Nature of radioactive decay

1. XNature of RadioactiveNature of Radioactive
DecayDecay
Physics  Module 5  Unit 7 Lesson 18 & 19
INTRODUCTION 2
OBJECTIVES 1
OBJECTIVES 2
Nature of
Radioactive Decay
Half Life
QUIZ
INTRODUCTION 1

2. X
Introduction 1: Nature of Radioactive
decay
Flip a ten and a twenty dollar coin two times and then answer the
following questions:
1. Half or 50%
2. No
QUESTIONS
1.What is the chance of getting a tail on
any flip?
2.Does the number of tails directly
proportional to the number of flips?
3.What is the meaning of random?
4.Do you think the flipping of coins is a
random movement?
2
3. Random means not expected
or unpredictable. It is anything
that is, has no pattern.
4. Yes
Click here to check your answers.

3. X
Introduction 2: Half Life
Read the scenario below and then answer the questions that follow:
A farmer has a herd of 1024 goats. He noticed that half of his remaining
herd of goats was stolen each week over a two month period.
1.Would he have any goat left at the end of the two months?
2.What number of goats remained each week of the two months?
Number of Goats
Remaining
Number
of Weeks
1024 0
512 1
256 2
128 3
64 4
32 5
16 6
8 7
4 8
Ans 1: Yes
Ans 2: The table
shows the result:
Click here to check your answers.
3

4. X
Introduction 2: Half Life Cont.
3. What would affect the number of goats remaining at the end each
week?
4. Does the pattern at which the goats were stolen from the farmer
remind you of radioactive decay?
5. Would the graph of the number of goat remaining vs. week number
be similar to that of a radioactive decay curve?
Ans 3: The number of goats which
died by natural cause or those sold or
killed for the family.
Ans 4: Yes
Ans 5: Yes
Click here to check your answers.
4

5. X
Objectives Part 1
As a result of this lesson, the student will be able to:
1.define nuclear notation in own words.
2.accurately represent the nuclear reactions of alpha, beta, and gamma
radiations in the standard form.
3.predict the types of nuclear radiations that will occur in certain
nuclear reactions in the standard form.
4.outline an activity to demonstrate the random nature of radioactive
decay.
5.state the differences between natural and artificial radioactivity.
6.explain that decay process is independent of the conditions external
to the nucleus.
7.actively participate in discussions.
5

6. X
Objectives Part 2
As a result of this lesson, the student will be able to:
1.define the term half life in your own words.
2.conduct a simple experiment on half life of a substance.
3.list the half lives of some radioactive materials.
4.determine the half life of a radioactive substance from a graph of mass of
radioactive substance against time.
5.draw the graph of a mass of radioactive isotopes against time for a
radioactive substance.
6.solve simple worded problems involving half lives.
7.respect the views of their peers.
8.work cooperatively in groups.
9.contribute meaningfully to open discussion.
6

7. X
Nature of Radioactive decay
Standard nuclear notation shows the chemical symbol, the mass
number and the atomic number of the radioactive element.
Where
A = mass or nucleon number
Z = charge or atomic number
X is the chemical symbol and
N = neutron number are related (A = Z + N).
For example:
XA
Z
C12
6
7

8. X
Alpha Particles
The characteristics of alpha radiation are :
1.The symbol of alpha is .
2.It occurs when the nucleus is too large.
3.An alpha particle is emitted, reducing the size of the
nucleus.
4.The daughter nucleus has an atomic number 2 less and an
atomic mass 4 less than the parent nucleus.
Example :
α
4 4
2 2
A A
Z Z
X Y He−
−
→ +
241 237 4
95 93 2
Am Np He.→ +
8

9. X
Beta Radiation
The characteristics of Beta radiation are :
1.The symbol of beta is .
2.It occurs when the nucleus has too many neutrons
relative to protons.
3.A beta particle is emitted, a neutron changes into a
proton and emits an electron.
4.The daughter nucleus has an atomic number 1 more
and an atomic mass the same as the parent nucleus.
For example:
β
0
1 1
A A
Z Z
X Y e+ −
→ +
14 0 14
6 1 7
C .e N−
→ +
9

