Encyclopedia of Environmental Science and Engineering, Volume I and II

(Ben Green) #1

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°


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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