The main improvement in caesium clock accuracy has been achieved through the use of laser-cooled caesium atoms in laboratory clocks known as 'caesium fountains'.
The current primary clock at the National Physical Laboratory, the caesium fountain NPL-CsF2, contributes to the international timescale UTC (coordinated universal time) and is the reference for the national timescale UTC.
However, while metals such as lithium, sodium and potassium caused blue fluorescence, its reaction with caesium
produced a distinctive green fluorescence.
By contrast, edible wild mushrooms (chanterelles, bay boletus, tooth fungus and other known mushrooms) and game (roe deer and stags, for example) and carnivorous freshwater fish (pike and perch for example, in which levels of contamination of 10,000 and 5,000 Bq/kg respectively are on occasion observed) continue to show levels of radioactive caesium
above 600 becquerels/kg (Bq).
The clock is one of an elite group of caesium
fountain clocks that have been built by the timing labs in Europe, the United States, and Japan as their national "primary frequency standard" for the measurement of time.
At present, Caesium
fountain clocks are the most accurate clocks in the world, furnishing the second accurate to 15 places after the decimal point.