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I. Beta First (it's easier)
In 1899, Rutherford had discovered alpha and beta "rays" from uranium. Now, today we know they are not rays, they are particles; alpha is a nucleus of helium and beta is an electron. How did they figure that out?
Well, Rutherford started off by being quite wrong:
"The cause and origin of the radiation continuously emitted by uranium and its salts still remain a mystery. All the results that have been obtained point to the conclusion that uranium gives out types of radiation which, as regards their effects on gases, are similar to Röntgen rays and the secondary radiation emitted by metals when Röntgen rays fall upon them. If there is no polarization or refraction the similarity is complete."
This idea that the uranium rays were like Röntgen rays went up in smoke when more powerful alpha & beta emitters (polonium and radium, discovered by the Curies) because available for use. Now, here I have to go off track for a second. Imagine a lump of radium. It's decaying radioactively. It's firing off alpha & beta particles in all directions, all 360°. This you do not want. What you want is a nice, narrow beam; say pencil size or smaller. You do this by enclosing the radium in a lead box with a hole in it and you use the beam which emerges.
So, the stronger the emitter, the stronger the beam you'll have and the easier it is to work with. The experiments I'm going to describe could not be done with the uranium samples Becquerel and Rutherford had at their disposal. There just were not enough alpha & beta particles in the beam to make it useful.
(Maybe now, you see why the work that Pierre and Marie Curie did as so important. No radium, no strong alpha & beta emitter, no results, nobody gets the Nobel Prize.)
By late 1899, a combined beam of alpha & beta had been exposed to a magnetic field. The alpha particles DID NOT bend in the field, but the beta did. (ALERT: the preceeding sentence was REAL IMPORTANT!) I think Becquerel had done this experiment, but I'm not sure. He had returned to a study of radioactivity when he realized how important his discovery was becoming.
Bending in a magnetic field in the hallmark, the infallible signal of a charged particle. That meant that the beta "ray" was actually a particle and was in no way similar to Röntgen radiation. Rutherford's ideas quoted above were wrong. Period!
In fact, using the magnet, one could determine the charge-to-mass ratio of the beta radiation. It turned out to be exactly the same of a beam of electrons. (Remember, the electron was discovered by J.J. Thomson in 1897.)
So, more-or-less a year after the discovery of beta radiation, it was shown to be a stream of electrons (or corpuscles, as J.J. might say, he never did get around to liking the word electron). Now, on to alpha.
II. Alpha as a Particle Happens in 1903
In 1900, the magnet could not make alpha radiation bend from its path.
So, instead, a study was made of how alpha particles were absorbed the deeper and deeper they went into matter (for example, into thicker and thicker layers of aluminum or even of air).
Pierre Curie found that alpha particles from radium traveled 6.7 cm through air and then he could detect them no longer. This needs some explanation. He detected the alpha radiation by its ability to ionize the air and create an electrical current he could measure between two plates. In modern terms, the alpha particle creates this ionization by hitting other molecules in the air and knocking electrons from them. In essence, the alpha particle stopped after 6.7 cm of travel through air. It also went from being +2 charged to zero charge by picking up two stray electrons. It became just another helium atom in the air.
At the same time, his wife studied the behavior of the ionization produced along the journey, say through a thin piece of aluminum in a vacuum or through air.. She found that the fractional decrease in radiation (dI / I) increased with the distance from the source of the rays. Another way to express this, the alpha radiation was more highly absorbed the further it went. Her comment on this contains the first hint that the alpha radiation was also a particle. It reminded her of the:
"behavior of a projectile, which loses kinetic energy in overcoming obstacles."
These results were announced in early 1900 and by the end of 1900, Robert J. Strutt (his father, J.W. Strutt, third Lord Rayleigh won the Nobel Prize in 1904 for his role in the discovery of the noble gases. R.J., the future fourth Lord Rayleigh was a student in 1900) conjectured in an article that the alpha radiation was a fast moving positively charged ion of atomic mass. The key, in retrospect, is that it convinced Rutherford to look into the matter.
So, sometime in 1901, Rutherford assembled the strongest magnetic field he could make and tried for over a year to bend the alpha radiation. He had no success. Then, late in 1902, he was successful in bending the alpha particle in both an electric and a magnetic field. He published two articles in 1903 on this work - one in January which summarized all the indirect evidence that the alpha radiation was really a stream of particles and one in February which announced his success in bending the alpha beam, thus proving that it was a particle.
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