1. Radioactive Decay
&
Half-life

2. Radioactive decay
We cannot predict when any particular nucleus will decay.

3. Radioactive decay
We cannot predict when any particular nucleus will decay.
But there is a fixed chance that any atom will decay in the next second.

4. Radioactive decay
We cannot predict when any particular nucleus will decay.
But there is a fixed chance that any atom will decay in the next second.
One tenth decay each second.

5. Radioactive decay
We cannot predict when any particular nucleus will decay.
But there is a fixed chance that any atom will decay in the next second.
One tenth decay each second.

6. Radioactive decay
We cannot predict when any particular nucleus will decay.
But there is a fixed chance that any atom will decay in the next second.
One tenth decay each second.

7. Radioactive decay
We cannot predict when any particular nucleus will decay.
But there is a fixed chance that any atom will decay in the next second.
One tenth decay each second.

8. Radioactive decay
We cannot predict when any particular nucleus will decay.
But there is a fixed chance that any atom will decay in the next second.
One tenth decay each second.

9. Decay graph
The time for the number left to halve is always the same. This time is called the half life.

10. What is the half-life of iodine-131?
Iodine-131 has a half-life of about 8 days (192 hours).
Iodine-131 has a half-life of about 8 days (192 hours).
What is the half-life of iodine-131?

11. What is the half-life of iodine-135?
Iodine-135 has a half-life of about 7 Hours.
The graph is the same shape as the previous graph (for iodine-131), but the time axis shows that the half-life is very different – about 7 hours.
What is the half-life of iodine-135?

12. Plotting both sets of data on the same graph shows the significance of different half lives; a short half-life means the activity decreases much more quickly – the radioactive source becomes safer sooner.
When used as a treatment for cancer of the thyroid, the longer-lived iodine-131 is used.
Plotting both sets of data on the same graph shows the significance of different half lives; a short half-life means the activity decreases much more quickly – the radioactive source becomes safer sooner.
When used as a treatment for cancer of the thyroid, the longer-lived iodine-131 is used.
Comparing half lives.

13. Should we still be concerned?
Caesium-137 was released into the atmosphere when the Chernobyl nuclear reactor exploded in 1986.
Should we still be concerned?
Caesium-137 has a half-life of about 30 years.
Traces of the isotope washed out of the clouds by heavy rain can still be found in the soil of the Welsh mountains.
Caesium-137 emits beta radiation.
So yes, we should be concerned; sheep from these Welsh hills are brought down to clean pastures so that the caesium they have taken in with contaminated grass can work its way out of their system, before the meat is eaten.

14. Radioactive decay Caesium-137 has a half-life of about 30 years.
Traces of the isotope washed out of the clouds by heavy rain can still be found in the soil around Europe.
Caesium-137 emits beta radiation.
So yes, we should be concerned.

15. ½ ½ ½ ½ ½ ½ ½ ½ ½ 16 8 4 2 1 16 x x x = After 1 Half life... ½ 16 x x x =
After 1 Half life... ½
After 2 Half lives... ¼
After 3 Half lives... 1/8
After 4 Half lives... 1/16
After 5 Half lives ...1/32
After 6 Half lives... 1/64

16. Questions
True or false? The activity of a source reduces to one-sixteenth of its original value after four half-lives.
2. The radioactive isotope sodium-25 has a half-life of 1 minute. What fraction of it remains after 3 minutes?
A) 1/3 B) 1/4 C) 1/6 D) 1/8
True

17. Questions
3. The medical tracer, technetium-99m, has a half-life of 6 hours. A sample gives a count rate of 2400 counts per second at 11:00 am on Monday.
How many half-lives will it take for the count to drop to 300 counts per second?
How long will it take for the count to drop to 300 counts per second?
c) What day and time will it be when the count is 300 counts per second?
3 Half lives x 6 hours = 18 Hours
11am Mon – 5 pm Mon – 11pm – Mon – 5 am Tues

