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Operation of the PMT System
Collecting the blue light; overview, fig. 1 from Doug's drawing.
Converting the blue light; PMT Internal
detail from Do ug's drawing, fig. 2
9–44
Collecting the Blue Light
When the red beam of the LASER strikes the PLATE, blue light is emitted in
random directions. The COLLECTOR and NOSEPIECE are designed to capture
as many of the rays of blue light as possible and direct them toward the face of
the PMTs.
Some of the red laser light striking the PHOSPHOR PLATE is reflected and
enters the COLLECTOR as well. Between the COLLECTOR and the PMTs is a
BLUE FILTER, which rejects most of the red light and passes most of the blue
light so that the PMTs see only the blue light from the PHOSPHOR PLATE.
Converting the Blue Light to Electrical Current
Inside the PMT are a number of elements that are connected to varying levels of
high voltage. The circuitry on the PMT/DAS BD divides the high voltage into a
descending series of voltages. These voltages are connected to various
structures within the PMT. In the illustration, example voltages are given to
illustrate the process.
The high voltage power supply actually produces a high negative voltage. The
PHOTOCATHODE is connected directly to the -600 volt source. The next
structure inside the PMT is the FOCUSING ELECTRODE, which is set to - 500
Volts. Further in are a series of DYNODES, which are set at decreasing voltages
until at the very end is an ANODE which is set at 0 volts.
The PHOTOCATHODE is made of a material that will emit an electron when it is
hit by a light photon. However, since the efficiency of the PHOTOCATHODE is
less than 100% the number of electrons will not be equal to the number of
incoming photons.
The negatively charged electron is influenced by the electric field that exists
between the PHOTOCAHTODE (at -600 V) and the more positive FOCUSING
ELECTRODE (at -500 V); therefore the electron is drawn toward the FOCUSING
ELECTRODE.
As it nears the FOCUSING ELECRODE, it is further drawn by the even more
positive DYNODE. At each DYNODE, the number of electrons increases so that
as the process continues, more and more electrons are added and all the
electrons are attracted to the next more positive DYNODE.
At the end of the PMT, all the electrons are attracted the ANODE which collects
the electrons and sends them to the PMT/DAS BD as a small current signal.
12/12/94 – 1234563
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