How Rutherford first split the atom
Following the success of their experiments on the scattering of particles by metal foils which gave undeniable proof of the existence of atomic nuclei Rutherford and his colleagues began to try the effect of firing ex-particles into various gases.
Marsden set up an evacuated tube containing an alpha· source and a zinc sulphide screen and noticed the scintillations produced when the ex-particles struck the screen. He then increased the distance of the screen from the source until it was too far away for the ex-particles to reach it. Then, on introducing a little hydrogen into the tube, the scintillations reappeared. Since these could not be caused by ex-particles, Marsden came to the conclusion that they were produced by the impact of hydrogen nuclei (protons) which had been struck by ex-particles and projected forward with sufficiently high velocity to reach the screen. Calculations based on the ordinary laws of conservation of momentum and energy confirmed this conclusion.
It was as though a ball of mass 4 units (ex-particle) moving at, say, 16000 km/s had hit a ball of mass I unit (proton) and projected it forward at about 24000 krn/s, being itself slowed up to about 8000 krn/s. Rutherford became very interested in these simple experiments and began to make further investigations of his own. Eventually he made the discovery that fast exparticles could be used not only to project a nucleus forwards but also to break it into two pieces.
Fig. 47.2 shows the apparatus he used. It consists of a metal tube containing an adjustable alpha source. At one end there is a window of silver foil, and two tubes are provided for the purpose of introducing various gases. Any particles which pass through the silver foil fall on a zinc sulohide screen and their scintillations can be observed through a microscope.
Rutherford first of all tried oxygen in the tube, but with no result, and in any case he did not expect projection of the oxygen nuclei, since they were far heavier than exparticles. But when nitrogen was allowed to enter the tube scintillations appeared on the screen. Rutherford made some tests and found that the particles which struck the screen were protons or hydrogen nuclei. Now the exceedingly small amount of hydrogen known to be present in the tube as an impurity was quite insufficient to account for the large number of scintillations observed. Rutherford came to the conclusion that the protons which caused the scintillations had been knocked out of the nitrogen nuclei by the fast ex-particles and, in the process, the nitrogen nuclei were transmuted into oxygen nuclei. It was an exciting discovery. Here, for the first time,
the alchemist’s dream of turning one substance into another had been realized. True, Rutherford had not turned lead into gold; he had only changed nitrogen into oxygen. But it was a vital link in the progress of research into the secrets of the atom, which has enabled man to transmute one substance into another, accompanied by the production of something of greater value than much gold, namely, energy.
The nuclear reaction which occurs in Rutherford’s nitrogen experiment is
represented as follows:
This may be interpreted thus. A nitrogen nucleus containing 7 protons and 7 neutrons is struck by an ex-particle (helium nucleus) consisting of 2 protons and 2 neutrons, forming an unstable collective mass of 9 protons and 9 neutrons. This ejects a single high-energy proton and becomes transmuted into an isotope of oxygen of mass number 17. The shorthand notation used to represent this reaction is
Following the nitrogen experiment, Rutherford and his colleague Chadwick
succeeded in disintegrating more than a dozen other elements by alpha bombardment and obtaining fast protons. Calculations made from the range of these protons showed that they had energies far greater than those of the ex-particles.
This was very strong evidence that the alphas were actually triggering off nuclear disintegration and thereby releasing nuclear energy. It was not a case of simple collision, whereby the exparticle conveyed part of its own energy to the proton, as had happened when Marsden first projected protons by alpha bombardment.