There are six different types of commonly used nuclear radiations: alpha particles, beta particles, positrons, gamma rays, protons, and neutrons. The first five of these radiations are detected directly by the ionization which they produce on passage through a medium, such as gas. Neutrons are unlike the other radiations in that they are nonionizing in nature, but when entering a nucleus they often convert it to a radioactive isotope whose emission is detectable (for instance, when they enter the nucleus of the boron-10 isotope, the nucleus disrupts, emitting an alpha particle and a lithium particle; the alpha and lithium particles can be detected). Neutrons, therefore, are detected by secondary means.