| Date | May 2011 | Marks available | 3 | Reference code | 11M.3.SL.TZ2.5 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Define and Determine | Question number | 5 | Adapted from | N/A |
Question
This question is about radioactive decay.
Nitrogen-13 \(\left( {{}_7^{13}{\rm{N}}} \right)\) is an isotope that is used in medical diagnosis. The decay constant of nitrogen-13 is 1.2×10–3s–1.
(i) Define decay constant.
(ii) A sample of nitrogen-13 has an initial activity of 800 Bq. The sample cannot be used for diagnostic purposes if its activity becomes less than 150 Bq. Determine the time it takes for the activity of the sample to fall to 150 Bq.
(i) Calculate the half-life of nitrogen-13.
(ii) Outline how the half-life of a sample of nitrogen-13 can be measured in a laboratory.
Nitrogen-13 undergoes β+ decay. Outline the experimental evidence that suggests another particle, the neutrino, is also emitted in the decay.
Markscheme
(i) probability that a nucleus decays in unit time;
(ii) \(150 = 800{e^{ - 1.2 \times {{10}^{ - 3}}t}}\);
1400s;
(i) 580 s;
(ii) activity/count rate measured at regular time intervals/for at least three half-lives;
plot graph activity/count rate versus time;
detail of determination of half-life from graph;
beta energy spectrum is continuous and associated gamma spectrum is discrete;
difference in energies accounted for by existence of another particle;
or
if another particle not present;
then momentum not conserved in beta decay;
