Rutherford's Discovery of Alpha Scattering

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I. The French Start First

In 1900, the Curies and Becquerel were studying radioactivity (Rutherford was in Canada, having just recently moved to McGill University in Montreal. He will return to England, but not for several years.) and they had occasion to determine if the alpha rays traveled in a straight line. Becquerel carried out the work.

He took some polonium (discovered by Pierre and Marie Curie) and made it into a straight line source, not the usual pencil of rays. He then used this long, narrow beam of alpha rays to make a photograph of a wire. He found that the shadows had sharp edges, even when the alpha rays had gone through a thin film of aluminum.

Marie Curie gave the interpretation of the experiment. She said that this showed that the alpha particles were not scattered, that is, able to be bent from their path. Alpha rays traveled in nice, straight lines ALL THE TIME.

They were wrong.

We jump to 1903 and Rutherford announces he has succeeded in bending alpha particles with a magnetic (and an electrical) field. Becquerel, using his different techniques, checks and confirms Rutherford's results.

What Becquerel does is aim his radioactive source (radium, now as well as polonium) at a narrow slit with a photographic plate on the other side. Of course, with no magnetic field, he gets a nice, crisp image of the narrow slit, directly in line with the radium source.

When he turned on the magnetic field, he got Rutherford's result, that is, the alpha particles bent in the magnetic field.

Part b of the figure shows the offset of the image on the photographic plate. It is not lined up directly with the open slit, labeled Y.

Becquerel also reversed the magentic field's direction during the exposure and got a double image.

The images were exactly the same distance from the center and the edges of the traces ALWAYS were sharp and crisp.

Up to this point, there was zero evidence of alpha particle scattering.

II. Now, the evidence starts to accumulate

The work described above was all done in the early part of 1903 and it was in June of 1903 that Becquerel made the first observation that led to alpha particle scattering.

He noticed, right at the end of his experiments, that the images from the doubled set (when he reversed the magnetic field during the experiment) moved closer together as he moved the photographic plate away from the slit (that is, increased the length b in the diagram above).

It seemed that the radius of curvature of the alpha particles path through the slit Y increased as b increased, thereby moving the two images closer together.

Now, Becquerel knew this equation:

RH = v (M/E)

R = the radius of curvature, H = the magnetic field strength, v is the velocity, and E/M is the charge-to-mass ratio of the alpha particle. (I know, I know, this equation just came out of nowhere. You'll have to trust me on this one, since I don't know how to derive the equation either.)

What Becquerel did is assume E is constant, so this left him with the unpleasant thought that either v or M had to increase. Since it is very had to visualize the alpha particle SPEEDING UP as it traveled from its source, Bequerel decided that M must increase, that the alpha particle picked up stray matter as it traveled along.

He could not have liked it, but there wasn't anything else he could think of. Since Rutherford was busy with other work, he did not take up any checking on Becquerel's results. However, one Rutherford result will have bearing on scattering. Rutherford found (in July 1905) that the alpha particle decreased in velocity, until at about 6,000 miles per second, it would not make its presence known by ionizing gases or darkening a photographic plate.

III. We're getting there

The importance of the slowing down of the alpha particle meant that R (the radius of the curvature in a magnetic field) should have gone down, not increased. This was fatal to Becquerel's conclusion, but in September 1905, he firmly rejected Rutherford's ideas.

Rutherford's response in January 1906 contains the discovery of alpha particle scattering (finally, we're there! Yay!). Rutherford compared the behavior of alpha particles in air and in vacuum, something Becquerel did not do.

As Rutherford himself writes so clearly:

We have seen that M. Becquerel early drew attention to the fact that the radius of curvature of the trace of the α rays obtained in air increased with increase of distance from the source. Assuming that the rays were homogeneous, he suggested as an explanation that the mass of the α particle increased as it passed through the air.

Proceeding on these assumptions, it is seen that the α rays from each product present in radium must increase in curvature with the distance of air traversed. In order to test this point definitely, I made the following experiment, with an active wire coated with radium C as a source of rays. Using the same apparatus employed to obtain the photograph in fig. 1, the part of the active wire on one side of the mica screen was covered with a sheet of metal of sufficient thickness to absorb all the α rays. The apparatus was filled with air at atmospheric pressure, and the trace of the rays obtained in a uniform magnetic field is seen in fig. 3 (Pl. IV.). After 40 minutes, the metal sheet was transferred to the other side of the screen and the apparatus was then exhausted. The trace of the rays then obtained in a vacuum is shown by the two narrower lines in the same figure. There is a striking difference between the two pairs of lines obtained in air and in vacuum.

The air-bands are more defected than the latter and in addition are broader. This experiment conclusively shows that the α rays from the active wire decrease in radius of curvature and consequently in velocity after passing through air. The greater width and lack of definition of the air-lines have been noticed in all other experiments, and show evidence of an undoubted scattering of the rays in their passage through air. The slight shift of the vacuum-lines in the figure, relative to the air-lines, was due to an accidental displacement of the plate in transferring the metal sheets in the middle of the experiment. Experiments are in progress to see whether this scattering also occurs in the passage of the rays through a solid substance. This scattering of the rays probably also occurs with aluminium, but it would not be as evident in the experiments where the layers of foil are placed over the active wire between the source and the slit.

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