Astronomy Principles and Practice Fourth Edition.pdf

The photomultiplier 295
Figure 18.7. Two arrangements of the dynodes within a photomultiplier. The high voltage is generated by a
stabilized power pack and the resistor chain provides successive dynodes with an increased positive potential.
photomultiplier is in the form of a series of venetian blinds. Both types of photomultiplier are depicted
in figure 18.7.
The amplification, or gain, provided by a photomultiplier depends upon two factors: one is the
number of dynodes or stages; the other is the number of electrons ejected at each stage by the arrival of
a single electron from the preceding stage. Hence, for a typical cell with twelve dynodes, each giving
an application of four, the gain, G,isgivenby
G = 4 × 4 × 4 × 4 ×···
= 4 12
≈ 1·68 × 10 7 .
By using the same figures given in the example of the estimate for the current produced by an
ordinary photocell, the output current provided by a typical photomultiplier receiving 200 photons s −1
is given by
3·2 × 10 −18 × 1·68 × 10 7 A ≈ 5 × 10 −11 A.
A current of this magnitude can be measured easily using subsidiary electronic systems.
The currents likely to be obtained from various telescope systems can be estimated as follows
if it is assumed that the spectral sensitivities of the eye and the photocathode are approximately the
same. Now the pupil of the eye, with a diameter of 8 mm, is capable of detecting the arrival of
200 photons s −1 , this energy being that which is provided by a sixth magnitude star. This is equivalent
to an energy arrival rate of
200 × 4
(8) 2 photons s −1 mm −2 .
π
For a telescope of diameter, D (mm), the energy collected by it is given by
200 × 4 × D 2 π
(8) 2 π × 4
= 200D2
64
photons s −1 .