Encyclopedia of Environmental Science and Engineering, Volume I and II

INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 585

DETECTOR TUBE

POWER SUPPLY

ANODE CATHODE

WINDOW INSULATOR

END SIDE

X-RAYS

• +2 kV

TO

PREAMPLIFIER

X-RAYS GAS

R

FIGURE 38 Structure of the gas-filled x-ray detector (proportional

and Geiger counters, and ionization chamber). The detector is shown to

have both end and side windows for illustration only.

APPLIED POTENTIAL

GAS AMPLIFICATION FACTOR

REGION OF LIMITEDPROPORTIONALITY

GLOW DISCHARGE

REGION

GEIGER COUNTER

REGION

ARC DISCHARGE

DISCHARGE

REGION OF UNSATURATION

PROPORTIONAL

COUNTER

REGION

IONIZATION

CHAMBER

REGION

MIGRATION

1 AVALANCHING

10

105

1010

FIGURE 39 Gas-amplification factor as a function of

applied potential for the gas-filled detector.

The proportional counter region yields a pulse height that

is proportional to the energy of the radiation or particle. The

amplification factor is 500 to 10,000 and the detector has a

dead or non-conducting time of 0.5  sec. Alpha and beta

particles can be measured separately by using two different

voltages and a pure alpha emitter in a proportional counter.

The limited proportional region is not useable because the

presence of secondary charges hinders the gas amplification

process.

In the GM tube region the amplification factor is 10^9 and

the pulse height is independent of the energy and type of

radiation. Therefore pulse height analysis can not be per-

formed using the GM tube but it can be used for general

counting. The plateau is 300 V in length and the counting

rate increases less than 3% for a 100 V increase in applied

voltage. The positive space charge formed in the detector

causes a non-conducting or dead time of 250 microseconds

leading to a loss in radiation events. Serious rate errors are

experienced when counting rates are larger than 10^4 cpm.

A correction to the rate may be calculated using the resolv-

ing time. This detector is useful for the analysis of separated

radionuclides.

(ii) Semiconductor detectors

There are several types of semiconductor detectors,

namely the surface barrier and p-n junction detectors and

the lithium drifted silicon or germanium detectors. They all

function, generally, according to the following model.

Semiconductor detectors are analogous to gas-filled detec-

tors in their principle of operation. In the semi-conductor

detector a radiation event leads to an ion pair formation of an

electron–hole pair, whereas in the gas-filled detector electron–

cation pair are formed. A model of a semiconductor detector,

a lithium drifted detector, is given in Figure 40. A central zone

of ultrapure intrinsic semiconductor is flanked by thin layers

of p and n type semiconductor material. A bias voltage is

imposed across the ensemble to form a high field. The radia-

tion event causes the formation in the central zone of a highly

energetic photoelectron which gives rise to a large number of

electron–hole pairs. The large number of highly mobile elec-

trons are raised to the conduction band due to the transfer of

kinetic energy from the photoelectron. The electrons and holes

“move” to the p and layers, respectively, and are collected under

the effect of the high field giving a current pulse. The size of

the current pulse is proportional to the energy of the radiation

event as in the proportional counter and has a pulse width due

to the randomness of the process as discussed previously.

In the lithium-drifted detectors lithium, an n-type sub-

stance, is used to form the ultrapure intrinsic semiconduc-

tor from p-type silicon or germanium. Ultimately lithium

becomes the doppant for the n-type semiconductor after

several involved processes. These detectors function more

efficiently if the detector and preamplifier are kept at liquid

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