Shielded background scan - some weird peaks?

[kotarak]

Hey guys,
I have a newbie question of what my background should look like?
I just completed my shielding and it is about 23 kg (50 lbs) of lead and some copper foil and it seems to be effective - my background count dropper dramatically from an average of 120-125 cps all the way down to 5-7 cps.
I completed a 9 hr sampling session and I do see a couple of peaks but I am not sure if they should be present - the usual suspect K40 bump is there but there are 2 other peaks that doesn't make much sense to me.
Can someone explain if what I see is normal or the lead is contaminated?



Thank you in advance!

[GigaBecquerel]

Hello Andrey,

The "Eu155" peak is just fluoresence from your lead at around 80 keV.
More copper will reduce that peak, but you'll never get rid of it.
The "Cs134" is most certainly 511 keV from positron annihilation in your lead, that's caused by interaction of cosmic particles. Real antimatter in your detector! Although the peak is a bit annoying I still think it's pretty cool.
The only way to get that out is a cosmic veto or literally tons more lead.
It's impressive that you have no radon background!
That and thorium usually bring the most peaks in an "empty" spectrum.