Radioactivity

Chapter 26 MCQs

Radioactivity

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1

Which type of radiation consists of helium nuclei?

A alpha
B beta
C gamma
D X-rays

2

Which type of radiation consists of high-speed electrons emitted from the nucleus?

A alpha
B beta
C gamma
D neutron radiation

3

Which type of radiation is an electromagnetic wave emitted by an unstable nucleus?

A alpha
B beta
C gamma
D proton radiation

4

Which row correctly compares the charges of alpha, beta and gamma radiation?

	alpha	beta	gamma
A	+2	−1	0
B	−2	+1	0
C	0	−1	+2
D	+1	0	−1

5

Which row correctly compares the relative penetrating powers?

	least penetrating	most penetrating
A	alpha	gamma
B	gamma	alpha
C	beta	alpha
D	alpha	beta

6

Which row correctly compares ionising power?

	most ionising	least ionising
A	alpha	gamma
B	gamma	alpha
C	beta	alpha
D	gamma	beta

7

A radioactive source emits alpha, beta and gamma radiation. A sheet of paper is placed between the source and detector.

Which radiation is mainly stopped?

A alpha
B beta
C gamma
D beta and gamma

8

A radioactive source emits alpha, beta and gamma radiation. Paper and aluminium are placed between the source and detector.

Which radiation is most likely still detected?

A alpha mainly
B beta mainly
C gamma mainly
D none, because aluminium stops all radiation

9

Which material is most suitable for significantly reducing gamma radiation intensity?

A thin paper
B a few millimetres of aluminium
C thick lead
D plastic film

10

A beta particle is deflected strongly by an electric field because it:

A has no charge and very large mass
B has charge and relatively small mass
C is an electromagnetic wave
D has twice the charge of an alpha particle

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11

Which radiation is not deflected by electric or magnetic fields?

A alpha
B beta
C gamma
D alpha and beta

12

An alpha particle is emitted from a nucleus.

What happens to the proton number and nucleon number?

	proton number	nucleon number
A	decreases by 2	decreases by 4
B	decreases by 4	decreases by 2
C	increases by 1	unchanged
D	unchanged	decreases by 4

13

A beta particle is emitted from a nucleus.

What happens to the proton number and nucleon number?

	proton number	nucleon number
A	decreases by 1	unchanged
B	increases by 1	unchanged
C	decreases by 2	decreases by 4
D	unchanged	unchanged

14

A gamma ray is emitted from a nucleus.

What happens to the proton number and nucleon number?

	proton number	nucleon number
A	decreases by 2	decreases by 4
B	increases by 1	unchanged
C	unchanged	unchanged
D	unchanged	decreases by 1

15

A nucleus ²³⁸₉₂U emits an alpha particle.

What is the daughter nucleus?

A ²³⁴₉₀Th
B ²³⁴₉₄Pu
C ²³⁸₉₀Th
D ²⁴²₉₄Pu

16

A nucleus ¹⁴₆C emits a beta particle.

What is the daughter nucleus?

A ¹⁴₅B
B ¹⁴₇N
C ¹⁰₄Be
D ¹⁴₆C

17

A nucleus ²¹⁰₈₄Po emits an alpha particle.

What is the daughter nucleus?

A ²⁰⁶₈₂Pb
B ²¹⁰₈₂Pb
C ²⁰⁶₈₆Rn
D ²¹⁴₈₆Rn

18

A nucleus ²³⁴₉₀Th emits a beta particle.

What is the daughter nucleus?

A ²³⁴₈₉Ac
B ²³⁴₉₁Pa
C ²³⁰₈₈Ra
D ²³⁸₉₂U

19

A radioactive nucleus emits one alpha particle followed by one beta particle.

What is the overall change in proton number and nucleon number?

	proton number	nucleon number
A	decreases by 1	decreases by 4
B	decreases by 2	decreases by 4
C	increases by 1	unchanged
D	unchanged	decreases by 4

20

A radioactive nucleus emits one alpha particle followed by two beta particles.

What is the overall change?

