Alpha Particles and the Atom
Rutherford at Manchester, 1907–1919
Ernest Rutherford discovered the nucleus of the atom in 1911. We read this in textbooks and in popular writings. But what does that statement mean? Geographical discovery usually means that one sees a place for the first time. But can discovery be the same for a realm hidden from sight? One cannot see an atom in that sense. So this hints that perhaps the story of the discovery of the nucleus was more complicated. The story as it unfolded in Rutherford's lab at the University in Manchester revolved around real people. It involved frustrations and triumphs. It involved hard work and perplexity and inspiration.
Rutherford arrived in Manchester in the summer of 1907, months before the university's term began. He had been named Langworthy Professor of Physics, successor to Arthur Schuster (1851–1934), who retired at age 56 to recruit Rutherford. Schuster had built a modern physics building, hired Hans Geiger, Ph.D. (1882–1945) because of his experimental skill, and endowed a new position in mathematical physics to round out a full physics program. Rutherford entered the center of the physics world. Researchers came to him by the dozen.
Rutherford arrived with many research questions in mind. He was not done with the puzzles of the decay families of thorium, radium, etc., but he was passing much of this work to Boltwood, Hahn, and Soddy. Boltwood and Hahn both worked with Rutherford in Manchester, Boltwood in 1909–1910 and Hahn in 1907–1908. Rutherford was gradually turning his attention much more to the α (alpha), β (beta), and γ (gamma) rays themselves and to what they might reveal about the atom. That is, he was leaving radio-chemistry to others and turning to physics.
Rutherford's early team at Manchester included Geiger and William Kay (1879–1961), junior laboratory assistant since 1894. Rutherford promoted Kay to laboratory steward in 1908, to manage lab equipment and to aid him in his research. In 1957, Kay thought back to his youth with Rutherford in an interview. The language is quaint, but the description is as close to Rutherford's approach as we get. The questioner was Samuel Devons (1914–2006), who was one of Rutherford's last students in the 1930s.
[Devons] “When you were here [in Manchester], during this period... did Rutherford actually make any apparatus himself?”
[Kay] “No, no, no, no. We used to, I used to set up nearly all his apparatus. You know, when he did his work, you know, oftener than not, he used to tell me and we did a rough experiment, re...”
[D.] “Did he sketch out what he wanted?”
[K.] “Well, he'd tell you what he wanted, roughly, you see, but he'd let you make what you wanted, you see, he'd tell you what he was going to do, which was very good, you see. It gives you......... it learnt you a lot and you knew what to do and what not to do. And then we would do a rough experiment, and get one or two curves you see, and then straight away button it on to somebody else to do the real work, and that's how he did his........ attacked these little things, you see.”
[D.] “He tried them out himself first?”
[K.] “He'd try a rough experiment himself on the little things, d'you see, and then he'd turn it over on to somebody...” (Quoted in Hughes, p. 104)
Rutherford and Hans Geiger worked closely in 1907 and 1908 on the detection and measurement of α particles. If they were to use α particles to probe the atom, they had first to know more about these particles and their behavior. Rutherford had tried and failed back at McGill to count α particles. He wrote Hahn in January 1907:
In regard to the α particle and helium, etc., and the fitting in of atomic weights, I am not worrying about it at present. ... The whole problem is very mixed. The difficulty lies in the fact that the alpha particle moves ‘in a peculiar way its wonders to perform.’... We want a new method of attack. (Rutherford to Hahn, 6 Jan. 1907, quoted in Eve, pp. 153–4.)
A year later in Manchester, he and Geiger succeeded with two methods of observing α particles. The first method involved scintillations excited by α particles on a thin layer of zinc sulfide. They observed these through a microscope and counted the scintillations at different angles of dispersion. They also developed an "electrometer" that could demonstrate the passage of an individual α particle to a large audience. The instrument, which evolved into the "Geiger counter," had a partially evacuated metal cylinder with a wire down its center. They applied a voltage between the cylinder and the wire high enough almost to spark. They admitted α particles through a thin mica window, where these particles collided with gasses, producing gas ions. These then collided with other molecules and produced more ions, and so on. Each α particle produced a cascade of ions, which partially discharged the cylinder and indicated the passage of an α particle. Geiger and Rutherford published several articles in 1908 and 1909 on these methods and their use.
