Question

Topic: Spectroscopy

Q: Why does the gain of a PMT (Photomultiplier Tubes) change so rapidly (non - linearly) with the voltage between dynodes?

Answer 1

In PMT(photo multiplier tubes), the photoelectrons emitted from a photocathode are accelerated by an electric field to strike the forst dynode and produce the secondary electron emissions. These secondary electrons further impinge on the next dynode to produce additional secondary electron emissions. This process is repeated over successive dynode stages and a high current amplification is obtained.

From this we can observe that a very high current amplification(gain) can be obtained from a small photo electric current from the photo cathode. Usually the number of dynode stages are 9 - 10 and so the anode output varies as high as 6 - 8 times. So, even a small photoelectric output from the photomultiplier tube is susceptible to high fluctuations.