Physics (AS & A Level) · Oct / Nov 2016 · Paper 41 · Question 12
9702
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Radon-222 (222 86 Rn) is a radioactive element found in atmospheric air. The decay constant of radon-222 is 2.1 × 10−6 s−1.
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12(a)(i)[2]
time for number of atoms/nuclei or activity to be reduced to one half reference to (number of…) original nuclide/single isotope or reference to half of original value/initial activity [2]
12(a)(ii)[2]
A = A0 exp(–λt) and either t = t½, A = ½A0 or ½A0 = A0 exp(–λt½) so ln 2 = λt½ (and ln 2 = 0.693), hence 0.693 = λt½ [2]
12(b)[4]
A = λN N = 200 / (2.1 × 10–6) = 9.52 × 107 mass = (9.52 × 107 × 222 × 10–3) / (6.02 × 1023) or mass = 9.52 × 107 × 222 × 1.66 × 10–27 = 3.5 × 10–17 kg [4]
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