Physics and Engineering of Radiation Detection

384 Chapter 6. Scintillation Detectors and Photodetectors

A.9 Sensitivity............................

Just like efficiency, sensitivity is also a measure to characterize the effectiveness of
a PMT. In the following we will discuss different types of sensitivities that are gen-
erally quoted by PMT manufacturers.

Radiant Sensitivity

Radiant sensitivity is a measure of the sensitivity of the photocathode and is
therefore closely related to the quantum efficiency we discussed earlier. Mathemat-
ically, radiant sensitivity is defined by the relation

Sr=

Ipe

P

, (6.5.10)

whereIpeis the photoelectric current andPis the incident radiant power. Radiant
sensitivity is also sometimes referred to as the PMTresponsivityand is generally
quoted in units of amperes per watt (A/W). Since the radiant sensitivity depends on
the incident light wavelength therefore most PMT manufacturers prefer to quote it
for the wavelength at which the sensitivity is maximum. However, the best method
to compare different PMTs is to look at their spectral response curves. A spectral
response curve shows the relationship between wavelength of the incident light and
the quantum efficiency or spectral sensitivity of a tube. Unfortunately it is not
practical for the manufactures to obtain such a curve for each tube they manufacture
and therefore they normally quote the radiant sensitivity only at certain wavelengths
together with some other sensitivity parameters that will be discussed shortly.
The reader would readily realize that the definition of radiation sensitivity is similar
to the one of quantum efficiency. This is true since radiant sensitivity and quantum
efficiency are related to each other through the relation

QE=

1237 Sr

λ

. (6.5.11)

where the wavelengthλis in units of nanometer (see the Example below for deriva-
tion).

Example:

Derive equation 6.5.11.

Solution:

We start with equation 6.5.10

Sr=

Ipe

P

.

The ionization currentIpemeasured in timetis given by

Ipe=

eNe

t

,

whereNeis the number of photoelectrons andeis the unit electrical charge.

The radiant powerPabsorbed by the photocathode in timetcan be calculated

from

P=Nγ

hc

λ

1

t

,