Rutherford wrote to Henry Bumstead (1870–1920), an American physicist, on 11 July 1908:
Geiger is a good man and worked like a slave. I could never have found time for the drudgery before we got things going in good style. Finally all went well, but the scattering is the devil. Our tube worked like a charm and we could easily get a throw of 50 mm. for each particle. ... Geiger is a demon at the work of counting scintillations and could count at intervals for a whole night without disturbing his equanimity. I damned vigorously and retired after two minutes. (Quoted in Eve, p. 180.)
Although Rutherford suspected as early as 1906 that α particles were helium atoms stripped of their electrons, he demanded a high standard of proof. One kind of experiment was not enough. One kind of detector was not enough. He wanted more proof. For this, Rutherford desired "big voltages" and big electromagnets to divert α particles, but this method was not yet ripe. Lab steward William Kay recalled in the cited oral history interview that Rutherford in 1908 insisted that strong electric and magnetic fields were needed to measure more directly the charge and mass of the α and β particles:
And that's what he was after all the time. That's what he got at Cambridge [after 1919], which we never got here, you see, because we'd got no money. (Hughes, “William Kay,” 2008, pp. 109–110.)
Kay said Rutherford wanted a big, water-cooled magnet, but that he “dropped it like a hot cake” when he learned its cost. So he needed a new line of attack. The new line was very simple, a chemical procedure mixed with physics. For this work Rutherford recruited Thomas Royds (1884–1955), who had earned his Physics Honours degree in 1906. They collected α particles in a sealed glass tube, compressed them, and passed an electric spark through. They studied the emitted light in a spectroscope and found it to be identical to the spectrum of helium. Within a few months, Rutherford was awarded the Nobel Prize for Chemistry, "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances." (Nobel citation) Rutherford and Royds had established the identity and primary properties of α particles. Rutherford next turned his attention to using them to probe the atom.
The autumn of 1908 began an important series of researches. Geiger had been passing beams of α particles through gold and other metallic foils, using the new detection techniques to measure how much these beams were dispersed by the atoms in the foils. Geiger thought Ernest Marsden (1889–1970), a 19-year-old student in Honours Physics, was ready to help on these experiments and suggested it to Rutherford. Since Rutherford often pushed third-year students into research, saying this was the best way to learn about physics, he readily agreed.
Geiger and Marsden began with small-angle dispersion and tried various thicknesses of foils, seeking mathematical relationships between dispersion and thickness of foil or number of atoms traversed. Marsden later recalled that Rutherford said to him amidst these experiments: "See if you can get some effect of alpha-particles directly reflected from a metal surface." (Reported by Marsden in Birks, 1962, p. 8). Marsden doubted that Rutherford expected back scatter of α particles, but as Marsden wrote
...it was one of those 'hunches' that perhaps some effect might be observed, and that in any case that neighbouring territory of this Tom Tiddler's ground might be explored by reconnaissance. Rutherford was ever ready to meet the unexpected and exploit it, where favourable, but he also knew when to stop on such excursions. (Birks, 1962, p. 8)
This was Rutherford's playful approach in action. His students and others tried out his ideas, many of which were dead-ends. This idea to look for backscattering of α particles, however, paid off. Rutherford wrote:
Experiment, directed by the disciplined imagination either of an individual or, still better, of a group of individuals of varied mental outlook, is able to achieve results which far transcend the imagination alone of the greatest philosopher. (Quoted in Eve, 1939, Frontmatter)
Sometime later in 1908 or 1909, Marsden said, he reported his results to Rutherford. Rutherford recalled this a little differently:
I remember ...later Geiger coming to me in great excitement and saying, 'We have been able to get some of the α-particles coming backwards...' It was quite the most incredible event that has ever happened to me in my life. It was almost incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. (Rutherford, 1938, p. 68)
Human memory is fallible. Whether Marsden or Geiger told Rutherford, the effect was the same. Rutherford said they should prepare a publication from this research, which they submitted in May 1909. Moreover, this started Rutherford thinking toward what ultimately, almost two years later, he published as a theory of the atom.
