Teledyne 200E, Instruction Manual, Page: 189 / 302

Theory of Operation Model 200E Instruction Manual
10.3.2.2. Photo Multiplier Tube (PMT)
The M200E uses a photo multiplier tube (PMT) to detect the chemiluminescence created by
the reaction of NO with O
3
in the reaction cell.
A typical PMT is a vacuum tube containing a variety of specially designed electrodes
(Figure 10-13). Photons from the reaction are filtered by an optical high-pass filter, enter
the PMT and strike a negatively charged photo cathode causing it to emit electrons. A high
voltage potential across these focusing electrodes directs the electrons toward an array of
high voltage dynodes. The dynodes in this electron multiplier array are designed so that
each stage multiplies the number of emitted electrons by emitting multiple, new electrons.
The greatly increased number of electrons emitted from one end of electron multiplier are
collected by a positively charged anode at the other end, which creates a useable current
signal. This current signal is amplified by the preamplifier board and then reported to the
motherboard.
Figure 10-13: Schematic of Basic PMT Design and Functionality
A significant performance characteristic of the PMT is the voltage potential across the
electron multiplier. The higher the voltage, the greater is the number of electrons emitted
from each dynode of the electron multiplier, making the PMT more sensitive and responsive
to small variations in light intensity but also increases random noise (dark noise). The gain
voltage of the PMT used in the M200E is usually set between 450 V and 800 V. This
parameter is viewable through the front panel as test function
HVPS (Section 6.2.1) and
does not need to be changed unless the PMT or the HVPS itself is changed. For information
on when and how to set this voltage, see Section 11.6.5.
The PMT is housed inside the sensor module assembly (Figure 11-7). This assembly also
includes the high voltage power supply required to drive the PMT, an LED used by the
instrument’s optical test function (Section 6.7.5), a thermistor that measures the tempera-
ture of the PMT and various components of the PMT cooling system including the thermo-
electric cooler (TEC).
10.3.2.3. PMT Cooling System
The performance of the analyzer’s PMT is significantly affected by temperature. The warmer
it is, the noisier its signal becomes and significant variations in PMT temperature will render
the concentration signal useless, as it will directly reflect those variations.
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