# Class 12 Physics MCQ – Nucleus – Radioactivity

This set of Class 12 Physics Chapter 13 Multiple Choice Questions & Answers (MCQs) focuses on “Nucleus – Radioactivity”.

1. Radioactive material decays by simultaneous emission of two particles with respective half-lives 1620 and 810 years. The time, in years, after which one-fourth of the material remains?
a) 1080
b) 2430
c) 3240
d) 4860
View Answer

Answer: a
Explanation: N = N0e-(λ1 + λ2) t.
4 = e(λ1 + λ2) t.
t = $$\frac {(2 \times 1620 \times 810)}{(2430)}$$ = 1080 year.

2. Which of the following substances cannot be emitted by radioactive substances during their decay?
a) Protons
b) Neutrinos
c) Helium nuclei
d) Electrons
View Answer

Answer: a
Explanation: Protons are not emitted by radioactive substances during their decay. They are positively charged subatomic particles found in the atomic nucleus. The others can be emitted by radioactive substances during their decay.

3. The electron emitted in β – radiation originates from where?
a) Inner orbits of atoms
b) Free electrons existing in nuclei
c) The decay of a neutron in nuclei
d) Photon escaping from the nucleus
View Answer

Answer: c
Explanation: In β-emission, a neutron of nucleus decays into a proton and a β-particle.
The reaction given as:
0n11H2 + -1e0.
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4. Find the probability that the nucleus of 87Ra221 undergoes decay after three half-lives if it’s a radioactive substance that has a half-life of 6 days.
a) 1/6
b) 5/6
c) 7/8
d) 1/2
View Answer

Answer: c
Explanation: After one half-life, N/2 sample remains and N/2 decays. After two half-lives, N/4 sample remains and N/4 decays out of the remaining N/2 sample. After three half-lives, N/8 sample remains and N/8 sample decays out of the remaining N/4 sample.

So, after three half-lives, in total, N/2 + N/4 + N/8 = 7N/8 decay happens. Hence, the probability that a nucleus undergoes decay after three half-lives is 7/8.

5. A 300-day old radioactive substance shows an activity of 5000 dps, 150 days later its activity becomes 2500 dps. What was its initial activity?
a) 25000 dps
b) 20000 dps
c) 32000 dps
d) 5000 dps
View Answer

Answer: b
Explanation: The expression is given as:
R0 = 4R
4R = 4 × 5000
R0 = 20000dps.

6. Emission of β-rays in radioactive decay results in the change of either mass or charge.
a) True
b) False
View Answer

Answer: b
Explanation: No, this is a false statement. The emission of β-rays can result in changes in the charge of the nucleus but not the mass of the nucleus. Mass of the nucleus is always conserved in radioactive decay.
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7. Which will be the unknown nucleus formed when 22Ne10 decays into two α-particles and an unknown nucleus?
a) Fluorine
b) Carbon
c) Neon
d) Oxygen
View Answer

Answer: b
Explanation: The reaction is given as:
22Ne10 ➔ 2 2He4 + 6X14.
Therefore, the unknown nucleus is 6C14.

8. What is the half-time of a radioactive sample (in minutes), if its mean life is 200 s?
a) 0.69 min
b) 2 min
c) 2.57 min
d) 2.31 min
View Answer

Answer: d
Explanation: T1/2 = 0.693t
T1/2 = 0.693 × 200s
T1/2 = $$\frac {138.6}{60}$$ min
T1/2 = 2.31 minutes
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9. What will happen in a time of 7 hours, if a radioactive substance has an average life of 7 hours?
a) Half of the active nuclei decay
b) Less half of the active nuclei decay
c) More than half of the active nuclei decay
d) All active nuclei decay
View Answer

Answer: c
Explanation: In one average life, i.e. at 7 hours, 63.2 % of the active nuclei will decay. Therefore, in a time of 7 hours, it can be considered that more than half of the active nuclei will decay.

10. A freshly prepared radioactive source of half-time 2h emits radiation of intensity which is 64 times the permissible safe level. Minimum time after which it would be possible to work safely with this source is which of the following?
a) 6 h
b) 12 h
c) 24 h
d) 20 h
View Answer

Answer: b
Explanation: $$\frac {N}{N_0} = \frac {1}{64}$$.
t = n T1/2
t = 6 × 2hours
t = 12hours.

Sanfoundry Global Education & Learning Series – Physics – Class 12.

To practice all chapters and topics of class 12 Physics, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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