In short, these arise in coincidence mode when any ADC makes a Dead time signal but no Data Ready before the Data Ready Timeout. The reason can be either a too short Data Ready Timeout, which can be set in the ADC Settings dialog, or noise signals which are larger than the ADC threshold (about 120 mV for 7072 ADCs) but below the Lower Level threshold. In case of noise it is recommended to turn the amplification down to assure that the noise signals are below 120 mV. A Lower Level Threshold higher than 120 mV is not recommended. It is possible to get rid of the zero events by crossing the checkbox "drop zero events" in the listmode box of the data operations dialog, but these events are then eliminated by software only, they are still in the data stream and produce dead time.