1. T ½ Time taken by radioactive substance for its : Activity of atoms
HALF LIFE CONCEPT
HALF LIFE
Time taken by radioactive substance for its :
Activity of atoms
Mass OR
Number OR
to decrease to half of its original value
T ½

2. HALF LIFE CONCEPT EXAMPLE 100 g 50 g 10 January 3000 20 January 3000
A radioactive decay for a radioactive sample
100 g
50 g
10 January 3000
20 January 3000
Half life,T1/2 =
10 days

3. HALF LIFE CONCEPT EXAMPLE 700/minutes 350/minutes Time 8.00 p.m
A radioactive decay for a radioactive sample
700/minutes
350/minutes
Time 8.00 p.m
Time p.m
Half life,T1/2 =
4 hours

4. HALF LIFE CONCEPT EXAMPLE 20 g 10 g 5 g
Half life of a radioactive sample is 2 days.Mass of the
radioactive sample on 11th January 3000 is 5 g.
What was it mass on 7th January 3000 ?
7th January 3000
20 g
9th January 3000
10 g
11th January 3000
5 g

5. HALF LIFE CONCEPT EXAMPLE 5 g 2.5 g 1.25 g
Half life of a radioactive sample is 2 days.Mass of the
radioactive sample on 11th January 3000 is 5 g.
What will it mass on 15th January 3000 ?
11th January 3000
5 g
13th January 3000
2.5 g
15th January 3000
1.25 g

6. HALF LIFE CONCEPT EXERCISE 0 day 1000 500 250 250/s
A sample of radioactive has a half life of 5 days.
What is the activity of 1000/s substance after 10 days ?
1000
0 day
5 days
500
5 days
250
250/s

7. HALF LIFE CONCEPT EXERCISE 200 g 150 g 100 g Year 3000 Year 4000
Below is a record of radioactive decay for a sample
recorded by a student
200 g
150 g
100 g
Year 3000
Year 4000
Year 5000
What is the half life of the radioactive sample ?
5000 – 3000 = 2000 years

8. HALF LIFE CONCEPT EXERCISE
A radioactive sample has a half life of 3 years.If mass of the
sample is 10 g.What was the mass of the sample 15 years ago ?
320 g
15 years ago
160 g
12 years ago
80 g
9 years ago
40 g
6 years ago
20 g
3 years ago
10 g
now

9. HALF LIFE CONCEPT EXERCISE 100 % T ½ 2T ½ = 5 hours 5 hours 50 %
A sample of radioactive left 25 % residual after 5 hours.
What is the half life of the sample ?
100 %
T ½
2T ½ = 5 hours
5 hours
50 %
T ½ = 5/2 hours
T ½
25 %

10. HALF LIFE CONCEPT EXERCISE 100 % 2T ½ = 6 days 75 % 50 % T ½ = 3 days
A radioactive isotope took 6 days to decay 75 % of its sample.
What was the mass of 5 g sample 9 days ago ?
9 days ago
40 g
100 %
6 days ago
20 g
2T ½ = 6 days
75 %
50 %
T ½ = 3 days
3 days ago
10 g
25 %
Now
5 g

11. HALF LIFE GRAPH(DECAY CURVE)
HALF LIFE CONCEPT
HALF LIFE GRAPH(DECAY CURVE)
A sample of radioactive has a half life of 2 days.
If its activity now is 50/s.What is its activity after 5 days ?
0 day
2 days
4 days
6 day
ANSWER CAN NOT BE OBTAINED BY CALCULATION.
THE ANSWER ONLY CAN BE FOUND BY USING
HALF LIFE GRAPH.

12. HALF LIFE GRAPH(DECAY CURVE)
HALF LIFE CONCEPT
HALF LIFE GRAPH(DECAY CURVE)
Activity 50 40 30 20 10
Time/day 1 2 3 4 5 6

13. HALF LIFE CONCEPT EXERCISE
If half life of a radioactive sample is 1 hour.
What is the activity of 25/s sample after 1.5 hours ?
Activity 25 20 15 10 5 1 2 3
Time/hour

14. What is the half life of the sample ?
HALF LIFE CONCEPT
EXERCISE
Activity
500
400
300
200
100 1 2 4 6
Time/hour
5 hours
What is the half life of the sample ?

15. IT IS NOT AN ANCIENT GOAT
HALF LIFE CONCEPT
EXERCISE
GM tube reads 2080/s of carbon-14 atoms in a fresh bone of a goat.
Background reading is 80/s.
A student from a school claimed that he has found a bone
of an ancient goat at the back yard of his house.
The reading of GM tube for the sample is 1080/s.
If the half life of carbon-14 is 5 years,estimate the age of the
ancient goat.
The actual reading of fresh bone = 2080 – 80 = 2000/s
The actual reading of ancient bone = 1080 – 80 = 1000/s
2000
1000
SORRY !
IT IS NOT AN ANCIENT GOAT
5 years

16. HALF LIFE CONCEPT
APPLICATIONS OF HALF LIFE
Understanding the half life for a radioactive substance in
medical field is very important.Radioactive substance with
longer half life tends to stay longer in human body and will
cause a side effect.
If the half life is too short,it will disappear too fast before
any test can be done.
2. Archeology.
Carbon-14 isotope is used to determine the age of the fossil
based on carbon-14 half life.

