N.O.R.M. spectra differences?

[RobertD]

Guys, this may be a stupid question:
I went to an abandoned uranium mine in the Czech Republic last weekend and collected some nice rocks from the remaining mining waste there. Some of them are pretty spicy (up to 350 uSv/hr, measured with a Radiacode).
When I took the gamma spectra of these finds at roughly the same cps, I noticed big differences, especially in the lower energy area. Can someone explain the reason for that?
Thanks very much.

P.S. The calibration of my setup is a little bit off.

[NuclearPhoenix]

Could it just be that some of these rock are slightly different kinds of waste? I.e. some contain maybe a bit natural U-235, some contain more Th, and so on. Just a wild guess, but I'd guess one possible reason is that it was waste from different stages of some production process?

[RobertD]

Could very well be, Matthias.
The cleanest spectrum is from a rock with really nice bubble pitchblende:

The second best is from this rock:

The "noisiest" spectrum is from this one, which is also the most spicy one of my finds with around 350 uSv/hr:

BTW, the last one was detected with the Open Gamma Detector from a couple of meters away and was buried around 30 cm below surface.

[Sparky]

The distribution of the radioactive material in the mineral can make a difference. If it is on the surface, you can expect better resolution on the low energy end than you would with a dense, homogeneous material.

[NuclearPhoenix]

The distribution of the radioactive material in the mineral can make a difference. If it is on the surface, you can expect better resolution on the low energy end than you would with a dense, homogeneous material.

Oh yes, I didn't even think about that! That's probably one big reason too, especially at low energies.

[RobertD]

Makes perfect sense, because the non radiactive material in the rock acts as an attenuator for lower energies. That's why it mainly affects the Ra und Pb peaks.

Thanks guys!