DEFINITION OF RADIOACTIVE DECAY

The nuclei of unstable radioactive elements

may spontaneously break down to become more stable,

emitting radiation as alpha particles, beta particles, or

gamma rays. When these particles and rays are emitted by

radioactive decay they move through matter and knock electrons

out of surrounding atoms, ionizing these atoms. Alpha

decay of an atomic nucleus produces alpha particles that are

the most ionizing form of radioactive radiation, consisting of

two neutrons and two protons with two positive charges, and

are heavy, slow-moving particles. Beta decay of a nucleus

converts a neutron into a proton, emitting a high-speed electron

and an electron-antineutrino, increasing the atomic

number by one and leaving the mass number the same. The

beta particles that are emitted are high-speed electrons and

are moderately ionizing, being more penetrating than alpha

particles. They can travel several meters in the air and are

easily deflected by electromagnetic fields. Gamma rays carry

no charge, are weakly ionizing, and consist of a very high-frequency

type of electromagnetic radiation emitted by the

nuclei of radioactive elements during decay, typically as part

of alpha or beta decay. Gamma rays may also form from the

interaction of high-energy electrons with matter. Gamma rays

are deeply penetrating and are not deflected by electromagnetic

fields, and they may be used to kill bacteria or to sterilize

surfaces. Space-based observatories have detected cosmic

gamma ray radiation coming from distant pulsars, quasars,

and radio galaxies, but this cosmic gamma ray radiation cannot

penetrate the Earth’s atmosphere.

When radioactive elements or radioactive isotopes of stable

isotopes decay to more stable elements, the atomic mass

number of the element is changed, transmuting the parent

element into a different element known as a daughter isotope

and emitting atomic radiation. For each radioactive element

or isotope, decay occurs at a constant rate known as the halflife,

determined by the time taken for half of any mass of that

isotope to decay from the parent isotope to the daughter iso-

tope. Radioactive decay is an exponential process, with half

of the original starting material decaying in the first step, half

of the remaining material (25 percent of the original material)

decaying after the second step, half of the remaining material

(12.5 percent of the original material) decaying after the third

step, and so on. The final product of all decay schemes is a

stable element.

Radioactive decay may occur in one step or, more commonly,

in a series of steps known as a decay series. In some

decay series the intermediate steps may be moderately or very

short-lived, and the daughter isotope may be more or less

radioactive than the parent isotope. There is a very wide

range in half-lives for different radioactive isotopes, ranging

from 4.4 × 10–22 sec for lithium-5, through 4.551 × 109 years

for 238U, to 1.5 × 1024 years for tellurium-128.

See also GEOCHRONOLOGY.

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