Optimizing PMT Circuit: Resistor Values for Secondary Electron Detection in SEM

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SUMMARY

The discussion focuses on optimizing resistor values for a 12-pin Photomultiplier Tube (PMT) used in secondary electron detection in Scanning Electron Microscopy (SEM). The recommended resistor values range from 100k ohms to 1M ohms, with a specific emphasis on maintaining a current of approximately 0.1 mA to minimize noise. The first resistor in the series should be 2 to 5 times larger than the subsequent resistors to enhance resolution in spectroscopic applications. This design adheres to guidelines from Hamamatsu's PMT handbook, ensuring effective signal processing with an op-amp at 1000 volts DC.

PREREQUISITES
  • Understanding of Photomultiplier Tube (PMT) operation
  • Knowledge of resistor configurations and Ohm's Law
  • Familiarity with signal processing techniques using operational amplifiers (op-amps)
  • Basic principles of electron detection in Scanning Electron Microscopy (SEM)
NEXT STEPS
  • Research resistor selection for PMT circuits in Hamamatsu's PMT handbook
  • Learn about the impact of resistor values on noise levels in electronic circuits
  • Study the design principles of signal processing with operational amplifiers
  • Explore advanced techniques for improving resolution in spectroscopic applications
USEFUL FOR

Electronics engineers, researchers in photonics, and professionals involved in Scanning Electron Microscopy (SEM) who are optimizing PMT circuits for secondary electron detection.

SupaVillain
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Hello, I have a 12 pin Photomultiplier tube and am wondering how many ohms should be on the resistors. There would be 11 resistors (1 between each pin) and no capacitors, anode grounding for DC operation, 1000volts, only to go off to an op amp for signal processing. This design is shown in Hamamatsu's PMT handbook but resistor values are not shown. They suggest between 100k ohms and 1M ohms for each resistor. Ib = 1000/(r*11). In some other types of circuits the first resistor is a different higher value. Any suggestions would be helpful. Purpose is for secondary electrons on phosphor in SEM.

I chose this part of the forum because of the usual purpose of PMT tubes, although question is for resistor values, more info than simple ohms law is required.
 
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I have some generic and somewhat dated info. The string of resistors is usually chosen to keep the current and thus heat generated by these resistors at a value of about 0.1 mA. Too much current and thus heat will increase the noise level of the signal. Too high of values for the string will will cause dynode to dynode voltage to vary too much if the signal current gets too high. The first resistor between the photo cathode and the first dynode is often chosen two to five time larger than the rest to improve resolution for spectroscopic applications. I hope this helps.
 
Yes! That helps greatly, I can make it now and understand how and why it works. Thanks so much.
 

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