1. Half-LIFE
It’s impossible to know exactly when an unstable atom will decay. We can however predict how many will decay in a period of time. A half-life is the time taken for half a group of unstable nuclei to decay. Half-lives vary according to the isotope that is decaying – these can range from microseconds, to thousands of millions of years.

2. Half-LIFE

3. Half-LIFE
The Half-life of an atom can be represented on a graph, known as a decay curve.
1st Half life –
the time it takes for 50% of the nuclei to decay
3rd Half life –
the time it takes for 50% of the remaining nuclei to decay
2nd Half life –
the time it takes for 50% of the remaining nuclei to decay
4th Half life –
the time it takes for 50% of the remaining nuclei to decay

4. The y-axis shows the number of Californium-252 atoms as a percentage
Half-LIFE
The Half-life of an atom can be represented on a graph, known as a decay curve.
The y-axis shows the number of Californium-252 atoms as a percentage X
To find the half-life, find 50% on the y-axis, ruling a line to the plot and match this up to the corresponding value on the x-axis
~2.645 yrs

5. Half-LIFE
The Half-life of an atom can be represented on a graph, known as a decay curve.
This tells us that the half-life of Californium-252 is approx years X
The second half-life
(when only 25% remain un-decayed –
ie. Half of the remaining 50%)
in this case, occurs in another 2.65 years, at approximately 5.3 years. X
2.65
5.33

6. Half-LIFE Half-LIFE X X X X When does the fourth half-life occur? 2.65
The third half-life
(when only 12.5% remain un-decayed –
ie. Half of the remaining 25%)
in this case, occurs in another 2.65 years, at approximately 7.95 years.
When does the fourth half-life occur? X
The fourth half-life
(when only 6.25% remain un-decayed –
ie. Half of the remaining 12.5%)
in this case, occurs in another 2.65 years, at approximately 10.6 years. X X X
2.65
5.33
7.95
10.6

7. Half-LIFE Use the decay curve to find: The Half-life of Uranium-235
b)The Second Half-life of Uranium-235
c) What fraction of the isotope will remain after 2840 million years?
d) What fraction of the isotope will remain after 4260 million years?

8. Half-LIFE 710 million years 1420 million years 6.25% 1.5625%
Use the decay curve to find:
The Half-life of Uranium-235
b)The Second Half-life of Uranium-235
c) What fraction of the isotope will remain after 2840 million years?
d) What fraction of the isotope will remain after 4260 million years?
710 million years
1420 million years
6.25%
1.5625%

9. Half-life ~ Simulation task
NOW DO
Half-life ~ Simulation task

10. Measuring decay We can measure the ionising radiation of a
radioactive source using a Geiger counter.
A Geiger counter detects Alpha, Beta and Gamma radiation.
The common unit for measuring radioactive decay is Becquerel (Bq).
Bq = number of decay’s per second.

11. Gradually decreasing over time
Measuring decay
Refer to the graph below, showing the decay curve of Thorium-234.
At the beginning when the decay is at large, the Geiger counter would of course be the most active, recording a high count rate
So, if we measured the decay of a radioactive source as graphed it, it would be the same as the decay curve
Gradually decreasing over time

12. Measuring decay
eg. A radioactive material is measured to have 600,000 decays per second. a) What is this equivalent to in Bq? b) After 3 half-lives, what will the activity be in Bq?
600,000 Bq
One Half-life
600, = 300,000 Bq
Two Half-lifes
300, = 150,000 Bq
Three Half-lifes
150, = 75,000 Bq

13. NOW TRY
Chapter one - Q 17; 23-29