A proton number unchanged, nucleon number decreases by 4
B proton number decreases by 4, nucleon number decreases by 4
C proton number increases by 2, nucleon number unchanged
D proton number decreases by 2, nucleon number unchanged

21

A nucleus changes from ²¹⁴₈₂Pb to ²¹⁴₈₃Bi.

Which radiation is emitted?

A alpha
B beta
C gamma
D alpha followed by gamma

22

A nucleus changes from ²²²₈₆Rn to ²¹⁸₈₄Po.

Which radiation is emitted?

A alpha
B beta
C gamma
D neutron

23

A nucleus emits beta radiation. Why does the nucleon number remain unchanged?

A a proton changes into a neutron
B a neutron changes into a proton
C two protons leave the nucleus
D an electron enters from outside the atom

24

A radioactive source has a corrected count rate of 1200 counts/min. Its half-life is 10 minutes.

What is the corrected count rate after 30 minutes?

A 150 counts/min
B 300 counts/min
C 400 counts/min
D 600 counts/min

25

A radioactive source has corrected count rate 960 counts/min. After 45 minutes, the corrected count rate is 120 counts/min.

What is the half-life?

A 5 minutes
B 10 minutes
C 15 minutes
D 22.5 minutes

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26

A sample has initial activity 6400 Bq. Its half-life is 4.0 hours.

What is its activity after 12 hours?

A 400 Bq
B 800 Bq
C 1600 Bq
D 3200 Bq

27

A radioactive isotope has half-life 6.0 days. A sample initially contains 8.0 × 10⁸ undecayed nuclei.

How many undecayed nuclei remain after 24 days?

A 5.0 × 10⁷
B 1.0 × 10⁸
C 2.0 × 10⁸
D 4.0 × 10⁸

28

A radioactive source has half-life 20 minutes. After 80 minutes, its corrected count rate is 25 counts/s.

What was its initial corrected count rate?

A 50 counts/s
B 100 counts/s
C 200 counts/s
D 400 counts/s

29

A detector records 580 counts/min with a source nearby. The background count rate is 40 counts/min.

What is the corrected count rate due to the source?

A 40 counts/min
B 540 counts/min
C 580 counts/min
D 620 counts/min

30

A source has total measured count rate 490 counts/min. Background count is 30 counts/min.

After one half-life of the source, what total count rate is measured?

A 230 counts/min
B 245 counts/min
C 260 counts/min
D 275 counts/min

31

A source has corrected count rate 800 counts/min. After 3 half-lives, background count rate 25 counts/min is added to the detector reading.

What total count rate is recorded?

A 100 counts/min
B 125 counts/min
C 200 counts/min
D 825 counts/min

32

A radioactive sample has corrected count rate 720 counts/min. After 18 minutes, it falls to 90 counts/min.

What is the half-life?

A 3 minutes
B 6 minutes
C 9 minutes
D 18 minutes

33

A radioactive source has corrected count rate 1600 counts/min. After 40 minutes, the corrected count rate is 100 counts/min.

What is the half-life?

A 5 minutes
B 10 minutes
C 20 minutes
D 40 minutes

34

A detector records 300 counts in 60 s from background radiation.

What is the background count rate?

A 0.20 counts/s
B 5.0 counts/s
C 60 counts/s
D 300 counts/s

35

A detector records 900 counts in 3.0 minutes with a source present. Background count rate is 50 counts/min.

What is the corrected count rate due to the source?

A 250 counts/min
B 300 counts/min
C 350 counts/min
D 850 counts/min

36

Why is background radiation measured before testing a radioactive source?

A to add it to the source count rate twice
B to subtract it from the measured count rate
C to increase the activity of the source
D to make the source decay faster

37

Which source is a natural source of background radiation?

A cosmic rays from space
B X-ray machines only
C nuclear power stations only
D medical tracers only

38

Which source is mainly artificial background radiation?

A rocks containing uranium
B cosmic rays
C medical X-rays
D radon gas from rocks

39

A student measures background count several times and obtains different readings.

Why?

A radioactive decay is random
B the detector is definitely broken
C background radiation is always zero
D atoms decay in a fixed repeating order

40

Which statement about radioactive decay is correct?

A the decay of a particular unstable nucleus can be predicted exactly
B radioactive decay is random and spontaneous
C heating a source always halves its half-life
D chemical reactions stop nuclear decay

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41

Which change can make a radioactive source decay faster?

