Radiation - Activity

Measuring the Source: A Guide to Radioactivity

Radioactivity, or simply 'activity', is the property of certain atomic nuclei that causes them to spontaneously 'decay' or disintegrate, in the process emitting ionizing radiation (such as alpha particles, beta particles, or gamma rays). The activity of a radioactive source is a measure of its decay rate—it quantifies how many atomic nuclei are decaying and emitting radiation per unit of time. It is a fundamental property of a radioactive substance that tells us how 'hot' or active it is. A source with high activity is emitting a large number of particles or photons per second. This is a crucial measurement in nuclear medicine, nuclear power, and environmental science for characterizing, handling, and shielding radioactive materials.

The standard international (SI) unit of activity is the Becquerel (Bq), named after Henri Becquerel, who discovered radioactivity. One Becquerel is defined as one atomic decay per second. Because this is a very small unit, prefixes like kilobecquerel (kBq) or megabecquerel (MBq) are often used. A much older, non-SI unit that is still widely used, particularly in the United States and in historical contexts, is the Curie (Ci), named after Marie and Pierre Curie. The Curie was originally based on the activity of one gram of radium-226 and is a very large unit of activity. This converter allows for easy translation between these different scales, which is essential for anyone working in the nuclear field to accurately quantify and communicate the strength of radioactive sources.

Relevant Formulas and Concepts

• Law of Radioactive Decay (Physics): The activity (A) of a sample decreases exponentially over time. The formula is A(t) = A₀ * e^(-λt), where A₀ is the initial activity, λ is the decay constant, and 't' is time.
• Decay Constant and Half-Life (Physics): The decay constant (λ) is related to the half-life (T₁/₂) of the isotope by the formula T₁/₂ = ln(2) / λ ≈ 0.693 / λ. The half-life is the time it takes for the activity of a sample to decrease by half.
• Activity, Moles, and Avogadro's Number (Chemistry/Physics): The activity of a sample can be calculated if you know the number of atoms (N) and the decay constant (λ): A = λN. The number of atoms can be found using the mass of the sample, its molar mass, and Avogadro's number.

A Deep Dive into Radioactivity Units

• Becquerel (Bq): The SI unit of radioactivity, defined as one nuclear decay per second (1 s⁻¹). It is the modern global standard for all scientific, regulatory, and medical applications involving radioactivity.
• Curie (Ci): The historical unit of radioactivity. 1 Curie is a large amount of activity, equal to exactly 37 billion Becquerels (3.7 x 10¹⁰ Bq). It was originally defined as the activity of one gram of the isotope radium-226. Due to its large size, smaller units like the millicurie (mCi) and microcurie (µCi) are very common.
• Rutherford (Rd): An even older, obsolete unit of radioactivity, defined as the activity of a quantity of radioactive material in which one million nuclei decay per second (10⁶ Bq). It is rarely used today but may be found in historical scientific papers.
• Disintegrations Per Minute (DPM): A common practical unit in lab settings using scintillation counters, which directly measure decay events. As the name implies, it is the number of atomic decays that occur per minute. 1 DPM = 1/60 Bq.