Coincidences

Coincidences are used to select the relevant pulses and the irrelevant ones. The purpose of a spark chamber is to see the trajectory of particles going through the whole chamber at once, not only 2 plates out of 25 for example. That's why we are looking f or coincidences between the signals coming from the photomultipliers. In order to do this, we use logic modules to obtain digital pulses out of analog signals and to make the coincidence between the two digital signals.

Discriminators are logic modules which output a digital pulse from an analog signal, as long as the voltage of the input is higher than an adjustable threshold. For our purpose, the threshold is fixed at 50 mV, since the noise generated by the photomultip liers is not higher than 30 mV. The width of the output pulse is also adjustable, which will we be useful for the data acquisition part of the system.

AND Gates are logical devices that output a digital pulse corresponding to a digital "1" (NIM) when both inputs are "1" also. Otherwise, they output a digital "0". It can be used with up to 4 input signals, but here, we need only 2 . The output pulse is the trigger pulse for the high voltage between the plates and is the reference pulse used in the data acquisition system.