Introduction Matter is composed of molecules which are specific arrangements of atoms. The atom consists mainly of three basic particles, electrons, neutrons, and protons. The nucleon consists of neutrons which have no charge, protons which have positive charge. Both the proton and neutrons have masses 1836 time that of an orbital electron.
Radioactive decay (emission of radiation) sometimes occurs whenever the number of neutrons and protons in a nucleus are unequal. For low atomic weight atoms the ratio is 1 to 1 and for high atomic weight atoms the ratio is 1.5 neutrons to 1 proton. This unstable condition sometimes results in a redistribution of the nucleus so that a more stable state is established. When known stable nuclides are displayed on a neutron number- atomic number plot, a smooth line of stability is obtained. There are some variations from the smooth curve but these are relatively small. Nuclides falling on a vertical line have equal Z numbers (atomic number or same number of protons) and are isotopes. Those on a horizontal line have equal N number (neutrons) and are isotones. Nuclides of equal A numbers (Atomic weight) lie on the 45-degree lines and are isobars.
For each value of Z there appears to be a corresponding small range of N values for stability. If the Z-N relation is upset by any means an unstable nuclide will result. Suppose, for example that an artificial nuclide was created by introducing an extra neutron into a stable configuration. The resulting structure would be above the stability line and would be expected to return to the line of stability by radioactive decay. It could do so by either a decrease in N or an increase in Z.
All one must do to see if a nucleus can decay is to compute the energy released (called the Q value) of the decay.
Q= (mass of the initial nucleus)- (mass of decay products). If Q is a positive number, decay can occur. If Q is a negative number, then decay cannot occur. In chemical terms, spontaneous nuclear decay must be exergonic. For example 235U emits an a particle.
In addition to determining if the decay will occur, the energetics also influence the rate at which the decay will take place. For example, the half live of the radioactive isotopes are inversely related to the decay energy.
Beta decay A beta particle is deflected by a strong magnetic field in the direction to be expected of negatively charged particle. Beta particles have a charge and mass equal to those of high speed electrons. Although beta particles are energetic electrons, ejected from radioactive nuclides, it must be emphasized that they do no come from the orbital electron structure surrounding the nucleus. Electrons that arise from nuclear decay are symbolized (b-) for negatrons and (b+) positrons. Both are referred to as beta particles.
Electrons that originate from extra nuclear processes, those outside the nucleus, are symbolized by (e-) for electrons and (e+) for positrons. Both are referred to as electrons. The Energies are expressed in MeV or KeV. The term MeV means million electron volts and KeV means thousand electron volts. Electron volt is denoted by eV and is defined as the kinetic energy acquired by an electron falling through a potential difference of one volt.
A neutron in the nucleus may change into a proton by emitting a beta particle plus antineutrino (v).
Unlike alpha particles and gamma rays emitted by nuclei, beta particles from radioactive nuclei exhibit a continuous energy distribution as shown in Figure 2.
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