In optical emission spectrometry, photomultipliers
are commonly used as detectors. They are photocell detectors. The
incident photons coming from the exit slit liberate electrons from
the photocathode and the electron flow is then amplified by a set
of dynodes. The final anode current is proportional to the incident
photon signal received by the photocathode.
The measurement dynamic range is very broad,
i.e. 1015, and sensitivity is high, as the dark current
is low. These detectors allow the detection of low intensities emitted
by trace elements, as well as strong signals from major elements.
They have very fast response times, typically 1-2 ns for a
10%-90% change in signal. The main inconvenience of photomultipliers
is their cost.
There are several types of photomultipliers,
which differ in the nature of the entrance window, either crystal
or fluoride, and in the nature of the sensitive layer on the photocathode.
Some are only sensitive in the far ultraviolet while others are
more sensitive in the visible. The type of photomultiplier to be
used is selected according to the wavelength of the line to be detected.
A fatigue lamp (a small incandescent light source)
is often used with photomultipliers to keep the temperature of the
tube and its associated electronics constant. The fatigue lamp is
switched on when the emission source is off and switched off when
the emission source is on.
Authors: Jean Charles Lefebvre
and Geoff Tyler, Jobin-Yvon Emission, France
First published on the web: 15 January 2000.