A-LevelPhysicsNuclear physics9702/42 · May/June 2019 · Q1210 Marks

State what is meant by the binding energy of a nucleus.

(a)State what is meant by the binding energy of a nucleus.[2]
(b)Some masses are shown in Fig. 12.1. Calculate the binding energy of a nucleus of lanthanum-141. binding energy = ................................................................ J[4]
(c)(i)The nuclide lanthanum-141 (¹⁴¹₅₇La) has a half-life of 3.9 hours. Initially, a radioactive source contains only lanthanum-141. The initial activity of the source is A₀. Calculate the time for the activity of the lanthanum-141 to be reduced to 0.40A₀. time = ................................................................ hours[3]
(c)(ii)Suggest why the total activity of the radioactive source measured at the time calculated in (i) may be greater than 0.40A₀.[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

12(a) energy required to separate the nucleons (in a nucleus) M1 to infinity A1 or energy released when nucleons come together (to form nucleus) (M1) from infinity (A1)
12(b) mass defect = 140.911 – (57 × 1.007) – (84 × 1.009) C1 = 140.911-142.155 = (-)1.244 (u) C1 energy = c²(A)m C1 = (3.00 × 108)2 × 1.244 × 1.66 × 10-27 = 1.9 × 10-10 J A1
12(c)(i) A = Aoe-ht and In 2 = At½ C1 0.40 = exp(-In 2 × t/3.9) C1 or (0.5)n = 0.40 (C1) n = 1.32 and t = 1.32 × 3.9 (C1) t = 5.2 hours A1
12(c)(ii) daughter product may be radioactive or random nature of decay B1

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