The
magnitude of the pulse from a radiation detector is
given by:
Qsignal
= Eabs
e
G
where Eabs is the energy of the absorbed
ionizing radiation, e is the ionization efficiency of
the detection material, and G is the gain (if any) of
the detector. If the detector is a scintillation
detector, then the quantum efficiency of the photodetector must also be factored in.
As an example, suppose you are detecting
0.662 MeV gamma-rays using a thallium doped cesium
iodide (CsI(Tl)) scintillator coupled to a
photomultiplier tube (PMT). CsI(Tl) has a scintillation
efficiency of 65,000 photons per MeV. You refer to the
specifications of your PMT and discover that the quantum
efficiency of the photocathode in your PMT is 0.15 (15%) at
the wavelength of emission for CsI(Tl) scintillators
(540 nm). Also, the specifications for the PMT indicate that the PMT
gain is 900 when set at the particular
operating voltage you are using. The
signal magnitude is calculated to be the product of
these factors: 5.8 x106 electrons. This
result can be
multiplied by the conversion factor of 1.6 x10-19 to
convert to Coulombs: =9.3 x10-13 Coulombs (=0.93 picoCoulombs).
Another
example is the detection of 60 keV gamma-rays in a
silicon
p-i-n photodiode detector (direct detection with no
scintillator). The ionization efficiency of silicon is
one electronic charge per 3.6 eV
(e
=1 / 3.6eV). p-i-n photodiodes
have only unity gain, so G=1. These factors combine to
form a charge signal of Qsignal
= 1.7 x104
electrons, or 2.7 femtoCoulombs.
Scintillation
efficiencies for various scintillator materials, as well
as ionization efficiencies for a number of semiconductor
materials can be found in Knoll GN, Radiation
Detection and Measurement, 3rd ed., John Wiley &
Sons, Inc. (2000).
