Curie

The curie (symbol: Ci) is a non-SI unit of radioactivity, named in honour of the French physicists Pierre Curie and Marie Curie, pioneers in the study of radioactivity. Originally defined in 1910 as the radioactivity of one gram of pure radium-226, the curie was later standardised as exactly 3.7 × 10¹⁰ nuclear disintegrations per second. Although the curie has been formally superseded by the becquerel (Bq), the SI unit of radioactivity adopted in 1975, it remains in widespread use, particularly in the United States, in nuclear medicine, radiological protection, and certain branches of industry and environmental science. One curie corresponds to 37 gigabecquerels (37 GBq), while one becquerel equals approximately 2.703 × 10⁻¹¹ curies.

Background

The curie takes its name from the husband-and-wife team of Pierre Curie (1859–1906) and Marie Curie (1867–1934), whose investigations of uranium rays—first observed by Henri Becquerel in 1896—led to the discovery of the radioactive elements polonium and radium in 1898. Marie Curie coined the term "radioactivity" to describe the spontaneous emission of radiation by certain heavy elements, and the Curies' meticulous work isolating radium from pitchblende ore established the quantitative foundations of the new science. Their achievements, together with Becquerel's initial discovery, were recognised with the Nobel Prize in Physics in 1903; Marie Curie later received a second Nobel Prize, in Chemistry, in 1911 for the discovery of polonium and radium and the isolation of pure radium.

Because radium was the most practically significant radioactive substance in the early twentieth century—used in early medical treatments, luminous paints, and scientific research—a need soon arose for a standard unit to quantify the amount of radioactive material and its emanation strength.

History

The curie was formally defined in 1910 at the International Congress of Radiology and Electricity in Brussels. The congress, at the suggestion of prominent scientists of the era, decided that a unit of radioactivity should be named after Pierre Curie, who had died in 1906. The original definition designated the quantity of radon (radium emanation) in radioactive equilibrium with one gram of radium as one curie.

This definition, based on radon's quantity relative to radium, proved cumbersome because it depended on the accuracy with which the atomic mass of radium was known and on the equilibrium conditions of radioactive decay. In 1950, an international commission revised the definition, redefining the curie as exactly 3.7 × 10¹⁰ disintegrations per second, regardless of the substance involved. This value was chosen because it closely approximated the measured activity of one gram of radium-226 while being an exact, round number applicable to any radionuclide.

The unit persisted in international use for several decades, but as the Système International d'Unités (SI) was developed and formalised in the mid-twentieth century, the desire for a coherent unit based on the fundamental quantity of events per second grew. In 1975, the General Conference on Weights and Measures adopted the becquerel, defined as one nuclear disintegration per second, in honour of Henri Becquerel. The curie was retained as a non-SI unit accepted for temporary use, and it continues to appear in legal, medical, and commercial contexts, especially in the United States.

Definition and Conversions

The curie is defined as exactly 3.7 × 10¹⁰ radioactive disintegrations per second:

• 1 Ci = 3.7 × 10¹⁰ disintegrations per second = 37,000,000,000 Bq = 37 GBq
• 1 Bq ≈ 2.703 × 10⁻¹¹ Ci

For many practical purposes, particularly older applications, one gram of radium-226 is still regarded as approximately one curie; more precisely, owing to the accepted value of the half-life of radium-226, one gram of radium corresponds to about 0.988 curies under the modern definition.

Because activities encountered in practice range over many orders of magnitude, the curie is commonly used with standard metric prefixes:

• 1 kilocurie (kCi) = 10³ Ci
• 1 megacurie (MCi) = 10⁶ Ci
• 1 millicurie (mCi) = 10⁻³ Ci = 37 MBq
• 1 microcurie (µCi) = 10⁻⁶ Ci = 37 kBq
• 1 nanocurie (nCi) = 10⁻⁹ Ci
• 1 picocurie (pCi) = 10⁻¹² Ci = 0.037 Bq

The picocurie is frequently encountered in environmental monitoring, notably in the measurement of radon concentrations in buildings, often expressed in picocuries per litre of air.

Applications and Usage

In nuclear medicine, the curie and its submultiples remain standard in many countries for expressing the activity of radiopharmaceuticals administered to patients. Diagnostic procedures typically involve doses measured in millicuries or microcuries, while therapeutic applications, such as radioiodine treatment of thyroid disorders, may involve tens or hundreds of millicuries.

In the nuclear power and fuel cycle industries, the curie is used to quantify the radioactivity of reactor materials, spent fuel, radioactive waste, and environmental releases. Large-scale inventories, such as the fission product content of a reactor core, are often expressed in megacuries. In radiological protection, contamination levels and effluent discharge limits have historically been specified in curies or microcuries per unit volume or mass.

In environmental science, the picocurie is a common unit for reporting radon levels and the activity of naturally occurring radioactive materials in water and soil. Regulatory standards in some jurisdictions continue to specify limits in curie-based units.

Distinction from Related Quantities

The curie measures activity—the rate at which radioactive decays occur—and should not be confused with quantities that describe the effects of radiation. The absorbed dose of radiation is measured in the rad or its SI counterpart, the gray; the equivalent dose, which accounts for differing biological effectiveness of radiation types, is measured in the rem or its SI counterpart, the sievert; and exposure to X-rays or gamma rays is measured in the roentgen. Activity, dose, and exposure are distinct physical quantities, and converting between them requires knowledge of the radionuclide involved, its decay energies, and the geometry of the irradiation.

Significance and Legacy

The curie holds a distinguished place in the history of science as one of the first standardised units in nuclear physics and as a tribute to the founders of radiochemistry. Its longevity reflects both the practical utility of a unit whose magnitude is well suited to the activities of common radioactive sources and the enduring influence of the Curie family's legacy. Marie Curie's daughter, Irène Joliot-Curie, together with her husband Frédéric Joliot-Curie, extended this legacy by discovering artificial radioactivity in 1934, for which they received the Nobel Prize in Chemistry in 1935.

Although international bodies have long recommended the becquerel as the preferred unit, the curie survives in customary use, historical literature, and regulatory frameworks. Understanding the curie and its relationship to SI units remains essential for interpreting older scientific documents, medical prescriptions, and environmental data, and for communicating across the communities that continue to employ both systems of measurement.

See also

• Becquerel (unit)
• Radium and radon
• Radioactivity and radioactive decay
• Marie Curie and Pierre Curie
• Rad (unit), rem (unit), and roentgen (unit)
• SI and non-SI units of radiation measurement