Rutherford's Experiment - Part I: 1906 to Early 1911

In Part II, Rutherford figures it all out

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I. January 1906

Rutherford announced the discovery of alpha particle scattering by air in Jan. 1906. He took a wire coated with radioactive material and passed the alpha rays through a narrow slit. This resulted in a narrow, rectangular beam rather than a narrow, circular pencil-shaped beam.

In vacuum, the narrow slit showed perfectly sharp edges on a photographic plate the alpha rays hit. However, when the beam was passes through air, the edges of the slit became diffuse and widened. Rutherford wrote ". . . the greater width and lack of definition of the air lines show evidence of an undoubted scattering of the rays in their passage through air."

II. January to June 1906

Rutherford did not work in a vacuum (although most of the experiments were!), but rather discussed his results with colleagues, including William H. Bragg (who, together with his son William L. Bragg, would win the 1915 Nobel Prize in Physics). Bragg was not happy with Rutherford's conclusion and suggested an alternative explanation. Rutherford's response was to perform a more definitive experiment, publishing the results in June 1906.

III. June 1906: Scattering by Mica

Rutherford used his wire coated with the alpha-emitting radioactive element, but he modified the slit the alpha rays traveled through. Half the slit was left open and half was covered by a thin (0.003 cm) mica plate. The experiment was done in the vacuum; the mica taking the place of the air. The open side of the resulting photographic image was sharp and the mica side was diffuse.

He also subjected the alpha beam to a magnetic field. This was in response to Bragg's alternative explanation which involved electrons (the exact details are not necessary). Since electrons bend the opposite way from alpha particles, any electrons produced by the mica as the beam went through it would be swept away. The result? The image was the same as when there was no magnetic field. The vacuum side was sharp and the mica side was diffuse.

Rutherford had proved conclusively the alpha particles could be scattered.

IV. More Results from the Mica Experiment

Careful measuring of the images allowed Rutherford to deduce that some alpha particles had been scattered by 2° from a straight-line path. In light of future events, he makes the interesting statement that other particles may have been deflected "through a considerably greater angle."

He also calculates that a field strength of 100 million volts per cm is required to bend the alpha particles through 2°. In another very interesting statement given future events, he says this result "brings out clearly the fact that the atoms of matter must be the seat of very intense electrical forces -- a deduction in harmony with the electronic theory of matter." By this he meant the Thomson model of the atom.

Keep in mind that the Thomson model (announced in early 1904, but speculated on in print by Thomson as early as 1899) is still the only model of the atom acceptable to the British. The "Saturnian model," announced by Nagaoka in May 1904, had been totally discredited by Thomson and his allies. Or so they thought!! Also, remember that Rutherford had been Thomson's student and the two were very close. (J.J.'s son George has many memories, from his childhood, of Rutherford and his wife over for dinner.) So it is quite natural for Rutherford to believe that J.J.'s model is correct.

V. Moving to Manchester

All this time, Rutherford had been in Canada (since 1898) at McGill University in Montreal. In 1907, a position came open at the University of Manchester and Rutherford applied it and was selected. Now he was back in England and, more importantly for our story, he hooked up with Hans Geiger.

VI. Manchester, 1908

One of the central goals of Rutherford's work was to determine the nature of the alpha particle. Was it a singly-charged hydrogen molecule or a doubly-charged helium atom? The e / m ratio was consistent with either. Although in 1907 he was confident it was the latter, he sought additional confirmation. To this end, while in Canada he had tried an experiment which needed to count the number of alpha particles. It was a failure. However, with Geiger now involved, a satisfactory counting device was designed and built.

However, the scattering of the alpha particle was wreaking havoc on their results. The counter worked fine, but as Rutherford put it in a letter to a friend, "the scattering is the devil." It was evident to both Rutherford and Geiger that an accurate picture of alpha particle scattering was required. Geiger started on this work even before the counting experiment was done, and in June 1908 he announced some preliminary results.

Above is Geiger's 1908 apparatus. R is the source, S is the thin metal foil which scatters the particles, Z is the zinc sulfide screen that flashed when struck (it was called the scintillation method), and M is a microscope to view the flashes with. It almost goes without saying, this work was done in a very dark room.

As this work proceeded, Geiger began to notice what he termed a "notable" scattering; in other words he was seeing scattering by greater angles than he had expected. In addition to this, he found that gold foil scattered through a greater angle than aluminum. As Geiger put it, "quite an appreciable angle." He proposed to continue with as many metals as possible "in the hope of establishing some connection between the scattering and stopping powers of these materials." In other words, what was first a problem now bcame the subject of study.

VII. Marsden Enters the Scene

To assist in these experiments, twenty-year-old Ernest Marsden joined Geiger. The tube was improved and lengthened, but experimental difficulties persisted. They could gain only a rough estimate of the most probable angle of deflection for a given metal. They (by the early spring of 1909) decided the trouble was somehow related to scattering by the tube walls and a series of washers installed in the tube solved the problem. According to Marsden (many years later), neither of them thought the alpha particles were being directly REFLECTED from the walls or the metal foil.

As Geiger himself says, some thirty years later:

"In the electric counting of alpha-particles it was seen that the small residuum of gas in the four-metre long tube . . . influenced the result. We attributed this to a slight scattering of the alpha-particles. Later on, I examined scattering quantitatively in several experiments. The most important observation was the appearance of isolated instances of extremely large angles of deflection, which were far outside the normal variations. At first we could not understand this at all."

