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