Nuclear radioactivity is the property of some atoms which, due to their nuclear instability, emit particles. The most commonly observed types of radioactivity are alpha (emission of alpha particles-a helium nucleus), beta (emission of an electron with transformation of a neutron into a proton) and gamma (emission of high energy photons). Radiation is uniformly distributed in all directions, and follows an inverse square law, as shown in Figure 1, where the intensity, I, at the surface of a sphere, centered at the radioactive sample, is a function of the intensity of the source, I₀, and the radius of the sphere.

Figure 1. Depiction of radial intensity.

If N is the number of radioactive nuclei in a given sample, then this sample will decay as radiation is emitted, following an exponential decay law:

N = N₀·e^-λtwhere N₀ is the initial number of radioactive nuclei, and λ is the decay constant of the radioactive substance.
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