where letters like this were more a quick way to get experimental results announced quickly, a rapid-publication device in the days when the scientific weekly was the quickest way of getting news out the fellow researchers.
But, no matter--it is still a short introduction to a long idea, not quite as romantic as the lede implies. Chadwick's "Possible Existence of a Neutron", appeared in Nature on 27 February 1932. It takes up a tidy and compact 1.25 columns, all on one page (312) of this issue of the journal. It is in a sort of imaginary "tradition" of great letters-to-the-editor, like the Alpher-Bethe-Gamow paper ("The Origin of Chemical Elements", on the stuff of the Big Bang, a 1948 paper published in the journal Physical Review that worked out the basis for the formation of particles in the universe), another instance of a major scientific announcement being made as a "simple" presentation to the journal's editor.
Anyway this first description of the neutron was made after only about two weeks of experimentation--his next paper is the more famous of the two, and was published in May with a much fuller account of the discovery and experimentation.
"In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons - elementary particles devoid of any electrical charge. In contrast with the helium nuclei (alpha rays) which are charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. Chadwick in this way prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb."--NobelPrize.org
His next paper1--and the more famous of the two--appeared a few months later in May and contained the proof of the existence of the neutron, and was an epochal achievement int he understanding of the nucleus.
For this epoch-making discovery he was awarded the Hughes Medal of the Royal Society in 1932, and then the Nobel Prize for Physics in 1935.
I think Chadwick may be the only recipient of the award who was also at one time a POW (1914-1918 in Chadwick's case, going from being a civilian prisoner of war in Germany, where he was studying physics, to a Ph.D. at Cambridge in 1921 and then to theGreat Kiwi Rutherford's assistant shortly thereafter).
1. Chadwick, J. (1932). "The Existence of a Neutron". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 136 (830): 692. Another paper appeared in the next year, (1933) "Bakerian Lecture. The Neutron", Proceedings of the Royal Society A: Mathematical, Physical and Engineering. The full text of the first paper appears further below:
James Chadwick, Nature, p. 312 (Feb. 27, 1932)
"It has been shown by Bothe and others that beryllium when bombarded by a-particles of polonium emits a radiation of great penetrating power, which has been an absorption coefficient in lead of about 0.3 (cm)¯1. Recently Mme. Curie-Joliot and M. Joliot found, when measuring the ionisation produced by this beryllium radiation in a vessel with a thin window, that the ionisation increased when matter containing hydrogen was placed in front of the window. The effect appeared to be due to the ejection of protons with velocities up to a maximum of nearly 3 x 109 cm. per sec. They suggested that the transference of energy to the proton was by a process similar to the Compton effect, and estimated that the beryllium radiation had a quantum energy of 50 x 106 electron volts. I have made some experiments using the valve counter to examine the properties of this radiation excited in beryllium. The valve counter consists of a small ionisation chamber connected to an amplifier, and the sudden production of ions by the entry of a particle, such as a proton or a-particle, is recorded by the deflexion of an oscillograph. These experiments have shown that the radiation ejects particles from hydrogen, helium, lithium, beryllium, carbon, air, and argon. The particles ejected from hydrogen behave, as regards range and ionising power, like protons with speeds up to about 3.2 x 109 cm. per sec. The particles from the other elements have a large ionising power, and appear to be in each case recoil atoms of the elements. If we ascribe the ejection of the proton to a Compton recoil from a quantum of 52 x 106 electron volts, then the nitrogen recoil atom arising by a similar process should have an energy not greater than about 400,000 volts, should produce not more than about 10,000 ions, and have a range in air at N.T.P. of about 1.3 mm. Actually, some of the recoil atoms in nitrogen produce at least 30,000 ions. In collaboration with Dr. Feather, I have observed the recoil atoms in an expansion chamber, and their range, estimated visually, was sometimes as much as 3 mm at N.T.P. These results, and others I have obtained in the course of the work, are very difficult to explain on the assumption that the radiation from beryllium is a quantum radiation, if energy and momentum are to be conserved in the collisions. The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons. The capture of the a-particle by the Be9 nucleus may be supposed to result in the formation of a C12 nucleus and the emission of the neutron. From the energy relations of this process the velocity of the neutron emitted in the forward direction may well be about 3 x 109 cm. per sec. The collisions of the neutron with the atoms through which it passes give rise to the recoil atoms, and the observed energies of the recoil atoms are in fair agreement with this view. Moreover, I have observed that the protons ejected from hydrogen by the radiation emitted in the opposite direction to that of the exciting a-particle appear to have a much smaller range than those ejected by the forward radiation. This again receives a simple explanation of the neutron hypothesis. If it be supposed that the radiation consists of quanta, then the capture of the a-particle by the Be9 nucleus will form a C13 nucleus. The mass defect of C13 is known with sufficient accuracy to show that the energy of the quantum emitted in this process cannot be greater than about 14 x 106 volts. It is difficult to make such a quantum responsible for the effects observed. It is to be expected that many of the effects of a neutron in passing through matter should resemble those of a quantum of high energy, and it is not easy to reach the final decision between the two hypotheses. Up to the present, all the evidence is in favour of the neutron, while the quantum hypothesis can only be upheld if the conservation of energy and momentum be relinquished at some point. J. Chadwick. Cavendish Laboratory, Cambridge, Feb. 17."