True Coincidence Summation occurs when two gamma-rays are emitted during the same decay event of the nucleus, so that they appear to be emitted instantaneously. This seemingly instantaneous emission of separate gamma-rays is known as coincidence. In this situation the detector will see both of the gamma-ray energies as one larger energy deposited in the detector. A tell-tale sign of coincidence is a summation peak that appears at the combined energy of the two characteristic gamma-rays of the source.
The Coincidence Summing Corrections++ application offers the possibility of calculating the coincidence summing correction factors for a large number of radionuclides and for virtually any detector and sample size, density and composition. The results are given gamma-line by gamma-line, with the unaffected lines having a correction factor of 1.
Coincidence Summing Corrections++ wiki page