10. X
Gamma Radiation
The characteristics of gamma radiation are :
1.The symbol of gamma is .
2.It occurs when a nucleus has excess energy.
3.A gamma ray is emitted is emitted from the nucleus.
4.The parent and daughter nuclides are the same.
For example:
γ
99 99
43 43
Tc Tc .→ + γ
10

11. X
Radioactive Decay Chart
Source 1: http://www.ccnr.org/decay_U238.html
11

12. X
Half Life Activity
Questions
1.What is the relationship
between the mass of
radioactive material and
half life?
2.What fraction does the
mass of substance decay
each half life?
Re
M ass
o f
active
material
Half life
m
1
t
2
m
16
m
3
t2
t
4
m
No of
Half life
Fraction of Mass
of Substance
remaining
0 1
1 ½
2 ¼
3 ¹₈̷
Answer 1: the mass of the
substance is inversely
proportional to the half
of the substance.
Answer 2: ½
Click here to check
your answers.
12

13. X
Half Life
The “half-life” (h) is the time taken for half the mass or atoms of a
radioactive substance to decay.
Example: Suppose a radioactive substance has 1,000 atoms or a mass
of 40g and an half-life of 1 hour, how many atoms or mass of that
substance would be left after:
1 hour (one life time)?
2 hours (two life times)?
3 hours (three life times)?
4 hours (four lifetimes)?
500 20 (50%)
250 10 (25%)
Time
mass
remaining
% of atoms
remaining
#atoms
remaining
62.5 2.5 (6.25%)
125 5 (12.5%)
SOLUTION
13

14. The grid figure represents a quantity of 92U239
. Each time you
click, one half-life goes by. Try it!
92U239
– gray 82Pb206
- red
As we begin notice that no time has gone by
and that 100% of the material is 92U239
Half
lives
% 92U239
%82Pb206 Ratio of
92U239
to
82Pb206
0 100% 0% no ratio
Age = 0 half lives (233000x 0 = 0 yrs)
14
CLICK

15. The grid figure represents a quantity of 92U239
. Each time you
click, one half-life goes by. Try it!
92U239
– gray 82Pb206
- red
Half
lives
% 92U239
%82Pb206 Ratio of
92U239
to
82Pb206
0 100% 0% no ratio
1 50% 50% 1:1
Age = 1 half lives (233000x 1 = 233000 yrs)
15
CLICK

16. The grid figure represents a quantity of 92U239
. Each time you
click, one half-life goes by. Try it!
92U239
– gray 82Pb206
- red
Half
lives
% 92U239
%82Pb206 Ratio of
92U239
to
82Pb206
0 100% 0% no ratio
1 50% 50% 1:1
2 25% 75% 1:3
Age = 2 half lives (233000x 2 = 466000 yrs)
16
CLICK

17. The grid figure represents a quantity of 92U239
. Each time you
click, one half-life goes by. Try it!
92U239
– gray 82Pb206
- red
Half
lives
% 92U239
%82Pb206 Ratio of
92U239
to
82Pb206
0 100% 0% no ratio
1 50% 50% 1:1
2 25% 75% 1:3
3 12.5% 87.5% 1:7
Age = 3 half lives (233000x 3 = 699000 yrs)
17

18. X
92U239
– gray 82Pb206
- red Question:
How can you find the age of a
sample without knowing how
much 92U239
was in it to begin
with?
Solution:
Use the ratio to determine how
many half lives have gone by since
the sample formed.
In the example above, the ratio is 1:3.
If the sample has a ratio of 1:3 that means it is 2 half lives old.
If the half life of 92U239
is 233000 years then the sample is 2 x 233000 or
233000 years old.
18
Activity
Remember, 1:1 ratio = 1 half life
1:3 ratio = 2 half lives
1:7 ratio = 3 half lives

19. X
Animation: Radioisotope decay
Click the play button to start animation
Screenshot taken from http://faculty.coloradomtn.edu/gcornwall/Starr%20Concepts
%20Animations%20and%20videos/chapter16/videos_animations/half_life.html
19

