Radioactivity is the spontaneous emission of energy and subatomic particles from the unstable nucleus of an atom, known as radioactive decay. Since this phenomenon is ubiquitous in the environment, food, and within our bodies, a precise international standard is required to measure its intensity. The becquerel is the metric unit adopted to quantify the rate of these nuclear events universally.
Defining the Becquerel: The SI Unit of Activity
The becquerel, symbolized as Bq, is the International System of Units (SI) measure for the activity of a radioactive material. It provides a precise quantification of a substance’s radioactivity, directly representing the frequency of nuclear transformations occurring within it. Specifically, one becquerel is defined as one nuclear disintegration, or one atom decaying, per second.
The unit is named after the French physicist Antoine Henri Becquerel, who discovered the phenomenon of radioactivity in 1896. It was officially adopted in 1975, replacing the older, non-SI unit called the curie (Ci). The curie represented a significantly larger quantity of activity, equaling 3.7 x 10¹⁰ becquerels (37 GBq). The shift to the becquerel provides a more granular and mathematically straightforward unit, which is beneficial when measuring the low levels of radiation found in environmental samples.
Contextualizing the Scale of Measurement
Because a single becquerel represents only one decay per second, it is a very small unit of measure, and measurements encountered in real-world scenarios often require the use of metric prefixes. Common multiples include the kilobecquerel (kBq), representing one thousand decays per second, and the megabecquerel (MBq), representing one million decays per second.
Measurements in Bq demonstrate that radioactivity is a constant component of our natural environment. For instance, the human body itself contains naturally occurring radioactive elements, primarily potassium-40 (\(^{40}K\)) and carbon-14 (\(^{14}C\)). A typical adult body contains between 3,000 and 6,000 becquerels of activity at any given time, mostly from the potassium that is essential to bodily function.
This natural activity extends to common household items and food sources. Many common foods contain trace amounts of radioactivity, with the activity from \(^{40}K\) ranging from approximately 40 to over 600 Bq per kilogram. Even building materials, like granite countertops, contain naturally occurring radioactive isotopes such as uranium, thorium, and potassium. The maximum activity concentrations in granite slabs can reach up to 2,715 Bq/kg for \(^{40}K\) and 450 Bq/kg for radium-226, illustrating that low Bq counts are normal and ubiquitous in our daily lives.
Practical Applications in Health and Safety
The becquerel is a foundational unit in public health and safety, providing the necessary basis for environmental monitoring and regulatory oversight. Governments and agencies rely on Bq measurements to continuously monitor radioactivity in the air, water, and soil surrounding nuclear facilities and other industrial sites. This ongoing surveillance establishes baseline levels of natural background radiation and allows for the detection of any abnormal releases.
The becquerel is also used to establish regulatory limits for contaminated substances, particularly food. Following nuclear incidents, standards are set in Bq per kilogram (Bq/kg) to ensure that food products entering the supply chain do not exceed safe consumption levels. A specific example is the monitoring of wild game for Cesium-137 (\(^{137}Cs\)) contamination, a long-lived radionuclide that entered the food chain after the Chernobyl accident.
Nuclear Medicine
In nuclear medicine, the becquerel is the standard unit for measuring the activity of radioactive tracers administered to patients for diagnostic imaging or therapeutic treatment. Specific doses of radiopharmaceuticals, such as those used in cancer therapy or heart scans, are carefully calibrated and measured in megabecquerels (MBq) or gigabecquerels (GBq). This accurate measurement ensures that the patient receives the exact amount of radioactive material needed for the intended medical procedure.