Calibrating with background

[Sesselmann]

This post is intended for beginners, who may not have access to a source.

I quite often have clients concerned about calibrating their gamma spectrometer, and confused about where to get calibration sources. I usually recommend Spectrum Techniques for sources as they are very professional and ship sources according to international shipping standards. http://www.spectrumtechniques.com

If your only requirement is to calibrate the energy axis, all you really need is background. This spectrum was taken with a 2x2" NaI(tl) detector and a simple GS-1100A, all I did was leave it on my workbench for 2 hours. The K40 peak stands out very clearly and provides your first calibration point, once K40 has been identified it's easy to pick out Bi214 and Ti208, which gives three calibration points with a good spread. If using PRA one would choose the interpolate option for multiple calibration points.

I live in a brick house so some of the peaks may be more obvious, but regardless of where you live in the world, K40 will be present and easy to see.

Background Spectrum

[Peter-1]

Hello Steven,
there is still a good source for a substance that gets Am241 and Cs137 as a calibrator.
On ebay (germany) you can get trinitite.
Peter

[Go-Figure]

I used background for calibration while doing a measurement in a basement bathroom (bathroom tiles never disappoint) where temperature was a bit colder than the room where the initial calibration was done so I noticed some thermal drift.
Since the calibration was already relatively right it was easy to spot all peaks of Lead, Bismuth and of course Potassium therefore I could do a second caibration using several points after which everything was right on the money.
I think that's particularly useful at high energies if (like me) you have only a source of Cesium 137 for calibration which (in my case at least) means the K40 peak is often 15-20 keV off without further calibration.

Massimo