Simple Thorium Linearity Check with Excel

[Sesselmann]

I know we have covered this before, but the many peaks all the way up to 2614 makes Thorium a good choice for calibrating and checking the linearity of probes, and it is also easily accessible.

There are a number of things that affect the linearity of a spectrum, PMT, divider, voltage and electronics, but the one you have most control over is the voltage, so to find the optimum voltage for a probe I made myself a simple spreadsheet (attached). Just run a Th spectrum at set voltages and plot the peaks in arbitrary units and repeat this for multiple voltages. The spreadsheet normalises the columns to KeV and plots the points on a chart, making it easy to see.

It doesn't need to be more complicated than this.

[luuk]

Hi Steven,
Nice tool, simple but effective!
I like it.
Luuk

[Svilen]

I did it in some more manual way long ago , but that's handy, thanks for posting it.

[Conor Whyte]

Thank you for the excellent graph. You have now inspired me to go get some Thorium dioxide today. 100g vial. :)

[Geoff]

When changing detector voltage, do you redo the pulse shape in PRA?

[Sesselmann]

When changing detector voltage, do you redo the pulse shape in PRA?

Geoff,

It depends, in the GS-USB-PRO I have the volume and shape trimmer pots interconnected, so changing one also changes the other, however in the earlier GS where the shape was fixed and the gain simply changed the volume it made less of a difference.

Steven

[Geoff]

Linearity? What's that? I've always known that my go-to probe had a problem, I didn't realize how bad it was!

linearity.JPG

specs.JPG

[Sesselmann]

Geoff,

Seems like your probe is struggling with the higher end, but I guess you wouldn't use a 1" detector for higher end gamma anyway.

Most detectors seem to display a convex curve, so it would be pretty neat if we had preamplifier with some kind of adjustable exponential gain, then we could straighten the darn things out :)

Steven