The nuclei of most familiar substances are stable. They retain their identities and remain unchanged indefinitely. However, some atomic nuclei change with time; they are unstable. As unstable atomic nuclei disintegrate, they emit high-energy photons and various subatomic particles. Radioactivity is a general term that refers to any of these types of radiation arising from the disintegration of unstable atoms.
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. A4 He 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. 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 near-light 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 .
In nature, radioactivity is produced by certain isotopes of elements with atomic numbers up to and including 92 (uranium). These are known as radioactive isotopes . An isotope of carbon, known as carbon-14 (14C), has eight neutrons. Atoms of 14 C are unstable; over time, they decay into carbon-12 (12C) atoms, which have six neutrons. Other examples of an unstable atoms include hydrogen-3 (3H), also known as tritium , which has a nucleus consisting of one proton and two neutrons; beryllium-7 (7Be), with a nucleus containing four protons and three neutrons; and 10Be, with a nucleus containing four protons and six neutrons.
In some instances, the most common isotope of a naturally occurring element also happens to be radioactive. Examples are radon, radium, and uranium. The barrage of cosmic particles from deep space can be considered a form of radioactivity, but these particles sometimes can create radioactive isotopes when they strike stable atoms in the Earth’s upper atmosphere.
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