K40 Confusion - Mislabelled?

[Ben H]

Hi Everyone!

I finally received my GS-USB-PRO and thanks to Karl I have a working detector to use while the one I plan to build finally makes it out of customs...

I've been recording spectra of everything I have, and everything seems to be working great.
This was until my K40 peak seemed to be moving about between samples...

I was recording a spectrum of a thoriated lens, and using the spectrum listed on the website here as a reference:

https://www.gammaspectacular.com/blue/g ... 2-spectrum

It was only after looking at several other ones on the forum that I realise I had K40 confused with Ac228 and this was because my spectrum matched the one linked above.

I believe that this spectrum has mislabelled K40 and calibrated according to that, as the Ac228 peak should be large enough to hide any K40 in background.

I may be wrong... I am new to this, but if I am not then that link should probably updated to stop other newbies getting confused.

I have included my recorded spectrum below and any comments would be welcome.

Ben

th232.PNG (58.18 KiB) Viewed 5891 times

[Sesselmann]

Ben,

The K40 at 1460 keV is normally visible to the left of the AC-228 peak, together they look like a double hump, but if you are using any form of lead shielding or background subtraction the K40 will obviously not be visible.

It probably shouldn't have been shown on my "thorium" sample spectrum, but as it was there I labelled it accordingly.

Steven

[Ben H]

Hi Steven,

You must have a lot of Potassium in your background compared to mine, In my background spectrum you can see the K40 peak at 1460 with calibration but then directly after, recording the Thorium the actinium peak blows the K40 out of the water and the K40 barely visible at all, just a slightly shallower fall off on the left side of the actinium peak.

Ben

[Sesselmann]

Ben,

K40 becomes more prominent in detectors with larger crystals, I can't recall what detector I used for that old spectrum but it might have been my 3" NaI. detector.

You can also turn on the sum qty feature to enhance the higher energy peaks, this effectively shows you energy per channel instead of counts.

Steven

[Go-Figure]

As far as I am concerned, if the sample is hot enough and it doesn't have some K40 in itself, the Potassium peak from my background is dwarfed to insignificance with my 2''x2'' NaI(Tl).
There are three more Th232 progeny peaks I usually identified and I think I can see then in your spectrum as well, two from Ac228 in the 400 keV region and another one from Bi212 at 727 keV.

Massimo