A heating it strongly
B cooling it strongly
C increasing pressure
D none of these ordinary changes

42

Which statement defines half-life?

A time taken for all nuclei in a sample to decay
B time taken for the count rate or activity to halve
C time taken for the nucleon number to halve
D time taken for background radiation to become zero

43

A radioactive source is used as a medical tracer inside the body.

Which property is most suitable?

A very long half-life and alpha emission
B short half-life and gamma emission
C very long half-life and strong beta emission only
D no radiation emission

44

Why is gamma radiation often suitable for medical tracers?

A it is easily detected outside the body
B it is stopped by skin immediately
C it has the highest ionising power
D it contains helium nuclei

45

Which radiation is most dangerous outside the body?

A alpha, because it cannot penetrate skin
B beta, because it has no charge
C gamma, because it penetrates deeply
D radio waves, because they are nuclear radiation

46

Which radiation is especially dangerous if the source is inside the body?

A alpha, because it is strongly ionising over a short distance
B gamma, because it has no ionising effect
C visible light, because it changes nucleon number
D radio waves, because they contain electrons from nuclei

47

A smoke detector uses alpha radiation.

Why is alpha radiation suitable?

A it ionises air strongly but is easily stopped by smoke particles
B it passes through thick lead easily
C it is not ionising
D it is an electromagnetic wave

48

In a smoke detector, smoke enters the detector chamber.

What happens to the ionisation current?

A it increases because smoke makes more ions
B it decreases because smoke absorbs alpha particles/ions
C it remains exactly constant
D it changes into alternating current only

49

A factory uses radiation to monitor paper thickness. The detector reading becomes too low.

What should the control system do?

A reduce paper thickness
B increase paper thickness
C switch to alpha radiation only because it penetrates all paper
D ignore the reading because radiation is never absorbed

50

Which radiation is usually most suitable for monitoring the thickness of aluminium foil?

A alpha
B beta
C gamma
D visible light

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Chapter 26 Answer Key

Radioactivity

Q	Ans	Q	Ans	Q	Ans	Q	Ans	Q	Ans
1	A	11	C	21	B	31	B	41	D
2	B	12	A	22	A	32	B	42	B
3	C	13	B	23	B	33	B	43	B
4	A	14	C	24	A	34	B	44	A
5	A	15	A	25	C	35	A	45	C
6	A	16	B	26	B	36	B	46	A
7	A	17	A	27	A	37	A	47	A
8	C	18	B	28	D	38	C	48	B
9	C	19	A	29	B	39	A	49	A
10	B	20	A	30	C	40	B	50	B

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Detailed Explanations

1. A

Alpha radiation consists of helium nuclei.
An alpha particle has:
- 2 protons
- 2 neutrons
- charge +2
- nucleon number 4

2. B

Beta radiation consists of high-speed electrons emitted from the nucleus.
In beta emission, a neutron changes into a proton and an electron is emitted.

3. C

Gamma radiation is an electromagnetic wave.
It has:
- no mass
- no charge
- very high penetrating power

4. A

Alpha has charge +2.
Beta has charge −1.
Gamma has charge 0.
Correct row = +2, −1, 0

5. A

Penetrating power from least to greatest:
- alpha
- beta
- gamma
Alpha is stopped by paper.
Gamma needs thick lead or concrete to reduce it significantly.

6. A

Ionising power from greatest to least:
- alpha
- beta
- gamma
Alpha is most ionising because it has large mass and charge.
Gamma is least ionising because it has no charge.

7. A

Paper stops alpha radiation.
Beta and gamma pass through paper more easily.
So alpha is mainly stopped.

8. C

Paper stops alpha.
Aluminium stops most beta.
Gamma is most likely still detected.
This is the classic absorption sequence.

9. C

Gamma radiation is highly penetrating.
It requires thick lead or concrete to reduce its intensity significantly.
Paper and thin aluminium are nowhere near enough.

10. B

Beta particles are charged and have very small mass.
Small mass means they are deflected strongly by electric or magnetic fields.
Alpha is also charged but much heavier, so it deflects less.

