In alpha decay the nucleus emits a He nucleus, an alpha particle. Alpha decay occurs most often in massive nuclei that have too large a proton to neutron ratio. An alpha particle, with its two protons and two neutrons, is a very stable configuration of particles. Alpha radiation reduces the ratio of protons to neutrons in the parent nucleus, bringing it to a more stable configuration. Nuclei, which are more massive than lead, frequently decay by this method.
Consider the example of
2 1 0
Po decaying by the emission of an alpha particle. The
reaction can be written
2 1 0
Po Æ
2 0 6
Pb +
4
He. This polonium nucleus has 84 protons and
126 neutrons. The ratio of protons to neutrons is Z/N = 84/126, or 0.667. A
2 0 6
Pb nucleus
has 82 protons and 124 neutrons, which gives a ratio of 82/124, or 0.661. This small
change in the Z/N ratio is enough to put the nucleus into a more stable state, and as shown
in Fig. 3-4, brings the “daughter” nucleus (decay product) into the region of stable nuclei
in the Chart of the Nuclides.
In alpha decay, the atomic number changes, so the original (or parent) atoms and the
decay-product (or daughter) atoms are different elements and therefore have different
chemical properties.
In the alpha decay of a nucleus, the change in binding energy appears as the kinetic
energy of the alpha particle and the daughter nucleus. Because this energy must be shared
between these two particles, and because the alpha particle and daughter nucleus must have
equal and opposite momenta, the emitted alpha particle and recoiling nucleus will each have
a well-defined energy after the decay. Because of its smaller mass, most of the kinetic
energy goes to the alpha particle.
