Sodium Iodide Detector (NaI) FWHM and applied Voltage

[abotiz]

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!

 

[Bob S]

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

 

[abotiz]

Hi Bob S!

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