11. C

Gamma radiation has no charge.
It is not deflected by electric or magnetic fields.
Alpha and beta are charged, so they can be deflected.

12. A

Alpha emission removes:
- 2 protons
- 2 neutrons
Proton number decreases by 2.
Nucleon number decreases by 4.

13. B

In beta emission:
- one neutron changes into one proton
- one electron is emitted
Proton number increases by 1.
Nucleon number remains unchanged.

14. C

Gamma emission changes only the energy of the nucleus.
It does not change:
- proton number
- nucleon number

15. A

Alpha emission:
- nucleon number decreases by 4
- proton number decreases by 2
²³⁸₉₂U → ²³⁴₉₀Th
Answer = ²³⁴₉₀Th

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

16. B

Beta emission:
- proton number increases by 1
- nucleon number stays same
¹⁴₆C → ¹⁴₇N
Answer = ¹⁴₇N

17. A

Alpha emission:
- nucleon number: 210 − 4 = 206
- proton number: 84 − 2 = 82
²¹⁰₈₄Po → ²⁰⁶₈₂Pb
Answer = ²⁰⁶₈₂Pb

18. B

Beta emission:
- nucleon number stays 234
- proton number increases from 90 to 91
²³⁴₉₀Th → ²³⁴₉₁Pa
Answer = ²³⁴₉₁Pa

19. A

One alpha emission:
- proton number −2
- nucleon number −4
One beta emission:
- proton number +1
- nucleon number unchanged
Overall:
- proton number decreases by 1
- nucleon number decreases by 4

20. A

One alpha emission:
- proton number −2
- nucleon number −4
Two beta emissions:
- proton number +2
- nucleon number unchanged
Overall:
- proton number unchanged
- nucleon number decreases by 4

21. B

²¹⁴₈₂Pb → ²¹⁴₈₃Bi
Nucleon number stays 214.
Proton number increases by 1.
This is beta emission.

22. A

²²²₈₆Rn → ²¹⁸₈₄Po
Nucleon number decreases by 4.
Proton number decreases by 2.
This is alpha emission.

23. B

In beta emission:
- a neutron changes into a proton
- an electron is emitted
Total number of nucleons stays the same.
That is why nucleon number does not change.

24. A

Half-life = 10 minutes.
Time = 30 minutes.
Number of half-lives = 30 / 10 = 3.
Corrected count rate:
- 1200 → 600 → 300 → 150
Answer = 150 counts/min

25. C

Count rate falls from 960 to 120.
960 → 480 → 240 → 120
This is 3 half-lives.
Total time = 45 minutes.
Half-life = 45 / 3 = 15 minutes

26. B

Half-life = 4.0 hours.
Time = 12 hours.
Number of half-lives = 12 / 4 = 3.
Activity:
- 6400 → 3200 → 1600 → 800
Answer = 800 Bq

27. A

Half-life = 6.0 days.
Time = 24 days.
Number of half-lives = 24 / 6 = 4.
Remaining nuclei:
- 8.0 × 10⁸ / 2⁴
- = 8.0 × 10⁸ / 16
- = 5.0 × 10⁷

28. D

Half-life = 20 minutes.
Time = 80 minutes.
Number of half-lives = 80 / 20 = 4.
Final corrected count rate = 25 counts/s.
Initial count rate:
- 25 × 2⁴ = 25 × 16
- = 400 counts/s

29. B

Corrected count rate = measured count rate − background count rate
Corrected count rate = 580 − 40
Corrected count rate = 540 counts/min

30. C

Total measured count = 490 counts/min.
Background = 30 counts/min.
Corrected source count = 490 − 30 = 460 counts/min.
After one half-life:
- source count = 460 / 2 = 230 counts/min
Total count = source + background
Total count = 230 + 30 = 260 counts/min

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. B

Corrected count rate = 800 counts/min.
After 3 half-lives:
- 800 → 400 → 200 → 100
Background = 25 counts/min.
Total recorded count = 100 + 25
Total = 125 counts/min

32. B

Count rate:
- 720 → 360 → 180 → 90
This is 3 half-lives.
Total time = 18 minutes.
Half-life = 18 / 3
Half-life = 6 minutes

