Becquerel as unit of activity

Physics · Activity

Drag the bar to change how much of one pure isotope you have. The masses and activities here are made-up numbers for one pure sample, so you can see the pattern.

Predict, then check

Think about the counter: what is it counting?

A pure sample of one radioactive isotope is left to decay. You measure its activity in Bq again later. What do you expect?

What you need to know

• Activity is the number of unstable nuclei that decay every second.
• We measure it in becquerels, Bq. One becquerel is one decay per second, so 100 Bq means 100 decays per second.
• Have a goSam looks at a sample marked 50 Bq and says: '50 decays, then it's finished.' What has Sam misread?

  50 Bq means 50 decays every second, not 50 decays in total.

  A becquerel is a rate, one decay per second, so the 'per second' is part of what the number means.

• Prefixes work on the becquerel like any unit. Kilo means thousand, so 1 kBq is 1000 decays per second.
• High activity means lots of decays per second. Low activity means fewer decays per second.
• Have a goJo is very sure: 'A sample at 5 kBq is weaker than one at 800 Bq, because 5 is smaller than 800.' Is Jo right?

  No. 5 kBq is 5000 decays per second, which is more than 800.

  Jo compared the numbers and ignored the k; kilo means thousand, so the units have to match before you compare.

• In a pure sample of one radioactive isotope, activity is proportional to the number of unstable nuclei.
• That makes it proportional to the mass of the isotope too: twice the mass gives twice the activity.
• Have a goA pure sample of one isotope has an activity of 600 Bq. Half of the isotope is taken away. What is the activity now?

  300 Bq.

  Half the isotope means half the unstable nuclei, and activity is proportional to that number, so it halves too.

• As a sample decays, fewer unstable nuclei remain, so its activity falls and the source becomes less radioactive.

The big picture

Activity is the number of unstable nuclei that decay each second, measured in becquerels (Bq): one becquerel is one decay per second, and a kilobecquerel is 1000. In a pure sample of one isotope, activity is proportional to the number of unstable nuclei, and it falls as the sample decays.

⚠ Watch out

Treating activity as a fixed label on the substance. Activity depends on how many unstable nuclei there are, so it scales with the mass of the isotope and falls as the sample decays. A second easy slip is reading kBq as Bq and losing the factor of 1000.

Memory hook

A becquerel is a click on the decay counter. Count the clicks in one second, and remember that kilo means a thousand clicks.

Check yourself

Without looking back: a sample reads 7 kBq. How many decays is that each second, and what would you expect its activity to do if you measured it again much later?