FORMS

Radioactivity, also called ionizing radiation because it can strip electrons from atoms, occurs in various forms. The most common are gamma rays (which we’ve already discussed), alpha (α) particles, beta (β) particles, and neutrinos. There are also some less common forms, such as high-speed protons and antiprotons, neutrons and antineutrons, and the nuclei of atoms heavier than helium.
Alpha particles are helium-4 (⁴He) nuclei traveling at high speeds. A4He nucleus consists of two protons and two neutrons. An alpha particle has a positive electric charge because there are no negatively charged electrons surrounding it. As such, all alpha particles are ions. They have significant mass, so if they attain high enough speeds, they can acquire considerable kinetic energy. An alpha particle traveling at a sizable fraction of the speed of light (known as relativistic speed) attains an increased mass because of relativistic effects; this gives it additional kinetic energy. You’ll learn about relativistic mass increase and other effects in Chapter 20. Most alpha particles can be blocked by modest barriers.
Beta particles are high-speed electrons or positrons. (Remember that a positron is the antimatter counterpart of the electron.) Any beta particle consisting of an electron, also called a negatron because it has a negative electric charge, is denoted β^-, and any β particle consisting of a positron, which carries a positive charge, is denoted β⁺. All beta particles have nonzero rest mass (their mass when not moving at relativistic speed). Their kinetic energies are increased by relativistic effects if they move at nearlight speeds.
Neutrinos are an entirely different sort of particle. They have no electric charge and no rest mass. They have tremendous penetrating power. The Earth is constantly being bombarded by neutrinos from space. These neutrinos have their origins in the cores of the Sun and distant stars. Most neutrinos pass through the entire planet unaffected. Sophisticated equipment is required to detect them. Neutrino detectors are placed far underground to block all other forms of radiation so that scientists can be sure the equipment is really detecting neutrinos and not stray particles of some other sort. The neutrino has a counterpart, known as the antineutrino.