What factors define the upper and lower detection limits of a photomultiplier?

Question

In atomic absorption spectroscopy, the photomultiplier detector is what obviously tells us the wavelength and hence type of metal we are analysing, but I'm not fully aware of the detector's response to photons in different situations. I'd just like to know a few of the factors/conditions under which the detector provides an alternative response ie its upper and lower detection limits.

Answer 1

well the photomultiplier is based on a series of photodynodes that have slightly increasing + potential so that after the photon strikes the first photocathode by the phtoelectrib effect some electrons are discharged these move toward the first dynode which is slightly positive relevant to the incident photocathode collector.THe electrons strike the dynode with more force then previously ( they have ben accelrated by the incerasing + potential ) and dislodge mor electrons that then move to the next dynode at a slightly higher + potential than the previous   sooo there is amplification of the number of electrons.At the exit anode there is additonal amplification so that thre initial signal is multiplied. The sensitivity can be increased by ampliying more voltage across the dynodes ...and by having blue sensitive or red sensitive photomultipliers wherein the energy necessary to dislodge an electron ( the work function ) is lower in the red sensitive and greater in the blue senstive photomultipliers. The  limit to sensitivity is the ability to amplify the signal...and the upward limitatio is that high energy photons like ultravioletinto blue ...may cause more rapid disintegration of the photocathose surface and decrease lifetime of the detector circuit.

Answer 2

Lower detection limit is the noise current of the system.  If you crank the voltage up inside the tube too much you will start to pull electrons off the surface of the last dynode -- even though there are no photon-induced electrons hitting it.  That is a false reading.

The high detection limit is when the dynodes get saturated and no more electrons can be knocked off because they are already all 'gone' (they cannot be replaced fast enough via the bias current power supply).  Now if more photons are coming in the front end, they will not register as a corresponding rise in current.  The current will plateau and not get any higher.

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