18. Questions
4. Xenon-133 is a radioactive gas used for diagnosing lung problems. In 15 days its activity falls to 1/8 of its original value. What is its half-life? =
= 3 Half lives X X 8
15 days = 5 days per half life

19. Questions
5. The half-life of the radioactive isotope sodium-24 is 15 hours. A sample has a count rate of 240 counts per minute (cpm). Its count rate 60 hours later will be:
A 15 cpm B 30 cpm C 40 cpm D 60 cpm
15 Hours
15 Hours
15 Hours
15 Hours
(30) Hours
(45) Hours
(60) Hours

20. Questions
6. A radioactive isotope of silver has a half-life of 20 minutes. A sample gives a count rate of 6400 counts per second at 9 am. At what time will the count rate be about 200 counts per second?
= 5 Half lives
9 am
9:20 am
9:40 am
10 am
10:20 am
10:40 am

21. Questions
7. A sample of bone from a living animal contains carbon-14. Its activity is 80 counts per minute.
The half-life of carbon-14 is 5730 years. How old is an antler with activity of 20 counts per minute?
= 2 Half lives
= 2 x 5730 = Years

22. A 1 half-life B 10 half-lives C 190 half-lives D 1000 half-lives
Questions
8. Cobalt-60 sources are used for sterilising medical instruments. It has a half-life of 5.27 years.
What percentage of a source remains after years?
b) After the activity drops too low to be used, the cobalt-60 source must be disposed of safely.
If the source must be stored until its activity has dropped to less than one-thousandth of its activity today, it would have to be stored for at least:
A 1 half-life B 10 half-lives C 190 half-lives D 1000 half-lives
c) How many years is this?

23. ½ ½ Questions 100% 50% 25% = 25% 5.27 years 10.54 years
8. Cobalt-60 sources are used for sterilising medical instruments. It has a half-life of 5.27 years.
What percentage of a source remains after years?
100% % %
= 25%
5.27 years
10.54 years

24. A 1 half-life B 10 half-lives C 190 half-lives D 1000 half-lives
Questions
b) After the activity drops too low to be used, the cobalt-60 source must be disposed of safely.
If the source must be stored until its activity has dropped to less than one-thousandth of its activity today, it would have to be stored for at least:
A 1 half-life B 10 half-lives C 190 half-lives D 1000 half-lives
1/1000 = 0.001
Using a calculator, 1 dived by 2 = 0.5, keep dividing by 2 until you get to or in this case which is 10 half lives.

25. Questions How many years is this?
1 Half life is 5.27 years so 10 Half lives = 52.7 Years.

26. Questions
9. All three of the radon isotopes in this table are emitted at the same rate from the walls of a room.
Which one will decay most quickly?
Which one are you least likely to breathe in? Explain your answer.
3. Which one has nuclei that are least likely to decay?
4. Of the two most likely to be breathed in, based on the information in the table, which is likely to be the most dangerous? Explain your answer.
Radon-220
Radon-220 because it doesn’t last very long, so there will be less of it in the room
Radon-226
Radon 222 because it is more likely to decay in the lungs than radon-226
isotope
half-life
emission
radon-220
55 seconds
alpha particles
radon-222
92 hours
radon-226
1602 years

27. Questions
10. a) Our food contains potassium-40. This has a half life of 1260 million years. It decays by emitting beta particles.
Explain how we can survive eating this isotope.
b) Strontium-90 was produced, and released into the atmosphere, by atomic bomb tests in the 1950s. It has a half-life of 28 years and is taken up by our bones instead of calcium. It also decays by emitting beta particles.
Explain whether strontium-90 in our bodies is likely to be more or less dangerous than potassium-40.
It is extremely unlikely to decay while its in our bodies because of its long half life.
More dangerous, it has a much shorter half life and can stay in our bones for a long duration and do damage to our cells. Its also more penetrating as its beta.