What was Rutherford doing for the rest of 1909 and all of 1910? For one thing, his close friend Boltwood was in Manchester for the academic year working with Rutherford on radioactive decay products of radium. He was also reviewing and speaking on earlier ideas about atomic structure. Most importantly, he was taking the phenomenon of the scattering of α particles apart systematically and testing each piece. Rutherford did not have his bold idea — the nuclear atom — instantly, but he came to it gradually by considering the problem from many sides.
In the autumn of 1910 he brought Marsden back to Manchester to complete rigorous experimental testing of his ideas with Geiger. They re-established rates of emission and the ranges of α particles by radioactive sources and they re-examined their statistical analyses. Rutherford tried to reconcile scattering results with different atomic models, especially that of J.J. Thomson, in which the positive electricity was considered as dispersed evenly throughout the whole sphere of the atom.
At some point in the winter of 1910–1911, Rutherford worked out the basic idea of an atom with a "charged center." As Geiger and Marsden pointed out in their 1909 article:
If the high velocity and mass of the α-particle be taken into account, it seems surprising that some of the α-particles, as the experiment shows, can be turned within a layer of 6 x 10-5 cm. of gold through an angle of 90°, and even more. To produce a similar effect by a magnetic field, the enormous field of 109 absolute units would be required. (Birks, p. 179)
Rutherford concluded in his May 1911 paper that such a remarkable deviation in the path of a massive charged particle could only be achieved if most of the mass of, say, an atom of gold and most of its charge were concentrated in a very small central body. Note: at this point in 1911, Rutherford did not call this a "nucleus."
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The first public announcement of the nuclear theory by Rutherford was made at a meeting of the Manchester Literary and Philosophical Society, and he invited us young boys to go to the meeting. He said he’d got some interesting things to say and he thought we’d like to hear them. We didn’t know what it was about at that time. The older people in the laboratory did, of course Geiger and Marsden knew because they were already doing the experiments. In fact, unless they had done some which were sufficient to be decisive, Rutherford never mentioned it publicly. And, of course, Darwin knew about it much earlier. But that must have been early in 1911, and we went to the meeting and he told us. And he mentioned then that there was some experimental evidence which had been obtained by Geiger and Marsden. He did not, as far as I remember, say more about the results than that they were quite decisive. And, as I said before, he would never have made a public announcement of that kind if he hadn’t had good evidence. And that is one of the characteristics that runs through all Rutherford’s work, particularly all his work up to the end of the Manchester period. If you look at some of his papers in the early days — I call McGill the early days — he was quite convinced that the alpha particles were atoms of helium, but he never said that in those words. He always said they were either atoms of helium or molecules of hydrogen or perhaps he may have said something else of that weight. It was quite characteristic of him that he would never say a thing was so unless he had experimental evidence for it that really satisfied him.
In fact, Rutherford was exceedingly cautious in drawing conclusions about this central charge: “A simple calculation shows that the atom must be a seat of an intense electric field in order to produce such a large deflexion at a single encounter.” (Birks, p. 183). He worked out quickly and roughly that several quantitative relationships should be true if this basic theory were correct. First, the number of α particles scattered through a given angle should be proportional to the thickness of the foil. Second, that number should be proportional to the square of the nuclear charge. Lastly, it should be inversely proportional to the fourth power of the velocity of the α particle. These three ideas laid out the experimental program of Geiger and Marsden for the next year.
