In radioactive processes, particles or electromagnetic radiation are emitted from the
nucleus. The most common forms of radiation emitted have been traditionally classified as
alpha (a), beta (b), and gamma (g ) radiation. Nuclear radiation occurs in other forms,
including the emission of protons or neutrons or spontaneous fission of a massive nucleus.
Of the nuclei found on Earth, the vast majority is stable. This is so because almost
all short-lived radioactive nuclei have decayed during the history of the Earth. There are
approximately 270 stable isotopes and 50 naturally occurring radioisotopes (radioactive
isotopes). Thousands of other radioisotopes have been made in the laboratory.
Radioactive decay will change one nucleus to another if the product nucleus has a
greater nuclear binding energy than the initial decaying nucleus. The difference in binding
energy (comparing the before and after states) determines which decays are energetically
possible and which are not. The excess binding energy appears as kinetic energy or rest
mass energy of the decay products
The Chart of the Nuclides, is a plot of nuclei as a function of proton number, Z, and neutron number, N . All stable nuclei and known radioactive nuclei, both naturally occurring and manmade, are shown on this chart, along with their decay properties. Nuclei with an excess of protons or neutrons in comparison
with the stable nuclei will decay toward the stable nuclei by changing protons into neutrons
or neutrons into protons, or else by shedding neutrons or protons either singly or in
combination. Nuclei are also unstable if they are excited, that is, not in their lowest energy
states. In this case the nucleus can decay by getting rid of its excess energy without
changing Z or N by emitting a gamma ray.
Nuclear decay processes must satisfy several conservation laws, meaning that the
value of the conserved quantity after the decay, taking into account all the decay products,
must equal the same quantity evaluated for the nucleus before the decay. Conserved
quantities include total energy (including mass), electric charge, linear and angular
momentum, number of nucleons, and lepton number (sum of the number of electrons,
neutrinos, positrons and antineutrinos—with antiparticles counting.
The number of nuclei in a sample that will decay in a given interval of time is
proportional to the number of nuclei in the sample. This condition leads to radioactive decay
showing itself as an exponential process, as shown in Fig. 3-2. The number, N, of the
original nuclei remaining after a time t from an original sample of N0
nuclei is
N = N0
e
-(t/T)
where T is the mean lifetime of the parent nuclei. From this relation, it can be shown that t1 / 2
= 0.693T.