Half-life Flashcards
activity of a radioactive source
the number of atoms that decay every second
measured in becquerel (Bq)
What happens when an atom decays?
it emits radiation but changes into a more stable isotope
what does the activity of a source depend on?
how many unstable atoms there are in a sample, and on the particular isotope
What happens to the activity as time goes on and why?
as more atoms in a sample decay, there are fewer unstable ones left, so the activity decreases
what is the half life of a radioactive isotope?
the time it takes for half of the unstable atoms to decay
it is also the time it takes for the activity to go down by half
Why is the half life useful?
we cannot predict when a particular nucleus will decay, but when there are a large number of nuclei, the half life gives a good prediction of the proportion of nuclei that will decay in a given time
The activity of a radioactive source is 240 Bq and the half life is 6 hours.
Describe its activity during the course of 24 hours
after 1 half life the activity halves to 120 Bq
after 2 half-lives it halves again to 60 Bq
after 3 it halves to 30 Bq
after 4 (24 hrs) it halves to 15 Bq
The activity of a source is 120 Bq. Four hours later it is found to be 30 Bq.
Calculate the half life of the source. [2]
120 Bq to 30 Bq is 1/4 of the activity/2 half lives in 4 hours
1 half life is half of four hours / 2 hours
The half-life of a source is 15 minutes.
Calculate the fraction that will remain after 1 hour. [3]
after 15 min: 1/2
after 30 min: 1/4
after 45 min: 1/8
after 60 min: 1/16
