Have you ever heard of the names Becquerel and Curie? They were renowned scientists in the field of Physics and Chemistry. The French Physicist, Henri Becquerel, discovered the phenomenon of radioactivity. He shared the Nobel Prize in Physics with Marie Curie and Pierre Curie (Marie's husband) in 1903 for their work in radioactivity ^[1] . Madame Curie and her husband coined the term radioactivity. The well-known radioactive elements radium and polonium were discovered by them. Madame Curie was not only the first woman who honoured with Nobel Prize, but also the first one with two Nobel Prizes, one in Physics in 1903 and the other in Chemistry in 1911 ^[2] .

A radionuclide will emit radiation through the process of radioactive decay. The activity of a radioactive material is the number of radioactive decays occurring within a unit of time. In radiation context, the SI unit for activity is becquerel (Bq), which was named in honour of Henri Becquerel. One becquerel is defined as the activity of one radioactive decay per second. In general, Bq is a unit for a small amount. Its multiples, such as kilobecquerel (kBq) or megabecquerel (MBq), are more commonly used.

Before 1975, the conventional unit for measuring the activity of a radioactive material was curie (Ci). It was named in honour of Madame Curie and her husband. One curie is defined as 3.7×10 ¹⁰ radioactive decays per second, or 1 Ci = 3.7 x 10 ¹⁰ Bq. The Ci is a unit for a relatively large amount. Its fractions, such as millicurie (mCi) or microcurie (µCi), are more commonly used. Curie is now still in use in some countries, such as the United States.

Do you know that many materials in our environment are radioactive such as containing the naturally occurring radionuclide potassium-40? Depending on the types of soil, the activity of potassium-40 in one kilogram of soil is roughly a few hundred Bq ^[3] . Our body is radioactive too. The activity of an adult is approximately a few thousand Bq ^[4] .