A-LevelPhysicsNuclear physics9702/41 · Oct/Nov 2021 · Q127 Marks
Define radioactive decay constant.
- (a)Define radioactive decay constant.[2]
- (b)A sample of radioactive iodine-131 (¹³¹₅₃I) of mass 5.87 x 10⁻¹⁰ kg has an activity of 2.92 x 10⁹ Bq. Determine the decay constant of iodine-131.[3]
- (c)Suggest two reasons why a detector placed near to the sample in (b) would record a count rate much less than 2.92 x 10⁹ counts per second.[2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.
- [M1] probability of decay (of a nucleus)
- [A1] per unit time
- [C1] A = λN
- [C1] N = mass / (nucleon number × u)
- [A1] λ = 1.08 × 10⁻⁶ s⁻¹
- [B1] sample emits radiation in all directions
- [B1] some radiation is absorbed by air/detector window
- [B1] self-absorption within the source
- [B1] dead time/inefficiency of detector
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