In practice, it is observed that the majority of gamma photons emitted have an energy of 511 keV and travel at exactly 180° to each other, regardless of the initial energy of the positron.
When fluorine-18 is used as a tracer, a very small proportion of the gamma photons have energies greater than 511 keV and do not travel at exactly 180° to each other.
When oxygen-15 is used as a tracer, a slightly larger proportion of the gamma photons have energies greater than 511 keV and do not travel at exactly 180° to each other.
(i)
Explain these observations.
[6]
(ii)
Suggest a potential issue in the detection of the gamma rays if the majority of photons possessed energies greater than 511 keV.
[2]