# SS: Characteristics of three types of emission

Show/Hide Sub-topics (Radioactivity and the nuclear atom | O Level)

α-particles consist of two protons and two neutrons; hence it is equivalent to a helium nucleus. It has a mass of 4u (atomic mass units, 4 $\times$ the mass of a proton) and a charge of +2 e. α-particles travel up to about one-tenth of the speed of light.

β-particles are electrons emitted with varying speeds which can be as high as nine-tenths of the speed of light. Like electrons, they carry negative charge of -e. β-particles are much lighter than α-particles. ($\frac{1}{1836} \times$ mass of a proton).

γ-rays are electromagnetic waves of very short wavelength. They are often emitted at the same time as α- or β- particles. γ-rays are massless.

Ionisation

Ion is a charged atom, formed by losing or gaining electrons. All three kinds of emissions are capable of creating ions.

• α-particles knock electrons out of nearby atoms, thus ionizing the atoms. The α-particles are much heavier than the electrons and can simply dislodge them from the atoms, thus they have the most powerful ionization ability.
• β-particles are less strongly ionising than α-particles, but more than γ-rays. It is less likely for a lighter β-particles to dislodge an electron.
• γ-rays are the least ionising of the nuclear radiation as it is uncharged.

Penetrating power

• α-particles are the least penetrating of the nuclear radiations and can be stopped by a sheet of paper. They can only pass through a few cm of air.
• β-particles are more penetrating and are stopped by a thin (1 mm) sheet of aluminium. The fastest β-particles have a range of a meter or so in air.
• γ-rays are the most penetrating nuclear radiation. Intensity of gamma radiation can be reduced to half by 1 or 2 cm thick lead.