Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 587

scintillator, emits light pulses in the range 330 to 400 nm.

Blue sensitive photomultiplier tubes employed in older model

counters necessitated the use of a secondary phosphor which

absorbed the light pulse of the primary phosphor and emitted

a light in the blue wavelength region. POPOP, the 2^0 scintilla-

tor, 2,2 p-phenylenebis(4-methyl-5-phenyl-oxazole), absorbs

at about 360 nm and emits fluorescence light at 410 to 420

nm. Therefore, PPO and POPOP are a good pair to provide an

effective liquid scintillator for the older blue sensitive photo-

multiplier tubes. A diagram of a scintillation counter is shown

in Figure 41.

(2) Signal processors and readout

Radiation detection instruments are able to measure two

characteristics; the number of particles or radiation events

occurring in a unit time and the energy of each particle or

event.

The counting of events is carried out in electronic coun-

tering devices. Counting circuits convert voltage pulses to

square waves, scale or reduce the number of input pulses,

and count the frequencies of pulses using binary digital cir-

cuits. A scaling circuit decreases the frequency of pulses

arriving from the detector by a known fraction in order to

accommodate the counting devices detection rate. Binary

digital circuits containing JK flip-flops accomplish this

reduction. A binary counter with four JK flip-flops produces

one output pulse for every sixteen input pulses. Arrangement

of the binary circuits to provide a decade counting unit pro-

vides a decimal readout.

In ionization chambers, proportional counters, semicon-

ductor detectors, and scintillation counters the size or height

of the pulse is proportional to the energy of the radiation

event. Pulse height analyzers are coupled to the output of

these detectors in order to form an energy dispersive instru-

ment. Pulse height analyzers employed to separate radiation

of differing energies are equivalent to monochrometers used

to disperse uv, vis, and ir radiation. Instruments are also

available that carry out wavelength dispersion. The radiation

is dispersed using a crystal for x- and gamma rays.

A description of the operation of a pulse height ana-

lyzer is as follows: the radiation event is converted to a

signal by the detector and amplified resulting in pulses as

large as 10 volts. A pulse height selector provides a narrow

voltage window by rejecting voltage pulses between mini-

mum and maximum values using electronic discriminator

circuits. This window can have a voltage range of 0.1 to 0.5

volts. A pulse height analyzer can consist of one or several

pulse height selectors. One pulse height selector comprises

a single channel pulse height analyzer. The voltage range of

10 volts can be scanned, automatically or by hand, using a

window of 0.1 volt yielding an energy dispersion spectrum.

Two to hundreds of channels comprise a multichannel ana-

lyzer. Each channel with its own counting circuit is set for a

specific voltage window. Therefore, the total spectrum can

be simultaneously counted and recorded.

Readout of counting rates can be displayed on solid state

devices, printers, etc. Spectra display on a potentiometric

recorder or storage in a computer with subsequent printout

is available.

(3) Applications

Liquid scintillation counting is used for a variety of

tasks: namely; measurement of the low energy beta emit-

ters^3 H,^14 C,^32 P and^35 S; gamma and x-ray energy disper-

sive spectrometry; and beta-gamma coincidence scintillation

counting for^131 I.

Standard Methods^2 has procedures for the analysis of

cesium-134 and 137, and iodine-129 through 135, total radio-

active strontium (^89 Sr and^90 Sr) and^90 Sr alone, tritium, radon-

222, uranium-234, 235 and 238, and total radium, radium-

226, radium-228, and total alpha and beta content of water.

REFERENCES

1. Kieth, L.H., ed. (1988), Principles of Environmental Sampling, Ameri-
   can Chemical Society, Washington, D.C.


2. Amer. Pub. Health Assoc., (1995), Standard Methods for the Examina-
   tion of Water and Wastewater, 19th Ed., Washington, D.C.


3. Mentink, A.F. (1968), Specifications for an Integrated Water Quality
   Data Acquisition System, U.S. Depart. of Interior, FWQO, Washing-
   ton, D.C.; STORET System Handbook (1973), U.S. Environmental
   Protection Agency, Washington, D.C.


4. Ciaccio, L.L., Raul R. Cardenas, Jr. and J.S. Jeris (1973), Automated
   and Instrumental Methods in Water Analysis in Water and Water Pollu-
   tion Handbook, Vol. 4, L.L. Ciaccio, ed., Chap. 27, Marcel Dekker Inc.,
   New York.


5. American Society for Testing Materials (1996), Book of ASTM Stan-
   dards, Vols. 11.01 and 11.02, American Society for Testing Materials,
   Philadelphia, PA.


6. EPA methods: Publications on the use of instruments in the analysis
   water by the U.S. Environmental Protection Agency are too numerous
   to list here. The Agency can supply a list of these publications.


7. Technical Practice Committee (1984), Process Instrumentation & Con-
   trol System, Manual of Practice No. OM-6, Water Pollution Control
   Federation, Washington, D.C.


8. Edmonds, T.E., editor (1988), Chemical Sensors, Blackie & Son,
   London (Chapman and Hall, New York).

Counter

housing

Preamplifier

Lead shield

Position

lock

Crystal

Well

Switch

Phototube

Magnetic

shielding

Internal lead

shielding

Lead shield

Removable lead cao

FIGURE 41 A well-type scintillation counter.

(Courtesy of TN Technologies, Inc.)

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