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Rutherford’s interest was then almost entirely in the research. He had done very little teaching in McGill. He was research professor. I suppose he gave some lectures but it would have been very few. And his interest was quite naturally on the research side. He did give some lectures, but elementary lectures, the kind of thing you would expect a man to know before he came to the University. They were the lectures to the engineers. They were a rowdy lot and Rutherford could keep them under control. There was perhaps only one other man in the department who could have done it, and he (Rutherford?) enjoyed them because he was able to show them the very interesting experiments one can perform in elementary courses.
It's often been said to me that Rutherford was a bad lecturer. I never heard such nonsense. It is quite true that on occasion he would be a bit dull, a bit mixed up, but that was only on very rare occasions. There were other occasions when he was really most stimulating. There was a tremendous enthusiasm about him.
Rutherford entertained the possibility that the charged center is negative. That sounds odd today, so what made it reasonable? First, it wasn't very different from Thomson's model. Second, since Rutherford knew that α particles carry a double + charge, he thought this might act the same way the Sun does on a comet sweeping near it. It would slingshot the α particle around and back towards its source. He also considered a nearly forgotten model suggested by Japanese physicist Hantaro Nagaoka (1865–1950) — the Saturnian model. Nagaoka and Rutherford were in contact in 1910 and 1911 and Rutherford mentioned Nagaoka's model of "a central attracting mass surround by rings of rotating electrons" (Birks, p. 203). The end result in this critical Rutherford paper, however, was Rutherford's announcement that whether the atom were a disk or a sphere, and indeed whether the central charge were positive or negative, would not affect the calculations. Rutherford was always careful not to claim more than his results could support.
Rutherford did see possible tests of the nature of the central charge. The absorption of β particles, he said, should be different with a negative center versus a positive one. A positive center would explain the great velocity that α particles achieve during emission from radioactive elements. But these were only hints.
Geiger and Marsden did indeed work systematically through the testable implications of Rutherford's central charge hypothesis. The first major publication of their results was in German in the Proceedings of the Vienna Academy of Sciences (Sitzungberichte der Wiener Akademie der Wissenschaften) in 1912. This 30-page version was followed by one in English in 1913 in the Philosophical Magazine: "The Laws of Deflexion of α Particles through Large Angles" The English version is the better known. Slight differences between the two lead one historian to suggest that Rutherford decided in favor of a positively charged center by August 1912 (Trenn, 1974). Rutherford's other team members, especially Charles Galton Darwin (1887–1962), H.G.J. Moseley (1887–1915), and Niels Bohr (1885–1962) figured prominently in the ultimate establishment of Rutherford's nuclear atom.
The ‘Great War’ totally disrupted work in Rutherford's Manchester department. Bohr returned to Denmark. Marsden accepted a professorship in New Zealand. Moseley died in the Battle of Gallipoli. James Chadwick (1891–1974), who was working with Geiger at the Technical University of Berlin when war broke out, spent several years interned in the Ruhleben camp for prisoners of war. Other students went off to war, too, and Rutherford devoted considerable energy to mobilizing science for the war effort and specifically to anti-submarine techniques.
Against this distracted background, Rutherford and his faithful lab steward, William Kay, began in 1917 to explore the passage of α particles through hydrogen, nitrogen, and other gases. Not surprisingly, α particles knocked the lighter hydrogen nuclei forward at great speed. Rutherford learned, however, that far more hydrogen nuclei than expected were accelerated. A much bigger surprise awaited him, however, when he passed α particles through nitrogen. Even though nitrogen atoms are heavier than α particles, Rutherford observed many long-range particles ejected by the collisions. Rutherford applied a magnetic field and thought that the particles behaved like hydrogen nuclei, even though no hydrogen had been present. He concluded that nitrogen atoms disintegrated when impacted, producing hydrogen atoms.
Rutherford's final research at Manchester in 1919 produced the artificial disintegration of the nitrogen nucleus. With this last brilliant accomplishment, Rutherford was called back to Cambridge to succeed J.J. Thomson as the Cavendish Professor and as director of the Cavendish Laboratory. He went in the fall of 1919.