Hi Guys,
I stumbled on this unusual behaviour while testing some Hamamatsu R980 PMT's with a regular 1.5" encapsulated NaI crystal. I have never seen anything quite like this with any of the detectors I have made previously. The PMT's appear to work normally up to a certain voltage and then almost like falling off a cliff the the resolution falls to 1.6%.
Would love to know if anyone can explain the physics of what is happening here.
Look at the attached image of the spectra, we can see a slight improvement in resolution each time I increase the voltage by 50V but then from 900 to 950 .... WOW!
Looking at the arb.u I am nowhere near the sound card threshold, so this is happening in the detector and not in the software.
Steven
Strange PMT Behaviour
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Sesselmann
- Posts: 1398
- Joined: 27 Apr 2015, 11:40
Strange PMT Behaviour
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luuk
- Posts: 313
- Joined: 01 May 2015, 21:00
Re: Strange PMT Behaviour
Hello Steven,
One main reason a pmt can get nonlinear behaver is that one(or more) of the dynodes are charged and can get rid of the charge build up.
This can happen for example : when there is for some reason a high resistance in a dynode so it cannot discharge properly(fast enough).
This can happen when the pmt has some mechanical damage internally, caused by a shock.
Or when the divider chain is not build correctly or has a bad connection to one of the dynodes.
You can get nonlinear behavior because of different reasons a good example is; when you use a detector with a very high resistance dynode chain you will see if the radiation is to strong peak a shift in the output of the pmt and then obvious you also get nonlinear behavior because the peak with the highest energy output will shift down de reason the charge on the dynodes is to high and cannot flow properly through the chain, A nice way to get great resolutions but unfortunately it is just fake of course.
In your case I think that maybe a low resistance chain may help you to get the pmt linear again and as you found out the use of a low voltage can also help.
Another test you can do is to expose the pmt/detector assembly when it is in the nonlinear state short to some(day) light while it is still on, we also do that that sometimes when we have problems with charge build up inside a pmt mostly because one of the dynodes is not connected (floating) it will charge and discharge the pmt and give for a short moment a normal spectrum until the dynode is fully charged again and the problem is back again.
Not sure if this has helped you, but it is the best I can think of.
Luuk
One main reason a pmt can get nonlinear behaver is that one(or more) of the dynodes are charged and can get rid of the charge build up.
This can happen for example : when there is for some reason a high resistance in a dynode so it cannot discharge properly(fast enough).
This can happen when the pmt has some mechanical damage internally, caused by a shock.
Or when the divider chain is not build correctly or has a bad connection to one of the dynodes.
You can get nonlinear behavior because of different reasons a good example is; when you use a detector with a very high resistance dynode chain you will see if the radiation is to strong peak a shift in the output of the pmt and then obvious you also get nonlinear behavior because the peak with the highest energy output will shift down de reason the charge on the dynodes is to high and cannot flow properly through the chain, A nice way to get great resolutions but unfortunately it is just fake of course.
In your case I think that maybe a low resistance chain may help you to get the pmt linear again and as you found out the use of a low voltage can also help.
Another test you can do is to expose the pmt/detector assembly when it is in the nonlinear state short to some(day) light while it is still on, we also do that that sometimes when we have problems with charge build up inside a pmt mostly because one of the dynodes is not connected (floating) it will charge and discharge the pmt and give for a short moment a normal spectrum until the dynode is fully charged again and the problem is back again.
Not sure if this has helped you, but it is the best I can think of.
Luuk
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Sesselmann
- Posts: 1398
- Joined: 27 Apr 2015, 11:40
Re: Strange PMT Behaviour
Luuk,
I agree it is a good example of how non linear spectra can fool us. It's easy to check, just take the first peak in this case around ab.u 5.0 divide it by 32 and multiply by 662 and we quickly see that the main peak should have been at 103 arb.u.
I also agree this particular PMT would perform better with lower value resistors.
Steven
I agree it is a good example of how non linear spectra can fool us. It's easy to check, just take the first peak in this case around ab.u 5.0 divide it by 32 and multiply by 662 and we quickly see that the main peak should have been at 103 arb.u.
I also agree this particular PMT would perform better with lower value resistors.
Steven
