Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 553

contact the succeeding dynodes accelerated by ever higher

voltages. A cascade of a large number of electrons is collected

by the anode of the ninth dynode. The final photocurrent can

be amplified, electronically, before readout. The gain, G, can

be calculated as follows:

G  ( fs )^ n (8)

where fs, the secondary emission factor for each stage,

depends on the dynode emissive coating and n is the

number of dynode stages. Using values for fs of 3 to 10 for

older dynode emissive coatings and 50 for newer coatings

and n equal to 9 results in gains of about 10^4 , 10^9 and 10^15 ,

respectively. The response times can vary from 0.5 to 2

nsec (nanosec, 10^ ^9 sec). The dark current can be decreased

considerably by cooling the photomultiplier detector. Since

the dark current is a fairly constant value it may be sub-

tracted or automatically nulled using a potentiometer. The

Cathode

Phototube

Anode

hn

R^

=^10

6 Ω

E = 1V

1 = 10 –6A

FIGURE 6 Simple phototube circuit.

(Reprinted from Ref. (176), p. 441

by permission of Prentice Hall, Inc.,

Englewood Cliffs, New Jersey.)

Incident radiation

Grill

Shield

Tube envelope

0 = Opaque photocathode

1–9 = Dynode = electron multiplier

10 = Anode

1

1

0

2

2

4

4

3

3

5

5

6

6

7

7

8

8

9

9

10

10

11

Focus ring

Semitransparent

photocathode

Internal conductive

coating

Incident

radiation

Faceplate

Focusing electrode

1–10 = Dynodes = Electron multiplier

11 = Anode (b)

(a)

FIGURE 7 Photomultiplier Design. (a) The Circular-Cage Multiplier Structure in a

Side-on Tube and (b) The Linear-Multiplier Structure in a Head-on Tube. (Courtesy of the

General Electric Company.)

C009_005_r03.indd 553C009_005_r03.indd 553 11/23/2005 11:12:21 AM11/23/2005 11:12:21