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Teledyne API - Model T200H/T200M Operation Manual
8.5.3. PHOTO MULTIPLIER TUBE (PMT)
07270D DCN7141
The T200H/M uses a photo multiplier tube (PMT) (shown in Figure 7-18) to detect the
amount of chemiluminescence created in the sample chamber.
A typical PMT is a vacuum tube containing a variety of specially designed electrodes.
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 8-17:
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 more noisy (dark noise). The
gain voltage of the PMT used in the T200H/M is usually set between 450 V and 800 V.
This parameter is viewable through the front panel as test function HVPS (see Section
6.2.1). For information on when and how to set this voltage, see Section 11.6.3.8.
The PMT is housed inside the PMT module assembly (see Figure 8-18). This assembly
also includes the high voltage power supply required to drive the PMT, an LED used by
the instrument's optical test function, a thermistor that measures the temperature of the
PMT and various components of the PMT cooling system including the thermo-electric
cooler (TEC).
Basic PMT Design
Principles of Operation
291
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