You will have heard radioactivity mentioned in everyday speech. It was discovered in 1896, when the Frenchman Henri Becquerel wrapped up a uranium compound in the dark with a photographic plate. When he developed the plate, it looked as if it had suffered exposure to light. Evidently, uranium compounds emit ‘rays’ or radiation which, although invisible, resemble light in affecting photographic film. Such substances are said to display radioactivity.
The radiation from uranium compounds is of three different types. This can be demonstrated by passing it between the poles of a powerful U-shaped magnet. The stream of radiation is split into three smaller streams, which we label with the Greek letters alpha (α), beta (β) and gamma (γ).
A schematic diagram of an experiment that shows that a uranium compound emits three kinds of radiation. The compound is placed in a horizontal shaft drilled in a block of lead. The radiation is stopped by lead, so it can emerge only along the line of the shaft in a horizontal direction. When it passes between the north and south poles of a powerful U shaped magnet, it is divided into three streams labelled α, β and γ
The γ-stream or γ-radiation is undeflected by the magnet and just goes straight on. This is what visible light would do, and indeed, γ-rays are just radiation at the short wavelength end of the electromagnetic spectrum. They resemble X-rays.
By contrast, the α- and β-streams in Figure 14.1 are deflected by the magnet. This indicates that they consist not of electromagnetic radiation like light, but of charged particles: moving charged particles are deflected by a magnetic field. Moreover, the α- and β-streams are deflected in opposite directions. This shows that they have opposite charges.
You may already know of one charged particle. It is the electron, and its charge is negative. The β-particles in the illustration are fast-moving electrons, so they have a negative charge.
What charge do the α-particles carry?
Because they have an opposite charge to the electrons, they are positively charged.
The essence of a particle is that it has mass: α- and β-particles are evidence that radioactive atoms are, quite spontaneously, firing off bits of charged matter. The mass of an electron (a β-particle) is very small; it is only about one two-thousandth of the mass of a hydrogen atom. But the mass of the positively charged α-particle is much greater; it is about four times that of a hydrogen atom.