20. X
The Half-Lives of Radioactive
Materials
Taken from http://www.atral.com/U2381.html
20

21. X
A Sample Calculation of Half Life
Question: The half-life of Plutonium -239 is 24110 years. If the original
mass of Plutonium -239 is 120 g, how much remains after 96440 years?
Solution: First, find the number of half-life's.
# Half-lifes =
4
120 120
7.5
2 16
g= =
21
120 ===> 60 ===> 30===> 15===> 7.5g Mass
1 2 3 4 half lives
96440 years
24110 years
= 4
OR
Now divide the gram sample in 1/2 four times.
Therefore the mass of Plutonium -239 after 96440 years is:

22. X
QUIZ
Are you ready to test yourself?
1.There are seven multiple choice items that you are
required to do.
2.For each item, click on the letter of the correct response
and then click the button ‘NEXT QUESTION’ to continue.
3.Click the BEGIN QUIZ button below to start the quiz.
22
BEGIN QUIZ!

23. X
QUIZ: Question 1
Given the nuclear reaction : , what name of
radiation emit as ‘X’
23
A. Alpha
B. Gamma
C. X-ray
D. Beta
FEEDBACKFEEDBACK
234 234
91 92
Pa U .X→ +

24. X
Quiz: Feedback to Question 1
Given the nuclear reaction : , what name of
radiation emit as ‘X’
24NEXT QUESTIONNEXT QUESTION
The daughter nucleus of 234Pa91
has an atomic number 1
more and an atomic mass the same as the parent
nucleus, hence the radiation has be beta.
234 234
91 92
Pa U .X→ +

25. X
QUIZ: Question 2
Given the nuclear reaction: , what name of
radiation emits as ‘Y’?
25
A. Beta
D. X-ray
C. Gamma
B. Alpha
FEEDBACKFEEDBACK
212 208
84 82
Po Pb .Y→ +

26. X
Quiz: Feedback to Question 2
Given the nuclear reaction : , what name of
radiation emit as ‘Y’?
26NEXT QUESTIONNEXT QUESTION
The daughter nucleus has an atomic number 2 less and an
atomic mass 4 less than the parent nucleus and the looks
like:
Hence Y is alpha radiation.
212 208
84 82
Po Pb .Y→ +
4 4
2 2
A A
Z Z
X Y He−
−
→ +

27. X
QUIZ: Question 3
The graph on the right
shows the half life decay of
a radioactive material.
What is the half life of the
substance?
27
A. 20 days
D. 24 days
B. 16 days
C. 8 days
FEEDBACKFEEDBACK

28. X
Quiz: Feedback to Question 3
The graph on the right
shows the half life decay of
a radioactive material.
What is the half life of the
substance?
28NEXT QUESTIONNEXT QUESTION
Half life is the taken for a radioactive material to decay by
half of its mass. Since the original mass of the substance is
40 and the time taken for its original mass to decay 20g is 8
days, them half life of the substance 8 days.

29. X
Quiz: Question 4
Use the graph to determine
the mass of the radioactive
substance after 40 days.
29
A. 2.5 g
D. 0.75g
C. 4.0 g
B. 1.25 g
FEEDBACKFEEDBACK

30. X
Quiz: Feedback to Question 4
Use the graph to
determine the mass of the
radioactive substance
after40 days.
30NEXT QUESTIONNEXT QUESTION
40
# 40 5
8
tan 40
40
1.25
52
days
of half lives after days is
days
Hence mass of subs ce after days
g
=
=

31. X
QUIZ: Question 5
A Physics institution requests 2 g of bismuth-214, which has
a half life of 20 min. How many grams of bismuth-214 must
be prepared if the travel time is 3 h?
31
B. 512g
D. 224g
C. 18g
A. 1024 g
FEEDBACKFEEDBACK

32. X
Quiz: Feedback to Question 5
A Physics institution requests 2 g of bismuth-214, which has
a half life of 20 min. How many grams of bismuth-214 must
be prepared if the travel time is 3 h?
32
Since
hence there are 9 half lives.
The set up is as follows:
END OF QUIZEND OF QUIZ
9
9
1
( )
2
1
( ) 2
2
2 2 1024
i fn
i
i
m Initial amount m
m g
m g g
=
=
= × =
3 60min
# 3 9
20min
s
of half lives after hours is
s
×
=

33. X
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