1. Alpha decay Alpha particles consist of two protons plus two neutrons.
They are emitted by some of the isotopes of the heaviest elements. 1

2. Example: The decay of Uranium 23892 Th
234 90 α 4 2 +
Uranium 238 decays to Thorium 234 plus an alpha particle.
Notes:
1. The mass and atomic numbers must balance on each side of the equation: (238 = AND 92 = 90 +2)
2. The alpha particle can also be notated as: He 4 2 2

3. Question
Show the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92). Pu
239 94 U
235 92 α 4 2 + 3

4. Beta decay Beta particles consist of high speed electrons.
They are emitted by isotopes that have too many neutrons.
One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle. 4

5. Example: The decay of Carbon 146 N 14 7 β- -1 +
Carbon 14 decays to Nitrogen 14 plus a beta particle.
Notes:
1. The beta particle, being negatively charged, has an effective atomic number of minus one.
2. The beta particle can also be notated as: e -1 5

6. Question
Show the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg). Na 25 11 Mg 12 β- -1 + 6

7. Gamma decay
Gamma decay is the emission of electromagnetic radiation from an unstable nucleus
Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle.
Example: Cobalt 60 Co 60 27 Co 60 27 γ +
Cobalt 60 with excess ENERGY decays to
Cobalt 60 with less ENERGY plus gamma radiation. 7

8. Do Now copy and complete Changing elements
Both alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number.
Decay type
Atomic number
Mass number
alpha
DOWN by 2
DOWN by 4
beta
UP by 1
NO CHANGE
gamma 8

9. Complete the decay equations below:Fe 59 26 Co 27 β- -1 + Ra
224 88 Rn
220 86 α 4 2 N 16 7 O 8
(a)
(c)
(b) 9

10. Write equations showing how Lead 202 could decay into Gold
Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!)
Element
Sym Z
Platinum Pt 78
Gold Au 79
Mercury Hg 80
Thallium Tl 81
Lead Pb 82
Bismuth Bi 83 Pb
202 82 Hg
198 80 α 4 2 + Hg
198 80 Pt
194 78 α 4 2 + Pt
194 78 Au 79 β- -1 +
There are other correct solutions 10

11. Choose appropriate words to fill in the gaps below:
When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______.
Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged.
Gamma rays consist of ______________ radiation that is emitted from a nucleus when it loses ________, often after undergoing alpha or beta decay.
two
four
neutrons
one
mass
electromagnetic
energy
WORD SELECTION:
four
one
energy
two
neutrons
mass
electromagnetic 11

12. Today’s lesson Title Half-life
Use the term half-life in simple calculations, including the use of information in tables or decay curves.
Give and explain examples of practical applications of isotopes.
Title Half-life

13. ½ - life – copy please
This is the time it takes for half the nuclei present in any given sample to decay
Number of nuclei undecayed
A graph of the count rate against time will be the same shape
time
half-life (t½)

14. Different ½ - lives Different isotopes have different half-lives
The ½-life could be a few milliseconds or 5000 million years!half life applet
Number of nuclei undecayed
time
half-life (t½)

15. Examples
A sample of a radioactive isotope of half life 2 hours has a count rate of counts per second. What will the count rate be after 8 hours?

16. Examples

17. Henri Becquerel discovered radioactivity in 1896
The activity of a radioactive source is equal to the number of decays per second.
Activity is measured
in bequerels (Bq)
1 becquerel
= 1 decay per second
Half life
Henri Becquerel discovered radioactivity in 1896 17

18. Question 1
At 10am in the morning a radioactive sample contains 80g of a radioactive isotope. If the isotope has a half-life of 20 minutes calculate the mass of the isotope remaining at 11am.
10am to 11am = 60 minutes
= 3 x 20 minutes
= 3 half-lives
mass of isotope = ½ x ½ x ½ x 80g
mass at 11 am = 10g 18

19. Question 2
Calculate the half-life of the radioactive isotope in a source if its mass decreases from 24g to 6g over a period of 60 days.
24g x ½ = 12g
12g x ½ = 6g
therefore TWO half-lives occur in 60 days
half-life = 30 days 19

