A-LevelPhysicsNuclear physics9702/41 · Oct/Nov 2020 · Q129 Marks
Iodine-131 (¹³¹₅₃I) is a radioactive isotope with a decay constant of 9.9 × 10⁻⁷s⁻¹.
- (a)(i)State what is meant by: radioactive[2]
- (a)(ii)decay constant.[2]
- (b)Some water becomes contaminated with iodine-131. The activity of the iodine-131 in 1.0kg of water is 560 Bq. Determine the number of iodine-131 atoms in 1.0kg of water.[2]
- (c)Regulations require that the activity of iodine-131 in 1.0kg of water is to be less than 170 Bq. Calculate the time, in days, for the activity of the contaminated water in (b) to be reduced to 170 Bq.[3]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.
- [B1] unstable nucleus
- [B1] emits ionising radiation or decays spontaneously
- [M1] probability of decay (of a nucleus)
- [A1] per unit time
- [C1] A = λΝ
- [A1] 560 = 9.9 x 10⁻⁷ × N = 5.7 × 10⁸
- [C1] A = A₀ e⁻λt
- [C1] 170 = 560 exp(-9.9 × 10⁻⁷ × t)
- [A1] = 14 days
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