Caesium in a sealed glass ampoule. (Source: © Kim / stock.adobe.com)

A chemical element with atomic number 55. It is fluid near room temperature; its boiling point is 28.5 °C. Caesium is a soft golden metal that explosively reacts with water and can spontaneously ignite in the air. It is a rare element but has wide usage in electronics, energy production, or chemistry. Caesium is used in scintillation detectors for gamma and X-ray detection. In medicine, radioisotopes of caesium are used for radioimaging or cancer treatment. The frequency of caesium radiation during the transition between two energy states is used as the definition of a second, and caesium atoms are used in precise atomic clocks.

Caesium 137 is one of the most common fission products, with a half-life of 30 years. Caesium radioisotopes in nature are mainly of anthropogenic origin (from atomic tests), and their presence can be used for dating and tracing in hydrology and geology.