Hi Charles, cool observation. If I may guess at it:
We know from experience that an atom will make XRF if excited by energy from alpha, beta (or electron stream) Gamma, X-Rays etc. Probably many other ways too.
So if you rely on internal energy to do this, it will do so in response to the available radiation from the rock. Some have LOTS, some have very little, some almost none.
One advantage to that way is all the atoms, even those deep within a big rock are being excited. If those XRF rays can get out depends only on the energy of the ray and the overburden.
Rays above 50 keV have little problem penetrating silicon-calcium-oxygen atoms.
This goes for adding external energy or gathering internally generated XRF.
So your method might show up something that an external exciter misses.
But a good exciter will work on a rock that has no discernable radioactivity, the thing is how radioactive is the rock and how radioactive is does the external exciter have to be to equal or better it.....
The pro units have a need for speed so they have X-Ray tubes, blasting the sample with as much as the law allows. We don't actually have a need for speed so we can use tiny little exciters, but as far as the elements at the surface and a little further
inside are concerned, it is much more "exciting" than the stuff coming from deep inside and spread over the entire surface of the rock.
In other words it may be small, but it is concentrated where it counts, we substitute long run times to get the results we need.
Say on another note, do you have enough Trinitite to do some Gamma Spec runs on it with your Si-PIN? You could precisely calibrate the detector with Am, the see if the X-Rays coming from Trinitite are from Np or U atoms, or both.
Geo
I recently picked up a specimen of melanocerite-Ce at the mineral show and did a scan without Am241 by depending on the internal radiation.? I was not surprised to see Th La Ce and Nd in the blue plot.
Then I wanted to see if I got the same results using Am241 as an additional exciter.? So I carefully moved the specimen just far enough from the detector to allow me to slide in the Am241 jig.? I was careful to keep the same spot on the rock even though
it looked quite uniform.
Curiously I am now getting additional strong Fe and Y peaks in the red plot.? What could account for this?? I am not surprised that the La Ce Nd are lit up as they are.
I have also attached both .mca files in case one wishes to analyze them.? However, I am not really asking for that.? I would just like to understand why the Am241 lights up the Fe and Y so strongly compared to the internal radiation.? It is kind of a pain
to weed through the Am241 background peaks but it appears that I may not get a complete picture with the internal radiation alone.
Charles
