State the meaning of a tracer.
For the decay of oxygen-15, $^{15}_{8}\mathrm{O} \rightarrow {}^{P}_{Q}X + {}^{R}_{S}\beta^{+} + Z$, where X is the nucleus produced in the decay and Z is another particle. State the values of the integers $P$, $Q$, $R$ and $S$.
State the identity of particle Z.
Define the activity of a sample.
Calculate the decay constant of oxygen-15. Include a unit in your answer.
Determine the rate at which positrons are produced in a sample of oxygen-15 with a mass of $2.85 \times 10^{-6}\,\mathrm{kg}$.
During PET scanning, the particles emitted from the body and picked up outside it are not positrons but a different kind of particle. State the name of the particles that are detected.
Explain how these particles are formed inside the body.