386 Chapter 6. Scintillation Detectors and Photodetectors
Anode Luminous Sensitivity
The definition of anode luminous sensitivity is similar to that of cathode luminous
sensitivity. It represents the average anode currentIanodeper incident photon flux
Φγfrom a tungsten filament lamp operated at a distribution temperature of 2856K,
that is
Sl,a=Ianode
Φγ. (6.5.13)
Both cathode and anode luminous sensitivities are generally quoted in dimensions
of amperes per lumen (A/lm).
Blue Sensitivity
Since most scintillators produce blue light therefore sometimes instead of the
luminous sensitivity the manufacturers provide blue sensitivity. The blue sensitivity
is defined as the average photoelectric current from the photocathode produced
per unit flux of blue light. The blue light is conventionally produced by passing
the light produced by a tungsten lamp operating at 2856Kthrough a blue filter.
Mathematically the blue sensitivity is given by
Sblue=Ipe
Φγ. (6.5.14)
Blue sensitivity is usually expressed in amperes per lumen-blue (A/lm-b).
A.10Gain...............................
The gain of a PMT characterizes how effectively it amplifies the incident number of
photons into a large enough signal at the output. It is generally described by the
ratio of the anode current to the photoelectric current.
PMT gain mainly depends on two factors: the number of dynodes and the voltage
applied to each of them. The gain of an individual dynode is described by the so
calledsecondary emission ratio,δ. This ratio has been empirically found to be
proportional to the potential at the dynode raised to a power between 0.7 and 0.8,
that is
δ ∝ Vdyα
= AVdya, (6.5.15)whereVdyrepresents the dynode voltage andα, having a usual value between 0.7 and
0.8, is a constant that depends on the geometric structure, orientation, and material
of the dynode. Ais the proportionality constant. The secondary emission ratio
can be different for each of the dynodes in the multiplication structure of a PMT.
However generally the difference is not very large and can be neglected. Assuming
theδto be the same for allndynodes of the multiplication structure, the overall