Most elements do not change unless a force is applied to them that causes them to join with another element. They are said to be stable. But some elements are not stable. Their nuclei are constantly breaking down, or decaying, as they shed particles in an attempt to become stable. This is radioactivity, and the particles that are given off are known as radiation. Three types of particles are known to be emitted: alpha, beta and gamma rays.
As its name implies, radioactivity is the act of emitting radiation spontaneously. This is done by an atomic nucleus that, for some reason, is unstable; it “wants” to give up some energy in order to shift to a more stable configuration. During the first half of the twentieth century, much of modern physics was devoted to exploring why this happens, with the result that nuclear decay was fairly well understood by 1960. Too many neutrons in a nucleus lead it to emit a negative beta particle, which changes one of the neutrons into a proton. Too many protons in a nucleus lead it to emit a positron (positively charged electron), changing a proton into a neutron. Too much energy leads a nucleus to emit a gamma ray, which discards great energy without changing any of the particles in the nucleus. Too much mass leads a nucleus to emit an alpha particle, discarding four heavy particles (two protons and two neutrons).
Radioactivity is a physical, not a biological, phenomenon. Simply stated, the radioactivity of a sample can be measured by counting how many atoms are spontaneously decaying each second. This can be done with instruments designed to detect the particular type of radiation emitted with each “decay” or disintegration. The actual number of disintegrations per second may be quite large. Scientists have agreed upon common units to use as a form of shorthand. Thus, a curie (abbreviated “Ci” and named after Pierre and Marie Curie, the discoverers of radium (87) is simply a shorthand way of writing “37,000,000,000 disintegrations per second,” the rate of disintegration occurring in 1 gram of radium. The more modern International System of Measurements (SI) unit for the same type of measurement is the Becquerel (abbreviated “Bq” and named after Henri Becquerel, the discoverer of radioactivity), which is simply a shorthand for “1 disintegration per second.”
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