Physics and Engineering of Radiation Detection

6.5. Photodetectors 385

whereNγrepresents the number of photons having wavelengthλ. Substituting

the expressions forIpeandPin equation 6.5.10 yields

Sr =

eNe/t

Nγhc/(λt)

=

Ne

Nγ

eλ

hc

SinceNe/Nγis the quantum efficiencyQE, we can write the above equation

as

Sr = QE

eλ

hc

⇒QE =

Sr

λ

hc

e

=

Sr

λ

(

6. 626 × 10 −^34

)(

2. 99 × 108

)

1. 602 × 10 −^19

≈

Sr

λ

1237 × 10 −^9.

If we take the wavelengthλin units of nanometer (10−^9 m), the above equation

can be written as

QE=

1237 Sr

λ

.

Both radiant sensitivity and quantum efficiency are interchangeably used to char-
acterize the sensitivity of a photomultiplier tube. In practical photomultiplier tubes
used in scintillation detectors, the incident light spectrum is very well known. Most
scintillators produce light with a spectrum that peaks at either blue or green wave-
length. Therefore radiant sensitivity can be effectively used to compare different
photomultiplier tubes to be used with a particular scintillator.

Cathode Luminous Sensitivity

The cathode luminous sensitivity is defined as the average photoelectric current
Ipefrom the photocathode per incident photon flux Φγ from a tungsten filament
lamp operated at a distribution temperature of 2856K.

Sl,c=

Ipe

Φγ

. (6.5.12)