20. THE JUBILEE OF THE ATOMIC NUCLEUS

by Sir Charles Darwin

It is now fifty years since the discovery of the nucleus of the atom by Rutherford, and the event is to be celebrated in September by the University of Manchester, where the discovery was made.

In his work at Montreal, Rutherford had discovered the three types of ray, alpha, beta and gamma, emitted by the radioactive elements. Of these, much the most remarkable was the alpha-particle because of its great weight: it was, in fact, the nucleus of a helium atom, as he showed soon after. When he came to Manchester in 1908, he started work using it to explore the interior of the atom. Its great mass and energy made it admirably suited for this, because it could nearly disregard the very much lighter electrons and so was fitted to reveal what the heavier things in the atom might be. The alpha-particle has also the practical merit that when it strikes a screen made of zinc sulphide a faint spark is emitted, so that for the first time in the world's history these scintillations revealed the behaviour of single atoms. It need hardly be said that a lot of work was needed to establish all this, but once the result was accepted it was evidently the best instrument for exploring the atom, and working mainly with Geiger and Marsden, Rutherford started to do this.

The principle of the experiments was very simple. A fine pencil of alpha-particles was made, by sending them from a source of radium through two small holes. This then fell on a sheet of gold foil, so thin that they could go right through it. Most of them went on in the straight line, but a few were deflected during their passage. A screen of zinc sulphide was set up on the other side, and by using a microscope the number of particles which had been deflected through various angles was counted.

In spite of the exciting results to be obtained, the actual experiments were very tedious to carry out. The scintillations were very faint, and the observer had to begin his day by sitting in the dark for half an hour so as to get his eyes adapted, and he must not come out of the dark until the day's work was ended.

The results were quite startling. Whereas in the previous studies of the scattering of electrons the results had clearly been the statistical effect of a large number of small scatterings, the distribution at different angles in the case of the alpha-particles did not look like that. Some of them were scattered to wide angles, indeed a few right back towards the source, and this could hardly be the cumulation of a number of small effects.

For some time Rutherford used to say that there must be very strong forces in the atom and to leave it at that, but I remember very well the first time when he came out with the whole theory: it was after supper, to which a few of us had been invited, on a Sunday evening. His starting-point was that the effect could not be a cumulative one, but must be due to a single scattering. He assumed that there was a heavy point charge in the atom, and that this deflected the alpha-particle into a hyperbola, when it happened to pass near by. He could calculate the chances of deflection to various angles, and therefore experiments were set up to test the law of distribution with angle that the theory suggested. It was brilliantly verified, and the results also told the strength of the electric charge which was doing the deflection. There was nothing in the experiments to show whether it was a positive or negative charge, but it was natural to assume it positive, so as to balance the charges of the electrons in the atom. At first, indeed, it was called the "central positive charge", but soon it was renamed the nucleus.

(from New Scientist, 13th July, 1961)