VIII. Rutherford makes the Critical Suggestion

Hans Geiger was a scientist in his own right. He held a Ph.D., was a teacher on staff, and did research with Rutherford's direction and collaboration. It was at this time that he said (according to Rutherford), "Don't you think that young Marsden whom I am training in radioactive methods ought to begin a small research?" Rutherford agreed and, as Marsden remembers it 50 years later, he came into the lab one day, turned to Marsden and said "See if you can get some effect from alpha particles directly reflected from a metal surface." Marsden goes on to say about his own thoughts:

"I did not think he expected any such result, but it was one of those hunches that perhaps some effect might be observed . . . ."

The year before he died, Rutherford himself recalled what then happened:

"Two or three days later Geiger came to me in great excitement and said: "We have been able to get some of the alpha particles coming backwards."

For Ernest Rutherford, winner of the 1908 Nobel Prize in Chemistry, his greatest scientific achievement still lay two years into the future. Sometime in late 1910/early 1911, he switched roles with Geiger, who remembers it from 27 years later:

"One day Rutherford, obviously in the best of spirits, came into my room and told me that he now knew what the atom looked like."

IX. The Experiment of 1909

Marsden first used the following set-up in his search for reflected alpha particles:

The alpha emitter was placed at A on top of a lead plate which prevented direct access of the particles to the counter located at S. With nothing placed at position R the counter did not record any hits. However, when one thin gold foil was placed at R, the counter came to life.

It was from this experimental set-up that Geiger reported two or thre days later that some alpha particles had been reflected back.

This is the opening paragraph of Geiger and Marsden's paper of May 1909:

When β-particles fall on a plate, a strong radiation emerges from the same side of the plate as that on which the β-particles fall. This radiation is regarded by many observers as a secondary radiation, but more recent experiments seem to show that it consists mainly of primary β-particles, which have been scattered inside the material to such an extent that they emerge again at the same side of the plate. For α-particles a similar effect has not previously been observed, and is perhaps not to be expected on account of the relatively small scattering which α-particles suffer in penetrating matter.

This is the data they reported in that same paper:

Atomic weight, A
Number of scintillations
per minute, Z.
Lead 207 62 30
Gold 197 67 34
Platinum 195 63 33
Tin 119 34 28
Silver 108 27 25
Copper 64 14·5 23
Iron 56 10·2 18·5
Aluminium. 27 3·4 12·5

For platinum, Geiger and Marsden reported:

"Three different determinations showed that of the incident α-particles about 1 in 8000 was reflected, under the described conditions."

X. 1909 Fades into 1910 and then 1911

Now, Ernest Rutherford was confronted with the challenge of explaining the results. His answer, to be published in May 1911, was the nuclear atom and it made Rutherford unique among all other Nobel Prize winners. You see, almost all historians of science call the discovery of the nucleus Rutherford's greatest scientific work. He is the only one to do his greatest work after receiving the Nobel Prize.

Here is what he says in a lecture at Clark University in September, 1909; just 6 months after the discovery of large-angle scattering:

"Geiger and Marsden observed the suprising fact that about one in eight thousand α particles incident on a heavy metal like gold is so deflected by its encounters with the molecules that it emerges again on the side of the incidence. Such a result brings to mind the enormous intensity of the electric field surrounding or within the atom."

Notice the word "encounters;" it's plural. Rutherford is thinking about multiple scattering. In other words, the alpha particle encounters one gold atom after another and each encounter deflects the alpha a bit more, so that the sum of all the deflections is to make the alpha particle come flying out 90° or more from the direction it went in.

In 1910, two events important to our story happen.

1) On Feb, 17, 1910, Geiger reads a paper in which he determines the most likely angle of deflection for any one alpha particle to be about 1°. However, in the same paper, he says, "It does not appear profitable at present to discuss the assumption that might be made to account for [it]." It seems safe to say that 10 months after the discovery of large-angle scattering, Geiger (and Rutherford) do not have a clue as to its explanation.

2) J.D. Crowther (a student of J.J. Thomson) publishes a theory of beta-particle scattering in March 1910. He follows up with more data in June and then December. He uses the 'multiple scattering' idea; that is, many small deflecions which add up to one large angle. Each deflecion is caused by the particle encountering an atom. Over time, Rutherford (in discussion with his friend W.H. Bragg) will become convinced that this is the incorrect explanation for large angle scattering.

It is not possible to put a precise date on when Rutherford hit on single scattering as the answer. The papers which bear his calculations are undated. However a series of letters he wrote at this time allow a glimse into when he started to put everything together. The very first mention of the atom occurs in a letter dated Dec. 14, 1910 (to B.B. Boltwood, an American chemist) in which Rutherford says he has been doing "a good deal of calculation on scattering." Scattering and the atom figure in nine letters between then and Feb. 19, 1911. To keep these dates in perspective, remember that Rutherford's published paper appeared in Philosophical Magazine for May 1911.

Just to review, 'single scattering' refers to one encounter of the alpha particle with an atom. One single incident is sufficient to deflect the alpha more than 90°. It does seem a bit fantastic that only one encounter with an atom was sufficient, but Rutherford was having serious problems making multiple scattering fit.

In Part II, Rutherford figures it all out

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