17. THE USES OF RADIOACTIVE SUBSTANCE
HALF LIFE CONCEPT
THE USES OF RADIOACTIVE SUBSTANCE
Detect the leaking pipes
Small amount of radioactive
is injected into the pipe
Geiger-Muller counter (G-M Counter)
will detect the radioactive at the leaking area

18. HALF LIFE CONCEPT
THE USES OF RADIOACTIVE SUBSTANCE
Explain with example the applications of radioactive
substances in the following fields :
1. medical
2. agriculture
3. industrial

19. THE HARMFULL EFFECTS OF RADIOACTICE SUBSTANCE
HALF LIFE CONCEPT
THE HARMFULL EFFECTS OF RADIOACTICE SUBSTANCE
The effects of radioactive depends on :
1. Types of radiation
2. Dosage
3. Exposure time
4. Parts of body
Common effects are :
1. Fatigue
4. Infertility in male
7. Genetic defect
- Down syndrome
- Klinefelter syndrome
- Turner syndrome
2. Vomiting
5. Severe skin burn
3. Hair loss
6. Leukemia
8. Death

20. HALF LIFE CONCEPT PRECAUTION STEPS IN HANDLING RADIOACTICE SUBSTANCE
1. Keep radioactive substance in a room enclosed by concrete
2. Wear protective suits and gloves
3. Use robotic arms to handle a strong radioactive
4. Shield of radioactive container has to be made of lead
5. Do not eat and drink in radioactive lab
6. Dispose radioactive substances in safe method
7. Do not make fun of radioactive substance to your friends
8. Put a radioactive sign or symbol at the place where radioactive is
handled
9. Wear photographic film to monitor the amount of radiation exposed.

21. PAST YEARS QUESTION A. P B. Q C. R D. S
Which isotope is suitable to detect the blood clotting ? A. P
P will disappear too fast
R and S will stay too long in blood stream
It is too dangerous.
Q is fine because it will not stay too long in blood
stream and enough time for it to travel in the
blood stream B. Q C. R D. S

22. PAST YEARS QUESTION A. J B. K C. L D. M
Which radioisotope is most suitable as a tracer in human body ? A. J
J,K and L are too long.Thus,they will stay longer in
human body.This will cause undesirable effect to
human body. B. K C. L D. M

23. PAST YEARS QUESTION A. 400 B. 200 C. 100 D. 50 E. 25
What is the activity after 1 hour ? A.
400
0 min min min min min B.
200 C.
100 D. 50 E. 25

24. What is the radioisotope that can be used to determine the
PAST YEARS QUESTION
What is the radioisotope that can be used to determine the
age of a fossil ? A.
Radon-222 B.
Uranium-238 C.
Carbon-14 D.
Iodine-131

25. PAST YEARS QUESTION A. 160 g 0 day 160 g B. 90 g 18 days 80 g C. 80 g
Half life of arsenic is 18 days.
After 72 days only 72 g arsenic still remains.
What is the initial mass of arsenic ? A.
160 g
0 day
160 g B.
90 g
18 days
80 g C.
80 g D.
72 g
36 days
40 g
54 days
20 g
72 days
10 g

26. PAST YEARS QUESTION a 2b a
How many alpha and beta particles are emitted in this process ?

27. PAST YEARS QUESTION a. What is meant by half life ?
Time taken by radioactive substances to reduce its activity to
half of its initial value.
b. The initial iodine-131 is 1600 counts per second.
Calculate the time taken for the iodine-131 activity to decrease
to 100 counts per second if half life of iodine-131 is 8 days.
4 half life
Time taken = 4 x 8 days
= 32 days

28. PAST YEARS QUESTION
Diagram shows the radioisotope detected in the kidneys of a patient.
A normal kidney can expel radioisotope in 20 minutes.
a. Which kidney is not function properly ? Give reason for your answer.
Right kidney because the radioisotope still can be detected in the
kidney after 20 minutes.

29. PAST YEARS QUESTION Explain the suitability of
characteristics of radioisotope
to be used in the sterilization of
medical equipments.
Choose the most suitable
radioisotope.Justify your choice.

30. PAST YEARS QUESTION Reason
characteristics
Reason
State of
matter is solid
Easy to handle
Type of ray
is gamma
High penetration
power
Half life is
long
Economical
Therefore,the most suitable
radioisotope is Cobalt-60 because
it is in solid,produces gamma ray and
has a long half life
a. Explain whether gamma ray is suitable for taking photograph of
human organ.
Not suitable because it has high penetrating power and able to kill
the body cells