40K Spectrum (Potassium)
The peak from 40K is easily recognizable, and very common to see when analysing low activity samples. 40K is a naturally occurring isotope and exists in all living organisms, bananas and brazil nuts have relatively high concentrations of Potassium. This spectrum was taken using the GS-1100A2 and a Ludlum 44-10, which is a 2" NaI detector. Due to the relatively high energy of the 40K gamma, it is not so easy to see with smaller detectors, so you need at least a 2" NaI to get a good look at it.
The following spectrum of 40K was submitted by Peter Daley, recorded using a GS-1100A and a 2" Alpha Spectra NaI detector and probably better shielding than what was used above. The result is excellent, showing a very pronounced peak at 1460 KeV and a very clear compton continuum. The differences betweeen these two spectra are largely due to the quality of the detector.