Real-time algorithm for robust coincidence search

In in-beam γ-ray spectroscopy experiments, we often look for coincident detection events. Among every N events detected, coincidence search is naively of principal complexity O(N2). When we limit the approximate width of the coincidence search window, the complexity can be reduced to O(N), permitting the implementation of the algorithm into real-time measurements, carried out indefinitely. We have built an algorithm to find simultaneous events between two detection channels. The algorithm was tested in an experiment where coincidences between X and γ rays detected in two HPGe detectors were observed in the decay of 61Cu. Functioning of the algorithm was validated by comparing calculated experimental branching ratio for EC decay and theoretical calculation for 3 selected γ-ray energies for 61Cu decay. Our research opened a question on the validity of the adopted value of total angular momentum of the 656 keV state (Jπ = 1/2−) in 61Ni.