The isotope $P_{15}^{30}$ decays to ${\mathrm{Si}}_{14}^{30}$. The mass number (30) remains the same, but the atomic number decreases by 1 (15 to 14). This indicates a positron emission, where a proton is converted into a neutron, emitting a positron ($e_1^0$) and a neutrino. The nuclear equation is: $P_{15}^{30}\to {\mathrm{Si}}_{14}^{30}+e_1^0+\nu$.

Final Answer: positron emission