NaI(Tl) Gamma Scintillation detector Voltage

[GetSirius]

I am just getting started experimenting with a modified Bicron 1.5M2.25/1.5L NaI(Tl) probe, using Theremino hardware and software. Just getting started with gamma spectroscopy other than using a Radiacode.

I was told that perhaps I could get a more linear response, not needing as much adjustment with the ThereminoMCA linearizer, if I lowered the voltage to the probe. I lowered from about 730 volts to 650 volts and it appeared that I lost some peaks when testing Am241 from a smoke detector. I did find that the linearizer did not need to be adjusted as far from center positions with the lower voltage.

Is such a thing normal or am I doing something wrong?

Any suggestion on what voltage should be used?

[ColoRad-o]

I ultimately gave up on seeking for very linear response (I assume you mean: how the location of gamma

peaks change as you change bias?
Here are several plots that may answer some of your questions. Perhaps the file Plateaux.pdf is the most important--ignore the Ludlum data (it was a bad detector). You could argue that you should select the bias voltage that keeps the response of the detector system (here, in counts per second, if I remember correctly, for some damn peak or another, presumably using a gamma source) as insensitive to bias voltage as possible. The right hand side of the plateaux plot is the derivative of the response with respect to bias.

Plateaux.pdf

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There was a gain boost within PRA (which I used for all of the above) that has a large impact on locations in the (arbitrary unit) spectrum

EndOfSpectrumWithBias.pdf

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.

Hope this helps!
DMW

[GetSirius]

Thank you for the information. It is helpful. I am finding that my probe is very sensitive to bias voltage. I am trying to get back to the arrangement that produced the "730V" graph with the nice low energy peak, finding that just 10-20 volts at ~730V makes a noticeable difference in peak location, throwing calibration off.

I am not sure the Theremino hardware I am using is all that (temperature?) stable, or my meter system is not stable. In 24 hours with probably a 10-15F drop in room temperature, I measure a 35 volt increase in PMT bias.

I understood soon after first starting to experiment with this probe that peaks move, gain varies, when I change PMT voltage. My main concern when first making this post was how my low energy peak disappeared when I lowered the bias from around 730 to 650 volts. So far, I have not been able to get back to that nice sharp 26KeV peak, it comes up rather wide while the higher peak stays sharp.

I think one thing I need to do is replace the voltage adjust pot on the Theremino board with a 10 turn pot to make adjustment easier and maybe more stable, it is so fiddly with the one turn pot.

[Sesselmann]

Chuck,

Right, linearity generally improves with lower voltage, not sure if this is caused by the PMT or the opamp, but you can test with a Thorium source and find a good compromise.

Here is a spreadsheet I made.

Steven