33. B

Count rate:
- 1600 → 800 → 400 → 200 → 100
This is 4 half-lives.
Total time = 40 minutes.
Half-life = 40 / 4
Half-life = 10 minutes

34. B

Count rate = counts / time
Count rate = 300 / 60
Count rate = 5.0 counts/s

35. A

Measured counts = 900 in 3.0 minutes.
Measured count rate = 900 / 3.0 = 300 counts/min.
Background = 50 counts/min.
Corrected count rate = 300 − 50
Corrected count rate = 250 counts/min

36. B

Background radiation is always present.
It must be subtracted from the measured count rate.
Corrected source count:
- source count = measured count − background count

37. A

Cosmic rays from space are a natural source of background radiation.
Other natural sources include:
- rocks
- soil
- radon gas
- food
- living things

38. C

Medical X-rays are mainly artificial background radiation.
Rocks, cosmic rays and radon are natural sources.
Artificial sources are human-made or technology-based.

39. A

Radioactive decay is random.
Background count readings fluctuate because random decay events occur at irregular times.
This does not automatically mean the detector is broken.

40. B

Radioactive decay is:
- random
- spontaneous
- unaffected by chemical conditions
You cannot predict exactly when one particular nucleus will decay.

41. D

Ordinary physical changes do not affect nuclear decay rate.
Heating, cooling and pressure do not make a radioactive source decay faster.
Half-life is not changed by normal temperature or pressure changes.

42. B

Half-life is the time taken for:
- activity to halve
- count rate to halve
- number of undecayed nuclei to halve
It is not the time for all nuclei to decay.

43. B

A good medical tracer should:
- have a short half-life to reduce dose
- emit gamma radiation so it can be detected outside the body
Alpha is too strongly ionising inside the body.

44. A

Gamma radiation is penetrating.
It can pass out of the body and be detected externally.
This makes it useful for medical tracers.

45. C

Outside the body:
- alpha is stopped by skin
- beta has moderate penetration
- gamma penetrates deeply
Therefore gamma is usually most dangerous outside the body.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

46. A

Inside the body, alpha radiation is very dangerous.
It is strongly ionising over a short distance.
It can damage cells badly if the source is inhaled or swallowed.

47. A

Smoke detectors use alpha radiation because:
- alpha strongly ionises air
- alpha does not travel far
- smoke reduces the ionisation current
Alpha is suitable because it is easy to block and strongly ionising.

48. B

In a smoke detector, alpha radiation ionises air.
This allows a small ionisation current.
Smoke absorbs alpha particles and/or ions.
The ionisation current decreases, triggering the alarm.

49. A

Detector reading too low means too much radiation is being absorbed.
That usually means the paper is too thick.
The control system should reduce paper thickness.
Thickness-control questions are sneaky little gremlins. Low count = too thick.

50. B

Beta radiation is usually suitable for monitoring aluminium foil thickness.
Alpha would be stopped too easily.
Gamma would penetrate too much and may not change enough with thickness.
Beta gives a useful change in detector reading.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Common Traps From This Chapter

Trap	Correct Rule
Alpha radiation	helium nuclei
Beta radiation	high-speed electrons from nucleus
Gamma radiation	electromagnetic wave
Alpha charge	+2
Beta charge	−1
Gamma charge	0
Most ionising	alpha
Most penetrating	gamma
Paper stops	alpha
Aluminium stops	beta
Thick lead/concrete reduces	gamma
Gamma deflection	not deflected
Beta deflection	strongest due to small mass
Alpha emission	proton number −2, nucleon number −4
Beta emission	proton number +1, nucleon number unchanged
Gamma emission	no change to proton/nucleon number
Beta reason	neutron changes into proton
Half-life	time for activity/count rate/nuclei to halve
Corrected count rate	measured − background
Total count rate	corrected source count + background
Decay	random and spontaneous
Heating/pressure	does not change half-life
Medical tracer	short half-life, gamma emitter
Alpha inside body	very dangerous
Gamma outside body	most dangerous
Smoke detector	alpha ionises air; smoke reduces current
Thickness gauge	low count means material too thick
Aluminium foil thickness	beta is usually suitable

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