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Q31: The activity of a radioactive sample is 1.6 curie and its half-life is 2.5 days. Its activity after 10 days will be:
A) 0.8 curie
B) 0.1 curie
C) 0.4 curie
D) 0.16 curie
Explanation: n=10/2.5=4 → A=1.6×(1/2)⁴=0.1 curie.
Q32: A radioactive element emits 200 particles/s. After 3h, 25 particles/s are emitted. Half-life is:
A) 50 min
B) 70 min
C) 60 min
D) 80 min
Explanation: 200→25 (1/8) in 3h=180 min → 3 half-lives → T½=60 min.
Q33: Half-life of Bi²¹⁰ is 5 days. Start: 50000 atoms. Atoms left after 10 days?
A) 5000
B) 12500
C) 20000
D) 25000
Explanation: n=2 → N=50000/4=12500.
Q34: A sample decayed after 5 half-lives. Remaining part is:
A) (1/32)N₀
B) N₀/31 - N₀/32
C) (31/32)N₀
D) N₀/32 - N₀/31
Explanation: N_D/N₀=(2⁵-1)/2⁵=31/32.
Q35: Relation for half-life of any radioactive element is:
A) T_y=λ ln2
B) λ=T_y ln2
C) T_y=(ln2)/λ
D) T_y=λ/(ln2)
Explanation: T½=(ln2)/λ.
Q36: Half-life of radium=1600 years. Average life is:
A) 3200 yr
B) 2319 yr
C) 4800 yr
D) 4217 yr
Explanation: Ï„=T½/0.693=1600/0.693≈2319 yr.
Q37: Decay constant=0.01/s. Half-life is:
A) 0.693 s
B) 6.93 s
C) 69.3 s
D) 693 s
Explanation: T½=0.693/0.01=69.3 s.
Q38: Ratio of un-decayed to decayed nuclei after 3 half-lives:
A) 1/7
B) 7/8
C) 7
D) 8/7
Explanation: N_R/N_D=(1/8)/(7/8)=1/7.
Q39: Percentage of original material left after 5 half-lives:
A) 3%
B) 5%
C) 10%
D) 20%
Explanation: (1/32)≈3% remains.
Q40: Which one can be preferred for the treatment of skin?
A) β-particles
B) x-rays
C) γ-radiations
D) α-rays
Explanation: γ-rays are used in radiotherapy for skin treatment.
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