BASIC INFORMATION ON PHOTOMULTIPLIER (PM) TUBE USED IN PET
Photomultiplier Tube - What you need to know about.
A photomultiplier (PM) tube is needed to convert the light photons produced in the detector as a result of g-ray interaction to an electrical pulse. The PM tube is a vacuum glass tube containing a photocathode at one end, 10 dynodes in the middle, and an anode at the other end.
The photocathode is usually an alloy of cesium and antimony that releases electrons after absorption of light photons.
The PM tube is fixed on to the detector by optical grease or optical light pipes.
A high voltage of ~1000 volts is applied between the photocathode and the anode, with about 100 - volt increments between the dynodes. When light photons from the detector strike the photocathode of the PM tube, electrons are emitted, which are accelerated toward the next closest dynode by the voltage difference between the dynodes.
Approximately 1 to 3 electrons are emitted per 7 to 10 light photons. Each of these electrons is again accelerated toward the next dynode and then more electrons are emitted.
The process of multiplication continues until the last dynode is reached and a pulse of electrons is produced, which is then attracted toward the anode.
The pulse is then delivered to the preamplifier. Next, it is amplified by an amplifier to a detectable pulse, which is then analyzed for its size by the pulse height analyzer, and finally delivered to a recorder or computer for storage or to a monitor for display.
A photomultiplier tube showing the photocathode at one end, several dynodes inside, and an anode at the other end.
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