Sodium Iodide Detector (NaI) FWHM and applied Voltage

In summary, the voltage applied to a NaI(Tl) detector has no direct effect on the crystal itself. It primarily affects the photomultiplier tube (PMT) connected to the crystal. Higher voltage can improve the collection quantum efficiency (QE) and gain of the PMT, resulting in a stronger signal and decreased FWHM. However, at too high of a voltage, there can be excessive noise and feedback that can worsen the energy resolution. A good fast amplifier is necessary for optimal results, and discriminators do not improve FWHM. The specific PMT used in this discussion is the CANBERRA 2007P.
  • #1
abotiz
72
0
Hi!

I have a question regarding the applied voltage on NaI(Tl) detectors, how does the voltage effect the FWHM ( energy resolution)?

For example, in a HPGe detector, the higher the applied voltage ( to some extent because of the leakage current) the more electron and holes are collected before recombination which results in lower FWHM.

Its seems to me that the voltage applied to a NaI detector has nothing to do with the crystal, but to the PM tube that usually is connected together with the crystal. Right?

What I know:

Photons knock out electron from the crystal and are reflected so they strike the photo cathode and multiply by striking dynodes and the multiplication factor is given by G=V^d, where V is the voltage and d is the number of Dynodes.

So is the answer to my question something like this:

Energy resolution improves with higher applied voltage because you get a stronger signal from the PM tube, but, to some extent because at too high voltage you would amplify electronic noise to much and that would worsen the energy resolution (but couldn't that be fixed with discriminators? )

Or am I missing something?

Thanks!
 
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  • #2
Hi abotiz-
You are correct. The dependence of the FWHM (full width half max) on voltage has nothing to do with the NaI crystal. The collection quantum efficiency (QE) of photoelectrons from the photomultiplier (PMT) cathode improves slightly with voltage, and the gain of the first dynodes. Higher dynode multiplication increases the number of electrons in the pulse and decreases FWHM due to better quantum statistics. Also, higher voltages will improve QE if there is a residual magnetic field. When the voltage is too high, you can get excessive noise (pulses) from thermal emission, and even light (feedback) from the last dynodes getting back to the photocathode. You need a good fast amplifier after the anode output, especially if you are using a 10 stage (or less) PMT. Discriminators do not improve FWHM.

What photomultiplier tube are you using?

Bob S
 
  • #3
Hi Bob S!

Thank you for your reply, I understand the realation between FWHM and voltage now.
The PMT I used was CANBERRA 2007P.
 

1. What is the role of applied voltage in a Sodium Iodide Detector (NaI)?

The applied voltage in a Sodium Iodide Detector (NaI) is crucial for its functioning. It creates an electric field that accelerates the charged particles produced by the interaction of radiation with the detector material. This helps in the efficient collection of the particles and results in a better resolution.

2. How does the applied voltage affect the Full Width at Half Maximum (FWHM) of a NaI Detector?

The applied voltage directly affects the FWHM of a NaI detector. As the voltage increases, the electric field strength also increases, resulting in better particle collection and a narrower peak. This leads to a lower FWHM and better energy resolution.

3. What is the relationship between applied voltage and energy resolution in a NaI Detector?

The applied voltage has a direct relationship with the energy resolution of a NaI detector. As the voltage increases, the energy resolution improves due to better particle collection. However, beyond a certain voltage, the resolution may start to degrade due to excessive noise and other factors.

4. Is there an optimal applied voltage for a NaI Detector?

Yes, there is an optimal applied voltage for a NaI detector. This voltage varies depending on the type and size of the detector, but it is usually in the range of 1000-1500 volts. This voltage provides the best compromise between energy resolution and noise level.

5. How does the FWHM of a NaI Detector change with different applied voltages?

The FWHM of a NaI detector decreases as the applied voltage increases, up to a certain point. Beyond that point, the FWHM may start to increase due to the limitations of the detector. Therefore, it is important to find the optimal voltage for each specific detector to achieve the best possible energy resolution.

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