Dear Colleagues,
I own an old NaI scintillation detector, which I do not have the datasheet for. How can I find out the correct operating voltage of the photomultiplier tube without destroying it?
If you have any additional information about the detector, I am also grateful for getting it.
Thank you for your help.
The general rule is to use the lowest possible voltage to extend the life of the pmt. You should test the pmt for counts vs voltage and find the plateau or region where the counts/voltage no longer increases. (not a true plateau, rate increase is lower) Use a voltage near the lowest voltage on the plateau. Typically the plateau starts about 700 V.
Thank you for this profound answer. Unfortunately I do not know if the NaI crystal is still okay so I don't know if the low signal that I get is because of an insufficient voltage or because of a broken crystal. However, I will try.
An old pmt may have electronic problems having nothing to do with the crystal. The crystal can have problems besides cracking.
The photo shows 6 pmts. You should test each individually. Use a small source to produce a better signal than you will get from background.
Dear Oliver,
just a remark different from voltage aspects:
from your photo it seems that the NaJ crystal might be a single cylindrical one having a diameter of about 20 cm and a height of 20 cm too and all six pmts look into the crystal. Large volume NaJ detectors are very sensitive to high energy environmental radiation such as radition from space.
Therefore please do not wonder if you have got strong dark countrate in the order of some 100 cts/s when you operate you detector without being in front of a radiation source.
Dear Oliver,
If you have access to the manufacturer information, I might suggest contacting them about this particular model of detector and operating conditions (ie PMT bias voltages). Do you know whether the PMT is set up to be positive or negative biased?
If that is not possible, I would suggest a conservative, experimental approach. From the photo, your detector looks to be large enough that the cosmic ray rate is large enough for you to see something without a source. However, if you happen to have access to a radioactive source (Fe(55) is good here) the procedure below will work better. There are PMTs that are spec'd to run at a few hundred volts others meant to run up to 2500 volts.
If you PMT is set up to be negatively biased (will produce negative signals and is most likely) you may connect up an oscilloscope. If the PMT is set to be positively biased (will produce positive signals), you may need a high voltage blocking capacitor to connect an oscilloscope. Hook up your oscilloscope (bandwidth >200MHz) to look at the signal from one PMT. If Set a very low triggering threshold (ie, 2mV) for the scope. Then adjust the HV up in 50 - 100 V steps until you see a signal on the scope, record the voltage for that PMT. Repeat for the remaining PMTs. Based on the size of the PMTs, if you don't see a signal by the time you reach 1500V, there may be a problem.
If you're looking at cosmic rays, you may turn on more than one PMT to the voltages previously recorded. Looking at the 2 or more signals on your oscilloscope, you should see occasional coincident signals from two or more PMTs. This would confirm operation of the PMTs. Calibrating the detector is a different matter.
Hope this helps.
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