In the typical case, a desired high-energy interaction or event occurs and is detected by some kind of detector, creating a fast electronic pulse in the associated nuclear electronics. But the desired events are mixed up with a significant number of other events, produced by other particles or processes, which create indistinguishable events in the detector. Very often it is possible to arrange other physical photon or particle detectors to intercept the unwanted background events, producing essentially simultaneous pulses that can be used with fast electronics to reject the unwanted background.