Encyclopedia of Environmental Science and Engineering, Volume I and II

552 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

across the electrodes, the radiant energy or photons cause

emission of photoelectrons. The photoelectrons are collected

at the anode giving rise to a photocurrent. The photocurrent is

proportional to the power or radiant energy of the light beam

and is independent of the applied potential (see Figure 6).

The eleven chemical compositions of various photoemissive

cathode coatings determine the wavelength range and sen-

sitivity varying from the uv to the near ir spectral regions.

The window in the tube must be transparent to wavelength

of interest. The dark current is a small current flowing when

no light falls on the cathode and is due to thermal energy and

electron emission from potassium-40,^40 K, in the glass tube.

It limits the sensitivity of the detector. Although this detector

has about one tenth the sensitivity of the photovoltaic cell,

its signal may be amplified because of its large internal elec-

trical resistance compared to the photovoltaic detector. The

photoemissive detector is used for higher intensity radiation

and lower wavelength scanning rates than used with other

detectors.

(iii) Photomultiplier tubes^20

A photomultiplier tube contains a photoemissive cathode

followed by a sequential, electron multiplying assemblage

of about nine dynodes (electrodes) as illustrated in Figure 7.

The voltage of each succeeded dynode increases by 75 to

100 volts. Photoelectrons from the photoemissive cathode are

accelerated by the voltage increase of the first dynode caus-

ing the release of several electrons for each impinging pho-

toelectron. This multiplier effect continues as the electrons

Sample

changer Samplespinner Rotation

of detector

Collimators

Detector

Diffracting

crystal

Goniometer

Phototube

Rotation

of crystal

100 kV

power supply

X-Ray tube

Phase

detector

Balance

indicator

Calibrated

attenuator

60-Hz

power

input Cooling water

30-Hz

generator

60-Hz

X-ray

unit

Synchronous

motor

Chopper

Cell

Fluorescent screen

Light collector

Dial

1800

rpm

75°

150°

0°

φ

φ

2

xs

θ 2

θ

(a)

(b)

θ

θ

Amplifier

FIGURE 5 (a) Geometry of a plane-crystal x-ray fluorescence spectrometer.

Note that the angle of the detector with respect to the beam, 2θ, is twice that of

the detector to the crystal face, θ. (Courtesy of Philips Electronic Instruments.)

(b) Nondispersive x-ray absorptiometer. (Courtesy of General Electric Co.)

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