Summation peaks occur when two gamma-rays are emitted in quick succession, such that they appear to emitted instantaneously. This seemingly instantaneous emission of separate gamma-rays is known as coincidence. In this situation, the detector will see both of those energies as one larger energy deposited in the detector. The result of this is a peak on the spectrum that is larger than any characteristic gamma-ray that a source gives off. A tell-tale sign of a summation peak is a prominent peak that has the same energy as two characteristic gamma-rays of the source.