Solution
= Solution
Let the incident positron total energy be $E$. For perpendicular photon <momenta>, real positive photon energies require
$(E+m_ec^2)^2-4m_ec^2(E+m_ec^2)\ge0$.
\b[Thus the threshold is $E=3m_ec^2$.]
= Solution
Let the incident positron total energy be $E$. For perpendicular photon <momenta>, real positive photon energies require
$(E+m_ec^2)^2-4m_ec^2(E+m_ec^2)\ge0$.
\b[Thus the threshold is $E=3m_ec^2$.]