Na-22 decays via B+ decay and electron capture. When it decays via B+, it releases a positron, which is then annihilated via electron-positron annihilation. This annihilation event causes 2 gamma rays of 511 keV to be emitted at 180 degrees angular separation. We can detect the 511 keV gamma ray on the spectrum of Na-22. There's also a 1274 keV peak though. What is the origin of this peak?