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3. A 6-stage photomultiplier tube has a dark current of 2 nA. The secondary emission ratio of each dynode is 10 and the external quantum efficiency of the photocathode is 0.4.+ (1) The electrons produced by the photocathode every second as the background noise.~ (2) The minimum detectable optical power for 600-nm light if the signal-to-noise ratio is 1.~