72.half life concept

Author : Khairi

HALF LIFE CONCEPT

HALF LIFE

Time taken by radioactive substance for its :

Activity of atoms

OR

OR

to decrease to half of its original value

T ½

Mass

Number

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EXAMPLE

A radioactive decay for a radioactive sample

10 January 3000 20 January 3000

Half life,T1/2 = 10 days

HALF LIFE CONCEPT

100 g 50 g

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Time 8.00 p.m Time 12.00 p.m

Half life,T1/2 = 4 hours

HALF LIFE CONCEPT

A radioactive decay for a radioactive sample

EXAMPLE

700/minutes 350/minutes

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HALF LIFE CONCEPT

Half life of a radioactive sample is 2 days.Mass of theradioactive sample on 11th January 3000 is 5 g.What was it mass on 7th January 3000 ?

EXAMPLE

11th January 3000 5 g



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HALF LIFE CONCEPT

Half life of a radioactive sample is 2 days.Mass of theradioactive sample on 11th January 3000 is 5 g.What will it mass on 15th January 3000 ?

EXAMPLE

15th January 3000 1.25 g

13th January 3000 2.5 g


Author : Khairi

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

500

5 days

250

5 days

250/s

HALF LIFE CONCEPT

EXERCISE

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Below is a record of radioactive decay for a sample recorded by a student

What is the half life of the radioactive sample ?

5000 – 3000 = 2000 years

200 g

150 g

100 g

Year 3000

Year 4000

Year 5000

HALF LIFE CONCEPT

EXERCISE

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A radioactive sample has a half life of 3 years.If mass of thesample is 10 g.What was the mass of the sample 15 years ago ?

10 g now

3 years ago20 g

6 years ago40 g

9 years ago80 g

12 years ago160 g

15 years ago320 g

HALF LIFE CONCEPT

EXERCISE

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A sample of radioactive left 25 % residual after 5 hours.What is the half life of the sample ?

100 %

50 %

25 %

T ½

T ½

2T ½ = 5 hours

T ½ = 5/2 hours

HALF LIFE CONCEPT

5 hours

EXERCISE

Author : Khairi

A radioactive isotope took 6 days to decay 75 % of its sample.What was the mass of 5 g sample 9 days ago ?

100 %

50 %

25 %

HALF LIFE CONCEPT

75 % 2T ½ = 6 days

T ½ = 3 days

Now

3 days ago

6 days ago

9 days ago

5 g

10 g

20 g

40 g

EXERCISE

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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 ?

ANSWER CAN NOT BE OBTAINED BY CALCULATION.THE ANSWER ONLY CAN BE FOUND BY USING

HALF LIFE GRAPH.

HALF LIFE CONCEPT

0 day 2 days 4 days 6 day

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Activity

Time/day1 2 3 4 5

50

40

30

20

10

6

HALF LIFE CONCEPT

HALF LIFE GRAPH(DECAY CURVE)

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EXERCISE

If half life of a radioactive sample is 1 hour.What is the activity of 25/s sample after 1.5 hours ?

Activity

Time/hour1 2 3

5

10

15

20

25

HALF LIFE CONCEPT

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Activity

Time/hour2 4 6

100

200

300

400

500

1

What is the half life of the sample ?5 hours

HALF LIFE CONCEPT

EXERCISE

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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 theancient goat.

The actual reading of fresh bone = 2080 – 80 = 2000/sThe actual reading of ancient bone = 1080 – 80 = 1000/s

2000 10005 years

SORRY ! IT IS NOT AN ANCIENT GOAT

HALF LIFE CONCEPT

EXERCISE

Author : Khairi

APPLICATIONS OF HALF LIFE

1. 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.

HALF LIFE CONCEPT

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THE USES OF RADIOACTIVE SUBSTANCE

HALF LIFE CONCEPT

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

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THE USES OF RADIOACTIVE SUBSTANCE

Explain with example the applications of radioactivesubstances in the following fields :

1. medical 2. agriculture 3. industrial

HALF LIFE CONCEPT

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THE HARMFULL EFFECTS OF RADIOACTICE SUBSTANCE

The effects of radioactive depends on :

HALF LIFE CONCEPT

1. Types of radiation

2. Dosage

3. Exposure time

4. Parts of body

Common effects are :

1. Fatigue

2. Vomiting

3. Hair loss

4. Infertility in male

5. Severe skin burn

6. Leukemia

7. Genetic defect - Down syndrome - Klinefelter syndrome - Turner syndrome

8. Death

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PRECAUTION STEPS IN HANDLING RADIOACTICE SUBSTANCE

HALF LIFE CONCEPT

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.

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Which isotope is suitable to detect the blood clotting ?

A. P

B. Q

C. R

D. S

P will disappear too fastR and S will stay too long in blood streamIt 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

PAST YEARS QUESTIONPAST YEARS QUESTION

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Which radioisotope is most suitable as a tracer in human body ?

A. J

B. K

C. L

D. M

J,K and L are too long.Thus,they will stay longer inhuman body.This will cause undesirable effect tohuman body.


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What is the activity after 1 hour ?

A. 400

B. 200

C. 100

D. 50

E. 25

0 min --- 15 min --- 30 min --- 45 min --- 60 min

800 --- 400 ---- 200 --- 100 -------- 50


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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


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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

B. 90 g

C. 80 g

D. 72 g


0 day

18 days

36 days

54 days

72 days 10 g

20 g

40 g

80 g

160 g

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How many alpha and beta particles are emitted in this process ?

α2β

α

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a. What is meant by half life ?

Time taken by radioactive substances to reduce its activity tohalf 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.

1600 ---- 800 ---- 400 ---- 200 ---- 100

4 half life

Time taken = 4 x 8 days

= 32 days


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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 thekidney after 20 minutes.


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Explain the suitability of characteristics of radioisotope to be used in the sterilization of

medical equipments.Choose the most suitable

radioisotope.Justify your choice.


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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 becauseit is in solid,produces gamma ray andhas 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 killthe body cells


Education

72.half life concept