Where does ionising radiation come from?
Teaching Guidance for 14-16
The discovery of alpha, beta and gamma radiation
The initial discovery and identification of alpha, beta and gamma radiation provides an interesting case for
how science works. The story takes us:
- From the initial discovery of the three kinds of radiation as a new phenomenon: this is something new!
- To the use of electric and magnetic fields in identifying the nature of the radiation: what is it?
- To theorising and experimenting on the origins of the new radiation: where does it come from and how is it produced?
Rutherford, Soddy, Villard and Becquerel: more depth
In 1898, Ernest Rutherford, then working at McGill University, Montreal, performed an experiment in which he placed successive layers of aluminium foil over a powdered uranium compound spread on a capacitor plate. A sensitive meter connected to the initially charged capacitor showed the rate of decay of charge from the capacitor as radiation from the uranium ionised the air between the plates. He found that the rate of ionisation decreased sharply on adding the first few layers of aluminium and then very slowly with additional layers. This finding led Rutherford to conclude that the radiation from the uranium consisted of two components: one capable of penetrating only a few centimetres of air or a few layers of aluminium foil and the other capable of penetrating much greater thicknesses of air or aluminium. Rutherford suggested that the two types of radiations should be designated alpha and beta. In 1900, the Frenchman, Villard, discovered a third, still more penetrating component, which he referred to as gamma radiation, following Rutherford's lead.
At this time the identity of these three kinds of radiations was unknown and investigations concentrated on finding out about the rays by passing them through electric and magnetic fields. In 1899, Becquerel showed that a component of the radiation from radium was deflected by electric and magnetic fields, as would be expected for negatively charged particles. Further experiments with electric and magnetic fields showed that this radiation was the same as Rutherford's beta rays, consisting of streams of high-velocity electrons. At first it was thought that neither alpha nor gamma radiations were deflected by electric and magnetic fields. In 1903, however, Rutherford showed that when using sufficiently strong electromagnets, alpha radiation is deflected as though positively charged. These alpha particles were later identified as being doubly ionised helium ions, while gamma radiation was identified as being high-frequency electromagnetic radiation.
Identification of the three forms of radiation, by using electric and magnetic fields, gave no insight into the origin of the radiation. The big question remained: how could the continuous emission of particles/radiation in the absence of chemical triggers or other alterations in the state of the radioactive materials be accounted for? It was well known that changing the ambient temperature and pressure of a chemical reaction changes the rate of reaction, and numerous efforts were made to determine whether such changes had any effect on the intensity of radiation from various sources. No effect was observed.
These findings led Rutherford and Soddy, in 1902, to the conclusion that:
Since…radioactivity is at once an atomic phenomenon and accompanied by chemical changes in which new types of matter are produced, these changes must be occurring within the atom, and the radioactive elements must be undergoing spontaneous transformation. The results have so far been obtained, which indicate that the velocity of this reaction is unaffected by the physical and chemical conditions, making it clear that the changes in question are different in character from any that have been before dealt with in chemistry. It is apparent that we are dealing with phenomena outside the sphere of known atomic forces. Radioactivity may therefore be considered as a manifestation of subatomic chemical change. (Philosophical Magazine, September, 1902).
In the following years, this theory, involving the spontaneous emission of radiation from the nuclei of certain atoms and the consequent transformation of those atoms from one element to another, was fully substantiated.