Physics and Engineering of Radiation Detection

210 Chapter 3. Gas Filled Detectors

whereNinis the number of incident photons andσ^2 mrepresents the standard devi-
ation of measurements. Note that this is not the standard deviation of the number
of incident particles, which is given byσin=

√

Nin.
For a quantum detector, where the charges are not integrated and individual pulses
are counted, this equation is not valid. For such detectors, the detective quantum
efficiency is given by (43)

DQEquant=QE

[

1 −

(QE)(τ)(Nin)

t

] 2

exp

(

−

(QE)(τ)(Nin)

t

)

, (3.8.9)

where, as before,Ninis the number of incident photons,τis detector’s dead time,
and tis the maesurement time.
Looking at equations 3.8.8 and 3.8.9 it becomes apparent that the two types of
detectors have fairly different behaviors. The detective quantum efficiency of an
integrating detector increases with incident photon intensity while the behavior of a
quantum detector is quite the opposite (see Fig.3.8.1). This can also be understood
by an intuitive argument: as the photon intensity increases, more and more photon
pulses arrive within the dead time of a quantum detector, thus decreasing its detec-
tion efficiency. On the contrary, an integrating detector sees more pulses within the
integration (measurement) time and hence itsDQEincreases.

Integrating

Detector

Quantum

Detector

log( )Nin

DQE

QE

Figure 3.8.1: Variation of detective

quantum efficiencies with respect to in-

cident number of photons for integrating

and quantum detectors.

The reader should again be reminded that quantum efficiency sets the physical
limit of any detector. That is why the maximumDQEpossible for integrating and
quantum detectors, according to equations 3.8.8 and 3.8.9, is actually given byQE
(cf. Fig.3.8.1).

Example:

An parallel plate ionization chamber is used to measure the intensity of a 5

keV photon beam. The detector, having an active length of 5cm, is filled

with dry air under standard conditions of temperature and pressure. For an

input number of photons of 10^5 , arriving at the detector within a specific

integration time, the standard deviation of the measurements turns out to

be 150 photons. Assuming that the absorption in the entrance window can

be safely ignored, compute the quantum efficiency and detective quantum