Nuclear Physics Test 1, Past Papers of MDCAT ECAT Physics

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Question 1: 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

Explanation: T₁/₂ = 0.693 × 200 s = 138.6 s = 2.31 min.

Question 2: 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 than half of the active nuclei decay

C) More than half of the active nuclei decay

D) All active nuclei decay

Explanation: In one average life (7 h), 63.2% nuclei decay → more than half.

Question 3: The difference between U²³⁵ and U²³⁸ atom is that

A) U²³⁸ contains 3 more protons

B) U²³⁸ contains 3 more neutrons and 3 more electrons

C) U²³⁸ contains 3 more protons and 3 more electrons

D) U²³⁸ contains 3 more neutrons

Explanation: Isotopes differ in neutrons; U²³⁸ has 3 more neutrons than U²³⁵.

Question 4: Which two nuclei contain the same number of neutrons?

A) ¹²C and ¹⁴C

B) ²³Na and ²⁴Mg

C) ¹⁶N and ¹⁵O

D) ³²Si and ³²P

Explanation: Neutrons in Na = 12, Mg = 12 → equal.

Question 5: What is the ratio of the nuclear densities of two nuclei having mass numbers in the ratio 1:4?

A) 1:1

B) 1:2

C) 1:4

D) 1:3

Explanation: Nuclear density is constant, independent of A → ratio 1:1.

Question 6: The radius for an atom is --- times the radius of the nucleus.

A) 10⁵

B) 10¹⁰

C) 10¹⁵

D) 10²⁰

Explanation: R_atom / R_nucleus = 10⁻¹⁰ / 10⁻¹⁵ = 10⁵.

Question 7: α, β and γ are emitted from a radioactive substance

A) Spontaneously

B) When it is heated

C) When it interacts with particles

D) When exposed to light

Explanation: Radioactive decay is spontaneous & random.

Question 8: The order of penetration power of α, β and γ-rays is

A) α > β > γ

B) α < β > γ

C) α > β < γ

D) α < β < γ

Explanation: Penetrating power: α (least) < β < γ (most).

Question 9: In a given reaction A/Z X → A/(Z-1) Y → (A-4)/(Z-1) K → (A-4)/(Z-2) M, radioactive radiations are emitted in the sequence

A) γ, β, γ

B) β, α, γ

C) γ, α, β

D) α, γ, β

Explanation: Sequence is β (Z+1), then α (A-4, Z-2), then γ emission.

Question 10: When boron ¹⁰B is bombarded by neutrons, α-particles are emitted. The resulting nucleus has the mass number

A) 11

B) 6

C) 7

D) 15

Explanation: ¹⁰B + ¹n → ⁷Li + ⁴He → Li nucleus has A=7.

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