caesium

British  

/ ˈsiːzɪəm /

noun

1. a ductile silvery-white element of the alkali metal group that is the most electropositive metal. It occurs in pollucite and lepidolite and is used in photocells. The radioisotope caesium-137 , with a half-life of 30.2 years, is used in radiotherapy. Symbol: Cs; atomic no: 55; atomic wt: 132.90543; valency: 1; relative density: 1.873; melting pt: 28.39±0.01°C; boiling pt: 671°C

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Example Sentences

In 1955, the NPL invented the first atomic clock of the sort that is used today, based on the frequency of radiation from an atom of the element caesium.

From BBC • Mar. 2, 2025

Within the sensors, the light passes through a small glass container containing a caesium gas, which absorbs the light at the right light frequencies.

From Science Daily • May 2, 2024

The small silver cylinder is a 19-becquerel caesium 137 ceramic source commonly used in radiation gauges.

From Seattle Times • Jan. 28, 2023

The radioactive strontium, caesium and plutonium mainly affected Ukraine and neighboring Belarus, as well as parts of Russia and Europe.

From Reuters • Feb. 25, 2022

It led Bunsen himself almost immediately to the isolation of two new elements of the alkali group, caesium and rubidium.

From Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary" by Various