Question 25.5: Positron Emission Oxygen-15 is a radioisotope often used in ...
Positron Emission
Oxygen-15 is a radioisotope often used in positron emission tomography (PET scanning; see Chapter 23). This isotope decays by emitting a positron a positive beta particle \beta^{+}, Identify the product nucleus and write this positive beta decay symbolically.
\text { In } \beta^{-} \text {decay, like that of }{ }^{14} C described above, the atomic number Z goes up one to compensate for the emitted negative charge. So in \beta^{+} emission Z must drop by one, in this case from oxygen’s Z=8 \text { to } Z=7 . \text { The positron }\left(\beta^{+}\right) has the same mass as the electron, so again the mass number doesn’t change. Thus the final nucleus has Z=7 \text { and } A=15 Element 7 is nitrogen, so the beta decay of { }^{15} O \text { is }
{ }^{15} O \rightarrow{ }^{15} N +\beta^{+}.
REFLECT { }^{15} N is one of nitrogen’s two stable isotopes, comprising just 0.37% of natural nitrogen.
The rest 99.63% is the common nitrogen-14. Positron emission is useful in medical imaging because the emitted positron soon annihilates with an electron, emitting two oppositely directed gamma-ray photons whose detection then localizes the emission site.