Detector Control and Overload Protection

The detector is placed behind a shutter. The detector output signal is connected to a preamplifier as described in Fig. 7.38. If the safe output current of the detector is exceeded the preamplifier delivers an overload signal, /ovld. The overload signal sets an overload flip-flop that closes the shutter. 

A photodiode in front of the shutter delivers a second overload signal. Thus the overload flip-flop cannot be reset as long as a potential overload situation persists. Of course, the photodiode is far less sensitive than a PMT and therefore not able to detect moderate overload. It does, however, detect a severe overload situation in which even a short opening of the shutter could damage the detector. 

The setup gives a reasonable safety against detector damage. However, it must be noted that it does not give absolute safety. If a microscope lamp is switched on when the shutter is open and the detector is active there is a delay of some milliseconds until the shutter closes. To avoid risk completely, another shutter must be placed in front of the lamp and operated exclusively with the detector shutter. 

A problem associated with shutters is often heat dissipation by the shutter coils. If the shutter warms up the detector, the dark eount rate may increase substantially. Excessive power dissipation of the shutter eoils can be avoided by reducing the coil current. In praetiee a shutter needs to be operated at its full switching current only in the moment when it opens. Some 100 ms later the current can be decreased by 60 to 70%, so that the power eonsumption is substantially reduced. 

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