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1. Spontaneous disintegration of atomic nuclei with emission of corpuscular or electromagnetic radiations. </dd>
The principal types of radioactivity are alpha decay, beta decay, and isomeric transition. To be considered as radioactive a process must have a measurable lifetime between approximately 10E-10 second and approximately 10E17 years. Radiations emitted within a time too short for measurement are called prompt. Prompt radiations such as gamma rays and X-rays are often associated with radioactive disintegrations. </dd>
2. The number of spontaneous disintegrations per unit mass and per unit time of a given unstable (radioactive) element, usually measured in curies. [[/a>|/a> ]]


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use