20. Example 2 – The decay of source Z
Time
Activity (Bq)
9 am
12 noon
3 pm
6 pm
9 pm
midnight
Source Z decays with a half-life of three hours.
At 9 am the source has an activity of Bq
The activity halves every three hours.
16000
8000
4000
2000
1000
500
When will the activity have fallen to 125 Bq?
6 am 20

21. Example 3 – The decay of isotope X
Isotope X decays to Isotope Y with a half-life of 2 hours.
At 2 pm there are 6400 nuclei of isotope X.
Time
Nuclei of X
Nuclei of Y
2 pm
4 pm
6 pm
8 pm
10 pm
midnight
6400
3200
3200
1600
4800
800
5600
400
6000
200
6200
When will the nuclei of isotope X fallen to 25?
6 am 21

22. Question 3 A radioactive source has a half-life of 3 hours.
At 8 am it has an activity of 600 Bq.
What will be its activity at 2 pm?
at 8 am activity = 600 Bq
2 pm is 6 hours later
this is 2 half-lives later
therefore the activity will halve twice
that is: 600  300  150
activity at 2 pm = 150 Bq 22

23. Question 4 – The decay of substance P
Substance P decays to substance Q with a half-life of 15 minutes. At 9 am there are 1280 nuclei of substance P.
Complete the table.
Time
Nuclei of X
Nuclei of Y
9 am
9:15
9:30
9:45
10 am
10:15
1280
640
640
320
960
160
1120 80
1200 40
1240
How many nuclei of substance X will be left at 11 am? 5 23

24. Question 5
A sample contains 8 billion nuclei of hydrogen 3 atoms. Hydrogen 3 has a half-life of 12 years. How many nuclei should remain after a period 48 years?
48 years = 4 x 12 years
= FOUR half-lives
nuclei left = ½ x ½ x ½ x ½ x 8 billion
nuclei left = 500 million 24

25. Experiment Dicium 25

26. Finding half-life from a graph
The half-life in this example is about 30 seconds.
A more accurate value can be obtained be repeating this method for a other initial nuclei numbers and then taking an average.
half-life 26

27. Question 6
Estimate the half-life of the substance whose decay graph is shown opposite.
half-life
The half-life is approximately 20 seconds 27

28. Question 7
The mass of a radioactive substance over a 8 hour period is shown in the table below.
Draw a graph of mass against time and use it to determine the half-life of the substance.
Time (hours) 1 2 3 4 5 6 7 8
Mass (g)
650
493
373
283
214
163
123 93 71
The half-life should be about 2 hours: 28

29. Choose appropriate words or numbers to fill in the gaps below:
The ________ of a radioactive substance is the average time taken for half of the _______of the substance to decay. It is also equal to the average time taken for the ________ of the substance to halve.
The half-life of carbon 14 is about _______ years. If today a sample of carbon 14 has an activity of 3400 Bq then in 5600 years time this should have fallen to ______ Bq years later the activity should have fallen to ____ Bq.
The number of carbon 14 nuclei would have also decreased by ______ times.
half-life
nuclei
activity
5600
1700
425
eight
WORD & NUMBER SELECTION:
5600
nuclei
eight
half-life
425
1700
activity 29

30. Revision Simulations
Half-Life - S-Cool section on half-life and uses of radioactivity including an on-screen half-life calculation and an animation showing thickness control.
BBC AQA GCSE Bitesize Revision:
Detecting radiation
Natural sources of background radiation
Artificial radiation
Half life
Alpha Decay - PhET - Watch alpha particles escape from a Polonium nucleus, causing radioactive alpha decay. See how random decay times relate to the half life.

31. Uses of radioactive isotopes

32. Smoke detection
Uses

33. Thickness control

34. Thickness control

35. Used as Tracers

36. Used as Tracers

37. Killing microbes

38. Killing microbes

39. Checking welds

40. Used as Tracers

41. Carbon dating – write notes using the book page 265

42. Summary sheet

43. “Can you………?”

44. Test!
Thursday 27th September 2012

45. Can you answer the questions on pages 261 and 265?