pulse pile-up 

https://doi.org/10.1351/goldbook.08755
Processing by a radiation counter of pulses resulting from the simultaneous absorption of independent particles or photons in a radiation detector, resulting in them being counted as one single particle or photon with energy between the individual energies and the sum of these energies.
Source:
PAC, 2021, 93, 69. (Vocabulary of radioanalytical methods (IUPAC Recommendations 2020)) on page 93 [Terms] [Paper]