@Paradigmnoia Thanks a lot for the link regarding the gamma spectra. I like it a lot its a very good summary of the different features of the gamma spectra resulting from gamma emission and I hadn't seen this particular link before.
It is true the beta emission curve looks like a Landau distribution. I suppose for a Bremsstrahlung radiation spectrum from beta decay we would need to integrate the linear electron beam type spectrum over all the energies and intensities of the beta emission spectrum.
In the case of K40 we would get only a few betas with energy around 1311 keV and much more at the lowered of the spectrum with a peak around 100 or 200 keV with an average around 560 keV, so I would not expect a cluster of X-rays around the 1311 keV value from the beta.
I think this would lead to a curve up towards the lower end similar to what has been seen in spectrum 7. It would be interesting to see if someone with better maths skills than me could derive this kind of integration over beta emission energies and see if the curve followed the same rate of increase and over all profile as seen in particular at lower energies.
There may be some interesting deviations at the lower energy. From the beta decay energy graph we see a maximum intensity in the landau type graph at a certain value after which we see it drop quite rapidly as the energy decreases further. From the beta decay example from wiki that I sent I think for a maximum energy in the 1.3 MeV region we would expect a peak intensity around 100 keV or so. I suppose the effect of this on the bremsstrahlung curve would be for the rate of change of increase in intensity to decrease from below the maximum intensity beta energy downwards. Interestingly in the curve for spectrum 7 it almost seems to continue increasing in rate up to the minimum sensitivity value of the spectrum, but maybe any change in rate is difficult to see as it is already increasing very rapidly with lower energy in this region.
I was also curious about the K40 decay… Its one of the few nuclei that have a ground state Q value for beta decay in the 1.3 to 1.4 MeV region. I'm not sure there was any extra Potassium in the device though?
I suppose a similar change in bremsstrahlung curve profile would occur from thermal electrons with a Boltzmann distribution of energies, but I think in this case we would probably see a different profile than that seen at much lower energies and I would be very surprised if we had thermal electrons with a maximum energy of around 1.3 or 1.4 MeV or so.
@Paradigmnoia@Thomas Clarke I'm also curious what Thomas comes up with regarding any possibility of an electrical problem causing the spectra we see. It is certainly true that we seem to see a broad effect over almost the complete spectrum but increasing very much in the low end. Although it looks like Bremsstrahlung of beta to me I think it is certainly very important to be sure there are no other possibilities causing some kind of systematic deviation to the spectra.