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

In summary, the conversation is about the resistor values for a 12 pin Photomultiplier tube. The suggested range is between 100k ohms and 1M ohms for each resistor. The purpose is for secondary electrons on a phosphor in SEM. The string of resistors is chosen to keep the current and heat generated at a value of about 0.1mA. The first resistor is often chosen to be larger to improve resolution for spectroscopic applications.
  • #1
SupaVillain
48
2
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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  • #2
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.
 
  • #3
Yes! That helps greatly, I can make it now and understand how and why it works. Thanks so much.
 

1. What is the purpose of optimizing PMT circuit resistor values in SEM?

The purpose of optimizing PMT circuit resistor values in SEM is to enhance the detection of secondary electrons generated by the sample. This allows for a more precise and accurate imaging of the sample, as the secondary electrons carry important information about its surface.

2. How do resistor values affect secondary electron detection in SEM?

The resistor values in the PMT circuit determine the gain and sensitivity of the detector. By adjusting these values, the signal-to-noise ratio can be improved, resulting in a clearer and more detailed image of the sample.

3. What factors should be considered when choosing resistor values for PMT circuit optimization?

There are several factors to consider when choosing resistor values for PMT circuit optimization, including the desired sensitivity, the type of sample being imaged, the electron beam energy, and the noise level of the system. It is important to find a balance between gain and noise to achieve the best results.

4. Can the same resistor values be used for all samples in SEM imaging?

No, the optimal resistor values may vary depending on the sample being imaged. Different samples may emit secondary electrons at different energies, so the resistor values must be adjusted accordingly to achieve the best results.

5. Are there any other methods for optimizing PMT circuit resistor values?

Yes, besides adjusting the resistor values, other methods such as using different PMT tubes or amplifiers, or implementing signal processing techniques can also help optimize the PMT circuit for secondary electron detection. It is important to experiment and find the best combination of methods for each specific sample and imaging conditions.

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