Radioactive Decay

The nucleus of each atom has a specific number of protons and neutrons and is either stable or unstable, depending on the relative number of each. The most stable atoms are those that have an equal number of protons and neutrons. Atoms that are unstable are radioactive. An atom that is radioactive can also be called a radionuclide. Of the known nuclides (approximately 2,000), only 264 are stable, and of the known radionuclides (approximately 1,700), only 70 occur in nature. The rest are man-made. Unstable atoms undergo a process called radioactive decay to reach a more stable state.

While a radionuclide is going through the process of decay, energy is released from the atom in one of three modes: alpha, beta, or gamma radiation. These modes may take several steps, involving only the nucleus or the entire atom. Each radionuclide has one or more characteristic modes of decay. The particular mode of decay determines the type of energy, or radiation, released from the atom, and consists of either subatomic particles, photons, or both.

Radionuclides are unstable to varying degrees. The more unstable a radionuclide is the faster it decays. The quantity of a radioactive substance is expressed as disintegrations per second, in units of Curies (Ci) named for Marie Curie, or if Système International is used, Becquerels (Bq) named for Henri Becquerel. The rate at which a radionuclide decays depends upon its half-life, the expected time required for half of the nuclei to decay to a stable state. The half-life is typically not affected by temperature, pres sure, or gravitational, magnetic, or electrical fields.

When radioactivity was first discovered, it was thought that all the energy given off by the radionuclide was basically the same, with differences only in penetrating power. However, research conducted by Becquerel and Pierre Curie proved that there were three distinct modes of radioactive decay, which differed not only in their ability to penetrate, but also in their velocity, as well as their susceptibility to magnetic fields.

Alpha and beta radioemissions are actually particulate matter that is thrown out from the nucleus. An alpha particle is two protons and two neutrons, or in other words, it is a helium atom without the electrons. After an alpha particle is emitted, the atomic mass decreases by four, and the number of protons and neutrons decrease by two. Alpha decay occurs in radionuclides with an atomic number greater than 83 and a mass number greater than 209. Alpha particles interact with negatively charged electrons in the environment, which consequently use up th e energy in the particle, slowing it down and greatly diminishing its penetrating power. Even a sheet of paper can stop an alpha particle. The direction of an alpha particle is only slightly affected by a magnetic field because the particle has a balanced change. When a radionuclide decays by a lpha radiation, it does not just disappear. Instead, the radionuclide transmutes into another radionuclide or nuclide. For example, uranium-238 transmutes into several other radionuclides, including radium-226 and radon-222, before ending up as lead-206, a stable nuclide.

Beta radiation, which also involves particulate emissions, can be either be negatively charged or positively charged. Beta particles are actually created in the nucleus by either a proton changing into a neutron (positron emission) or a neutron changing into a p roton (negatron emission). A beta particle has a higher velocity than an alpha particle, and its path is markedly deflected by a magnetic field. When a negatron is emitted from an atom, the atomic mass of the atom is unchanged, the number of protons increases by one, and the number of neutrons decreases by one. The mass remains unchanged when a positron is emitted, the number of neutrons increases by one, and the number of protons decreases by one.

An atom usually becomes excited from either of the above-mentioned decay processes and sheds excess energy in the form of a gamma ray photon. With gamma emissions, the atomic mass, number of protons (atomic number), or the number of neutrons, remains unchanged. The velocity of a gamma ray is almost that of light and is not affected by magnetic fields.