The becquerel (Bq) is a unit of measurement that quantifies the rate of decay of unstable isotopes. One becquerel is defined as the decay of one atomic nucleus per second. This unit of measurement is used to measure the rate of radioactivity in a sample. A small amount of highly radioactive material will have a much higher Bq rating than a more inert sample of the same size. This figure will change over time, as radioactive elements gradually decay into stable elements, and the time of measurement must be noted when a reading in Bq is taken or predicted mathematically.
Antoine Henrie Becquerel worked with Pierre and Marie Curie in the early days of research into radioactivity. He is typically credited with being the original discoverer of radioactivity, which he stumbled upon during research into the phosphorescent properties of uranium. His name was later given to a unit of measurement, a traditional method of honoring great scientists.
Modern scientific analysis relies on a standardized system of measurements called the International System of Units (SI). This system consists of two tiers: a basic tier of defined units, such as the meter and the second, and an additional tier of derived units based on those defined units. The becquerel is one of the derived units in this system.
The measurement of radiation is a complicated process, and different units measure different aspects of radioactivity. Some units, such as the rem or the sievert, attempt to measure the potential impact of radiation on organic tissue. Others measure the total energy in a sample or an area. The becquerel measures the radioactivity, or raw number of atomic destructions per second.
Not all atoms decay in the same fashion, which makes the direct conversion from the becquerel to a measure of absorbed or dangerous radiation somewhat difficult. Alpha particles, for example, are very dangerous in internal exposure, but interact strongly with other matter and typically cannot penetrate the skin. Gamma rays and neutrons, however, travel easily through matter, and can be very dangerous as external radiation.
The Bq is a very small unit of measurement and is typically given a prefix in order to provide useful information. As an example, a typical human being experiences around 5,000 Bq of radiation, or 5,000 destructions per second, from naturally occurring radioactive isotopes within the body. Major radiation events are many orders of magnitude more energetic than this. The first unit devised to measure radioactivity, the curie, is 37 billion Bq, which gives a sense of how small a unit the Bq actually is.